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OH formation dynamics in 193 nm photolysis of 2-methoxyethanol: A laser induced fluorescence study
Chemical Physics 443 (2014) 8–16
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OH formation dynamics in 193 nm photolysis of 2-methoxyethanol: A laser induced fluorescence study Sumana SenGupta, Hari P. Upadhyaya ⇑, Awadhesh Kumar, Prakash D. Naik Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Received 26 May 2014 In final form 17 August 2014 Available online 27 August 2014 Keywords: Laser induced fluorescence Translational energy release Spin–orbit ratio Lambda-doublet ratio Exit barrier Methoxyethanol
a b s t r a c t Dynamics of OH radical formation in the 193 nm photolysis of 2-methoxyethanol is studied using Laser Photolysis–Laser Induced Fluorescence technique. The nascent state distribution of the OH radical is measured. The OH fragments are formed vibrationally cold, characterized by a Boltzmann-like single rotational temperature of 450 100 K. The spin–orbit and K-doublet ratios of OH fragments are measured. The relative average translational energy of the OH channel is determined to be 17.0 3.0 kcal/mol. The experimental studies along with theoretical calculations suggest a complex mechanism for OH formation consisting of at least three pathways. The prominent pathway at shorter timescale (<50 ns) involves crossing over to the nearby repulsive state, whereas, at longer timescale (>1 ms) involves a series of reaction with initial H3C–OCH2CH2OH bond cleavage, followed by rearrangement of OCH2CH2OH to CH2OCH2OH, and a final concerted step to generate OH and ethylene epoxide. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The photodissociation dynamics of small polyatomic molecules following electronic excitation, wherein dissociation occurs on a time scale comparable to that of molecular vibrations, has been an active area of research in recent years. However, photodissociation may also involve complex dynamics, resulting in dissociation on different or often longer time scales in the ground electronic state, which is produced via internal conversion. This situation ensures that the energy acquired in the reactant is statistically distributed. The mechanism and the time scale of the dissociation process in these situations can be predicted with the various statistical theories along with the help of advanced theoretical calculations. Moreover, determination of the energy disposal into the fragments reveals dynamical information about the dissociation pathways, and the nature of the potential energy surface involved in the process. Recently, studies on photodissociation dynamics of small molecules belonging to different class, especially various volatile organic compounds (VOC), have been pursued intensely with an aim to understand their dissociation mechanism in atmosphere. These molecules become even more significant if they generate the OH radical in the dissociation process. In this context we have chosen 2-methoxyethanol (CH3OCH2CH2OH), a similar VOC, for
⇑ Corresponding author. E-mail address: [email protected] (H.P. Upadhyaya). http://dx.doi.org/10.1016/j.chemphys.2014.08.005 0301-0104/Ó 2014 Elsevier B.V. All rights reserved.
the present study which generates OH radical upon excitation in the ultra-violet (UV) region. 2-methoxyethanol is the smallest and simplest member of the series of compounds containing ethylene glycol (H–OCH2CH2OH, EG) unit. This group of compounds containing EG unit is wellknown for its interesting molecular properties and aggregation patterns in solutions [1–9]. Apart from being widely used in the industry as detergents, emulsifiers, or dispersants [10], they are very useful for making surfaces resistant to protein adsorption and cell adhesion [11]. The vibrational spectrum of this compound has been studied both experimentally [12–14] and theoretically [6,12,15,16]. There are a number of reports on the detailed conformational analysis of 2-methoxyethanol in different conditions [12,16–21]. All of them demonstrate that in the ground state several rotational conformations exist, which are quite close in energy. The conformers with intramolecular hydrogen bonding directed from the hydroxyl H-atom to one of the lone pairs of the O-atom of the ether linkage are reported to be the most stable. These conformers are stabilized by about 3.5 kcal/mol compared to the non-hydrogen bonded conformers. Hence, in gaseous phase at low pressure and room temperature, the compound is expected to exist predominantly in the most stable hydrogen bonded form [12]. The hydrogen bonding was found to be strong enough to attenuate solvent influences even at higher dielectric constant (e > 10) [22]. The presence of the strong intramolecular hydrogen bond is expected to play a significant role in the photodissociation dynamics of 2-methoxyethanol, particularly, the reaction channels
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which involve the OH functional group. Earlier, our group has studied the OH formation dynamics in compounds, having intramolecular hydrogen bonding, like enolic acetyl acetone [23] and cyclohexadione [24]. These compounds stay predominantly in the enolic form in the gas-phase due to strong hydrogen bond stabilization. The OH formation dynamics in the photodissociation of these compounds in the UV region are well documented. In almost all of these cases, the nascent OH radical showed the preference for population in a particular spin orbit and K-doublet states. A detailed analysis of these experimental results, in combination with results from theoretical calculations, showed the importance of strong hydrogen bonding, even in the excited states. Also, in some cases, the conversion from a hydrogen bonded to non-hydrogen bonded conformer has been observed in the excited states. Apart from the importance of the hydrogen bond in the ground state, it is interesting to know whether there exists a dynamical constraint due to strong hydrogen bond in the excited states or in the pathways for the OH radical formation in 2-methoxyethanol. In the case of 2-methoxyethanol, with possible involvement of more than one conformers, hydrogen bonded and non-hydrogen bonded, having comparable energies, may introduce complexities in the dissociation mechanism. Apart from imposing dynamical constraints in the potential energy surfaces, the strong intramolecular hydrogen bond can have a more profound impact on the dissociation process by altering the energetics of different reaction channels with respect to each other. This may facilitate the formation of the OH radical by some indirect pathway rather than the simple C–OH bond scission. To address these queries, it is required to have detailed studies on its photodissociation dynamics. Though the H2O elimination channel from ionized 2-methoxyethanol to produce C3H6O+ has been studied extensively [25–29], the photodissociation of the neutral molecule is not well-documented in literature. In the present work, we have studied the dynamics of the nascent OH radical formation in the photodissociation of 2-methoxyethanol at 193 nm, using LP-LIF technique. Detailed theoretical calculations have been carried out for understanding the nature of excitation of this compound at 193 nm, as well as for identifying the intermediates, products and transition states involved in the probable OH reaction channels. To get the complete picture of the photodissociation of 2-methoxyethanol at 193 nm, we have also identified the stable products by FTIR. The comparative discussion on the various reaction channels as well as the detailed characterization of the OH formation dynamics is expected to supplement general understanding on the photoexcitation and photodissociation in the UV region of molecules containing intramolecular hydrogen bonding.
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50 mm, diameter 38 mm) to collect the fluorescence, a photomultiplier tube (Hamamatsu, R 928P) to detect it, and a band pass filter (kcenter = 310 nm, FWHM = 10 nm, %T310 nm = 10) placed between them to cut off the scattering from the photolysis laser. The fluorescence signal was gate integrated by a boxcar (SRS 250), averaged over 30 laser shots, and fed into an interface (SRS 245) and data were collected through a GPIB interface, using a control and data acquisition program. A PC was used to control the scan of the dye laser via an RS232 interface. LIF intensities were normalized with respect to both pump and probe laser energies, to correct for the laser intensity fluctuations. The vapor of the compound was flowed through the reaction chamber at few mTorr level of pressure (10–50 mTorr) ensuring collisionless condition and was photolysed by ArF laser at 193 nm. The OH fragment was probed state selectively by exciting its A2 R X2 P (0, 0) and (1, 1) transition of OH, and monitoring the subsequent A ! X fluorescence. Both the laser beams were unfocussed and attenuated to prevent any saturation effect or multiphotonic event. The LIF intensities of the nascent OH fragments formed in the photolysis of 2-methoxyethanol were found to be directly proportional to the photolysis and probe lasers intensity, and their log–log plot yielded a straight line with a slope of 1.0 0.1, showing that OH formation is a monophotonic process and there is no saturation effect in LIF process. For identification of the stable products of photolysis using FTIR, a stainless steel cell fitted with a pair of CaF2 windows was filled with a known amount of 2-methoxyethanol (5 Torr pressure), and photolysed by 10,000 pulses of 193 nm laser. 3. Results and analysis On photolysis of 2-methoxyethanol at 193 nm, the OH radical is detected by its characteristic LIF spectrum. Though both (0, 0) and (1, 1) transitions of OH were probed, signal could be detected only from the vibrational ground state, v0 = 0, which means that the nascent OH radical formed is mostly vibrationally cold. Fig. 1 shows a part of the fluorescence excitation spectra of the (0, 0) band of the A2 Rþ X2 P system of OH. The standard nomenclature is used for designating the observed transitions [33]. Briefly, the symbols P, Q, and R denote rotational transitions with DN = 1, 0, and 1, respectively. The subscripts 1 and 2 represent the transitions satisfying both DJ and DN, from P3=2 and P1=2 spin orbit states respectively, which form the main branches of the transition, while 21 and 12 represents the transition where only DN is
2. Experimental section The present studies on photodissociation dynamics are carried out in a flow reactor, at mTorr level pressure (10 mTorr), using the Laser Photolysis–Laser Induced Fluorescence setup (LP–LIF). 2-methoxyethanol sample (98% purity, Aldrich) was used after degassing by several freeze–pump–thaw cycles. The experimental set up is the same as that described in our previous papers [23,24,30–32]. Briefly, the photolysis was initiated by an excimer laser (Lambda Physik, Model Compex-102, Fluorine version), and the product OH was probed by a dye laser, with frequency doubling and the mixing module (Quantel, TDL 90), pumped by a seeded Nd:YAG laser (Quantel, YG 980 E-20). The glass reaction chamber had crossed arms at right angles to allow the entrance of pump and probe laser beams to intersect at the center of the chamber. The detection system was attached to the bottom window to capture a view of the intersection volume of the photolysis and the probe lasers. This system consisted of a lens (focal length
Fig. 1. A typical portion of LIF spectrum of OH after photodissociation of 2-methoxyethanol (25 mTorr) at 193 nm photodissociation. (pump–probe delay 50 ns).
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satisfied and are satellite to the main branches represented by subscript 1 and 2. The Q and P/R branches originate respectively from 00 0 K doublet states P (A ) and Pþ (A ) due to the parity selection rule (+ $ ) [34]. The relative populations of the OH fragments are determined by normalizing the area under the peaks of the rotational lines with respect to pump and probe laser intensities, pressure change, if any, and the respective Einstein absorption coefficients [33]. The spin orbit and the K doublet ratios are calculated from the relative populations of different rotational states. The translational energy associated with the OH fragment is calculated from the Doppler profiles of the rotational lines. The detailed results are presented below. 3.1. Rotational energy distribution The nascent rotational state population of OH radicals, generated on photodissociation of 2-methoxymethanol at 193 nm, is used to construct a Boltzmann plot for obtaining the rotational temperature of nascent OH fragments. The normalized area under the curve of a rotational line, which represents the population in that particular rotational state, was used to construct such a plot 00 for OH (v = 0) and shown in Fig. 2. The points could be fitted reasonably well with a straight line. Hence, the rotational distribution can be described by a single rotational temperature 450 100 K. The OH (1, 1) transition was also scanned to deter00 mine the population of the OH fragment in v = 1, if any, but no LIF signal was observed. Based on the experimental detection limit and Frank–Condon factors relative to the OH (0, 0) transition, it is estimated that less than 5% of the total OH yield is formed in the 00 v = 1 state. 3.2. Spin orbit and K doublet ratio The spin–orbit ratios are obtained from the measured ratio of the population of the P3=2 state, probed by P1 and Q1 rotational lines, to that of the P1=2 states, probed by P2 and Q2 rotational lines, after multiplying by the appropriate statistical weights. In Fig. 3, this ratio is depicted by the red circles as a function of the rotational level, N, and its value is close to unity at all the rotational levels up to N = 7, indicating a statistical distribution of the nascent OH radicals in the two spin–orbit levels. The relative population of the spin–orbit states gives a clue on coupling between the initially prepared excited states with a nearby dissociating state. The observed statistical distribution suggests that the initially excited state probably does not interact directly with a triplet dissociating
Fig. 3. The statistically weighted spin–orbit (red filled circle) and K-doublet (blue 00 filled squares) ratios of nascent OH(v =0) as a function of the rotational quantum number (N) at 193 nm photolysis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
state. However, a non-statistical distribution indicates probably a role of a triplet state in the photodissociation process. 00 0 The ratios of populations of the K-doublets P (A ) and Pþ (A ) of OH fragments are obtained from the normalized intensities of the Q and P rotational lines, respectively, and depicted in Fig. 3. The ratio shows a slight decreasing trend with an increase in the rotational quantum number, indicating a nonstatistical distribution. In the high J limit, in the Pþ (A0 ) state, the p lobe lies in the plane of rotation, while in the P (A00 ) state, the p lobe is perpendicular to the plane of rotation. The relative populations of the K-doublets provide information about the exit channel dynamics during the breaking of a chemical bond. The present result implies a role for an exit channel interaction in formation of OH most probably due to presence of H-bonding in the photodissociation process. 3.3. Translational energy of the fragments From the OH Doppler profiles, the average kinetic energy of the OH radical in the laboratory frame, Elab T (OH), can be determined. The Doppler profiles reflect the distribution, f(vz), of the velocity component vz of the absorbing species along the propagation direction of the probe laser beam via the linear Doppler shift Dm ¼ m m0 ¼ vz m0 /c. For an isotropic velocity distribution, f(vz ) = f(vx) = f(vy), the average translational energy in the labora2 2 tory frame is given by Elab T ðOHÞ ¼ 3=2mOH hvZ iOH , where hvZ iOH for a Gaussian Doppler profile, is represented by the equation:
hv2Z iOH ¼
Fig. 2. Boltzmann plots of rotational distributions of the nascent OH radical generated on 193 nm photolysis. The solid red line is the fit to the data points. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1 2 ln 2
2 FWHM c2 2m0
ð1Þ
where, c is the speed of light and FWHM is the full width at half maximum of the normalized Doppler profile of the OH radical with transition at m0 . In the present case, Doppler profile is a Gaussian function as depicted in Fig. 4 for the P1(2) line, which implies that the translational energy follows the Maxwell–Boltzmann distribution. The width and the shape of the Doppler broadened LIF line include contributions from the fragment molecular velocity, the thermal motion of the parent molecule and the finite probe laser line width. Thus, the actual Doppler width is calculated using deconvolution procedure using the width (FWHM) of the laser spectral profile of the probe laser beam, which is obtained from the OH Doppler profile measured in a thermalized condition. All the rotational lines exhibit the same width within an experimental error. More than 50 rotational line profiles were evaluated to estimate the average kinetic energy of the OH fragment. For a two-body
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alcohol (CH3OH) and ethylene epoxide with an endothermicity of 28.9 kcal/mol. However we were unable to locate the transition state for this molecular rearrangement. Also, we were able to locate the transition state for the elimination of methanol with co-product acetaldehyde (4b) with the lowest barrier and DH value. H3 COCH2 CH2 OH ! CH3 OH þ cyclic-CH2 OCH2 DH ¼ 28:9 kcal=mol ð4aÞ ! CH3 OH þ CH3 CHO DH ¼ 2:3 kcal=mol
ð4bÞ
Though the DH values of all these molecular elimination channels are not very high, these channels may involve multi steps with high activation barriers. However, we expect that all these channels are possible at 193 nm with 148 kcal/mol of photon energy. 3.5. Absorption spectra, absolute cross-section and quantum yield measurements Fig. 4. Doppler profile of the P1 (2) line (blue filled circles) in the spectrum for 248 nm photodissociation of CHD. The red line is the Gaussian fit to the data points. The dotted green line represents instrument function that includes laser band width also. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
direct dissociation mechanism, the average kinetic energy in the centre-of-mass (CM) frame, ECM T (OH) is found to be 17.0 3.0 kcal/mol. The fraction of the available energy released as translational energy, f T ¼ ECM T /Eavail, is calculated as 0.33. 3.4. Stable product analysis The stable product analysis of photodissociation of 2-methoxyethanol at 193 nm was done by FTIR absorption technique, and CH3OH (methanol), H2C = CH2 (ethylene) and H2C = O (formaldehyde) could be detected. These products can be formed by a number of low energy channels opening up from the ground state of 2-methoxyethanol depending upon their respective relative energy. Molecular orbital (MO) calculations were carried out to generate the relative potential energy diagram for the dissociation channels, of 2-methoxyethanol on its ground state, accessible on laser excitation at 193 nm. Calculations were done at G3B3 level of theory, using Gaussian suite of program for the DH evaluation. The transition state geometries were optimized at HF/6-31 + G⁄ and the barrier heights were calculated at QCISD(T)/6-311++G⁄⁄ level of theory. All the stationary points were calculated in the ground state, for the various dissociation channels. A number of such channels are presented in Fig. 5 with their respective barrier heights and the transition states. Our calculations show the elimination of water, having DH of 8.5 kcal/mol (reaction 2).
H3 COCH2 CH2 OH ! H2 O þ CH2 CHOCH3
DH ¼ 8:5 kcal=mol
ð2Þ
Elimination of formaldehyde (reaction 3) also has a low endothermicity of 18.6 kcal/mol and 6.8 kcal/mol with respective co-product dimethyl ether (3a) and ethanol (3b).
H3 COCH2 CH2 OH ! CH2 O þ CH3 OCH3
DH ¼ 18:6 kcal=mol ð3aÞ
! CH2 O þ CH3 CH2 OH DH ¼ 6:8 kcal=mol ð3bÞ Both these channels, though less endothermic, can have higher energy transition states in their potential energy surfaces. From our theoretical calculations, we could locate another possible molecular elimination channel leading to stable products (reaction 4). In this channel, the 2-methoxyethanol molecule undergoes intramolecular rearrangement, where the –CH3 group migrates from the O-atom of the ether linkage to the O-atom of the terminal –OH group, which breaks free from the rest of the molecule, and simultaneously, the O-atom of the former ether linkage binds with the now free –CH2 group. This results in the formation of methyl
The absorption spectra were measured using a commercial uv spectrophotometer at different pressures of 2-methoxyethanol, and presented in Fig. 6 with its absolute cross-section. To verify it further, the absorption cross-sections for 2-methoxyethanol at 193 nm was measured using a 50-cm long absorption cross-section cell. For this purpose the ratios of intensities, I/I0, were measured, where I is the laser intensity with the sample and I0 without the sample in the cell. The sample pressure in the cell was measured with a capacitance manometer. The laser light was attenuated heavily by suitable substrate window, and was collimated by the iris to get a very low-intensity laser beam (<1 lJ). The intensity of the laser beam was measured by photodiodes (Becker and Heckl, PDM-400). The absorption cross-section of 2-methoxyethanol at 193 nm thus measured was found to be (1.3 0.3) 1019 cm2 molecule1. This value was used for the determination of the quantum yield of OH formation channel from 2-methoxyethanol on excitation at 193 nm. We calculated the quantum yield of the OH generation from 2-methoxyethanol by a relative method. The concentration of OH produced in 193 nm photolysis of 2-methoxy193 ethanol is proportional to, ½OHME / r193 abs ðMEÞ ½ME UOH ðMEÞ, where r; U and [ME] are the absorption cross-section, quantum yield and concentration of 2-methoxyethanol, respectively. Similar equation can be written for a reference compound also. In the present study, acetic acid was taken as a reference compound with the measured values of the absorption cross-section and the quantum yield of OH generation channel to be 1.1 1019 cm2 molecule1 and 0.8, respectively, which was determined in our earlier study [31]. Hence, the quantum yield for ME photolysis for the OH channel, using acetic acid as a reference, is given by
U193 OH ðMEÞ ¼
r193 ½acetic acid ½OHME abs ðacetic acidÞ ½ME ½OH r193 ðMEÞ acetic acid abs U193 OH ðacetic acidÞ
ð5Þ
The ratio [OH]ME/[OH]acetic acid was determined from the area of a particular rotational line of OH in thermalized condition, which ensure the same rotational temperature of the OH radical formed in the photolysis of both the compounds. In the present study, the quantum yield determined to be 0.20 0.07 for the OH formation channel in the 193 nm photolysis of 2-methoxyethanol. 4. Discussion 4.1. Nature of the ground state and the excited states Ab initio molecular orbital (MO) calculations are performed to investigate the potential energy surface (PES) of the ground and excited electronic states of the most stable conformer of 2-methoxyethanol, using the Gaussian suite of program. A number of different conformers can exist in the ground state of the
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Fig. 5. Relative energy diagram in kcal/mol, along with the structures, for the various transition states and products in the photodissociation of 2-methoxyethanol at 193 nm.
Fig. 6. uv absorption spectrum of 2-methoxyethanol at room temperature.
2-methoxyethanol molecule, and intramolecular hydrogen bonding plays a significant role in the relative stabilities of these conformers. Ab initio MO calculations found out that in the most stable conformer there exists a strong hydrogen bond between the H-atom of the hydroxyl moiety and O-atom of the ether linkage [12,15]. The hitherto reported calculation were done at MP2/631G⁄ level of theory. The difference in energy between the most stable hydrogen-bonded conformer and the non-hydrogen bonded conformer is 5.7 kcal/mol. We have optimized and calculated the relative energies of different conformers of 2-methoxyethanol in the ground state at G3B3 level of theory, and arrived at a similar lowest energy hydrogen bonded conformer (see Fig. 7). At G3B3 level of calculation, the difference between the hydrogen bonded and the non-hydrogen bonded conformers was found to be 4.0 kcal/mol. Hence, it is inferred that in the vapor phase, the 2-methoxyethanol exist predominantly as hydrogen-bonded conformer. The gas phase UV spectrum of 2-methoxyethanol shows onset of optical absorption at 205 nm (6.0 eV, see Fig. 6). To understand the nature of the transitions responsible for the optical absorption spectra, ab initio MO calculations were performed, in detail. We optimized the ground state geometries of the most stable hydrogen-bonded conformer of 2-methoxyethanol, at MP2/6-311++G⁄⁄
level of theory. The augmented basis sets, with diffuse and triple zeta functions, namely, aug–cc–pVTZ, were then used for obtaining the vertical excitation energies for various transitions, using timedependent density functional theory (TD-DFT) with MPW1PW91 correlation and exchange functionals. Although the calculated vertical transition energies slightly differ as compared to the experimental results, the nature of transitions and that of the orbitals involved are accurately predicted, using this method. The orbitals participating in different electronic transitions were visualized for better understanding of the process, and can be seen in Fig. 7. The vertical excitation energies and the respective oscillator strengths of several low–lying singlet and triplet states are calculated, and a total of four singlet excited states and two triplet states is considered. These states are primarily due to excitations from the n (HOMO-1) and n (HOMO) orbitals, to 3s orbitals which are LUMO, LUMO+1 and LUMO+2. The two triplet states, T1 and T2, are mainly Rydberg state at 6.46 eV (192 nm) and 6.58 eV (189 nm), respectively. The T1 state involves the transition from HOMO (non bonding electrons of oxygen at the ether linkage) to LUMO and the T2 state involves that from HOMO-1 (non bonding electrons of oxygen at the OH moiety) to LUMO. The S1 state is mainly a Rydberg state with considerable oscillator strength (0.0225) at 6.59 eV (188 nm). The transition is from HOMO (non bonding electrons of oxygen at the ether linkage) to LUMO. The S2 state is also Rydberg in nature with the oscillator strength of 0.0092, at 6.77 eV (183 nm). This state involves the transition from HOMO-1 to LUMO. The other singlet states, namely S3 and S4 are Rydberg states too with vertical excitation energies of 7.08 eV (175 nm) and 7.12 eV (174 nm) and oscillator strengths of 0.0165 and 0.0055, respectively. Considering corresponding wavelengths for vertical excitation energies and the respective oscillator strengths of various transitions, it is evident that at 193 nm, 2-methoxyethanol is excited to Rydberg state, which involves the non-bonding electrons of oxygen atom at the ether linkage. The Rydberg state of the molecule can also be calculated using the Rydberg formula En,l = IP ½13:606=ðn dl Þ2 ], where IP is ionization potential, n = 3 for 3 s Rydberg transition, and dl is l-dependent quantum defect, typically dl = 1 for s, 0.6 for p and 0.1 for d Rydberg transition. The ionization potential of 2-methoxyethanol is estimated using G3B3 method to be 9.54 eV. The approximate position of the origin for the first Rydberg transition involving a 3 s as the upper state is about 6.14 eV (202 nm).
S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
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Fig. 7. Computed MOs involved in the transition of 2-methoxyethanol. The green and the red color of the MO lobes represent two opposite phases of the MO wavefunction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
This value matches reasonably well with our calculated value of 6.59 eV (188 nm) using TD-DFT method, considering the fact that this method gives vertical transition energy whereas the Rydberg formula gives the minimum energy. 4.2. Average energies of the fragments The partitioning of the available energy into various degrees of freedom of the fragments is mainly governed by the nature of transition state and its location on the dissociative potential energy surface. Dissociation of 2-methoxyethanol at 193 nm occurs with a considerable amount of translational energy (ECM T = 17.0 3.0 kcal/ mol, f T = 0.33) being imparted into the OH channel. Since the statistical model predicts well the energy partitioning of a dissociation process only on the ground state with no barrier, we have employed different models, e.g. impulsive model, to understand the dissociation process at different wavelengths. In the statistical model, the available energy, Eavl, is distributed among all the accessible states with equal probabilities under the constraint of conservation of energy. A statistical dissociation process occurs predominantly, if the photo-excited parent molecule is so longlived that the excess energy is partitioned statistically amongst the available degrees of freedom of the products. This includes a molecular process where a rapid internal conversion to the ground electronic state takes place, followed by slow dissociation. Under these events, in a large molecule with many low frequency modes, a relatively small amount of the excess energy is partitioned into translational motion of the products. For this kind of dissociation process, a priori calculations [35–37] were adopted, along with a simple analytical expression established by Klots [38], relating the mean translational energy release, ET, and the Eavail, for a statistical barrierless dissociation process. The statistical model puts 5% of the available energy into the relative translational mode of the photofragments. Thus, the measured f T value of 0.33 is underestimated using statistical model (f T = 0.05). Hence, the statistical model fails to explain the observed partitioning of the available energy. The large amount of translational energy released into the OH photofragment indicates the dissociation is either from a repulsive state or across a barrier. If the dissociation takes place from a repulsive state the energy partitioning can be explained by an impulsive model. In the impulsive model, the distribution of energy among product states is governed by the dissociative
event, i.e., by the repulsive force acting during the breaking of the parent molecule into the products. The impulsive model puts 52% of the available energy into the product translational mode. Like the statistical model, the impulsive model is also unable to explain the experimentally obtained translational energy partitioning between the fragments. However, both the statistical and impulsive models allocate low rotational and vibrational energies into the OH radical. Thus, the failure of both the statistical and impulsive models in explaining the partitioning of the available energy implies the dissociation process may have an exit barrier. This observation prompted us to investigate the ground state PES in detail, which will be discussed in the following section. 4.3. Mechanism of the dissociation process Since the parent compound, 2-methoxyethanol, contains an OH moiety in its structure, there is a possibility of formation of the OH radical by direct dissociation of H3COCH2CH2–OH bond, through the following channel:
H3 COCH2 CH2 —OH ! H3 COCH2 CH2 þ OH DH ¼ 96:1 kcal=mol
ð6Þ
At G3B3 level of theory, our calculations show this channel to be barrierless with high reaction enthalpy (see Fig. 5). The OH radical has high translational energy, which indicates high probability for involvement of either an exit barrier in its formation channel or the dissociation to take place on the repulsive PES. The non-statistical K-doublets distribution indicates the involvement of an excited state PES. In this scenario, we suggest that at least one of the pathways may involve the cross-over of the initially prepared Rydberg state to nearby r⁄ state along the C–OH bond and subsequent dissociation from the repulsive PES. Secondly, if the dissociation is believed to occur with an exit barrier, then the exit barrier can be either on the excited or on the ground electronic state. In such scenario, the initially prepared Rydberg state must cross over to nearby lower electronic states, including the ground state. The lower excited states are mainly triplet states (T1 and T2) which are Rydberg state, like S1. A number of different high energy reaction channels are available from the energised molecule for dissociating from the ground state. Molecular orbital (MO) calculations were carried out to generate the relative potential energy diagram for the ground state dissociation channels of 2-methoxyethanol, which are accessible
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at the excitation energy corresponding to 193 nm. Our calculation at the G3B3 level of theory is very accurate, and the theory has the fewest convergence problems. In fact, the G3B3 level of theory calculates DH0298 , within an error of 1 kcal/mol. We also locate all the transition states involved in the PES, and studied in detail various reactions apart from the reactions which are relevant to the OH radical formation. Hence, in Fig. 5, the stationary points and the transition states for the various dissociation channels from the ground state of 2-methoxyethanol have been presented with the energy of each species marked in kcal/mol. In addition to the direct OH forming channel (reaction 6), we have optimized at least two other possible indirect channels for formation of the OH radical from the ground state of 2-methoxyethanol. Our calculation suggests (Fig. 9) the reaction enthalpies corresponding to the dissociation of the H3C–OCH2CH2OH bond (84.3 kcal/mol) and the H3CO–CH2CH2OH bond (87.9 kcal/mol) are comparable. It is possible to generate the OH radical from both these channels. Our calculations predict that once the H3CO– CH2CH2OH bond breaks, the generated CH2CH2OH radical may undergo the barrierless CH2CH2–OH bond scission to generate ethylene molecule and the OH radical, the overall mechanism for this channel being:
H3 COCH2 CH2 OH ! H3 CO þ CH2 CH2 OH DH ¼ 87:9 kcal=mol
ð7Þ
ð8Þ
CH2 CH2 OH ! CH2 CH2 þ OH DH ¼ 27:2 kcal=mol
The overall reaction, represented by reactions (7) and (8) leads to total endothermicity of 115.1 kcal/mol for formation of OH radical. The absorbed energy of 148 kcal/mol, as supplied by a photon of wavelength 193 nm, is sufficient to facilitate this channel energetically. However, this reaction pathway is throughout barrierless throughout in both the steps, and, hence, cannot explain the high translational energy of OH radical. However, this channel can be present in our experimental condition with low translational energy distribution, buried in the high translational energy component depicted by Doppler profile. In the second possible pathway of the OH formation (reaction 9–11), after the weakest H3C–OCH2CH2OH bond breaks, the OCH2CH2OH radical (RAD 1) formed still retains the hydrogen bond of the parent molecule. In this radical, the distance between the O-atom of the ether linkage and the hydroxyl H-atom is 2.26 Å, and very similar to that in the parent molecule. The radical (RAD 1) may undergo an intramolecular rearrangement involving simultaneous C–C bond scission and C–O bond formation to generate the OHCH2O CH2 radical (RAD 2). This radical, then can lose the OH radical and simultaneously rearrange to ethylene epoxide.
H3 COCH2 CH2 OH ! CH3 þ OCH2 CH2 OH DH ¼ 84:3 kcal=mol
OCH2 CH2 OH ! OHCH2 O CH2
DH ¼ 4:0 kcal=mol
OHCH2 O CH2 ! OH þ c-CH2 OCH2
DH ¼ 39:5 kcal=mol
ð9Þ ð10Þ ð11Þ
The detailed energetics of this pathway is shown in Fig. 8, with the total calculated endothermicity of 119.8 kcal/mol. Fig. 8 shows that subsequent to RAD 1 formation, both the steps leading to the formation of the OH radical involve exit barriers. We could optimize the transition states involved in these two steps, depicted as TS1 and TS2 in Fig. 8, at MP2/6-311++G⁄⁄ level of theory, and their single point energies calculated at QCISD(T)/6-311++G(2df,pd) level of theory. These energies at QCISD(T)/6-311++G(2df,pd) level are in good agreement with the G3B3 energies. At this level of theory, QCISD(T)/6-311++G(2df,pd), TS1 lies at around 49.8 kcal/mol higher than RAD 1, and its structure clearly shows that the hydrogen bond is still almost intact, the O......H distance being 2.25 Å. The rearranged radical, RAD 2 is more stable than RAD 1 by about 4.0 kcal/mol, though the hydrogen bond becomes somewhat
weaker, with the O.....H distance lengthening to 2.53 Å. The transition state for the OH formation step, TS2, lies at 58.5 kcal/mol higher than RAD 1, whereas the product is at 35.5 kcal/mol higher. The structure of TS2 suggests that while the OH radical is ejected from RAD 2, its H atom again comes closer the O-atom of the ether linkage, with the O.....H distance of only 2.34 Å. The presence of a high exit barrier in the final step of this mechanism can result in the release of a large amount of energy in the relative translational mode of the fragments. In addition to the OH formation RAD 1 can have other possible decomposition pathways (reactions 12 and 13)
OCH2 CH2 OH ! OCH2 CH2 ðtripletÞ þ OH DH ¼ 95:0 kcal=mol
ð12Þ
OCH2 CH2 OH ! CH2 CH2 OH þ O DH ¼ 140:0 kcal=mol
ð13Þ
However, at 193 nm photolysis, the energy is insufficient for the above decomposition pathways to occur. Hence, the only pathway that is possible is the rearrangement of the OCH2CH2OH radical to RAD1, RAD2 and subsequently giving rise to ethylene oxide and OH radical as discussed above, and shown in Fig. 8. The different mechanisms for the dissociation pathways leading to the formation of OH radical were further studied by calculating their respective rate constants using Rice–Ramsperger–Kassel– Marcus (RRKM) method. The RRKM calculated rate constants are used to predict the relative yields of various initial direct radical channels from the ground state potential energy surface of the 2-methoxyethanol. Once the parent molecule relaxes to the ground state PES, following reaction channels occur which lead to formation of OH radicals, and each will be discussed in following paragraph: k6
CH3 OCH2 CH2 OH ! CH3 OCH2 CH2 þ OH k7
CH3 OCH2 CH2 OH ! CH3 O þ CH2 CH2 OH k8
CH2 CH2 OH ! CH2 CH2 þ OH
ð6Þ ð7Þ ð8Þ
k10
k9
CH3 OCH2 CH2 OH ! OCH2 CH2 OH
ðRAD 1Þ
O
k11
CH2 OCH2 OH ! ðRAD 2Þ
H2C
CH2
k10
þ OH
Various input parameters such as rotational constants, vibrational frequencies and critical energies for above reactions are already calculated as described in the previous section. These parameters are used as inputs in the program UNIMOL [39] which calculates the RRKM rate constants as a function of the available energy. The initial steps which involve the rate constants namely, k6, k7 and k8, are calculated as 3.0 108, 5.5 108 and 1.4 109 s1 respectively at the energy of 148 kcal/mol. These rate constants give a ratio of 13:25:62 (1:2:5) for probabilities of initial reactions. Although the direct formation of the OH radical (reaction 6) is a minor channel, it can not be ruled out in our present experimental conditions. However, the formation time of 3 ns for this channel implies that the OH formation is within the bandwidth of laser pulse. The second pathway for formation of the OH radical (reaction 7 and 8) has two steps whereas the initially CH2CH2OH radical is formed with a rate constant k7, and subsequently dissociates with almost no barrier giving rise to the OH radical and ethylene with rate k8. The highest rate constant for second step i.e. k8, is 2.4 1011 s1, assuming a zero energy portioning into translational and internal degrees of freedom in the initial step. Even if we assume a statistical distribution of energy, which imparts about 5% of the available energy into the translational mode in the first
S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
15
Fig. 8. Relative energy diagram in kcal/mol, along with the various transition states and stationary point structures, for decomposition pathway of OCH2CH2OH (RAD 1, see text).
Fig. 9. Schematic potential energy diagram for various pathways for the OH formation process in the photodissociation of 2-methoxyethanol at 193 nm.
step, the rate constant for this reaction is estimated to be 1.8 1011 s1. In this scenario the whole formation pathway follows a consecutive first-order reaction type where k8 k7 and hence the overall reaction rate can be taken as k7 which is 5.5 108 s1. The contribution from this pathway is almost the double of the first pathway wherein the OH radical is formed as a primary product. Similar to the first pathway for OH formation, this reaction pathway also suggests a fast formation time of 2 ns. Moreover, a third pathway for OH formation is also suggested (reactions 9,10 and 11). In this pathway, the weakest H3C–OCH2CH2OH bond breaks and OCH2CH2OH radical (RAD 1) is formed, which still retains the hydrogen bond of the parent molecule. Subsequently, this radical (RAD 1) undergoes an intramolecular rearrangement step to generate CH2OCH2OH radical
(RAD 2). This radical, then can lose OH radical and simultaneously rearrange to ethylene epoxide as discussed earlier and shown in Fig. 8. Here also, all the relevant rate constants such as k9, k10, k10, and k11 have been estimated using RRKM program at the highest available energy. The calculated rate constants are k9 = 1.4 109 s1, k10 = 4.2 107 s1, k10 = 3.7 106 s1, and k11 = 2.4 104 s1. Here also, these rate constants are the upper limit when there is no energy partition into the translational mode and internal energy of H3C–OCH2CH2OH photofragment pair. The rate limiting step is the unimolecular dissociation RAD 2 with rate constant k11. If we assume 5% of available energy goes into the translational mode then the k11 = 1.0 103 s1 with a formation time 1 ms. Although, this mechanism is quite probable, the OH detection could be not possible because of its slow formation time,
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S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
as the diffusion of OH radical from the probe zone is much faster than its formation time. Also, in our experimental condition, the pump-probe delay was always kept at 50 ns to ensure collision less condition. In this scenario, we strongly feel that the detection of the OH radical formed through this mechanism is not detected in our present experimental condition. As discussed above, it is well known that the Rydberg state, in most of the cases, crosses over to some nearby r⁄ state and can directly dissociate to give OH radical on the repulsive PES. To investigate this issue, we have mapped the potential energy (PE) curves for various excited states including the ground state. We carried out theoretical calculations similar to our recent studies [24,40]. In this study, we have calculated PE curves as a function of the C–OH bond length for various excited electronic states, such as ground, S1 and S2 states of 2-methoxyethanol. The molecular geometries in the ground state for different discrete values of the C–OH bond lengths were optimized at ROHF/6-311++G⁄⁄ level of theory, and the energies of such fully optimized configurations were used to generate the PE curve for the ground state. For excited states, the curves were generated by plotting the calculated vertical transition energies corresponding to different ground state optimized geometries using TD-DFT method with B3LYP functional, employing cc-pVTZ basis sets. However, it should be noted that the energies shown in the figure are not those along the excited state reaction coordinate following the minimum energy path, but rather much higher energies. The C–OH bond length of 1.42 Å has the minimum energy structure. From the calculation, we can easily infer that at around C–OH bond length of 1.60 Å there is a cross-over from the S1 state, which is a Rydberg state, to a repulsive surface. In this case the repulsive surface seems to be low lying r⁄ surface arising from n-r⁄ transition. Hence, we attribute dissociation from the r⁄ repulsive surface along the C–OH bond as the major pathway for the OH formation in our present experimental condition. However, the concluding remark on the different mechanism operating in the photolysis of 2-methoxyethanol at 193 nm can be visualized in following schematic (Fig. 9) which clearly shows the complexities of the dissociation mechanisms for OH formation.
to be 0.20 0.07 for the OH formation channel in the 193 nm photolysis of 2-methoxyethanol. Conflict of interest The authors declare no conflict of interest of any kind. Acknowledgments The authors thank Drs. D.K. Palit and B.N. Jagatap, Chemisrty Group, BARC, for their constant guidance and keen interest throughout this work and Department of Chemistry, Pune University, for assistance in theoretical calculations. We also thank Prof. Chi-Kung Kenny Ni, IAMS, Taiwan for his suggestion and discussion during TSRP-2014 meet. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
5. Conclusion In summary, 2-methoxyethanol (H3COCH2CH2OH) generates the OH radical on excitation at 193 nm, which prepares the molecule in a Rydberg state. The nascent state of the photofragment OH is probed with LIF. At 193 nm photolysis, the rotational population is fairly described by Boltzmann–like distribution, which is characterized by a rotational temperature of 450 100 K. The average translational energy partitioned into the OH channel is determined to be 17.0 3.0 kcal/mol in CM frame. The high percentage of translational energy partitioned into the products is assigned to dissociation from a nearby repulsive surface. The spin orbit and K doublet ratios are also measured to gain insights into dynamics of the OH formation. Detailed ab initio quantum calculations suggest that the initially excited Rydberg state crosses over to a nearby r⁄ repulsive state and also to the ground state, and the dissociation occurs on both these surfaces. The present experimental studies along with theoretical calculations suggest that OH is formed through a complex mechanism consisting of, at least, three pathways. The prominent pathway at initial timescale (<50 ns) is crossover from the initially excited Rydberg state to the nearby r⁄ state and dissociation from that repulsive state. The major pathway that dominates at longer timescale (>1 ms) is through initial H3C–OCH2 CH2OH bond cleavage, followed by rearrangement of OCH2CH2OH to CH2OCH2OH, and a final concerted step to generate OH and ethylene epoxide. In the present study, the quantum yield determined
[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
[35] [36] [37] [38] [39]
[40]
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OH formation dynamics in 193 nm photolysis of 2-methoxyethanol: A laser induced fluorescence study Sumana SenGupta, Hari P. Upadhyaya ⇑, Awadhesh Kumar, Prakash D. Naik Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Received 26 May 2014 In final form 17 August 2014 Available online 27 August 2014 Keywords: Laser induced fluorescence Translational energy release Spin–orbit ratio Lambda-doublet ratio Exit barrier Methoxyethanol
a b s t r a c t Dynamics of OH radical formation in the 193 nm photolysis of 2-methoxyethanol is studied using Laser Photolysis–Laser Induced Fluorescence technique. The nascent state distribution of the OH radical is measured. The OH fragments are formed vibrationally cold, characterized by a Boltzmann-like single rotational temperature of 450 100 K. The spin–orbit and K-doublet ratios of OH fragments are measured. The relative average translational energy of the OH channel is determined to be 17.0 3.0 kcal/mol. The experimental studies along with theoretical calculations suggest a complex mechanism for OH formation consisting of at least three pathways. The prominent pathway at shorter timescale (<50 ns) involves crossing over to the nearby repulsive state, whereas, at longer timescale (>1 ms) involves a series of reaction with initial H3C–OCH2CH2OH bond cleavage, followed by rearrangement of OCH2CH2OH to CH2OCH2OH, and a final concerted step to generate OH and ethylene epoxide. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The photodissociation dynamics of small polyatomic molecules following electronic excitation, wherein dissociation occurs on a time scale comparable to that of molecular vibrations, has been an active area of research in recent years. However, photodissociation may also involve complex dynamics, resulting in dissociation on different or often longer time scales in the ground electronic state, which is produced via internal conversion. This situation ensures that the energy acquired in the reactant is statistically distributed. The mechanism and the time scale of the dissociation process in these situations can be predicted with the various statistical theories along with the help of advanced theoretical calculations. Moreover, determination of the energy disposal into the fragments reveals dynamical information about the dissociation pathways, and the nature of the potential energy surface involved in the process. Recently, studies on photodissociation dynamics of small molecules belonging to different class, especially various volatile organic compounds (VOC), have been pursued intensely with an aim to understand their dissociation mechanism in atmosphere. These molecules become even more significant if they generate the OH radical in the dissociation process. In this context we have chosen 2-methoxyethanol (CH3OCH2CH2OH), a similar VOC, for
⇑ Corresponding author. E-mail address: [email protected] (H.P. Upadhyaya). http://dx.doi.org/10.1016/j.chemphys.2014.08.005 0301-0104/Ó 2014 Elsevier B.V. All rights reserved.
the present study which generates OH radical upon excitation in the ultra-violet (UV) region. 2-methoxyethanol is the smallest and simplest member of the series of compounds containing ethylene glycol (H–OCH2CH2OH, EG) unit. This group of compounds containing EG unit is wellknown for its interesting molecular properties and aggregation patterns in solutions [1–9]. Apart from being widely used in the industry as detergents, emulsifiers, or dispersants [10], they are very useful for making surfaces resistant to protein adsorption and cell adhesion [11]. The vibrational spectrum of this compound has been studied both experimentally [12–14] and theoretically [6,12,15,16]. There are a number of reports on the detailed conformational analysis of 2-methoxyethanol in different conditions [12,16–21]. All of them demonstrate that in the ground state several rotational conformations exist, which are quite close in energy. The conformers with intramolecular hydrogen bonding directed from the hydroxyl H-atom to one of the lone pairs of the O-atom of the ether linkage are reported to be the most stable. These conformers are stabilized by about 3.5 kcal/mol compared to the non-hydrogen bonded conformers. Hence, in gaseous phase at low pressure and room temperature, the compound is expected to exist predominantly in the most stable hydrogen bonded form [12]. The hydrogen bonding was found to be strong enough to attenuate solvent influences even at higher dielectric constant (e > 10) [22]. The presence of the strong intramolecular hydrogen bond is expected to play a significant role in the photodissociation dynamics of 2-methoxyethanol, particularly, the reaction channels
S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
which involve the OH functional group. Earlier, our group has studied the OH formation dynamics in compounds, having intramolecular hydrogen bonding, like enolic acetyl acetone [23] and cyclohexadione [24]. These compounds stay predominantly in the enolic form in the gas-phase due to strong hydrogen bond stabilization. The OH formation dynamics in the photodissociation of these compounds in the UV region are well documented. In almost all of these cases, the nascent OH radical showed the preference for population in a particular spin orbit and K-doublet states. A detailed analysis of these experimental results, in combination with results from theoretical calculations, showed the importance of strong hydrogen bonding, even in the excited states. Also, in some cases, the conversion from a hydrogen bonded to non-hydrogen bonded conformer has been observed in the excited states. Apart from the importance of the hydrogen bond in the ground state, it is interesting to know whether there exists a dynamical constraint due to strong hydrogen bond in the excited states or in the pathways for the OH radical formation in 2-methoxyethanol. In the case of 2-methoxyethanol, with possible involvement of more than one conformers, hydrogen bonded and non-hydrogen bonded, having comparable energies, may introduce complexities in the dissociation mechanism. Apart from imposing dynamical constraints in the potential energy surfaces, the strong intramolecular hydrogen bond can have a more profound impact on the dissociation process by altering the energetics of different reaction channels with respect to each other. This may facilitate the formation of the OH radical by some indirect pathway rather than the simple C–OH bond scission. To address these queries, it is required to have detailed studies on its photodissociation dynamics. Though the H2O elimination channel from ionized 2-methoxyethanol to produce C3H6O+ has been studied extensively [25–29], the photodissociation of the neutral molecule is not well-documented in literature. In the present work, we have studied the dynamics of the nascent OH radical formation in the photodissociation of 2-methoxyethanol at 193 nm, using LP-LIF technique. Detailed theoretical calculations have been carried out for understanding the nature of excitation of this compound at 193 nm, as well as for identifying the intermediates, products and transition states involved in the probable OH reaction channels. To get the complete picture of the photodissociation of 2-methoxyethanol at 193 nm, we have also identified the stable products by FTIR. The comparative discussion on the various reaction channels as well as the detailed characterization of the OH formation dynamics is expected to supplement general understanding on the photoexcitation and photodissociation in the UV region of molecules containing intramolecular hydrogen bonding.
9
50 mm, diameter 38 mm) to collect the fluorescence, a photomultiplier tube (Hamamatsu, R 928P) to detect it, and a band pass filter (kcenter = 310 nm, FWHM = 10 nm, %T310 nm = 10) placed between them to cut off the scattering from the photolysis laser. The fluorescence signal was gate integrated by a boxcar (SRS 250), averaged over 30 laser shots, and fed into an interface (SRS 245) and data were collected through a GPIB interface, using a control and data acquisition program. A PC was used to control the scan of the dye laser via an RS232 interface. LIF intensities were normalized with respect to both pump and probe laser energies, to correct for the laser intensity fluctuations. The vapor of the compound was flowed through the reaction chamber at few mTorr level of pressure (10–50 mTorr) ensuring collisionless condition and was photolysed by ArF laser at 193 nm. The OH fragment was probed state selectively by exciting its A2 R X2 P (0, 0) and (1, 1) transition of OH, and monitoring the subsequent A ! X fluorescence. Both the laser beams were unfocussed and attenuated to prevent any saturation effect or multiphotonic event. The LIF intensities of the nascent OH fragments formed in the photolysis of 2-methoxyethanol were found to be directly proportional to the photolysis and probe lasers intensity, and their log–log plot yielded a straight line with a slope of 1.0 0.1, showing that OH formation is a monophotonic process and there is no saturation effect in LIF process. For identification of the stable products of photolysis using FTIR, a stainless steel cell fitted with a pair of CaF2 windows was filled with a known amount of 2-methoxyethanol (5 Torr pressure), and photolysed by 10,000 pulses of 193 nm laser. 3. Results and analysis On photolysis of 2-methoxyethanol at 193 nm, the OH radical is detected by its characteristic LIF spectrum. Though both (0, 0) and (1, 1) transitions of OH were probed, signal could be detected only from the vibrational ground state, v0 = 0, which means that the nascent OH radical formed is mostly vibrationally cold. Fig. 1 shows a part of the fluorescence excitation spectra of the (0, 0) band of the A2 Rþ X2 P system of OH. The standard nomenclature is used for designating the observed transitions [33]. Briefly, the symbols P, Q, and R denote rotational transitions with DN = 1, 0, and 1, respectively. The subscripts 1 and 2 represent the transitions satisfying both DJ and DN, from P3=2 and P1=2 spin orbit states respectively, which form the main branches of the transition, while 21 and 12 represents the transition where only DN is
2. Experimental section The present studies on photodissociation dynamics are carried out in a flow reactor, at mTorr level pressure (10 mTorr), using the Laser Photolysis–Laser Induced Fluorescence setup (LP–LIF). 2-methoxyethanol sample (98% purity, Aldrich) was used after degassing by several freeze–pump–thaw cycles. The experimental set up is the same as that described in our previous papers [23,24,30–32]. Briefly, the photolysis was initiated by an excimer laser (Lambda Physik, Model Compex-102, Fluorine version), and the product OH was probed by a dye laser, with frequency doubling and the mixing module (Quantel, TDL 90), pumped by a seeded Nd:YAG laser (Quantel, YG 980 E-20). The glass reaction chamber had crossed arms at right angles to allow the entrance of pump and probe laser beams to intersect at the center of the chamber. The detection system was attached to the bottom window to capture a view of the intersection volume of the photolysis and the probe lasers. This system consisted of a lens (focal length
Fig. 1. A typical portion of LIF spectrum of OH after photodissociation of 2-methoxyethanol (25 mTorr) at 193 nm photodissociation. (pump–probe delay 50 ns).
10
S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
satisfied and are satellite to the main branches represented by subscript 1 and 2. The Q and P/R branches originate respectively from 00 0 K doublet states P (A ) and Pþ (A ) due to the parity selection rule (+ $ ) [34]. The relative populations of the OH fragments are determined by normalizing the area under the peaks of the rotational lines with respect to pump and probe laser intensities, pressure change, if any, and the respective Einstein absorption coefficients [33]. The spin orbit and the K doublet ratios are calculated from the relative populations of different rotational states. The translational energy associated with the OH fragment is calculated from the Doppler profiles of the rotational lines. The detailed results are presented below. 3.1. Rotational energy distribution The nascent rotational state population of OH radicals, generated on photodissociation of 2-methoxymethanol at 193 nm, is used to construct a Boltzmann plot for obtaining the rotational temperature of nascent OH fragments. The normalized area under the curve of a rotational line, which represents the population in that particular rotational state, was used to construct such a plot 00 for OH (v = 0) and shown in Fig. 2. The points could be fitted reasonably well with a straight line. Hence, the rotational distribution can be described by a single rotational temperature 450 100 K. The OH (1, 1) transition was also scanned to deter00 mine the population of the OH fragment in v = 1, if any, but no LIF signal was observed. Based on the experimental detection limit and Frank–Condon factors relative to the OH (0, 0) transition, it is estimated that less than 5% of the total OH yield is formed in the 00 v = 1 state. 3.2. Spin orbit and K doublet ratio The spin–orbit ratios are obtained from the measured ratio of the population of the P3=2 state, probed by P1 and Q1 rotational lines, to that of the P1=2 states, probed by P2 and Q2 rotational lines, after multiplying by the appropriate statistical weights. In Fig. 3, this ratio is depicted by the red circles as a function of the rotational level, N, and its value is close to unity at all the rotational levels up to N = 7, indicating a statistical distribution of the nascent OH radicals in the two spin–orbit levels. The relative population of the spin–orbit states gives a clue on coupling between the initially prepared excited states with a nearby dissociating state. The observed statistical distribution suggests that the initially excited state probably does not interact directly with a triplet dissociating
Fig. 3. The statistically weighted spin–orbit (red filled circle) and K-doublet (blue 00 filled squares) ratios of nascent OH(v =0) as a function of the rotational quantum number (N) at 193 nm photolysis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
state. However, a non-statistical distribution indicates probably a role of a triplet state in the photodissociation process. 00 0 The ratios of populations of the K-doublets P (A ) and Pþ (A ) of OH fragments are obtained from the normalized intensities of the Q and P rotational lines, respectively, and depicted in Fig. 3. The ratio shows a slight decreasing trend with an increase in the rotational quantum number, indicating a nonstatistical distribution. In the high J limit, in the Pþ (A0 ) state, the p lobe lies in the plane of rotation, while in the P (A00 ) state, the p lobe is perpendicular to the plane of rotation. The relative populations of the K-doublets provide information about the exit channel dynamics during the breaking of a chemical bond. The present result implies a role for an exit channel interaction in formation of OH most probably due to presence of H-bonding in the photodissociation process. 3.3. Translational energy of the fragments From the OH Doppler profiles, the average kinetic energy of the OH radical in the laboratory frame, Elab T (OH), can be determined. The Doppler profiles reflect the distribution, f(vz), of the velocity component vz of the absorbing species along the propagation direction of the probe laser beam via the linear Doppler shift Dm ¼ m m0 ¼ vz m0 /c. For an isotropic velocity distribution, f(vz ) = f(vx) = f(vy), the average translational energy in the labora2 2 tory frame is given by Elab T ðOHÞ ¼ 3=2mOH hvZ iOH , where hvZ iOH for a Gaussian Doppler profile, is represented by the equation:
hv2Z iOH ¼
Fig. 2. Boltzmann plots of rotational distributions of the nascent OH radical generated on 193 nm photolysis. The solid red line is the fit to the data points. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1 2 ln 2
2 FWHM c2 2m0
ð1Þ
where, c is the speed of light and FWHM is the full width at half maximum of the normalized Doppler profile of the OH radical with transition at m0 . In the present case, Doppler profile is a Gaussian function as depicted in Fig. 4 for the P1(2) line, which implies that the translational energy follows the Maxwell–Boltzmann distribution. The width and the shape of the Doppler broadened LIF line include contributions from the fragment molecular velocity, the thermal motion of the parent molecule and the finite probe laser line width. Thus, the actual Doppler width is calculated using deconvolution procedure using the width (FWHM) of the laser spectral profile of the probe laser beam, which is obtained from the OH Doppler profile measured in a thermalized condition. All the rotational lines exhibit the same width within an experimental error. More than 50 rotational line profiles were evaluated to estimate the average kinetic energy of the OH fragment. For a two-body
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S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
alcohol (CH3OH) and ethylene epoxide with an endothermicity of 28.9 kcal/mol. However we were unable to locate the transition state for this molecular rearrangement. Also, we were able to locate the transition state for the elimination of methanol with co-product acetaldehyde (4b) with the lowest barrier and DH value. H3 COCH2 CH2 OH ! CH3 OH þ cyclic-CH2 OCH2 DH ¼ 28:9 kcal=mol ð4aÞ ! CH3 OH þ CH3 CHO DH ¼ 2:3 kcal=mol
ð4bÞ
Though the DH values of all these molecular elimination channels are not very high, these channels may involve multi steps with high activation barriers. However, we expect that all these channels are possible at 193 nm with 148 kcal/mol of photon energy. 3.5. Absorption spectra, absolute cross-section and quantum yield measurements Fig. 4. Doppler profile of the P1 (2) line (blue filled circles) in the spectrum for 248 nm photodissociation of CHD. The red line is the Gaussian fit to the data points. The dotted green line represents instrument function that includes laser band width also. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
direct dissociation mechanism, the average kinetic energy in the centre-of-mass (CM) frame, ECM T (OH) is found to be 17.0 3.0 kcal/mol. The fraction of the available energy released as translational energy, f T ¼ ECM T /Eavail, is calculated as 0.33. 3.4. Stable product analysis The stable product analysis of photodissociation of 2-methoxyethanol at 193 nm was done by FTIR absorption technique, and CH3OH (methanol), H2C = CH2 (ethylene) and H2C = O (formaldehyde) could be detected. These products can be formed by a number of low energy channels opening up from the ground state of 2-methoxyethanol depending upon their respective relative energy. Molecular orbital (MO) calculations were carried out to generate the relative potential energy diagram for the dissociation channels, of 2-methoxyethanol on its ground state, accessible on laser excitation at 193 nm. Calculations were done at G3B3 level of theory, using Gaussian suite of program for the DH evaluation. The transition state geometries were optimized at HF/6-31 + G⁄ and the barrier heights were calculated at QCISD(T)/6-311++G⁄⁄ level of theory. All the stationary points were calculated in the ground state, for the various dissociation channels. A number of such channels are presented in Fig. 5 with their respective barrier heights and the transition states. Our calculations show the elimination of water, having DH of 8.5 kcal/mol (reaction 2).
H3 COCH2 CH2 OH ! H2 O þ CH2 CHOCH3
DH ¼ 8:5 kcal=mol
ð2Þ
Elimination of formaldehyde (reaction 3) also has a low endothermicity of 18.6 kcal/mol and 6.8 kcal/mol with respective co-product dimethyl ether (3a) and ethanol (3b).
H3 COCH2 CH2 OH ! CH2 O þ CH3 OCH3
DH ¼ 18:6 kcal=mol ð3aÞ
! CH2 O þ CH3 CH2 OH DH ¼ 6:8 kcal=mol ð3bÞ Both these channels, though less endothermic, can have higher energy transition states in their potential energy surfaces. From our theoretical calculations, we could locate another possible molecular elimination channel leading to stable products (reaction 4). In this channel, the 2-methoxyethanol molecule undergoes intramolecular rearrangement, where the –CH3 group migrates from the O-atom of the ether linkage to the O-atom of the terminal –OH group, which breaks free from the rest of the molecule, and simultaneously, the O-atom of the former ether linkage binds with the now free –CH2 group. This results in the formation of methyl
The absorption spectra were measured using a commercial uv spectrophotometer at different pressures of 2-methoxyethanol, and presented in Fig. 6 with its absolute cross-section. To verify it further, the absorption cross-sections for 2-methoxyethanol at 193 nm was measured using a 50-cm long absorption cross-section cell. For this purpose the ratios of intensities, I/I0, were measured, where I is the laser intensity with the sample and I0 without the sample in the cell. The sample pressure in the cell was measured with a capacitance manometer. The laser light was attenuated heavily by suitable substrate window, and was collimated by the iris to get a very low-intensity laser beam (<1 lJ). The intensity of the laser beam was measured by photodiodes (Becker and Heckl, PDM-400). The absorption cross-section of 2-methoxyethanol at 193 nm thus measured was found to be (1.3 0.3) 1019 cm2 molecule1. This value was used for the determination of the quantum yield of OH formation channel from 2-methoxyethanol on excitation at 193 nm. We calculated the quantum yield of the OH generation from 2-methoxyethanol by a relative method. The concentration of OH produced in 193 nm photolysis of 2-methoxy193 ethanol is proportional to, ½OHME / r193 abs ðMEÞ ½ME UOH ðMEÞ, where r; U and [ME] are the absorption cross-section, quantum yield and concentration of 2-methoxyethanol, respectively. Similar equation can be written for a reference compound also. In the present study, acetic acid was taken as a reference compound with the measured values of the absorption cross-section and the quantum yield of OH generation channel to be 1.1 1019 cm2 molecule1 and 0.8, respectively, which was determined in our earlier study [31]. Hence, the quantum yield for ME photolysis for the OH channel, using acetic acid as a reference, is given by
U193 OH ðMEÞ ¼
r193 ½acetic acid ½OHME abs ðacetic acidÞ ½ME ½OH r193 ðMEÞ acetic acid abs U193 OH ðacetic acidÞ
ð5Þ
The ratio [OH]ME/[OH]acetic acid was determined from the area of a particular rotational line of OH in thermalized condition, which ensure the same rotational temperature of the OH radical formed in the photolysis of both the compounds. In the present study, the quantum yield determined to be 0.20 0.07 for the OH formation channel in the 193 nm photolysis of 2-methoxyethanol. 4. Discussion 4.1. Nature of the ground state and the excited states Ab initio molecular orbital (MO) calculations are performed to investigate the potential energy surface (PES) of the ground and excited electronic states of the most stable conformer of 2-methoxyethanol, using the Gaussian suite of program. A number of different conformers can exist in the ground state of the
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Fig. 5. Relative energy diagram in kcal/mol, along with the structures, for the various transition states and products in the photodissociation of 2-methoxyethanol at 193 nm.
Fig. 6. uv absorption spectrum of 2-methoxyethanol at room temperature.
2-methoxyethanol molecule, and intramolecular hydrogen bonding plays a significant role in the relative stabilities of these conformers. Ab initio MO calculations found out that in the most stable conformer there exists a strong hydrogen bond between the H-atom of the hydroxyl moiety and O-atom of the ether linkage [12,15]. The hitherto reported calculation were done at MP2/631G⁄ level of theory. The difference in energy between the most stable hydrogen-bonded conformer and the non-hydrogen bonded conformer is 5.7 kcal/mol. We have optimized and calculated the relative energies of different conformers of 2-methoxyethanol in the ground state at G3B3 level of theory, and arrived at a similar lowest energy hydrogen bonded conformer (see Fig. 7). At G3B3 level of calculation, the difference between the hydrogen bonded and the non-hydrogen bonded conformers was found to be 4.0 kcal/mol. Hence, it is inferred that in the vapor phase, the 2-methoxyethanol exist predominantly as hydrogen-bonded conformer. The gas phase UV spectrum of 2-methoxyethanol shows onset of optical absorption at 205 nm (6.0 eV, see Fig. 6). To understand the nature of the transitions responsible for the optical absorption spectra, ab initio MO calculations were performed, in detail. We optimized the ground state geometries of the most stable hydrogen-bonded conformer of 2-methoxyethanol, at MP2/6-311++G⁄⁄
level of theory. The augmented basis sets, with diffuse and triple zeta functions, namely, aug–cc–pVTZ, were then used for obtaining the vertical excitation energies for various transitions, using timedependent density functional theory (TD-DFT) with MPW1PW91 correlation and exchange functionals. Although the calculated vertical transition energies slightly differ as compared to the experimental results, the nature of transitions and that of the orbitals involved are accurately predicted, using this method. The orbitals participating in different electronic transitions were visualized for better understanding of the process, and can be seen in Fig. 7. The vertical excitation energies and the respective oscillator strengths of several low–lying singlet and triplet states are calculated, and a total of four singlet excited states and two triplet states is considered. These states are primarily due to excitations from the n (HOMO-1) and n (HOMO) orbitals, to 3s orbitals which are LUMO, LUMO+1 and LUMO+2. The two triplet states, T1 and T2, are mainly Rydberg state at 6.46 eV (192 nm) and 6.58 eV (189 nm), respectively. The T1 state involves the transition from HOMO (non bonding electrons of oxygen at the ether linkage) to LUMO and the T2 state involves that from HOMO-1 (non bonding electrons of oxygen at the OH moiety) to LUMO. The S1 state is mainly a Rydberg state with considerable oscillator strength (0.0225) at 6.59 eV (188 nm). The transition is from HOMO (non bonding electrons of oxygen at the ether linkage) to LUMO. The S2 state is also Rydberg in nature with the oscillator strength of 0.0092, at 6.77 eV (183 nm). This state involves the transition from HOMO-1 to LUMO. The other singlet states, namely S3 and S4 are Rydberg states too with vertical excitation energies of 7.08 eV (175 nm) and 7.12 eV (174 nm) and oscillator strengths of 0.0165 and 0.0055, respectively. Considering corresponding wavelengths for vertical excitation energies and the respective oscillator strengths of various transitions, it is evident that at 193 nm, 2-methoxyethanol is excited to Rydberg state, which involves the non-bonding electrons of oxygen atom at the ether linkage. The Rydberg state of the molecule can also be calculated using the Rydberg formula En,l = IP ½13:606=ðn dl Þ2 ], where IP is ionization potential, n = 3 for 3 s Rydberg transition, and dl is l-dependent quantum defect, typically dl = 1 for s, 0.6 for p and 0.1 for d Rydberg transition. The ionization potential of 2-methoxyethanol is estimated using G3B3 method to be 9.54 eV. The approximate position of the origin for the first Rydberg transition involving a 3 s as the upper state is about 6.14 eV (202 nm).
S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
13
Fig. 7. Computed MOs involved in the transition of 2-methoxyethanol. The green and the red color of the MO lobes represent two opposite phases of the MO wavefunction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
This value matches reasonably well with our calculated value of 6.59 eV (188 nm) using TD-DFT method, considering the fact that this method gives vertical transition energy whereas the Rydberg formula gives the minimum energy. 4.2. Average energies of the fragments The partitioning of the available energy into various degrees of freedom of the fragments is mainly governed by the nature of transition state and its location on the dissociative potential energy surface. Dissociation of 2-methoxyethanol at 193 nm occurs with a considerable amount of translational energy (ECM T = 17.0 3.0 kcal/ mol, f T = 0.33) being imparted into the OH channel. Since the statistical model predicts well the energy partitioning of a dissociation process only on the ground state with no barrier, we have employed different models, e.g. impulsive model, to understand the dissociation process at different wavelengths. In the statistical model, the available energy, Eavl, is distributed among all the accessible states with equal probabilities under the constraint of conservation of energy. A statistical dissociation process occurs predominantly, if the photo-excited parent molecule is so longlived that the excess energy is partitioned statistically amongst the available degrees of freedom of the products. This includes a molecular process where a rapid internal conversion to the ground electronic state takes place, followed by slow dissociation. Under these events, in a large molecule with many low frequency modes, a relatively small amount of the excess energy is partitioned into translational motion of the products. For this kind of dissociation process, a priori calculations [35–37] were adopted, along with a simple analytical expression established by Klots [38], relating the mean translational energy release, ET, and the Eavail, for a statistical barrierless dissociation process. The statistical model puts 5% of the available energy into the relative translational mode of the photofragments. Thus, the measured f T value of 0.33 is underestimated using statistical model (f T = 0.05). Hence, the statistical model fails to explain the observed partitioning of the available energy. The large amount of translational energy released into the OH photofragment indicates the dissociation is either from a repulsive state or across a barrier. If the dissociation takes place from a repulsive state the energy partitioning can be explained by an impulsive model. In the impulsive model, the distribution of energy among product states is governed by the dissociative
event, i.e., by the repulsive force acting during the breaking of the parent molecule into the products. The impulsive model puts 52% of the available energy into the product translational mode. Like the statistical model, the impulsive model is also unable to explain the experimentally obtained translational energy partitioning between the fragments. However, both the statistical and impulsive models allocate low rotational and vibrational energies into the OH radical. Thus, the failure of both the statistical and impulsive models in explaining the partitioning of the available energy implies the dissociation process may have an exit barrier. This observation prompted us to investigate the ground state PES in detail, which will be discussed in the following section. 4.3. Mechanism of the dissociation process Since the parent compound, 2-methoxyethanol, contains an OH moiety in its structure, there is a possibility of formation of the OH radical by direct dissociation of H3COCH2CH2–OH bond, through the following channel:
H3 COCH2 CH2 —OH ! H3 COCH2 CH2 þ OH DH ¼ 96:1 kcal=mol
ð6Þ
At G3B3 level of theory, our calculations show this channel to be barrierless with high reaction enthalpy (see Fig. 5). The OH radical has high translational energy, which indicates high probability for involvement of either an exit barrier in its formation channel or the dissociation to take place on the repulsive PES. The non-statistical K-doublets distribution indicates the involvement of an excited state PES. In this scenario, we suggest that at least one of the pathways may involve the cross-over of the initially prepared Rydberg state to nearby r⁄ state along the C–OH bond and subsequent dissociation from the repulsive PES. Secondly, if the dissociation is believed to occur with an exit barrier, then the exit barrier can be either on the excited or on the ground electronic state. In such scenario, the initially prepared Rydberg state must cross over to nearby lower electronic states, including the ground state. The lower excited states are mainly triplet states (T1 and T2) which are Rydberg state, like S1. A number of different high energy reaction channels are available from the energised molecule for dissociating from the ground state. Molecular orbital (MO) calculations were carried out to generate the relative potential energy diagram for the ground state dissociation channels of 2-methoxyethanol, which are accessible
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S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
at the excitation energy corresponding to 193 nm. Our calculation at the G3B3 level of theory is very accurate, and the theory has the fewest convergence problems. In fact, the G3B3 level of theory calculates DH0298 , within an error of 1 kcal/mol. We also locate all the transition states involved in the PES, and studied in detail various reactions apart from the reactions which are relevant to the OH radical formation. Hence, in Fig. 5, the stationary points and the transition states for the various dissociation channels from the ground state of 2-methoxyethanol have been presented with the energy of each species marked in kcal/mol. In addition to the direct OH forming channel (reaction 6), we have optimized at least two other possible indirect channels for formation of the OH radical from the ground state of 2-methoxyethanol. Our calculation suggests (Fig. 9) the reaction enthalpies corresponding to the dissociation of the H3C–OCH2CH2OH bond (84.3 kcal/mol) and the H3CO–CH2CH2OH bond (87.9 kcal/mol) are comparable. It is possible to generate the OH radical from both these channels. Our calculations predict that once the H3CO– CH2CH2OH bond breaks, the generated CH2CH2OH radical may undergo the barrierless CH2CH2–OH bond scission to generate ethylene molecule and the OH radical, the overall mechanism for this channel being:
H3 COCH2 CH2 OH ! H3 CO þ CH2 CH2 OH DH ¼ 87:9 kcal=mol
ð7Þ
ð8Þ
CH2 CH2 OH ! CH2 CH2 þ OH DH ¼ 27:2 kcal=mol
The overall reaction, represented by reactions (7) and (8) leads to total endothermicity of 115.1 kcal/mol for formation of OH radical. The absorbed energy of 148 kcal/mol, as supplied by a photon of wavelength 193 nm, is sufficient to facilitate this channel energetically. However, this reaction pathway is throughout barrierless throughout in both the steps, and, hence, cannot explain the high translational energy of OH radical. However, this channel can be present in our experimental condition with low translational energy distribution, buried in the high translational energy component depicted by Doppler profile. In the second possible pathway of the OH formation (reaction 9–11), after the weakest H3C–OCH2CH2OH bond breaks, the OCH2CH2OH radical (RAD 1) formed still retains the hydrogen bond of the parent molecule. In this radical, the distance between the O-atom of the ether linkage and the hydroxyl H-atom is 2.26 Å, and very similar to that in the parent molecule. The radical (RAD 1) may undergo an intramolecular rearrangement involving simultaneous C–C bond scission and C–O bond formation to generate the OHCH2O CH2 radical (RAD 2). This radical, then can lose the OH radical and simultaneously rearrange to ethylene epoxide.
H3 COCH2 CH2 OH ! CH3 þ OCH2 CH2 OH DH ¼ 84:3 kcal=mol
OCH2 CH2 OH ! OHCH2 O CH2
DH ¼ 4:0 kcal=mol
OHCH2 O CH2 ! OH þ c-CH2 OCH2
DH ¼ 39:5 kcal=mol
ð9Þ ð10Þ ð11Þ
The detailed energetics of this pathway is shown in Fig. 8, with the total calculated endothermicity of 119.8 kcal/mol. Fig. 8 shows that subsequent to RAD 1 formation, both the steps leading to the formation of the OH radical involve exit barriers. We could optimize the transition states involved in these two steps, depicted as TS1 and TS2 in Fig. 8, at MP2/6-311++G⁄⁄ level of theory, and their single point energies calculated at QCISD(T)/6-311++G(2df,pd) level of theory. These energies at QCISD(T)/6-311++G(2df,pd) level are in good agreement with the G3B3 energies. At this level of theory, QCISD(T)/6-311++G(2df,pd), TS1 lies at around 49.8 kcal/mol higher than RAD 1, and its structure clearly shows that the hydrogen bond is still almost intact, the O......H distance being 2.25 Å. The rearranged radical, RAD 2 is more stable than RAD 1 by about 4.0 kcal/mol, though the hydrogen bond becomes somewhat
weaker, with the O.....H distance lengthening to 2.53 Å. The transition state for the OH formation step, TS2, lies at 58.5 kcal/mol higher than RAD 1, whereas the product is at 35.5 kcal/mol higher. The structure of TS2 suggests that while the OH radical is ejected from RAD 2, its H atom again comes closer the O-atom of the ether linkage, with the O.....H distance of only 2.34 Å. The presence of a high exit barrier in the final step of this mechanism can result in the release of a large amount of energy in the relative translational mode of the fragments. In addition to the OH formation RAD 1 can have other possible decomposition pathways (reactions 12 and 13)
OCH2 CH2 OH ! OCH2 CH2 ðtripletÞ þ OH DH ¼ 95:0 kcal=mol
ð12Þ
OCH2 CH2 OH ! CH2 CH2 OH þ O DH ¼ 140:0 kcal=mol
ð13Þ
However, at 193 nm photolysis, the energy is insufficient for the above decomposition pathways to occur. Hence, the only pathway that is possible is the rearrangement of the OCH2CH2OH radical to RAD1, RAD2 and subsequently giving rise to ethylene oxide and OH radical as discussed above, and shown in Fig. 8. The different mechanisms for the dissociation pathways leading to the formation of OH radical were further studied by calculating their respective rate constants using Rice–Ramsperger–Kassel– Marcus (RRKM) method. The RRKM calculated rate constants are used to predict the relative yields of various initial direct radical channels from the ground state potential energy surface of the 2-methoxyethanol. Once the parent molecule relaxes to the ground state PES, following reaction channels occur which lead to formation of OH radicals, and each will be discussed in following paragraph: k6
CH3 OCH2 CH2 OH ! CH3 OCH2 CH2 þ OH k7
CH3 OCH2 CH2 OH ! CH3 O þ CH2 CH2 OH k8
CH2 CH2 OH ! CH2 CH2 þ OH
ð6Þ ð7Þ ð8Þ
k10
k9
CH3 OCH2 CH2 OH ! OCH2 CH2 OH
ðRAD 1Þ
O
k11
CH2 OCH2 OH ! ðRAD 2Þ
H2C
CH2
k10
þ OH
Various input parameters such as rotational constants, vibrational frequencies and critical energies for above reactions are already calculated as described in the previous section. These parameters are used as inputs in the program UNIMOL [39] which calculates the RRKM rate constants as a function of the available energy. The initial steps which involve the rate constants namely, k6, k7 and k8, are calculated as 3.0 108, 5.5 108 and 1.4 109 s1 respectively at the energy of 148 kcal/mol. These rate constants give a ratio of 13:25:62 (1:2:5) for probabilities of initial reactions. Although the direct formation of the OH radical (reaction 6) is a minor channel, it can not be ruled out in our present experimental conditions. However, the formation time of 3 ns for this channel implies that the OH formation is within the bandwidth of laser pulse. The second pathway for formation of the OH radical (reaction 7 and 8) has two steps whereas the initially CH2CH2OH radical is formed with a rate constant k7, and subsequently dissociates with almost no barrier giving rise to the OH radical and ethylene with rate k8. The highest rate constant for second step i.e. k8, is 2.4 1011 s1, assuming a zero energy portioning into translational and internal degrees of freedom in the initial step. Even if we assume a statistical distribution of energy, which imparts about 5% of the available energy into the translational mode in the first
S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
15
Fig. 8. Relative energy diagram in kcal/mol, along with the various transition states and stationary point structures, for decomposition pathway of OCH2CH2OH (RAD 1, see text).
Fig. 9. Schematic potential energy diagram for various pathways for the OH formation process in the photodissociation of 2-methoxyethanol at 193 nm.
step, the rate constant for this reaction is estimated to be 1.8 1011 s1. In this scenario the whole formation pathway follows a consecutive first-order reaction type where k8 k7 and hence the overall reaction rate can be taken as k7 which is 5.5 108 s1. The contribution from this pathway is almost the double of the first pathway wherein the OH radical is formed as a primary product. Similar to the first pathway for OH formation, this reaction pathway also suggests a fast formation time of 2 ns. Moreover, a third pathway for OH formation is also suggested (reactions 9,10 and 11). In this pathway, the weakest H3C–OCH2CH2OH bond breaks and OCH2CH2OH radical (RAD 1) is formed, which still retains the hydrogen bond of the parent molecule. Subsequently, this radical (RAD 1) undergoes an intramolecular rearrangement step to generate CH2OCH2OH radical
(RAD 2). This radical, then can lose OH radical and simultaneously rearrange to ethylene epoxide as discussed earlier and shown in Fig. 8. Here also, all the relevant rate constants such as k9, k10, k10, and k11 have been estimated using RRKM program at the highest available energy. The calculated rate constants are k9 = 1.4 109 s1, k10 = 4.2 107 s1, k10 = 3.7 106 s1, and k11 = 2.4 104 s1. Here also, these rate constants are the upper limit when there is no energy partition into the translational mode and internal energy of H3C–OCH2CH2OH photofragment pair. The rate limiting step is the unimolecular dissociation RAD 2 with rate constant k11. If we assume 5% of available energy goes into the translational mode then the k11 = 1.0 103 s1 with a formation time 1 ms. Although, this mechanism is quite probable, the OH detection could be not possible because of its slow formation time,
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S. SenGupta et al. / Chemical Physics 443 (2014) 8–16
as the diffusion of OH radical from the probe zone is much faster than its formation time. Also, in our experimental condition, the pump-probe delay was always kept at 50 ns to ensure collision less condition. In this scenario, we strongly feel that the detection of the OH radical formed through this mechanism is not detected in our present experimental condition. As discussed above, it is well known that the Rydberg state, in most of the cases, crosses over to some nearby r⁄ state and can directly dissociate to give OH radical on the repulsive PES. To investigate this issue, we have mapped the potential energy (PE) curves for various excited states including the ground state. We carried out theoretical calculations similar to our recent studies [24,40]. In this study, we have calculated PE curves as a function of the C–OH bond length for various excited electronic states, such as ground, S1 and S2 states of 2-methoxyethanol. The molecular geometries in the ground state for different discrete values of the C–OH bond lengths were optimized at ROHF/6-311++G⁄⁄ level of theory, and the energies of such fully optimized configurations were used to generate the PE curve for the ground state. For excited states, the curves were generated by plotting the calculated vertical transition energies corresponding to different ground state optimized geometries using TD-DFT method with B3LYP functional, employing cc-pVTZ basis sets. However, it should be noted that the energies shown in the figure are not those along the excited state reaction coordinate following the minimum energy path, but rather much higher energies. The C–OH bond length of 1.42 Å has the minimum energy structure. From the calculation, we can easily infer that at around C–OH bond length of 1.60 Å there is a cross-over from the S1 state, which is a Rydberg state, to a repulsive surface. In this case the repulsive surface seems to be low lying r⁄ surface arising from n-r⁄ transition. Hence, we attribute dissociation from the r⁄ repulsive surface along the C–OH bond as the major pathway for the OH formation in our present experimental condition. However, the concluding remark on the different mechanism operating in the photolysis of 2-methoxyethanol at 193 nm can be visualized in following schematic (Fig. 9) which clearly shows the complexities of the dissociation mechanisms for OH formation.
to be 0.20 0.07 for the OH formation channel in the 193 nm photolysis of 2-methoxyethanol. Conflict of interest The authors declare no conflict of interest of any kind. Acknowledgments The authors thank Drs. D.K. Palit and B.N. Jagatap, Chemisrty Group, BARC, for their constant guidance and keen interest throughout this work and Department of Chemistry, Pune University, for assistance in theoretical calculations. We also thank Prof. Chi-Kung Kenny Ni, IAMS, Taiwan for his suggestion and discussion during TSRP-2014 meet. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
5. Conclusion In summary, 2-methoxyethanol (H3COCH2CH2OH) generates the OH radical on excitation at 193 nm, which prepares the molecule in a Rydberg state. The nascent state of the photofragment OH is probed with LIF. At 193 nm photolysis, the rotational population is fairly described by Boltzmann–like distribution, which is characterized by a rotational temperature of 450 100 K. The average translational energy partitioned into the OH channel is determined to be 17.0 3.0 kcal/mol in CM frame. The high percentage of translational energy partitioned into the products is assigned to dissociation from a nearby repulsive surface. The spin orbit and K doublet ratios are also measured to gain insights into dynamics of the OH formation. Detailed ab initio quantum calculations suggest that the initially excited Rydberg state crosses over to a nearby r⁄ repulsive state and also to the ground state, and the dissociation occurs on both these surfaces. The present experimental studies along with theoretical calculations suggest that OH is formed through a complex mechanism consisting of, at least, three pathways. The prominent pathway at initial timescale (<50 ns) is crossover from the initially excited Rydberg state to the nearby r⁄ state and dissociation from that repulsive state. The major pathway that dominates at longer timescale (>1 ms) is through initial H3C–OCH2 CH2OH bond cleavage, followed by rearrangement of OCH2CH2OH to CH2OCH2OH, and a final concerted step to generate OH and ethylene epoxide. In the present study, the quantum yield determined
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