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Resonant two-photon ionization spectroscopy of the 35Cl and 37Cl isotopomers of cis and trans 3-chloro-4-fluoroanisole
Journal of Molecular Structure 1000 (2011) 92–98
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Resonant two-photon ionization spectroscopy of the of cis and trans 3-chloro-4-fluoroanisole
35
Cl and
37
Cl isotopomers
Dan Yu, Changwu Dong, Lijuan Zhang, Min Cheng, Lili Hu, Yikui Du ⇑, Qihe Zhu, Cunhao Zhang Beijing National Laboratory of Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 2 April 2011 Received in revised form 27 May 2011 Accepted 28 May 2011 Available online 12 June 2011 Keywords: R2PI 3-Chloro-4-fluoroanisole Conformational effect Isotopic effect
a b s t r a c t The effects of conformation and isotopic substitution on the properties of 3-chloro-4-fluoroanisole (3C4FA) were studied by mass-analyzed resonant two-photon ionization (R2PI) technique and theoretical S0 electronic transitions (00 bands) calculations. In the one color R2PI spectra, the band origins of the S1 of cis 35Cl–3C4FA and cis 37Cl–3C4FA were found to be equivalent at 34,703 ± 3 cm1, while the 00 bands of trans 35Cl–3C4FA and trans 37Cl–3C4FA were found to be equivalent at 34,747 ± 3 cm1. Assignments of the observed vibrational bands of R2PI spectra were made mainly based on the 10-electron, 8-orbital CASSCF/6-31g calculations and on conformity with the available data of the similar aromatic molecules in the literature. With the two color R2PI technique, the adiabatic ionization energies (IEs) of cis 35Cl– 3C4FA and cis 37Cl–3C4FA were determined to be equivalently 67,349 ± 15 cm1, while the IEs of trans 35 Cl–3C4FA and trans 37Cl–3C4FA were determined to be equivalently 67,595 ± 15 cm1. The conformational effect on the transition energies, ionization energies and vibrational frequencies of 3C4FA is greater than the isotopic effect. The hetero-dihalogen-substitution effect on the transition energies is also discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The spectroscopic studies on benzene derivatives have been extensively reported over the past few decades because of their important role in developing the concept of aromaticity and their significance in industry and environment. Containing methoxyl group and halogen atom, halogen-substituted anisoles can serve as a model system for studying the substitution effects on aromatic molecules. In addition, halogen-substituted anisoles are of substantial interest due to their high environmental impact [1–3]. The halogen-substituted anisoles have been the subjects of many spectroscopic studies, for example the monofluoroanisoles (including o-, m-, and p-fluoroanisole) have been investigated with fluorescence spectroscopy [4], vibrational spectroscopy [5], emission spectra [6,7], resonance-enhanced multi-photon ionization (REMPI) spectroscopy [8] and laser induced fluorescence (LIF) spectroscopy [9]. A REMPI spectroscopic study on p-chloroanisole was also reported recently [10]. However, the REMPI spectroscopic study on the multihalogen-substituted anisoles is still lacking. The homo-dihalogen-substituted aromatic molecules, such as difluorobenzenes [11–13], dichlorobenzenes [14] and difluoroanisoles [15,16], have been studied by experimental and theoretical method. But there is no publication concerning the hetero-dihalo⇑ Corresponding author. Tel.: +86 10 62635054; fax: +86 10 62563167. E-mail address: [email protected] (Y. Du). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.05.058
gen-substituted aromatic molecules in the excited S1 state. The 3-chloro-4-fluoroanisole (3C4FA) seems a good model compound for studying the hetero-multihalogen-substitution effect. In addition, the investigation of the conformational and isotopic effects on transition energy, vibrational frequencies and other molecular properties is also feasible. Intramolecular isomerism plays an important role in biologically relevant molecular systems such as the proteins [17,18] and molecular machines [19]. In understanding the function of the complicated large biological molecules, it is important to know the conformational properties, excited and ionic state dynamics of their local units. It is known that ortho or meta halogen substituted anisole has two stable planar conformers, cis and trans, which are raised from the orientation of the methoxy group with respect to the halogen atom. Due to the natural abundance of chlorine isotopes, each of the two conformers, cis 3C4FA and trans 3C4FA, has two isotopomers: 35Cl isotopomer and 37Cl isotopomer. The REMPI spectroscopic and theoretical study on the four species of 3C4FA (35Cl isotopomer of cis 3C4FA, 37Cl isotopomer of cis 3C4FA, 35Cl isotopomer of trans 3C4FA and 37Cl isotopomer of trans 3C4FA) will help to elucidate the conformational and isotopic effects on the molecular properties of 3C4FA in the excited S1 state. In this paper, we report the resonant two-photon ionization (R2PI) spectra, the photo-ionization efficiency (PIE) spectra and the theoretically predicted molecular geometries of the 35Cl and 37 Cl isotopomers of cis and trans 3C4FA. The first electronic
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transition energies (E1s) and adiabatic ionization energies (IEs) are determined, and the assignments of the active vibrations in the S1 state are attempted. The conformation and isotopic substitution effects on the geometric structures, transition energies and vibrational frequencies are discussed.
2. Experimental and computational methods The experimental system consisting of a time-of-flight mass spectrometer (TOF-MS) and a pulsed supersonic molecular beam source has been described in previous publications [8,10]. 3C4FA (Alfa, 97% purity) was used without further purification. To acquire sufficient vapor pressure, the sample was heated to about 336 K. Carried by argon (approximately 1.5 atm), the sample was expanded into the source chamber through a pulsed valve (general valve) with an orifice 0.25 mm in diameter to form a supersonically cooled molecular beam. After being collimated by a 1-mmdiameter orifice skimmer, the molecular beam entered the ionization chamber. The ionization of 3C4FA was generated 70 mm downstream from the nozzle orifice by the UV laser beam perpendicular to the molecular beam. The generated cations were accelerated by two DC electric fields of 200 and 3500 V/cm. After being focused by the einzel lens with a DC electric field of 850 V, the cations passed a 1.0-m-long field-free region and were detected by a dual-stacked micro-channel plate (MCP) detector. The ion signals were amplified by a preamplifier (SR445A, Stanford Research System) and then collected and analyzed by a multi-channel scaler (MCS, Stanford Research System, SR430). During the experiment, the pressures of the source and ionization chambers were maintained at approximately 2.0 103 and 1.8 105 Pa, respectively. The one-color resonant two-photon ionization (1C-R2PI) technique [20,21] was applied for acquiring the excitation spectra. The UV laser was produced by doubling the frequency of the output of the dye laser (Sirah Dye Laser-CSTR), pumped by the third harmonic of the Nd:YAG laser (Quanta-Ray, PRO-Series, 355 nm) at a repetition rate of 10 Hz. Rhodamine 590 and Coumarin 153 dyes were used. Whereas the two-color resonant two-photon ionization (2C-R2PI) technique was applied for the photo-ionization efficiency (PIE) experiments [22]. Two UV lasers were produced by doubling the frequencies of the output of two independent dye lasers (Sirah Dye Laser-CSTR), pumped by the second and third harmonic (532 and 355 nm, respectively) of the Nd:YAG laser (Quanta-Ray, PRO-Series). Rhodamine 590 and 640 dyes were used. The delay time between the two dye lasers was adjusted by changing the light path difference between the two lasers. To optimize the contrast between the one- and two-color signals, the energy ratio of the exciting and ionizing lasers was kept at 1:37. The diameter of excitation laser spot was 2.0 mm, while the diameter of the ionization laser spot was 3.0 mm. The synchronization of the whole system was controlled by a pulse delay generator (DG535, Stanford Research System). The calculations were performed by the Gaussian 09 W program package [23]. HF, MP2 and B3LYP methods were performed to optimize the molecular structures and to calculate the frequencies in the ground S0 state and the ionic ground D0 state. CIS, TDB3LYP and CASSCF methods were performed to optimize the molecular structures and to calculate the frequencies in the first excited state. For each calculation method, basis sets of 6-31g and cc-PVDZ (correlation consistent-polarized valence double zeta) were used. To search for stable molecular structures, different fixed torsional angles around the C–OCH3 bond in this paper were used to optimize the geometry. The vibrational frequencies in the S1 state were calculated using the 10-electron, 8-orbital CASSCF/ 6-31g method, which was denoted as CAS(10,8)/6-31g. The active space consists of six p-electron orbitals of the benzene ring, one
oxygen lone pair having p symmetry with respect to the molecular plane, and one chlorine lone pair having p symmetry [24]. The calculated vibrational frequencies quoted in this paper were scaled by the proper factors to approximately correct the combined errors stemming from the basis-set incompleteness and vibrational anharmonicity. 3. Results 3.1. Calculated results Fig. 1 shows the molecular structures of the cis and trans 3C4FA conformers. The carbon atoms of 3C4FA are numbered 1–6 around the phenyl ring, and the substituted portions are labeled as C1–O7– CH3, C3–Cl and C4–F. Since there is no theoretical or experimental data of the molecular structure for 3C4FA in the literature, calculations by HF, MP2 and DFT methods with the cc-PVDZ basis set were employed to optimize the geometric structures in the S0 and D0 state. The CIS and TD-DFT methods with the cc-PVDZ basis set were used to optimize the molecular structures in the S1 state. To search for all the possible proper structures of 3C4FA, different fixed torsional angles around the C1–O7 bond in this paper were used during the theoretical calculation. The calculated potential functions for the dihedral angle of C13O7C1C2 are shown in Fig. 2. Because the chlorine isotopic effect hardly influences the structures of the 3C4FA, only the calculated data of 35Cl–3C4FA in the S0, S1 and D0 state are shown in Fig. 2a–c, and listed in Table 1. In Fig. 2a, there are two global minima related to the two planar structures, cis (\C13O7C1C2 = 0°) and trans (\C13O7C1C2 = 180°) 3C4FA. The energy of trans 3C4FA is higher than that of cis 3C4FA by less than 0.2 kcal/mol. An additional minimum for the perpendicular conformer (\C13O7C1C2 = 90°) is predicted using the HF/ cc-PVDZ method. Its energy with the zero-point correction is about 0.9 kcal/mol higher than that of cis 3C4FA. A very shallow minimum at \C13O7C1C2 = 90° is also predicted by the MP2/cc-PVDZ method, which is too shallow to be considered. The perpendicular structure corresponds to a transition state according to calculated results using the B3LYP/cc-PVDZ method. Its energy with the zeropoint correction is about 2.5 kcal/mol higher than that of cis 3C4FA. Thus there are two planar conformers in the S0 state, cis and trans 3C4FA conformers, which are similar to previous results for 3-fluorophenol [25], 3-chlorophenol [26] and 3,4-difluoroanisole [27]. The energy curves in Fig. 2b are not as regular as those in Fig. 2a. But it is obvious that cis and trans conformers still exist in the S1 state. An additional minimum for the non-planar conformer
cis (∠C13O7C1C2=0°)
trans (∠C13O7C1C2=180°)
Fig. 1. Molecular structures of cis and trans 3C4FA.
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D. Yu et al. / Journal of Molecular Structure 1000 (2011) 92–98 Table 1 The calculated geometric parameters of cis and trans 3C4FA in the S0, S1 and D0 states using the basis set of cc-PVDZ. cis S0/DFT
trans S1/TD-DFT
D0/DFT
S0/DFT
S1/TD-DFT
D0/DFT
Bond length (Å) O–CH3 1.419 O–C1 1.363 C3–Cl 1.751 C4–F 1.346 C1–C2 1.401 C2–C3 1.400 C3–C4 1.393 C4–C5 1.394 C5–C6 1.390 C6–C1 1.405
1.412 1.316 1.724 1.307 1.402 1.414 1.428 1.384 1.416 1.437
1.452 1.311 1.712 1.307 1.420 1.380 1.440 1.407 1.373 1.444
1.419 1.363 1.751 1.346 1.401 1.400 1.393 1.394 1.390 1.405
1.412 1.314 1.729 1.307 1.430 1.409 1.399 1.414 1.409 1.416
1.455 1.310 1.715 1.306 1.429 1.374 1.435 1.413 1.374 1.439
Dihedral angle (°) \C13O7C1C2 0 \C13O7C1C6 180
0 180
0 180
180 0
180 0
180 0
that of cis 3C4FA by about 0.4 kcal/mol for HF method, 1.0 kcal/mol for DFT method, and 2.4 kcal/mol for MP2 method. The perpendicular structure corresponds to a transition state according to the data calculated using HF, DFT and MP2 method. An additional minimum for the conformer with \C13O7C1C2 = 170° is predicted using the MP2/cc-PVDZ method. Its energy with the zero-point correction is about 2.4 kcal/mol above that of cis 3C4FA. But there is an imaginary frequency corresponding to the out-of-plane bending vibration of the O–CH3 bond. Its value is about 258 cm1. Therefore, this structure might be a saddle point in the two-dimensional potential energy surface. The geometric parameters of cis and trans 3C4FA in the S0, S1 and D0 state are listed in Table 1. 3.2. TOF mass spectrum Fig. 3 shows the TOF mass spectrum of 3C4FA recorded at the laser wavelength of 288.16 nm. The peaks at mass 160 and 162 result from the ion signal of the 35Cl and 37Cl isotopomers of 3C4FA, respectively. The flight times of 35Cl and 37Cl isotopomers of 3C4FA were 22.390 and 22.530 ls, respectively. The full widths at half maximum (FWHM) of the two peaks were 0.033 and 0.030 ls, respectively. This leads to an estimated t/Dt value of about 680, which corresponds to a mass resolution m/Dm of about 340. The high resolution of the TOF mass spectrometer makes the detection of the selected 3C4FA isotopomer possible.
Fig. 2. Potential functions for the dihedral angle of \C13O7C1C2 in the (a) S0, (b) S1 and (c) D0 state, calculated using ab initio and DFT methods with cc-PVDZ basis sets. The solid circle, square and triangle frames designate the calculated potential energies without the zero-point correction. The open circle, square and triangle frames designate the calculated potential energies with the zero-point correction (zero-point level, ZPL).
(\C13O7C1C2 = 10°) is predicted using the CIS/cc-PVDZ method with the zero-point correction. However, for this conformer, the benzene ring is non-planar, and the length of C–Cl bond is 2.275 Å, which is not a normal single bond. This conformer is too different from the two conformers of 3C4FA in the S0 state, so it can not be consistent with the experimental observation that the 00 peak is the most intense one in the REMPI spectra. Therefore, the cis and trans conformers are also the two optimized molecular structures in the S1 state. Fig. 2c shows two global minima related to the cis and trans 3C4FA conformers in the D0 state. The energy of trans 3C4FA is higher than
Fig. 3. TOF spectrum of 3C4FA recorded at a laser wavelength of 288.16 nm.
D. Yu et al. / Journal of Molecular Structure 1000 (2011) 92–98
3.3. 1C-R2PI spectra Fig. 4a–c show the 1C-R2PI spectra of the 35Cl and 37Cl isotopomers of 3C4FA in the energy range of 33,800–36,350 cm1. Because 3C4FA exists as cis and trans conformers in the S0 state, each 1CR2PI spectrum is believed to originate from two conformers of 3C4FA. PIE spectroscopy helps us to make sure of this, which will be discussed below. In Fig. 4a – 35Cl, the intense peaks located at 34,703 and 34,747 cm1 are respectively assigned as the band origins of the S1 S0 electronic transitions of the cis and trans
95
35
Cl–3C4FA. Similarly, the intense peaks located at 34,703 and 34,747 cm1 in Fig. 4a – 37Cl are respectively assigned to the band origins of the S1 S0 electronic transitions of the cis and trans 37 Cl–3C4FA. The difference between the first electronic transition energies of the cis and trans 3C4FA shows that the conformational effect brings changes in the electronic transition energy. The similarity of the first electronic transition energies for 35Cl and 37Cl isotopomers indicates no chlorine isotopic effect on the electronic transition energy. The same phenomenon was observed for the 35 Cl and 37Cl isotopomers of p-chlorophenol [28] and p-chloroaniline [29]. The observed bands, except the 00 band, in the 1C-R2PI spectra of 3C4FAs, should originate from the excitation of the vibrations in the S1 state. 3C4FA has 42 normal mode vibrations. Because the intensity of the vibronic band is proportional to the Franck–Condon factor [30], not all vibrations are active for the 1C-R2PI process. Table 2 lists the frequencies of the observed REMPI bands of cis and trans conformer of 35Cl–3C4FA and 37Cl–3C4FA, the calculated vibrational frequencies of the S1 state and the possible assignments. The spectral assignments were made according to the available vibrational frequencies of the similar molecules such as p-fluoroanisole [8] and m-chlorophenol [26], and the calculated results using the CAS(10,8)/6-31g method. Most of the vibrational bands in the REMPI spectra of 3C4FAs are related to the in-plane deformation of the aromatic ring. The bands at 400 (35,103), 469 (35,172), 870 (35,573) and 1304 (36,007) cm1 for cis 35Cl–3C4FA and 400 (35,103), 469 (35,172), 870 (35,573) and 1292 (35,995) cm1 for cis 37Cl–3C4FA are related 1 to the in-plane ring deformation of 6b0 , 6a10 , 110 and m(C–Cl) + b(C– H), respectively. The bands at 182 (34,885) cm1 for cis 35Cl–3C4FA and 180 (34,883) cm1 for cis 37Cl–3C4FA are then assigned to the vibration of b(O–CH3) + b(C–Cl), involving the in-plane bending vibration of the O–CH3 and C–Cl bond. The band at 116 (34,819) cm1 for cis 35Cl–3C4FA is assigned to the vibration of c(C–Cl), involving the out-of-plane bending vibration of the C–Cl bond. The out-of-plane bending of C–O–CH3 and C–F bonds is found to have frequencies of 87 (34,790) cm1 for the cis 35Cl isomer and 85 (34,788) cm1 for the cis 37Cl isomer. The bands at 383 (35,130), 864 (35,611) and 1302 (36,049) cm1 for trans 35Cl– 3C4FA and 386 (35,133), 864 (35,611) and 1290 (36,037) cm1 for trans 37Cl–3C4FA are related to the in-plane ring deformation 1 of 6b0 , 110 , and m(C–Cl) + b(C–H), respectively. The bands at 574 (35,321) cm1 for trans 35Cl–3C4FA and 577 (35,324) cm1 for trans 1 37 Cl–3C4FA are then assigned to the vibration of 17b0 . The out-ofplane bending of O–CH3 bond is found to have frequencies of 60 (34,807) cm1 for the trans 35Cl isomer and 60 (34,807) cm1 for the trans 37Cl isomer. 3.4. PIE curves
Fig. 4. 1C-R2PI spectra of the 35Cl and 37Cl isotopomers of cis and trans 3C4FA in the energy ranges of (a) 33,800–34,770 cm1, (b) 34,770–35,700 cm1 and (c) 35,700– 36,400 cm1.
The conformational isomers, cis and trans 3C4FA conformers, may coexist in a chemical sample. Species selection can be accomplished easily by applying the two-color resonant two-photon ionization (2C-R2PI) for successive excitation/ionization process. This is accomplished by detecting the prompt ions formed in the photoionization process while tuning the frequency of the ionization laser across the ionization limit and keeping the frequency of the excitation laser at a particular vibronic level of the chosen species. Fig. 5 shows the PIE curves of cis and trans 3C4FA conformers, which are recorded via their individual vibrationless levels of the S1 state. The distinct steps correspond to the ionization energies (IE) of these species. The IEs of the cis 35Cl–3C4FA and the cis 37 Cl–3C4FA are both determined to be 67,349 ± 15 cm1, whereas the IEs of the trans 35Cl–3C4FA and trans 37Cl–3C4FA are both determined to be 67,595 ± 15 cm1. The difference in the IE between two structural conformers is about 250 cm1. Similar results
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Table 2 Observed bands (cm1) in the REMPI spectrum of 3C4FAs and their possible assignments. S1 cis-Cl35
S1 cis-Cl37 a
Exp
Calc
34,703 34,790 34,819 34,885 35,103
0 87 116 182 400
108 112 168 404
35,172
469
Calc 0 85
108
c(C–O–CH3) + c(C–F) c(C–Cl)
34,883 35,103
180 400
166 403
b(O–CH3) + b(C–Cl)
460
35,172
469
459
6a10 110 m(C–Cl) + b(C–H)
35,573
870
865
35,573
870
864
36,007
1304
1290
35,995
1292
1290
1
6b0
S1 trans-Cl37 a
Exp
b
Exp 34,703 34,788
S1 trans-Cl35
a
Assignment and approximate descriptionb a
Calc
Calca
Exp
34,747 34,807 35,130
0 60 383
57 385
34,747 34,807 35,133
0 60 386
57 384
6b0
35,321
574
586
35,324
577
586
17b0
35,611
864
860
35,611
864
859
36,049
1302
1290
36,037
1290
1289
110 m(C–Cl) + b(C–H)
c(O–CH3) 1 1
Obtained from the CAS(10,8)/6–31 g calculation, scaled by 0.95. Varsanyi and Szoke’s notations [35] are applied for the ring vibrational modes; c denotes out-of-plane bending; b denotes in-plane bending; m denotes stretching.
the same conformer. Although the calculations underestimate by a few thousands of wavenumbers, they successfully predict the order of the IEs of the cis and trans 3C4FA, and the similarity of the IEs of 35Cl and 37Cl isomer for the same conformer. 4. Discussion 4.1. Molecular structure
Fig. 5. PIE spectra of
35
Cl and
37
Cl isotopomers of cis and trans 3C4FA.
have been reported for the structural isomers of m-chlorophenol [26]. We have also performed the ab initio and DFT calculations to predict the IEs. Calculations using the HF, DFT and MP2 method with the basis set of cc-PVDZ gave the adiabatic IE values of 57,204, 64,244 and 68,107 cm1 for the cis conformer, which are lower than the measured values by 15%, 5% and 1%, respectively. Calculations using the HF, DFT and MP2 method with the basis set of ccPVDZ gave the adiabatic IE values of 57,308, 64,535 and 69,223 cm1 for the trans conformer, which are lower than the measured values by 15%, 4% and 2%, respectively. There is no difference between the calculated IE values of 35Cl and 37Cl isomer for
With the hydrogen atoms of the methyl group bent out of the ring-plane by approximately 61°, the 3C4FA is non-planar and belongs to Cs symmetry point group in the S0 state. In the S1 state, 3C4FA also belongs to Cs symmetry point group with the two hydrogen atoms of the methyl group bent out of the ring by approximately 61°. Most of the C–C bonds of the aromatic ring in the S1 state are respectively longer than those in the S0 state, which means that the aromatic ring expands during the S1 S0 transition. The ring expansion is caused by the electron transfer from a bonding p orbital, the highest occupied molecular orbital (HOMO), to an anti-bonding p⁄ orbital, the lowest unoccupied molecular orbital (LUMO), during the S1 S0 transition. The C1–O bond is shortened during the S1 S0 transition, which exhibits partial double bond character in the S1 state. This may result from the enhanced p–p conjugation in the S1 state. The C3–Cl and C4–F bond are also shortened during the S1 S0 transition. This indicates that the interactions between the halogen atoms and aromatic ring in the S1 state are greater than those in the S0 state. The expansion of the ring and the shrinkage of C1–O, C3–Cl and C4–F bonds are consistent with the changes for p-fluoroanisole [8] and p-chloroanisole [10]. Compared to 3C4FA in the S1 state, the perimeter of the aromatic ring in the D0 state is shortened. This indicates that the aromatic ring shrinks during the D0 S1 transition. It may result from that the excited electron is removed from the anti-bond orbital (HOMO, p⁄). It is interesting that the C2–C3 and C5–C6 bonds are shorter than the other four C–C bonds in the D0 state. This indicates that the 3C4FA isomers in the D0 state have the quinone-like structures. 4.2. Transition and ionization energy Table 3 lists the origins of the S1 S0 electronic transition energies (E1s) and the adiabatic ionization energies (IEs) of fluoroben-
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zene [31], chlorobenzene [31,32], difluorobenzenes [12,13], dichlorobenzenes [14], anisole [33] and some anisole derivatives [8]. The E1s of fluorobenzene and chlorobenzene are respectively 37,819 and 37,048 cm1. The IEs of fluorobenzene and chlorobenzene are respectively 74,238 and 73,170 cm1. This indicates that the chloro-substitution shifts the first electronic transition energy and the ionization energy to the red with respect to the fluorosubstitution. According to the previous studies [34], substitution with the electron-donating groups usually shifts the S1 S0 transition energies of the aromatic molecules to the red, while electron-withdrawing groups usually shift the S1 S0 transition energies of the aromatic molecules to the blue. For halogen atoms, the conjugation effects between their p orbitals and the benzene p orbitals make them the electron-donating substituted groups, while the inductive effects through their r bonds make them to be the electron-withdrawing groups. Therefore, the final effect resulting from the halogen atom is decided by the competition of the two electronic effects. Since the chlorine atom is bigger and more polarizable than the fluorine atom, and then the electronegativity of the chlorine atom is weaker than that of the fluorine atom. The conjugation effect is stronger for chlorine atom than for fluorine atom, and the inductive effect is weaker for chlorine atom than for fluorine atom. Therefore, the chlorine atom is more like an electron-donating substituted group than the fluorine atom. As shown in Table 3, the E1s and IEs of o-difluorobenzene and m-difluorobenzene are blue shifted compared to those of fluorobenzene. The E1s of o-dichlorobenzene, m-dichlorobenzene and p-dichlorobenzene, as well as the IE of p-dichlorobenzene are respectively red shifted compared to those of chlorobenzene. These are consistent with the discussion above. But the E1 and IE of p-difluorobenzene are red shifted compared to those of fluorobenzene, and the IEs of o-dichlorobenzene and m-dichlorobenzene are blue shifted compared to that of chlorobenzene. We can see that the shifts of the energies caused by the dihalogen-substitution on benzenes are more complicated. It seems that, besides the property of the halogen atoms, the relative locations of the two halogen atoms on the benzene ring could influence the direction of the energy-shift. The E1 of p-fluoroanisole is red shifted compared to that of anisole, and the IE of p-fluoroanisole is blue shifted compared to that of anisole. The E1s of the cis and trans 3C4FA are red shifted compared to that of p-fluoroanisole, and the IEs of the cis and trans
3C4FA are blue shifted compared to that of p-fluoroanisole. It is clear that, for the hetero-dihalogen-substitute benzenes, the halogen-substitute effects on the first electronic transition energies and the ionization energies are consistent with the previous conclusions of studies on the monohalogen-substitute benzenes. These data also indicate that a cumulative effect exists in the transition energies of 3C4FA. As shown in Table 3, the E1 and IE of the cis 3C4FA are less than those of the trans 3C4FA. And the E1s and IEs of the 35Cl–3C4FA are the same as those of the 37Cl–3C4FA, respectively. It indicates that the conformational effect rather than the isotopic effect can substantially influence the transition and ionization energies of 3C4FA. The similarity of 35Cl and 37Cl isotopomers of each conformer in transition energy and the ionization energy measured could be explained by the calculated results. Though, due to the deficiency of the theoretical calculations, the HF, DFT and MP2 calculations may lead to a deviation in predicting the total energy, the general trend derived from the calculated results may be useful for comparison with the experimental results in terms of the isotopic effect. Although the structures of the 35Cl and 37Cl isotopomers of each conformer are identical, the energies at various states are expected to be different. Table 4 shows that, for each of the cis or trans conformers, the zero point level of the 37Cl isotopomer is lower than that of the 35Cl isotopomer for each state. The calculated results show that the differences in the zero point energy in the S0, S1 and D0 state have the same value of 7 cm1. Thus, as the energy gap among the electronic states, the electronic transition energies and the ionization energies are the same for the 35Cl and 37Cl isotopomers of each conformer. These calculated results provide a reasonable explanation of the experimental findings. 4.3. Molecular vibrations 1
For the 35Cl–3C4FA, the measured frequencies of the 6b0 , 110 and m(C–Cl) + b(C–H) vibration are 400, 870 and 1304 cm1 for the cis conformer and 383, 864 and 1302 cm1 for the trans conformer, respectively. It is clear that the measured frequencies of the three vibration modes of trans conformer are red shifted with respect to those of cis conformer. This phenomenon is also observed for 37Cl– 3C4FA. Those red-shifts of the vibrational frequencies of the trans conformer, compared with those of the cis conformer, may result from the conformational effect.
Table 3 Measured transition energies (in cm1) of fluorobenzene, chlorobenzene, difluorbenzenes, dichlorobenzenes, anisole and some of anisole-derivatives. Molecule c
Fluorobenzene o-Difluorobenzened m-Difluorobenzened p-Difluorobenzened Chlorobenzenee o-Dichlorobenzenef m-Dichlorobenzenef p-Dichlorobenzenef Anisoleg p-Fluoroanisoleh 3-Chloro-4-fluoroanisole, cisi 3-Chloro-4-fluoroanisole, transi a b c d e f g h i
E1a
DE1a
IEb
DIEb
37,819 37,824 37,909 36,840 37,048 36,238 36,193 35,752 36,383 35,149 34,703 34,747
0 5 90 979 0 810 855 1328 0 1234(0) 1680(446) 1636(402)
74,238 75,003 75,332 73,861 73,170 73,237 73,776 72,191 66,399 66,621 67,349 67,595
0 765 1094 377 0 67 606 979 0 222(0) 950(728) 1196(974)
E1 is the first electronically excited transition energy; DE1 is the shift in E1 with respect to the corresponding original molecule. IE is the ionization energy; DIE is the shift in IE with respect to the corresponding original molecule. Ref. [31]. Ref. [12,13]. Ref. [32]. Ref. [14]. Ref. [33]. Ref. [8]. This experiment.
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D. Yu et al. / Journal of Molecular Structure 1000 (2011) 92–98
Table 4 Calculated energies in (Eh) of the the S0, S1 and D0 statesa.
35
Cl and
37
Cl isotopomers of cis and trans 3C4FA in
cis 37
902.380174 0.123362 902.256812
902.380174 0.123328 902.256846
902.379895 0.123327 902.256568
902.379895 0.123294 902.256602
902.167993 0.118528 902.049465
902.167994 0.118497 902.049497
902.167262 0.118623 902.048639
902.167263 0.118592 902.048671
902.119533 0.122131 901.997402
902.119533 0.122097 901.997436
902.118775 0.122047 901.996727
902.118775 0.122013 901.996762
Cl
S0 ERHF ZPC E00 S1 ECIS ZPC E0 D0 EUHF ZPC E+
trans
35
Cl
35
Cl
37
Cl
a
Calculations are at the RHF/cc-PVDZ, CIS/cc-PVDZ and UHF/cc-PVDZ levels for the S0, S1 and D0 states, respectively. ZPC is the zero-point correction and E00 , E0 and E+ are the energies at the zero-point level. 1 Hartree = 27.211 eV = 219474 cm1.
For most vibrational modes, the measured frequency differences between the 35Cl and 37Cl isotopomer of cis or trans 3C4FA are less than 3 cm1 (as shown in Table 2). Because the error limit of the measured vibrational frequency in our REMPI spectrum is ±3 cm1, the conclusion may be that the isotopic effect on the most vibrations is not strong enough to be measured by our REMPI spectrum. For vibrations involving C–Cl bond, such as m(C– Cl) + b(C–H), the frequencies of the 37Cl isomers are red shifted by 12 cm1, compared to those of the 35Cl isomers. The relatively greater isotopic effect may be caused by the higher degree of the involvement of the chorine atom in this vibration. The analysis indicates that the isotopic effect on the vibration involving the C–Cl stretching is more intense than that on the vibration involving the C–Cl bending. 5. Conclusion The 1C-R2PI spectroscopic method was applied to record the vibronic spectra of the four species, the 35Cl and 37Cl isotopomers of cis and trans 3C4FA in the S1 state. The transition energies of both the 35Cl and 37Cl isotopomers of cis 3C4FA were found to be the same, with value of 34,703 ± 3 cm1. The transition energies of both the 35Cl and 37Cl isotopomers of trans 3C4FA were found to be the same, with value of 34,747 ± 3 cm1. The general features in the vibronic spectra of both isotopomers of any conformer are similar, though the frequencies of some vibrational modes of isotopomers are slightly different by a few wavenumbers. This frequency shift is somewhat related to the degree of involvement of Cl atom in the vibrations. The general features in the vibronic spectra of the two conformers are quite different. With 2C-R2PI technique, the PIE spectra of the four species were achieved, and the adiabatic ionization energies were determined to be 67,349 ± 15 cm1 for the 35Cl and 37Cl isotopomers of the cis conformers, and 67,595 ± 15 cm1 for the 35Cl and 37Cl isotopomers of the trans conformers. Detailed analysis on the E1s, IEs and active vibrations of the four species leads to the following insights: (i) the conformational effect on the E1s and IEs of 3C4FA is remarkable, whereas the isotopic effect on the E1s and IEs of 3C4FA is negligible; (ii) the conforma-
tional effect on the active modes and the frequencies of vibrations of 3C4FA in the S1 state is greater than the isotopic effect; (iii) the isotopic effect on the vibration involving the C–Cl stretching is more intense than that on the vibration involving the C–Cl bending. Acknowledgment We gratefully acknowledge the financial support from the Center of Molecular Sciences of ICCAS under Contract No. CMS-LX200915 and the National Natural Science Foundation of China under Grant 20973180. References [1] L.H. Aung, J.L. Smilanick, P.V. Vail, P.L. Hartsell, E. Gomez, J. Agric. Food Chem. 44 (1996) 3294. [2] U. Fuhrer, K. Ballschmiter, Environ. Sci. Technol. 32 (1998) 2208. [3] P. Chatonnet, S. Bonnet, S. Boutou, M.D. Labadie, J. Agric. Food Chem. 52 (2004) 1255. [4] T. Isozaki, K. Sakeda, T. Suzuki, T. Ichimura, J. Chem. Phys. 132 (2010). [5] P.D. Singh, Indian J. Pure Appl. Phys. 7 (1969) 430. [6] K.N. Upadhya, J.N. Rai, Indian J. Pure Appl. Phys. 3 (1965) 100. [7] B.J. Ansari, D. Sharma, S.L. Srivasta, J. Mol. Spectrosc. 34 (1970) 468. [8] D.Q. Xiao, D. Yu, X.L. Xu, Z.J. Yu, Y.K. Du, Z. Gao, Q.H. Zhu, C.H. Zhang, J. Mol. Struct. 882 (2008) 56. [9] T. Isozaki, K. Sakeda, T. Suzuki, T. Ichimura, unpublished. [10] D. Yu, C. Dong, M. Cheng, L. Hu, Y. Du, Q. Zhu, C. Zhang, J. Mol. Spectrosc. 265 (2011) 86. [11] L.A. Zotti, G. Teobaldi, K. Palotas, W. Ji, H.J. Gao, W.A. Hofer, J. Comput. Chem. 29 (2008) 1589. [12] Y. Tsuchiya, K. Takazawa, M. Fujii, M. Ito, J. Phys. Chem. 96 (1992) 99. [13] C.H. Kwon, H.L. Kim, M.S. Kim, J. Chem. Phys. 118 (2003) 6327. [14] A. Gaber, M. Riese, J. Grotemeyer, J. Phys. Chem. A 112 (2008) 425. [15] O.V. Dorofeeva, Y.V. Vishnevskiy, A.N. Rykov, N.M. Karasev, N.F. Moiseeva, L.V. Vilkov, H. Oberhammer, J. Mol. Struct. 789 (2006) 100. [16] N.I. Giricheva, G.V. Girichev, J.S. Levina, H. Oberhammer, J. Mol. Struct. 703 (2004) 55. [17] F.Z. Zhang, A. Zarrine-Afsar, M.S. Al-Abdul-Wahid, R.S. Prosser, A.R. Davidson, G.A. Woolley, J. Am. Chem. Soc. 131 (2009) 2283. [18] R.P. Jakob, F.X. Schmid, J. Mol. Biol. 377 (2008) 1560. [19] Y. Norikane, N. Tamaoki, Org. Lett. 6 (2004) 2595. [20] C.E.H. Dessent, K. Müller-Dethlefs, Chem. Rev. 100 (2000) 3999. [21] B. Brutschy, Chem. Rev. 100 (2000) 3891. [22] C. Lifshitz, Int. Rev. Phys. Chem. 16 (1997) 113. [23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, GAUSSIAN 09 (Revision A.02), Gaussian, Inc., Wallingford CT, 2009. [24] S. Hirokawa, T. Imasaka, J. Phys. Chem. A 105 (2001) 9252. [25] K. Remmers, W.L. Meerts, A. Zehnacker-Rentien, K. Le Barbu, F. Lahmani, J. Chem. Phys. 112 (2000) 6237. [26] M.C.R. Cockett, M. Takahashi, K. Okuyama, K. Kimura, Chem. Phys. Lett. 187 (1991) 250. [27] J. Han, R.L. Deming, F.M. Tao, J. Phys. Chem. A 108 (2004) 7736. [28] J.G. Huang, J.L. Lin, W.B. Tzeng, Chem. Phys. Lett. 422 (2006) 271. [29] J.L. Lin, W.B. Tzeng, J. Chem. Phys. 113 (2000) 4109. [30] F.T. Chau, J. Mol. Spectrosc. 103 (1984) 66. [31] K. Walter, K. Scherm, U. Boesl, J. Phys. Chem. 95 (1991) 1188. [32] T.G. Wright, S.I. Panov, T.A. Miller, J. Chem. Phys. 102 (1995) 4793. [33] M. Pradhan, C. Li, J.L. Lin, W.B. Tzeng, Chem. Phys. Lett. 2005 (2005) 100. [34] J.L. Lin, W.B. Tzeng, Chem. Phys. Lett. 370 (2003) 44. [35] G. Varsányi, S. Szoke, Vibrational Spectra of Benzenes Derivatives, Academic Press, New York, 1969.
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Resonant two-photon ionization spectroscopy of the of cis and trans 3-chloro-4-fluoroanisole
35
Cl and
37
Cl isotopomers
Dan Yu, Changwu Dong, Lijuan Zhang, Min Cheng, Lili Hu, Yikui Du ⇑, Qihe Zhu, Cunhao Zhang Beijing National Laboratory of Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 2 April 2011 Received in revised form 27 May 2011 Accepted 28 May 2011 Available online 12 June 2011 Keywords: R2PI 3-Chloro-4-fluoroanisole Conformational effect Isotopic effect
a b s t r a c t The effects of conformation and isotopic substitution on the properties of 3-chloro-4-fluoroanisole (3C4FA) were studied by mass-analyzed resonant two-photon ionization (R2PI) technique and theoretical S0 electronic transitions (00 bands) calculations. In the one color R2PI spectra, the band origins of the S1 of cis 35Cl–3C4FA and cis 37Cl–3C4FA were found to be equivalent at 34,703 ± 3 cm1, while the 00 bands of trans 35Cl–3C4FA and trans 37Cl–3C4FA were found to be equivalent at 34,747 ± 3 cm1. Assignments of the observed vibrational bands of R2PI spectra were made mainly based on the 10-electron, 8-orbital CASSCF/6-31g calculations and on conformity with the available data of the similar aromatic molecules in the literature. With the two color R2PI technique, the adiabatic ionization energies (IEs) of cis 35Cl– 3C4FA and cis 37Cl–3C4FA were determined to be equivalently 67,349 ± 15 cm1, while the IEs of trans 35 Cl–3C4FA and trans 37Cl–3C4FA were determined to be equivalently 67,595 ± 15 cm1. The conformational effect on the transition energies, ionization energies and vibrational frequencies of 3C4FA is greater than the isotopic effect. The hetero-dihalogen-substitution effect on the transition energies is also discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The spectroscopic studies on benzene derivatives have been extensively reported over the past few decades because of their important role in developing the concept of aromaticity and their significance in industry and environment. Containing methoxyl group and halogen atom, halogen-substituted anisoles can serve as a model system for studying the substitution effects on aromatic molecules. In addition, halogen-substituted anisoles are of substantial interest due to their high environmental impact [1–3]. The halogen-substituted anisoles have been the subjects of many spectroscopic studies, for example the monofluoroanisoles (including o-, m-, and p-fluoroanisole) have been investigated with fluorescence spectroscopy [4], vibrational spectroscopy [5], emission spectra [6,7], resonance-enhanced multi-photon ionization (REMPI) spectroscopy [8] and laser induced fluorescence (LIF) spectroscopy [9]. A REMPI spectroscopic study on p-chloroanisole was also reported recently [10]. However, the REMPI spectroscopic study on the multihalogen-substituted anisoles is still lacking. The homo-dihalogen-substituted aromatic molecules, such as difluorobenzenes [11–13], dichlorobenzenes [14] and difluoroanisoles [15,16], have been studied by experimental and theoretical method. But there is no publication concerning the hetero-dihalo⇑ Corresponding author. Tel.: +86 10 62635054; fax: +86 10 62563167. E-mail address: [email protected] (Y. Du). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.05.058
gen-substituted aromatic molecules in the excited S1 state. The 3-chloro-4-fluoroanisole (3C4FA) seems a good model compound for studying the hetero-multihalogen-substitution effect. In addition, the investigation of the conformational and isotopic effects on transition energy, vibrational frequencies and other molecular properties is also feasible. Intramolecular isomerism plays an important role in biologically relevant molecular systems such as the proteins [17,18] and molecular machines [19]. In understanding the function of the complicated large biological molecules, it is important to know the conformational properties, excited and ionic state dynamics of their local units. It is known that ortho or meta halogen substituted anisole has two stable planar conformers, cis and trans, which are raised from the orientation of the methoxy group with respect to the halogen atom. Due to the natural abundance of chlorine isotopes, each of the two conformers, cis 3C4FA and trans 3C4FA, has two isotopomers: 35Cl isotopomer and 37Cl isotopomer. The REMPI spectroscopic and theoretical study on the four species of 3C4FA (35Cl isotopomer of cis 3C4FA, 37Cl isotopomer of cis 3C4FA, 35Cl isotopomer of trans 3C4FA and 37Cl isotopomer of trans 3C4FA) will help to elucidate the conformational and isotopic effects on the molecular properties of 3C4FA in the excited S1 state. In this paper, we report the resonant two-photon ionization (R2PI) spectra, the photo-ionization efficiency (PIE) spectra and the theoretically predicted molecular geometries of the 35Cl and 37 Cl isotopomers of cis and trans 3C4FA. The first electronic
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D. Yu et al. / Journal of Molecular Structure 1000 (2011) 92–98
transition energies (E1s) and adiabatic ionization energies (IEs) are determined, and the assignments of the active vibrations in the S1 state are attempted. The conformation and isotopic substitution effects on the geometric structures, transition energies and vibrational frequencies are discussed.
2. Experimental and computational methods The experimental system consisting of a time-of-flight mass spectrometer (TOF-MS) and a pulsed supersonic molecular beam source has been described in previous publications [8,10]. 3C4FA (Alfa, 97% purity) was used without further purification. To acquire sufficient vapor pressure, the sample was heated to about 336 K. Carried by argon (approximately 1.5 atm), the sample was expanded into the source chamber through a pulsed valve (general valve) with an orifice 0.25 mm in diameter to form a supersonically cooled molecular beam. After being collimated by a 1-mmdiameter orifice skimmer, the molecular beam entered the ionization chamber. The ionization of 3C4FA was generated 70 mm downstream from the nozzle orifice by the UV laser beam perpendicular to the molecular beam. The generated cations were accelerated by two DC electric fields of 200 and 3500 V/cm. After being focused by the einzel lens with a DC electric field of 850 V, the cations passed a 1.0-m-long field-free region and were detected by a dual-stacked micro-channel plate (MCP) detector. The ion signals were amplified by a preamplifier (SR445A, Stanford Research System) and then collected and analyzed by a multi-channel scaler (MCS, Stanford Research System, SR430). During the experiment, the pressures of the source and ionization chambers were maintained at approximately 2.0 103 and 1.8 105 Pa, respectively. The one-color resonant two-photon ionization (1C-R2PI) technique [20,21] was applied for acquiring the excitation spectra. The UV laser was produced by doubling the frequency of the output of the dye laser (Sirah Dye Laser-CSTR), pumped by the third harmonic of the Nd:YAG laser (Quanta-Ray, PRO-Series, 355 nm) at a repetition rate of 10 Hz. Rhodamine 590 and Coumarin 153 dyes were used. Whereas the two-color resonant two-photon ionization (2C-R2PI) technique was applied for the photo-ionization efficiency (PIE) experiments [22]. Two UV lasers were produced by doubling the frequencies of the output of two independent dye lasers (Sirah Dye Laser-CSTR), pumped by the second and third harmonic (532 and 355 nm, respectively) of the Nd:YAG laser (Quanta-Ray, PRO-Series). Rhodamine 590 and 640 dyes were used. The delay time between the two dye lasers was adjusted by changing the light path difference between the two lasers. To optimize the contrast between the one- and two-color signals, the energy ratio of the exciting and ionizing lasers was kept at 1:37. The diameter of excitation laser spot was 2.0 mm, while the diameter of the ionization laser spot was 3.0 mm. The synchronization of the whole system was controlled by a pulse delay generator (DG535, Stanford Research System). The calculations were performed by the Gaussian 09 W program package [23]. HF, MP2 and B3LYP methods were performed to optimize the molecular structures and to calculate the frequencies in the ground S0 state and the ionic ground D0 state. CIS, TDB3LYP and CASSCF methods were performed to optimize the molecular structures and to calculate the frequencies in the first excited state. For each calculation method, basis sets of 6-31g and cc-PVDZ (correlation consistent-polarized valence double zeta) were used. To search for stable molecular structures, different fixed torsional angles around the C–OCH3 bond in this paper were used to optimize the geometry. The vibrational frequencies in the S1 state were calculated using the 10-electron, 8-orbital CASSCF/ 6-31g method, which was denoted as CAS(10,8)/6-31g. The active space consists of six p-electron orbitals of the benzene ring, one
oxygen lone pair having p symmetry with respect to the molecular plane, and one chlorine lone pair having p symmetry [24]. The calculated vibrational frequencies quoted in this paper were scaled by the proper factors to approximately correct the combined errors stemming from the basis-set incompleteness and vibrational anharmonicity. 3. Results 3.1. Calculated results Fig. 1 shows the molecular structures of the cis and trans 3C4FA conformers. The carbon atoms of 3C4FA are numbered 1–6 around the phenyl ring, and the substituted portions are labeled as C1–O7– CH3, C3–Cl and C4–F. Since there is no theoretical or experimental data of the molecular structure for 3C4FA in the literature, calculations by HF, MP2 and DFT methods with the cc-PVDZ basis set were employed to optimize the geometric structures in the S0 and D0 state. The CIS and TD-DFT methods with the cc-PVDZ basis set were used to optimize the molecular structures in the S1 state. To search for all the possible proper structures of 3C4FA, different fixed torsional angles around the C1–O7 bond in this paper were used during the theoretical calculation. The calculated potential functions for the dihedral angle of C13O7C1C2 are shown in Fig. 2. Because the chlorine isotopic effect hardly influences the structures of the 3C4FA, only the calculated data of 35Cl–3C4FA in the S0, S1 and D0 state are shown in Fig. 2a–c, and listed in Table 1. In Fig. 2a, there are two global minima related to the two planar structures, cis (\C13O7C1C2 = 0°) and trans (\C13O7C1C2 = 180°) 3C4FA. The energy of trans 3C4FA is higher than that of cis 3C4FA by less than 0.2 kcal/mol. An additional minimum for the perpendicular conformer (\C13O7C1C2 = 90°) is predicted using the HF/ cc-PVDZ method. Its energy with the zero-point correction is about 0.9 kcal/mol higher than that of cis 3C4FA. A very shallow minimum at \C13O7C1C2 = 90° is also predicted by the MP2/cc-PVDZ method, which is too shallow to be considered. The perpendicular structure corresponds to a transition state according to calculated results using the B3LYP/cc-PVDZ method. Its energy with the zeropoint correction is about 2.5 kcal/mol higher than that of cis 3C4FA. Thus there are two planar conformers in the S0 state, cis and trans 3C4FA conformers, which are similar to previous results for 3-fluorophenol [25], 3-chlorophenol [26] and 3,4-difluoroanisole [27]. The energy curves in Fig. 2b are not as regular as those in Fig. 2a. But it is obvious that cis and trans conformers still exist in the S1 state. An additional minimum for the non-planar conformer
cis (∠C13O7C1C2=0°)
trans (∠C13O7C1C2=180°)
Fig. 1. Molecular structures of cis and trans 3C4FA.
94
D. Yu et al. / Journal of Molecular Structure 1000 (2011) 92–98 Table 1 The calculated geometric parameters of cis and trans 3C4FA in the S0, S1 and D0 states using the basis set of cc-PVDZ. cis S0/DFT
trans S1/TD-DFT
D0/DFT
S0/DFT
S1/TD-DFT
D0/DFT
Bond length (Å) O–CH3 1.419 O–C1 1.363 C3–Cl 1.751 C4–F 1.346 C1–C2 1.401 C2–C3 1.400 C3–C4 1.393 C4–C5 1.394 C5–C6 1.390 C6–C1 1.405
1.412 1.316 1.724 1.307 1.402 1.414 1.428 1.384 1.416 1.437
1.452 1.311 1.712 1.307 1.420 1.380 1.440 1.407 1.373 1.444
1.419 1.363 1.751 1.346 1.401 1.400 1.393 1.394 1.390 1.405
1.412 1.314 1.729 1.307 1.430 1.409 1.399 1.414 1.409 1.416
1.455 1.310 1.715 1.306 1.429 1.374 1.435 1.413 1.374 1.439
Dihedral angle (°) \C13O7C1C2 0 \C13O7C1C6 180
0 180
0 180
180 0
180 0
180 0
that of cis 3C4FA by about 0.4 kcal/mol for HF method, 1.0 kcal/mol for DFT method, and 2.4 kcal/mol for MP2 method. The perpendicular structure corresponds to a transition state according to the data calculated using HF, DFT and MP2 method. An additional minimum for the conformer with \C13O7C1C2 = 170° is predicted using the MP2/cc-PVDZ method. Its energy with the zero-point correction is about 2.4 kcal/mol above that of cis 3C4FA. But there is an imaginary frequency corresponding to the out-of-plane bending vibration of the O–CH3 bond. Its value is about 258 cm1. Therefore, this structure might be a saddle point in the two-dimensional potential energy surface. The geometric parameters of cis and trans 3C4FA in the S0, S1 and D0 state are listed in Table 1. 3.2. TOF mass spectrum Fig. 3 shows the TOF mass spectrum of 3C4FA recorded at the laser wavelength of 288.16 nm. The peaks at mass 160 and 162 result from the ion signal of the 35Cl and 37Cl isotopomers of 3C4FA, respectively. The flight times of 35Cl and 37Cl isotopomers of 3C4FA were 22.390 and 22.530 ls, respectively. The full widths at half maximum (FWHM) of the two peaks were 0.033 and 0.030 ls, respectively. This leads to an estimated t/Dt value of about 680, which corresponds to a mass resolution m/Dm of about 340. The high resolution of the TOF mass spectrometer makes the detection of the selected 3C4FA isotopomer possible.
Fig. 2. Potential functions for the dihedral angle of \C13O7C1C2 in the (a) S0, (b) S1 and (c) D0 state, calculated using ab initio and DFT methods with cc-PVDZ basis sets. The solid circle, square and triangle frames designate the calculated potential energies without the zero-point correction. The open circle, square and triangle frames designate the calculated potential energies with the zero-point correction (zero-point level, ZPL).
(\C13O7C1C2 = 10°) is predicted using the CIS/cc-PVDZ method with the zero-point correction. However, for this conformer, the benzene ring is non-planar, and the length of C–Cl bond is 2.275 Å, which is not a normal single bond. This conformer is too different from the two conformers of 3C4FA in the S0 state, so it can not be consistent with the experimental observation that the 00 peak is the most intense one in the REMPI spectra. Therefore, the cis and trans conformers are also the two optimized molecular structures in the S1 state. Fig. 2c shows two global minima related to the cis and trans 3C4FA conformers in the D0 state. The energy of trans 3C4FA is higher than
Fig. 3. TOF spectrum of 3C4FA recorded at a laser wavelength of 288.16 nm.
D. Yu et al. / Journal of Molecular Structure 1000 (2011) 92–98
3.3. 1C-R2PI spectra Fig. 4a–c show the 1C-R2PI spectra of the 35Cl and 37Cl isotopomers of 3C4FA in the energy range of 33,800–36,350 cm1. Because 3C4FA exists as cis and trans conformers in the S0 state, each 1CR2PI spectrum is believed to originate from two conformers of 3C4FA. PIE spectroscopy helps us to make sure of this, which will be discussed below. In Fig. 4a – 35Cl, the intense peaks located at 34,703 and 34,747 cm1 are respectively assigned as the band origins of the S1 S0 electronic transitions of the cis and trans
95
35
Cl–3C4FA. Similarly, the intense peaks located at 34,703 and 34,747 cm1 in Fig. 4a – 37Cl are respectively assigned to the band origins of the S1 S0 electronic transitions of the cis and trans 37 Cl–3C4FA. The difference between the first electronic transition energies of the cis and trans 3C4FA shows that the conformational effect brings changes in the electronic transition energy. The similarity of the first electronic transition energies for 35Cl and 37Cl isotopomers indicates no chlorine isotopic effect on the electronic transition energy. The same phenomenon was observed for the 35 Cl and 37Cl isotopomers of p-chlorophenol [28] and p-chloroaniline [29]. The observed bands, except the 00 band, in the 1C-R2PI spectra of 3C4FAs, should originate from the excitation of the vibrations in the S1 state. 3C4FA has 42 normal mode vibrations. Because the intensity of the vibronic band is proportional to the Franck–Condon factor [30], not all vibrations are active for the 1C-R2PI process. Table 2 lists the frequencies of the observed REMPI bands of cis and trans conformer of 35Cl–3C4FA and 37Cl–3C4FA, the calculated vibrational frequencies of the S1 state and the possible assignments. The spectral assignments were made according to the available vibrational frequencies of the similar molecules such as p-fluoroanisole [8] and m-chlorophenol [26], and the calculated results using the CAS(10,8)/6-31g method. Most of the vibrational bands in the REMPI spectra of 3C4FAs are related to the in-plane deformation of the aromatic ring. The bands at 400 (35,103), 469 (35,172), 870 (35,573) and 1304 (36,007) cm1 for cis 35Cl–3C4FA and 400 (35,103), 469 (35,172), 870 (35,573) and 1292 (35,995) cm1 for cis 37Cl–3C4FA are related 1 to the in-plane ring deformation of 6b0 , 6a10 , 110 and m(C–Cl) + b(C– H), respectively. The bands at 182 (34,885) cm1 for cis 35Cl–3C4FA and 180 (34,883) cm1 for cis 37Cl–3C4FA are then assigned to the vibration of b(O–CH3) + b(C–Cl), involving the in-plane bending vibration of the O–CH3 and C–Cl bond. The band at 116 (34,819) cm1 for cis 35Cl–3C4FA is assigned to the vibration of c(C–Cl), involving the out-of-plane bending vibration of the C–Cl bond. The out-of-plane bending of C–O–CH3 and C–F bonds is found to have frequencies of 87 (34,790) cm1 for the cis 35Cl isomer and 85 (34,788) cm1 for the cis 37Cl isomer. The bands at 383 (35,130), 864 (35,611) and 1302 (36,049) cm1 for trans 35Cl– 3C4FA and 386 (35,133), 864 (35,611) and 1290 (36,037) cm1 for trans 37Cl–3C4FA are related to the in-plane ring deformation 1 of 6b0 , 110 , and m(C–Cl) + b(C–H), respectively. The bands at 574 (35,321) cm1 for trans 35Cl–3C4FA and 577 (35,324) cm1 for trans 1 37 Cl–3C4FA are then assigned to the vibration of 17b0 . The out-ofplane bending of O–CH3 bond is found to have frequencies of 60 (34,807) cm1 for the trans 35Cl isomer and 60 (34,807) cm1 for the trans 37Cl isomer. 3.4. PIE curves
Fig. 4. 1C-R2PI spectra of the 35Cl and 37Cl isotopomers of cis and trans 3C4FA in the energy ranges of (a) 33,800–34,770 cm1, (b) 34,770–35,700 cm1 and (c) 35,700– 36,400 cm1.
The conformational isomers, cis and trans 3C4FA conformers, may coexist in a chemical sample. Species selection can be accomplished easily by applying the two-color resonant two-photon ionization (2C-R2PI) for successive excitation/ionization process. This is accomplished by detecting the prompt ions formed in the photoionization process while tuning the frequency of the ionization laser across the ionization limit and keeping the frequency of the excitation laser at a particular vibronic level of the chosen species. Fig. 5 shows the PIE curves of cis and trans 3C4FA conformers, which are recorded via their individual vibrationless levels of the S1 state. The distinct steps correspond to the ionization energies (IE) of these species. The IEs of the cis 35Cl–3C4FA and the cis 37 Cl–3C4FA are both determined to be 67,349 ± 15 cm1, whereas the IEs of the trans 35Cl–3C4FA and trans 37Cl–3C4FA are both determined to be 67,595 ± 15 cm1. The difference in the IE between two structural conformers is about 250 cm1. Similar results
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Table 2 Observed bands (cm1) in the REMPI spectrum of 3C4FAs and their possible assignments. S1 cis-Cl35
S1 cis-Cl37 a
Exp
Calc
34,703 34,790 34,819 34,885 35,103
0 87 116 182 400
108 112 168 404
35,172
469
Calc 0 85
108
c(C–O–CH3) + c(C–F) c(C–Cl)
34,883 35,103
180 400
166 403
b(O–CH3) + b(C–Cl)
460
35,172
469
459
6a10 110 m(C–Cl) + b(C–H)
35,573
870
865
35,573
870
864
36,007
1304
1290
35,995
1292
1290
1
6b0
S1 trans-Cl37 a
Exp
b
Exp 34,703 34,788
S1 trans-Cl35
a
Assignment and approximate descriptionb a
Calc
Calca
Exp
34,747 34,807 35,130
0 60 383
57 385
34,747 34,807 35,133
0 60 386
57 384
6b0
35,321
574
586
35,324
577
586
17b0
35,611
864
860
35,611
864
859
36,049
1302
1290
36,037
1290
1289
110 m(C–Cl) + b(C–H)
c(O–CH3) 1 1
Obtained from the CAS(10,8)/6–31 g calculation, scaled by 0.95. Varsanyi and Szoke’s notations [35] are applied for the ring vibrational modes; c denotes out-of-plane bending; b denotes in-plane bending; m denotes stretching.
the same conformer. Although the calculations underestimate by a few thousands of wavenumbers, they successfully predict the order of the IEs of the cis and trans 3C4FA, and the similarity of the IEs of 35Cl and 37Cl isomer for the same conformer. 4. Discussion 4.1. Molecular structure
Fig. 5. PIE spectra of
35
Cl and
37
Cl isotopomers of cis and trans 3C4FA.
have been reported for the structural isomers of m-chlorophenol [26]. We have also performed the ab initio and DFT calculations to predict the IEs. Calculations using the HF, DFT and MP2 method with the basis set of cc-PVDZ gave the adiabatic IE values of 57,204, 64,244 and 68,107 cm1 for the cis conformer, which are lower than the measured values by 15%, 5% and 1%, respectively. Calculations using the HF, DFT and MP2 method with the basis set of ccPVDZ gave the adiabatic IE values of 57,308, 64,535 and 69,223 cm1 for the trans conformer, which are lower than the measured values by 15%, 4% and 2%, respectively. There is no difference between the calculated IE values of 35Cl and 37Cl isomer for
With the hydrogen atoms of the methyl group bent out of the ring-plane by approximately 61°, the 3C4FA is non-planar and belongs to Cs symmetry point group in the S0 state. In the S1 state, 3C4FA also belongs to Cs symmetry point group with the two hydrogen atoms of the methyl group bent out of the ring by approximately 61°. Most of the C–C bonds of the aromatic ring in the S1 state are respectively longer than those in the S0 state, which means that the aromatic ring expands during the S1 S0 transition. The ring expansion is caused by the electron transfer from a bonding p orbital, the highest occupied molecular orbital (HOMO), to an anti-bonding p⁄ orbital, the lowest unoccupied molecular orbital (LUMO), during the S1 S0 transition. The C1–O bond is shortened during the S1 S0 transition, which exhibits partial double bond character in the S1 state. This may result from the enhanced p–p conjugation in the S1 state. The C3–Cl and C4–F bond are also shortened during the S1 S0 transition. This indicates that the interactions between the halogen atoms and aromatic ring in the S1 state are greater than those in the S0 state. The expansion of the ring and the shrinkage of C1–O, C3–Cl and C4–F bonds are consistent with the changes for p-fluoroanisole [8] and p-chloroanisole [10]. Compared to 3C4FA in the S1 state, the perimeter of the aromatic ring in the D0 state is shortened. This indicates that the aromatic ring shrinks during the D0 S1 transition. It may result from that the excited electron is removed from the anti-bond orbital (HOMO, p⁄). It is interesting that the C2–C3 and C5–C6 bonds are shorter than the other four C–C bonds in the D0 state. This indicates that the 3C4FA isomers in the D0 state have the quinone-like structures. 4.2. Transition and ionization energy Table 3 lists the origins of the S1 S0 electronic transition energies (E1s) and the adiabatic ionization energies (IEs) of fluoroben-
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zene [31], chlorobenzene [31,32], difluorobenzenes [12,13], dichlorobenzenes [14], anisole [33] and some anisole derivatives [8]. The E1s of fluorobenzene and chlorobenzene are respectively 37,819 and 37,048 cm1. The IEs of fluorobenzene and chlorobenzene are respectively 74,238 and 73,170 cm1. This indicates that the chloro-substitution shifts the first electronic transition energy and the ionization energy to the red with respect to the fluorosubstitution. According to the previous studies [34], substitution with the electron-donating groups usually shifts the S1 S0 transition energies of the aromatic molecules to the red, while electron-withdrawing groups usually shift the S1 S0 transition energies of the aromatic molecules to the blue. For halogen atoms, the conjugation effects between their p orbitals and the benzene p orbitals make them the electron-donating substituted groups, while the inductive effects through their r bonds make them to be the electron-withdrawing groups. Therefore, the final effect resulting from the halogen atom is decided by the competition of the two electronic effects. Since the chlorine atom is bigger and more polarizable than the fluorine atom, and then the electronegativity of the chlorine atom is weaker than that of the fluorine atom. The conjugation effect is stronger for chlorine atom than for fluorine atom, and the inductive effect is weaker for chlorine atom than for fluorine atom. Therefore, the chlorine atom is more like an electron-donating substituted group than the fluorine atom. As shown in Table 3, the E1s and IEs of o-difluorobenzene and m-difluorobenzene are blue shifted compared to those of fluorobenzene. The E1s of o-dichlorobenzene, m-dichlorobenzene and p-dichlorobenzene, as well as the IE of p-dichlorobenzene are respectively red shifted compared to those of chlorobenzene. These are consistent with the discussion above. But the E1 and IE of p-difluorobenzene are red shifted compared to those of fluorobenzene, and the IEs of o-dichlorobenzene and m-dichlorobenzene are blue shifted compared to that of chlorobenzene. We can see that the shifts of the energies caused by the dihalogen-substitution on benzenes are more complicated. It seems that, besides the property of the halogen atoms, the relative locations of the two halogen atoms on the benzene ring could influence the direction of the energy-shift. The E1 of p-fluoroanisole is red shifted compared to that of anisole, and the IE of p-fluoroanisole is blue shifted compared to that of anisole. The E1s of the cis and trans 3C4FA are red shifted compared to that of p-fluoroanisole, and the IEs of the cis and trans
3C4FA are blue shifted compared to that of p-fluoroanisole. It is clear that, for the hetero-dihalogen-substitute benzenes, the halogen-substitute effects on the first electronic transition energies and the ionization energies are consistent with the previous conclusions of studies on the monohalogen-substitute benzenes. These data also indicate that a cumulative effect exists in the transition energies of 3C4FA. As shown in Table 3, the E1 and IE of the cis 3C4FA are less than those of the trans 3C4FA. And the E1s and IEs of the 35Cl–3C4FA are the same as those of the 37Cl–3C4FA, respectively. It indicates that the conformational effect rather than the isotopic effect can substantially influence the transition and ionization energies of 3C4FA. The similarity of 35Cl and 37Cl isotopomers of each conformer in transition energy and the ionization energy measured could be explained by the calculated results. Though, due to the deficiency of the theoretical calculations, the HF, DFT and MP2 calculations may lead to a deviation in predicting the total energy, the general trend derived from the calculated results may be useful for comparison with the experimental results in terms of the isotopic effect. Although the structures of the 35Cl and 37Cl isotopomers of each conformer are identical, the energies at various states are expected to be different. Table 4 shows that, for each of the cis or trans conformers, the zero point level of the 37Cl isotopomer is lower than that of the 35Cl isotopomer for each state. The calculated results show that the differences in the zero point energy in the S0, S1 and D0 state have the same value of 7 cm1. Thus, as the energy gap among the electronic states, the electronic transition energies and the ionization energies are the same for the 35Cl and 37Cl isotopomers of each conformer. These calculated results provide a reasonable explanation of the experimental findings. 4.3. Molecular vibrations 1
For the 35Cl–3C4FA, the measured frequencies of the 6b0 , 110 and m(C–Cl) + b(C–H) vibration are 400, 870 and 1304 cm1 for the cis conformer and 383, 864 and 1302 cm1 for the trans conformer, respectively. It is clear that the measured frequencies of the three vibration modes of trans conformer are red shifted with respect to those of cis conformer. This phenomenon is also observed for 37Cl– 3C4FA. Those red-shifts of the vibrational frequencies of the trans conformer, compared with those of the cis conformer, may result from the conformational effect.
Table 3 Measured transition energies (in cm1) of fluorobenzene, chlorobenzene, difluorbenzenes, dichlorobenzenes, anisole and some of anisole-derivatives. Molecule c
Fluorobenzene o-Difluorobenzened m-Difluorobenzened p-Difluorobenzened Chlorobenzenee o-Dichlorobenzenef m-Dichlorobenzenef p-Dichlorobenzenef Anisoleg p-Fluoroanisoleh 3-Chloro-4-fluoroanisole, cisi 3-Chloro-4-fluoroanisole, transi a b c d e f g h i
E1a
DE1a
IEb
DIEb
37,819 37,824 37,909 36,840 37,048 36,238 36,193 35,752 36,383 35,149 34,703 34,747
0 5 90 979 0 810 855 1328 0 1234(0) 1680(446) 1636(402)
74,238 75,003 75,332 73,861 73,170 73,237 73,776 72,191 66,399 66,621 67,349 67,595
0 765 1094 377 0 67 606 979 0 222(0) 950(728) 1196(974)
E1 is the first electronically excited transition energy; DE1 is the shift in E1 with respect to the corresponding original molecule. IE is the ionization energy; DIE is the shift in IE with respect to the corresponding original molecule. Ref. [31]. Ref. [12,13]. Ref. [32]. Ref. [14]. Ref. [33]. Ref. [8]. This experiment.
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Table 4 Calculated energies in (Eh) of the the S0, S1 and D0 statesa.
35
Cl and
37
Cl isotopomers of cis and trans 3C4FA in
cis 37
902.380174 0.123362 902.256812
902.380174 0.123328 902.256846
902.379895 0.123327 902.256568
902.379895 0.123294 902.256602
902.167993 0.118528 902.049465
902.167994 0.118497 902.049497
902.167262 0.118623 902.048639
902.167263 0.118592 902.048671
902.119533 0.122131 901.997402
902.119533 0.122097 901.997436
902.118775 0.122047 901.996727
902.118775 0.122013 901.996762
Cl
S0 ERHF ZPC E00 S1 ECIS ZPC E0 D0 EUHF ZPC E+
trans
35
Cl
35
Cl
37
Cl
a
Calculations are at the RHF/cc-PVDZ, CIS/cc-PVDZ and UHF/cc-PVDZ levels for the S0, S1 and D0 states, respectively. ZPC is the zero-point correction and E00 , E0 and E+ are the energies at the zero-point level. 1 Hartree = 27.211 eV = 219474 cm1.
For most vibrational modes, the measured frequency differences between the 35Cl and 37Cl isotopomer of cis or trans 3C4FA are less than 3 cm1 (as shown in Table 2). Because the error limit of the measured vibrational frequency in our REMPI spectrum is ±3 cm1, the conclusion may be that the isotopic effect on the most vibrations is not strong enough to be measured by our REMPI spectrum. For vibrations involving C–Cl bond, such as m(C– Cl) + b(C–H), the frequencies of the 37Cl isomers are red shifted by 12 cm1, compared to those of the 35Cl isomers. The relatively greater isotopic effect may be caused by the higher degree of the involvement of the chorine atom in this vibration. The analysis indicates that the isotopic effect on the vibration involving the C–Cl stretching is more intense than that on the vibration involving the C–Cl bending. 5. Conclusion The 1C-R2PI spectroscopic method was applied to record the vibronic spectra of the four species, the 35Cl and 37Cl isotopomers of cis and trans 3C4FA in the S1 state. The transition energies of both the 35Cl and 37Cl isotopomers of cis 3C4FA were found to be the same, with value of 34,703 ± 3 cm1. The transition energies of both the 35Cl and 37Cl isotopomers of trans 3C4FA were found to be the same, with value of 34,747 ± 3 cm1. The general features in the vibronic spectra of both isotopomers of any conformer are similar, though the frequencies of some vibrational modes of isotopomers are slightly different by a few wavenumbers. This frequency shift is somewhat related to the degree of involvement of Cl atom in the vibrations. The general features in the vibronic spectra of the two conformers are quite different. With 2C-R2PI technique, the PIE spectra of the four species were achieved, and the adiabatic ionization energies were determined to be 67,349 ± 15 cm1 for the 35Cl and 37Cl isotopomers of the cis conformers, and 67,595 ± 15 cm1 for the 35Cl and 37Cl isotopomers of the trans conformers. Detailed analysis on the E1s, IEs and active vibrations of the four species leads to the following insights: (i) the conformational effect on the E1s and IEs of 3C4FA is remarkable, whereas the isotopic effect on the E1s and IEs of 3C4FA is negligible; (ii) the conforma-
tional effect on the active modes and the frequencies of vibrations of 3C4FA in the S1 state is greater than the isotopic effect; (iii) the isotopic effect on the vibration involving the C–Cl stretching is more intense than that on the vibration involving the C–Cl bending. Acknowledgment We gratefully acknowledge the financial support from the Center of Molecular Sciences of ICCAS under Contract No. CMS-LX200915 and the National Natural Science Foundation of China under Grant 20973180. References [1] L.H. Aung, J.L. Smilanick, P.V. Vail, P.L. Hartsell, E. Gomez, J. Agric. Food Chem. 44 (1996) 3294. [2] U. Fuhrer, K. Ballschmiter, Environ. Sci. Technol. 32 (1998) 2208. [3] P. Chatonnet, S. Bonnet, S. Boutou, M.D. Labadie, J. Agric. Food Chem. 52 (2004) 1255. [4] T. Isozaki, K. Sakeda, T. Suzuki, T. Ichimura, J. Chem. Phys. 132 (2010). [5] P.D. Singh, Indian J. Pure Appl. Phys. 7 (1969) 430. [6] K.N. Upadhya, J.N. Rai, Indian J. Pure Appl. Phys. 3 (1965) 100. [7] B.J. Ansari, D. Sharma, S.L. Srivasta, J. Mol. Spectrosc. 34 (1970) 468. [8] D.Q. Xiao, D. Yu, X.L. Xu, Z.J. Yu, Y.K. Du, Z. Gao, Q.H. Zhu, C.H. Zhang, J. Mol. Struct. 882 (2008) 56. [9] T. Isozaki, K. Sakeda, T. Suzuki, T. Ichimura, unpublished. [10] D. Yu, C. Dong, M. Cheng, L. Hu, Y. Du, Q. Zhu, C. Zhang, J. Mol. Spectrosc. 265 (2011) 86. [11] L.A. Zotti, G. Teobaldi, K. Palotas, W. Ji, H.J. Gao, W.A. Hofer, J. Comput. Chem. 29 (2008) 1589. [12] Y. Tsuchiya, K. Takazawa, M. Fujii, M. Ito, J. Phys. Chem. 96 (1992) 99. [13] C.H. Kwon, H.L. Kim, M.S. Kim, J. Chem. Phys. 118 (2003) 6327. [14] A. Gaber, M. Riese, J. Grotemeyer, J. Phys. Chem. A 112 (2008) 425. [15] O.V. Dorofeeva, Y.V. Vishnevskiy, A.N. Rykov, N.M. Karasev, N.F. Moiseeva, L.V. Vilkov, H. Oberhammer, J. Mol. Struct. 789 (2006) 100. [16] N.I. Giricheva, G.V. Girichev, J.S. Levina, H. Oberhammer, J. Mol. Struct. 703 (2004) 55. [17] F.Z. Zhang, A. Zarrine-Afsar, M.S. Al-Abdul-Wahid, R.S. Prosser, A.R. Davidson, G.A. Woolley, J. Am. Chem. Soc. 131 (2009) 2283. [18] R.P. Jakob, F.X. Schmid, J. Mol. Biol. 377 (2008) 1560. [19] Y. Norikane, N. Tamaoki, Org. Lett. 6 (2004) 2595. [20] C.E.H. Dessent, K. Müller-Dethlefs, Chem. Rev. 100 (2000) 3999. [21] B. Brutschy, Chem. Rev. 100 (2000) 3891. [22] C. Lifshitz, Int. Rev. Phys. Chem. 16 (1997) 113. [23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, GAUSSIAN 09 (Revision A.02), Gaussian, Inc., Wallingford CT, 2009. [24] S. Hirokawa, T. Imasaka, J. Phys. Chem. A 105 (2001) 9252. [25] K. Remmers, W.L. Meerts, A. Zehnacker-Rentien, K. Le Barbu, F. Lahmani, J. Chem. Phys. 112 (2000) 6237. [26] M.C.R. Cockett, M. Takahashi, K. Okuyama, K. Kimura, Chem. Phys. Lett. 187 (1991) 250. [27] J. Han, R.L. Deming, F.M. Tao, J. Phys. Chem. A 108 (2004) 7736. [28] J.G. Huang, J.L. Lin, W.B. Tzeng, Chem. Phys. Lett. 422 (2006) 271. [29] J.L. Lin, W.B. Tzeng, J. Chem. Phys. 113 (2000) 4109. [30] F.T. Chau, J. Mol. Spectrosc. 103 (1984) 66. [31] K. Walter, K. Scherm, U. Boesl, J. Phys. Chem. 95 (1991) 1188. [32] T.G. Wright, S.I. Panov, T.A. Miller, J. Chem. Phys. 102 (1995) 4793. [33] M. Pradhan, C. Li, J.L. Lin, W.B. Tzeng, Chem. Phys. Lett. 2005 (2005) 100. [34] J.L. Lin, W.B. Tzeng, Chem. Phys. Lett. 370 (2003) 44. [35] G. Varsányi, S. Szoke, Vibrational Spectra of Benzenes Derivatives, Academic Press, New York, 1969.