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Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of the 35Cl and 37Cl isotopomers of p-chloroanisole
Journal of Molecular Spectroscopy 265 (2011) 86–91
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Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of the 37 Cl isotopomers of p-chloroanisole
35
Cl and
Dan Yu, Changwu Dong, 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 16 September 2010 In revised form 23 November 2010 Available online 4 December 2010 Keywords: R2PI p-Chloroanisole Ab initio Isotope effect
a b s t r a c t The geometric structures and vibrations of p-chloroanisole isotopomers in the first electronically excited state were studied by mass-analyzed resonance-enhanced two-photon ionization spectroscopy and by S0 electronic transitions of 35Cl and 37Cl isotopomers theoretical calculations. The band origins of the S1 were found to be equivalent at 34 859 ± 3 cm1. Assignments of the observed vibrational bands of the two isotopomers were made mainly based on the CIS/cc-PVDZ calculations and on conformity with the available data in the literature. Although the general spectral features of these two isotopomers are similar, the frequencies of some vibrational modes are different. This frequency shift partially depends on the degree of involvement of the chlorine atom in the molecular vibrations. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Chloroanisoles are of substantial interest due to their high environmental impact. They have been extensively found to exist in the air, in marine and freshwater fish, in sediments and even in drinking water treated by the chlorination process [1,2]. Chloroanisoles also cause serious problems in the food industry, especially in the enological industry, and can result in large economic losses [3,4]. For example, the presence of certain anisole chlorine derivatives are related to generally tainted foods and the musty odors of wine [5]. Therefore, the determination and degradation of chloroanisoles has been the subject of many experimental and theoretical studies [6–9]. As a model compound of chloroanisoles, p-chloroanisole has caught much attention from researchers. Its detection and degradation have been extensively studied [8–10]. However, there are few publications on its molecular properties in different electronic states [11]. Studies on the geometric structures, energies and vibrations of p-chloroanisole in the electronically excited S1 state can elucidate its photochemical and catalytic degradation. In this paper, we report REMPI spectroscopic and theoretical studies on p-chloroanisole. Due to the natural abundance of chlorine isotopes, the 35Cl and 37Cl isotopomers of p-chloroanisole were detected by high-resolution time-of-flight (TOF) mass spectrometry, and thus, the resonance-enhanced two-photon ionization (R2PI) spectra [12,13] of the two isotopomers were measured. Because the R2PI process occurs when the laser wavelength is tuned to an intermediate state of the molecule, the R2PI spectrum of p-
⇑ Corresponding author. Fax: +86 10 62563167. E-mail address: [email protected] (Y. Du). 0022-2852/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2010.11.006
chloroanisole not only provides useful information about the molecular properties, but also presents a promising method for the determination of p-chloroanisole [14,15]. Calculations by ab initio and density functional theory (DFT) methods were performed to predict the molecular geometry of p-chloroanisole and to interpret the experimental results. Generally, the REMPI technique combined with the theoretical method was used to study the optimized structures, energies and the vibrational levels of 35Cl and 37 Cl p-chloroanisole in the S1 state. Moreover, based on the experimental results, the halogen substitution effect, large atom effect and isotope effect on the molecular properties were examined.
2. Experimental and computational methods p-Chloroanisole (Aldrich, 99% purity) was used without further purification. To acquire sufficient vapor pressure, the sample was heated to 353 K. The experimental system consisted of a time-offlight mass spectrometer (TOF-MS) and a pulsed supersonic molecular beam source, which has been described in previous publications [16,17]. The p-chloroanisole was carried by argon (approximately 2 atm) and expanded into the source chamber through a pulsed valve (General Valve) with an orifice 0.25 mm in diameter. After being collimated by a 1-mm-diameter orifice skimmer, the molecular beam entered the ionization chamber. Then, the ionization of p-chloroanisole 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
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3. Results 3.1. TOF mass spectrum Fig. 1 shows the TOF mass spectrum of p-chloroanisole recorded at the laser wavelength of 280.59 nm. The peaks at mass 142 and 144 result from the ion signal of the 35Cl and 37Cl isotopomers of p-chloroanisole, respectively. The flight times of 35Cl and 37Cl isotopomers of p-chloroanisole were 21.070 and 21.215 ls, respectively. The full widths at half maximum (FWHM) of the two peaks were 0.031 and 0.034 ls, respectively. This leads to an estimated t/Dt value of 630, which corresponds to a mass resolution m/Dm of 315. The high resolution of the TOF mass spectrometer
makes the detection of the 1C-R2PI spectrum of the selected pchloroanisole isotopomer possible. Two small peaks at mass 143 and 145 are observable in Fig. 1 and can be assigned to the 13C isotopomers of 35Cl- and 37Cl-pchloroanisole, respectively. Due to the natural abundance of carbon isotopes, there are 7.02% p-chloroanisole molecules that contain one 13C atom and six 12C atoms. Because the 13C atom can replace any of the seven 12C atoms in the p-chloroanisole molecule, there are seven 13C isotopomers of 35Cl- or 37Cl-p-chloroanisole that are related to each of the two small peaks. This effect severely complicates the measurement of the R2PI spectrum of the 13C isotopomers. However, the average effect of the 13C isotopomers on the R2PI spectroscopy of the 35Cl and 37Cl isotopomers is negligible. 3.2. 1C-R2PI spectra Fig. 2a and b shows the 1C-R2PI spectra of the 35Cl and 37Cl isotopomers of p-chloroanisole in the energy range of 0–1750 cm1 relative to their S1 S0 electronic transition origins. Because pchloroanisole exists as a single conformer in the neutral ground S0 state and excited S1 state (as discussed below), each of the two 1C-R2PI spectra is believed to originate from one species of p-chloroanisole. In the 1C-R2PI spectrum of the 35Cl isotopomer, the intense peak located at 34 859 cm1 is assigned as the band
(a)
00
35
Cl
1 0
6a
7a10
Ion Intensity
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). The UV laser was produced by the double frequency of the dye laser (Sirah Dye LaserCSTR), pumped by the second harmonic or the third harmonic of the Nd:YAG laser (Quanta-Ray, PRO-Series) at a repetition rate of 10 Hz. The synchronization of the Nd:YAG laser with the pulsed valve was controlled by a pulse delay generator (DG535, Stanford Research System). Rhodamine 590 (532-nm pump) and Coumarin 153 (355-nm pump) dyes were used in the experiments. During the operation, the pressures of the source and ionization chambers were maintained at approximately 2.0 103 and 4.0 105 Pa, respectively. The calculations were performed by the Gaussian 09W program package [18]. The carbon atoms of p-chloroanisole are labeled 1–6 around the phenyl ring, and the substituted portions are numbered C1AOACH3 and C4ACl. Both HF 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. Both CIS and TD-B3LYP methods were performed to optimize the molecular structures and to calculate the frequencies in the first excited state. To search for stable molecular structures, various basis sets, namely, 6-31G+(d,p), 6-311G++(d,p) and cc-PVDZ (correlation consistent-polarized valence double zeta), and various starting geometric structures were used to optimize the geometry. The calculated vibrational frequencies quoted in this paper were scaled by the proper factor to approximately correct the combined errors stemming from the basis-set incompleteness and vibrational anharmonicity.
1 0
16b
37
00
Cl
7a10
-100
0
100
200
300
6a10
1 0
16b
400
500
600
700
800
900
-1
Wavenumber / cm
(b) 35
Cl
1 0
3 1310
35
Ion Intensity
Ion Intensity
Cl
37
Cl
6a
Cl 1310 900
mass Fig. 1. TOF spectrum of p-chloroanisole recorded at a laser wavelength of 280.59 nm.
6a
2 0
37
115 120 125 130 135 140 145 150 155 160 165 170
2 0
310
1000 1100 1200 1300 1400 1500 1600 1700
Wavenumber / cm-1 Fig. 2. (a) 1C-R2PI spectra of 35Cl and 37Cl isotopomers of p-chloroanisole in the energy range of 100 to 900 cm1 from the band origin. (b) 1C-R2PI spectra of 35Cl and 37Cl isotopomers of p-chloroanisole in the energy range of 850–1800 cm1 from the band origin.
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D. Yu et al. / Journal of Molecular Spectroscopy 265 (2011) 86–91
origin of the S1 S0 electronic transition. Similarly, the intense peak located at 34 859 cm1 in the 1C-R2PI spectrum of the 37Cl isotopomer can be assigned to the band origin of the S1 S0 transition. The similarity of the first electronic transition energies for 35 Cl and 37Cl isotopomers indicates no chlorine isotope effect on the electronic transition energy. The same phenomenon was observed for the 35Cl and 37Cl isotopomers of p-chlorophenol [19] and p-chloroaniline [20]. The observed bands, except the 00 band in the REMPI spectra of p-chloroanisole, should originate from the excitation of the vibrations in the S1 state. p-chloroanisole has 42 normal mode vibrations, including 30 benzene-like vibrations and 12 OCH3 vibrations. Because the intensity of the vibronic band is proportional to the Franck–Condon factor, not all vibrations are active for the 1C-R2PI process. Table 1 lists the frequencies of the observed REMPI bands of both the 35Cl and 37Cl isotopomers of pchloroanisole, the vibrational frequencies of p-chloroanisole in the S0 state, the calculated vibrational frequencies of the S1 state and the possible assignments. The spectral assignments were made according to the available vibrational data of p-chloroanisole in the S0 state [21], the vibrational frequencies of the similar molecules [19,20,22,23] and our theoretical calculations. Most of the vibrational bands in the REMPI spectra of 35Cl- and 37 Cl-p-chloroanisole are related to the in-plane deformation of the aromatic ring. The intense bands at 344 and 780 cm1 for 35Cl-pchloroanisole and 341 and 780 cm1 for 37Cl-p-chloroanisole are related to the in-plane ring deformation of 7a10 and 6a10 , respectively. The relatively weak bands at 623, 1004, 1055, 1123, 1169, 1276 and 1396 cm1 for 35Cl-p-chloroanisole and 623, 1003, 1053, 1120, 1169, 1276 and 1394 cm1 for 37Cl-p-chloroanisole are then assigned to the transitions of 1210 , 18a10 , 110 , 1310 , 1410 , 1 310 and 19b0 , respectively. These vibrations are also related to the in-plane vibrations of the aromatic ring. The bands at 480 cm1 for both 35Cl and 37Cl isotopomers result from the transition of 1 16b0 , involving the out-of-plane vibration mode of the aromatic ring. The two bands at 1260 and 1558 cm1 for both 35Cl and 37 Cl isotopomers are tentatively assigned to the combination bands. The weak peaks at 188 and 207 cm1 for 35Cl-p-chloroanisole and 166 and 183 cm1 for 37Cl-p-chloroanisole are then assigned to the bending vibrations of the CAOCH3 and the CACl bonds.
4. Discussion 4.1. Molecular structure According to the Franck–Condon principle, the probability of a vibrational transition and, thus, the relative intensity of a vibrational band is proportional to the Franck–Condon factor, which is governed by the structural difference between two electronic states. Therefore, the structures of p-chloroanisole in the S0 and S1 state are useful for the analysis of the 1C-R2PI spectra. Because there are no experimental data available in the literature, ab initio and DFT calculations were performed to optimize the structures of p-chloroanisole in the different states. HF and B3LYP calculations with various basis sets were performed to predict the structures of p-chloroanisole in the S0 and D0 states, whereas CIS and TDB3LYP calculations with various basis sets were performed to predict the structure of p-chloroanisole in the S1 state. Table 2 lists the calculated parameters with the basis set of cc-PVDZ. Because there are no differences between the structures of the 35Cl isotopomer in the S0, S1 and D0 states and those of the 37Cl isotopomer in the respective states calculated with the same methods and basis sets, only the geometric parameters of the 35Cl isotopomer in the S0, S1 and D0 states are listed in Table 2. In the S0 state, p-chloroanisole is non-planar with the hydrogen atoms of the methyl group bent out of the plane of the ring by approximately 61° and possesses a symmetry point group of Cs. In the S1 state, p-chloroanisole also possesses a symmetry point group of Cs with the two hydrogen atoms of the methyl group bending out of the ring by 61°. The bond lengths of the CAC bonds of the aromatic ring in the S1 state are increased when compared to those in the S0 state. This indicates that the aromatic ring expands during the S1 S0 transition. The ring expansion is caused by the electron transfer from the highest occupied molecular orbital (HOMO, p) to the lowest unoccupied molecular orbital (LUMO, p⁄) during the S1 S0 transition. The C1AO bond is shortened during the S1 S0 transition, which exhibits partial double bond character in the excited S1 state. This may result from the enhanced p– p conjugation in the S1 state. The C4ACl bond is also shortened during the S1 S0 transition. This indicates that the interaction between the chlorine atom and aromatic ring in the S1 state is stronger than that in the S0 state. The expansion of the ring and the
Table 1 Observed bands (cm1) in the REMPI spectrum of p-chloroanisole and their possible assignments. S0a
S1 Cl35 Exp.
S1 Cl37 Calc.
b
Exp.
50
16a10
166
123
207 344
200 334
183 341
200 331
506
480
481
480
481
16b0
636
623
609
623
609
1210 6a10
16a10 , c(O–CH3) + c(C–Cl) b(O–CH3) + b(C–Cl) 7a10 1
798
780
771
780
771
1005
1004
955
1003
955
1094
1055
1018
1053
1018
1248
1123
1054
1120
1054
1310
1280
1169
1127
1169
1127
1410
1293
1276
1303
1276
1303
310
1442
1396
1406
1394
1406
19b0
1558
c
50
123
1260
a
Calc.
188 365
b
Assignment and approximate descriptionc b
1
1260
1558
18a10 110
6a10 6b0 1
6a20
Ref. [22]. Obtained from the CIS/cc-PVDZ calculation, scaled by 0.90. Varsanyi and Szoke’s notations are applied for the ring vibrational modes; c denotes out-of-plane bending; b denotes in-plane bending.
89
D. Yu et al. / Journal of Molecular Spectroscopy 265 (2011) 86–91 Table 2 The calculated geometric parameters of p-chloroanisole in the S0, S1 and D0 states using the basis set of cc-PVDZ. S0 RHF
S1 RB3LYP
CIS
D0 TD-B3LYP
UHF
Table 3 The deviation of the calculated (a) E1 and (b) IE with respect to the experimental result. (a)
UB3LYP
Bond length (Å) OACH3 1.400 OAC1 1.346 C4ACl 1.750 C1AC2 1.388 C2AC3 1.393 C3AC4 1.378 C4AC5 1.390 C5AC6 1.379 C6AC1 1.396
1.419 1.363 1.762 1.402 1.400 1.392 1.399 1.391 1.405
1.409 1.320 1.725 1.417 1.412 1.415 1.403 1.411 1.424
1.426 1.352 1.739 1.421 1.430 1.418 1.414 1.429 1.424
1.450 1.267 1.706 1.438 1.365 1.422 1.418 1.368 1.442
1.454 1.309 1.704 1.431 1.374 1.426 1.422 1.371 1.437
Bond angle (°) \C1OCH3 119.8 \C2C1O 124.8 \C6C1O 115.8 \C3C4Cl 120.0 \C5C4Cl 119.7 \C6C1C2 119.3 \C1C2C3 120.0 \C2C3C4 120.1 \C3C4C5 120.3 \C4C5C6 119.7 \C5C6C1 120.6
118.2 124.9 115.7 119.8 119.6 119.4 120.0 120.0 120.5 119.5 120.6
121.4 123.6 114.5 118.0 118.8 121.9 118.6 118.8 123.2 118.2 119.3
120.2 123.0 113.7 117.6 118.0 123.3 118.1 118.0 124.4 117.6 118.6
124.7 124.4 115.4 119.3 119.8 120.2 118.9 120.6 120.9 119.6 119.8
122.3 124.8 114.8 119.2 119.6 120.4 119.2 120.0 121.2 119.2 120.1
35
E1
shrinkage of C1AO and C4ACl bonds are consistent with the changes in p-chlorophenol [24–26]. This result indicates that pchlorophenol can be used for the comparative analysis of pchloroanisole. As shown in Table 1, the frequency of each vibrational mode of the ring deformation in the S1 state is less than that in the S0 state. This indicates that the aromatic ring of p-chloroanisole in the S1 state is not as rigid as that in the S0 state. This argument is supported by the calculated results, in which the perimeter of the aromatic ring in the S1 state is slightly greater than that in the S0 state. Compared to p-chloroanisole in the S1 state, the bond lengths of the CAC bonds of the aromatic ring in the D0 state are shortened. This indicates that the aromatic ring shrinks during the D0 S1 transition. This results from the excited electron being removed from the anti-bond orbital (HOMO, p⁄), so that the aromatic ring reverts. The bond lengths of the C1AO and C4ACl are shortened during the D0 S1 transition. This indicates that the interaction between the aromatic ring and substituted groups in the D0 state is stronger than that in the S1 state. 4.2. Transition energy Table 3 lists the transition energy of 35Cl- and 37Cl-p-chloroanisole determined by experiments and calculations. The deviation in the calculated first electronically excited transition energy (E1) with respect to the experimental result is more than 20% for the CIS method, while the deviation is less than 5% for the TD-B3LYP method. It seems that the TD-B3LYP method is more accurate for the prediction of E1, compared to the CIS method. For the ionization energy (IE), the deviation in the calculated results is approximately 15% for the HF method and approximately 4% for the DFT method. It seems that the DFT method is more accurate for the prediction of the IE when compared to the HF method. As shown in Table 3, the calculated E1 of 35Cl-p-chloroanisole is the same as that of the 37Cl-p-chloroanisole for the same method with the same basis set. The similarity of the calculated E1s is consistent with that of the experimental results. Table 4 lists the origins of the S1 S0 electronic transition energies (E01 s) of phenol [27–29], anisole [30–32], aniline [33–35] and
IE
E1 (cm1)
Deviation (%)
34 859 42 436 42 287 45 349 34 704 34 787
– 21.7 21.3 30.1 0.4 0.2
34 859 42 436 42 287 45 349 34 704 34 787
– 21.7 21.3 30.1 0.4 0.2
36 520
4.8
36 520
4.8
)
35
37
Cl isotopomer
IE (cm1) Experimental result HF/631+dp HF/6311++dp HF/cc-PVDZ B3LYP/631+dp B3LYP/6311 ++dp B3LYP/ccPVDZ
1
Cl isotopomer
Deviation (%)
E1 (cm Experimental result CIS/631+dp CIS/6311++dp CIS/cc-PVDZ TD-B3LYP/631+dp TD-B3LYP/ 6311++dp TD-B3LYP/cc-PVDZ (b)
37
Cl isotopomer
IE (eV)
Cl isotopomer
Deviation (%)
8.25
–
56 187 56 226 55 662 64 077 64 427
6.97 6.97 6.90 7.94 7.99
15.1 15.1 16.4 3.8 3.1
63 135
7.83
5.1
IE (cm1)
IE (eV)
Deviation (%)
8.25
–
56 187 56 226 55 662 64 077 64 427
6.97 6.97 6.90 7.94 7.99
15.1 15.1 16.4 3.8 3.1
63 135
7.83
5.1
Table 4 Measured transition energies (in cm1) of phenol, anisole and some of its parahalogen derivativesa. Molecule
E1
4E1
Phenolb p-Fluorophenolc p-Chlorophenold Anisolee p-Fluoroanisolef p-Chloroanisoleg Anilineh p-Fluoroanilineh p-Chloroanilinei
36 349 35 117 34 813 36 383 35 149 34 859 34 029 32 652 32 572
0 1232 1536 0 1234 1524 0 1377 1457
a E1 is the first electronically excited transition energy; DE1 is the shift in E1 with respect to the corresponding original molecule. b Ref. [27]. c Ref. [36]. d Ref. [19]. e Ref. [30]. f Ref. [16]. g This experiment. h Ref. [33]. i Ref. [20].
their para-halogen substituted derivatives [16,20,22,33,36]. The band origins of the S1 S0 vibrationless transitions of p-fluoroanisole and p-chloroanisole are 35 146 and 34 859 cm1, which are red-shifted by 1234 and 1524 cm1, respectively, relative to that of anisole. Similarly, the E1s of p-fluorophenol and p-chlorophenol are red-shifted by 1232 and 1536 cm1 compared to that of phenol. The E1s of p-fluoroaniline and p-chloroaniline are red-shifted by 1377 and 1456 cm1 relative to that of aniline. This indicates that the para-substitution of the halogen atom on the aromatic ring causes the E1 red-shift. The degree of the red-shift induced by the chlorine substitution is larger than that induced by the fluorine substitution. As mentioned in the previous sections, the S1 S0 excitation is mainly subject to a p⁄ p transition [19,20,30,33,36]. Therefore, the change in the p-electron density around the ring influences the transition energy of the molecule. It was found that the substitution of a functional group on a phenyl
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D. Yu et al. / Journal of Molecular Spectroscopy 265 (2011) 86–91
Table 5 Calculated energies in (Eh) of the S0, S1 and D0 states.a 35
35
Cl and
37
Cl isotopomers of p-chloroanisole in the
Cl
37
Cl
S0 ERHF ZPC E00
803.530048 0.131734 803.398314
803.530048 0.131703 803.398345
S1 ECIS ZPC E0
803.316703 0.126572 803.190131
803.316703 0.126541 803.190162
D0 EUHF ZPC E+
803.276434 0.130450 803.145984
803.276434 0.130418 803.146017
the two isotopomers of p-chloroanisole are identical, the energies at various states are expected to be different. Table 5 shows that 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 states have the same value (7 cm1). Thus, the electronic transition energy and the ionization energy are the same for these two isotopomers. Because the chlorine atom is relatively far away from the methoxy group in p-chloroanisole, there is no sterically hindered effect between the chlorine atom and the methoxy group, and the isotope effect on the electronic transition and ionization energy is insignificant. These calculated results provide a reasonable explanation of the experimental findings. 4.3. Isotope effect on molecular vibrations
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 = 219 474 cm1.
ring can cause a significant change in the nearby p-electron density and the molecular geometry, which led to a decrease in the ZPL (zero-point level) energy of an electronic state. The extent of the ZPL decrease reflects the strength of the interaction of the substituent and the ring. If the ZPL energy decrease of the upper electronic state is greater than that of the lower electronic state, it gives rise to a red shift in the transition energy. In the opposite case, it causes a blue shift. The interaction between the halogen atom and the aromatic ring may result from the inductive effect through the r bond and the conjugation effect through the p orbitals. The former depends on the electronegativity of the main atom in the substituted group, while the latter reflects the overtone of the p system of the aromatic ring and the p orbital of the main atom in the substituted group. The observed red-shift in the first electronic transition energy suggests that the interaction between the halogen atom and the ring is stronger in the S1 state than in the S0 state. The larger chlorine atom has a greater p-electron overlap with the ring than the fluorine atom. Thus, the red-shift in the E1 induced by chlorine substitution is greater than that induced by fluorine substitution. Due to the deficiency of the theoretical calculations, the ab initio and DFT calculations may lead to a deviation in the prediction of the total energy. However, the general trend obtained from the calculated results may be useful for comparison with the experimental results in terms of the isotope effect. Although the structures of
For the ring deformation vibrations, the frequency difference between the 35Cl and 37Cl isotopomers of p-chloroanisole is less than 3 cm1. Because the error limit of the measured vibrational frequency in our REMPI spectrum is ± 3 cm1, it is difficult to confirm the existence of the frequency differences between the 35Cl and 37Cl isotopomers. The conclusion may be that the isotope effect on the normal vibrations of the aromatic ring is not strong enough to be measured by our REMPI spectrum. For the substituted group vibrations, the frequencies of the 16a10 vibration, c(OACH3) + c(CACl) vibration and b(OACH3) + b(CACl) vibration are 188 and 207 cm1 for the 35Cl isotopomer and 166 and 183 cm1 for the 37Cl isotopomer, respectively. The frequencies of the 16a10 vibration, c(OACH3) + c(CACl) vibration and b(OACH3) + b(CACl) vibration of the 37Cl-p-chloroanisole are redshifted by 22 and 24 cm1 compared to those of the 35Cl-p-chloroanisole. It can be deduced that the red-shift of the two vibrational frequencies is caused by the isotope effect of the chorine atom. The analysis indicates that the isotope effect on the c(OACH3) + c(CACl) and b(OACH3) + b(CACl) vibrations is stronger than that on the normal ring deformation vibrations. This might be related to the higher degree of the involvement of the chlorine atom in the two vibrations than that in the normal ring deformation vibrations, as shown in Fig. 3. 5. Conclusion The REMPI spectroscopic method was applied to record the vibronic spectra of the 35Cl and 37Cl isotopomers of p-chloroanisole in
Fig. 3. Vibrational structures of 35Cl and 37Cl isotopomers of p-chloroanisole in the S1 state. The open circles designate the original locations of the carbon atoms, and the solid dots mark the displacements.
D. Yu et al. / Journal of Molecular Spectroscopy 265 (2011) 86–91
the S1 state. The transition energies of both isotopic species were found to be equivalent, at 34 859 cm1. Comparing the E1 and IE of p-chloroanisole with those of p-fluoroanisole led to the following insights: (i) the p-electron overlap between the halogen atom and the aromatic ring is responsible for the observed red-shift in the origin of the S1 S0 excitation; and (ii) because the chlorine atom is larger than the fluorine atom, the red-shift in the E1 of pchloroanisole is greater than that of p-fluoroanisole. The analysis of the frequencies of the vibrations for 35Cl and 37Cl p-chloroanisole in the S1 state shows that the frequency shifts of the vibrational modes of isotopomers partially depends on the degree of the involvement of chlorine atom in the molecular motion. These experimental findings are supported by ab initio and DFT calculations. Acknowledgments We gratefully acknowledge the financial support from the Center of Molecular Sciences of ICCAS under Contract No. CMSLX200915 and the National Natural Science Foundation of China under Grant 20973180. References [1] U. Fuhrer, K. Ballschmiter, Environ. Sci. Technol. 32 (1998) 2208–2215. [2] C. Flodin, F.B. Whitfield, Phytochemistry 53 (2000) 77–80. [3] P. Chatonnet, S. Bonnet, S. Boutou, M.D. Labadie, J. Agric. Food Chem. 52 (2004) 1255–1262. [4] S. Insa, V. Salvado, E. Antico, J. Chromatogr. A 1122 (2006) 215–221. [5] L.H. Aung, J.L. Smilanick, P.V. Vail, P.L. Hartsell, E. Gomez, J. Agric. Food Chem. 44 (1996) 3294–3296. [6] R.M. Callejon, A.M. Troncoso, M.L. Morales, Talanta 71 (2007) 2092–2097. [7] A. Diaz, F. Ventura, T. Galceran, J. Chromatogr. A 1064 (2005) 97–106. [8] Y. Ukisu, Appl. Catal. A 349 (2008) 229–232. [9] J.P. Da Silva, L. Ferreira, I. Osipov, I. Machado, J. Hazard. Mater. 179 (2010) 187– 191. [10] J.P. Da Silva, S. Jockkusch, J.M.G. Martinho, M.F. Ottaviani, N.J. Turo, Org. Lett. 12 (2010) 3062–3065. [11] J.N. Rai, K.N. Upadhya, Spectrochim. Acta 22 (1966) 1427–1430. [12] C.E.H. Dessent, K. Müller-Dethlefs, Chem. Rev. 100 (2000) 3999–4021. [13] B. Brutschy, Chem. Rev. 100 (2000) 3891–3920. [14] R. Tembreull, D.M. Lubman, Anal. Chem. 56 (1984) 1962–1967.
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Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of the 37 Cl isotopomers of p-chloroanisole
35
Cl and
Dan Yu, Changwu Dong, 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 16 September 2010 In revised form 23 November 2010 Available online 4 December 2010 Keywords: R2PI p-Chloroanisole Ab initio Isotope effect
a b s t r a c t The geometric structures and vibrations of p-chloroanisole isotopomers in the first electronically excited state were studied by mass-analyzed resonance-enhanced two-photon ionization spectroscopy and by S0 electronic transitions of 35Cl and 37Cl isotopomers theoretical calculations. The band origins of the S1 were found to be equivalent at 34 859 ± 3 cm1. Assignments of the observed vibrational bands of the two isotopomers were made mainly based on the CIS/cc-PVDZ calculations and on conformity with the available data in the literature. Although the general spectral features of these two isotopomers are similar, the frequencies of some vibrational modes are different. This frequency shift partially depends on the degree of involvement of the chlorine atom in the molecular vibrations. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Chloroanisoles are of substantial interest due to their high environmental impact. They have been extensively found to exist in the air, in marine and freshwater fish, in sediments and even in drinking water treated by the chlorination process [1,2]. Chloroanisoles also cause serious problems in the food industry, especially in the enological industry, and can result in large economic losses [3,4]. For example, the presence of certain anisole chlorine derivatives are related to generally tainted foods and the musty odors of wine [5]. Therefore, the determination and degradation of chloroanisoles has been the subject of many experimental and theoretical studies [6–9]. As a model compound of chloroanisoles, p-chloroanisole has caught much attention from researchers. Its detection and degradation have been extensively studied [8–10]. However, there are few publications on its molecular properties in different electronic states [11]. Studies on the geometric structures, energies and vibrations of p-chloroanisole in the electronically excited S1 state can elucidate its photochemical and catalytic degradation. In this paper, we report REMPI spectroscopic and theoretical studies on p-chloroanisole. Due to the natural abundance of chlorine isotopes, the 35Cl and 37Cl isotopomers of p-chloroanisole were detected by high-resolution time-of-flight (TOF) mass spectrometry, and thus, the resonance-enhanced two-photon ionization (R2PI) spectra [12,13] of the two isotopomers were measured. Because the R2PI process occurs when the laser wavelength is tuned to an intermediate state of the molecule, the R2PI spectrum of p-
⇑ Corresponding author. Fax: +86 10 62563167. E-mail address: [email protected] (Y. Du). 0022-2852/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2010.11.006
chloroanisole not only provides useful information about the molecular properties, but also presents a promising method for the determination of p-chloroanisole [14,15]. Calculations by ab initio and density functional theory (DFT) methods were performed to predict the molecular geometry of p-chloroanisole and to interpret the experimental results. Generally, the REMPI technique combined with the theoretical method was used to study the optimized structures, energies and the vibrational levels of 35Cl and 37 Cl p-chloroanisole in the S1 state. Moreover, based on the experimental results, the halogen substitution effect, large atom effect and isotope effect on the molecular properties were examined.
2. Experimental and computational methods p-Chloroanisole (Aldrich, 99% purity) was used without further purification. To acquire sufficient vapor pressure, the sample was heated to 353 K. The experimental system consisted of a time-offlight mass spectrometer (TOF-MS) and a pulsed supersonic molecular beam source, which has been described in previous publications [16,17]. The p-chloroanisole was carried by argon (approximately 2 atm) and expanded into the source chamber through a pulsed valve (General Valve) with an orifice 0.25 mm in diameter. After being collimated by a 1-mm-diameter orifice skimmer, the molecular beam entered the ionization chamber. Then, the ionization of p-chloroanisole 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
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3. Results 3.1. TOF mass spectrum Fig. 1 shows the TOF mass spectrum of p-chloroanisole recorded at the laser wavelength of 280.59 nm. The peaks at mass 142 and 144 result from the ion signal of the 35Cl and 37Cl isotopomers of p-chloroanisole, respectively. The flight times of 35Cl and 37Cl isotopomers of p-chloroanisole were 21.070 and 21.215 ls, respectively. The full widths at half maximum (FWHM) of the two peaks were 0.031 and 0.034 ls, respectively. This leads to an estimated t/Dt value of 630, which corresponds to a mass resolution m/Dm of 315. The high resolution of the TOF mass spectrometer
makes the detection of the 1C-R2PI spectrum of the selected pchloroanisole isotopomer possible. Two small peaks at mass 143 and 145 are observable in Fig. 1 and can be assigned to the 13C isotopomers of 35Cl- and 37Cl-pchloroanisole, respectively. Due to the natural abundance of carbon isotopes, there are 7.02% p-chloroanisole molecules that contain one 13C atom and six 12C atoms. Because the 13C atom can replace any of the seven 12C atoms in the p-chloroanisole molecule, there are seven 13C isotopomers of 35Cl- or 37Cl-p-chloroanisole that are related to each of the two small peaks. This effect severely complicates the measurement of the R2PI spectrum of the 13C isotopomers. However, the average effect of the 13C isotopomers on the R2PI spectroscopy of the 35Cl and 37Cl isotopomers is negligible. 3.2. 1C-R2PI spectra Fig. 2a and b shows the 1C-R2PI spectra of the 35Cl and 37Cl isotopomers of p-chloroanisole in the energy range of 0–1750 cm1 relative to their S1 S0 electronic transition origins. Because pchloroanisole exists as a single conformer in the neutral ground S0 state and excited S1 state (as discussed below), each of the two 1C-R2PI spectra is believed to originate from one species of p-chloroanisole. In the 1C-R2PI spectrum of the 35Cl isotopomer, the intense peak located at 34 859 cm1 is assigned as the band
(a)
00
35
Cl
1 0
6a
7a10
Ion Intensity
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). The UV laser was produced by the double frequency of the dye laser (Sirah Dye LaserCSTR), pumped by the second harmonic or the third harmonic of the Nd:YAG laser (Quanta-Ray, PRO-Series) at a repetition rate of 10 Hz. The synchronization of the Nd:YAG laser with the pulsed valve was controlled by a pulse delay generator (DG535, Stanford Research System). Rhodamine 590 (532-nm pump) and Coumarin 153 (355-nm pump) dyes were used in the experiments. During the operation, the pressures of the source and ionization chambers were maintained at approximately 2.0 103 and 4.0 105 Pa, respectively. The calculations were performed by the Gaussian 09W program package [18]. The carbon atoms of p-chloroanisole are labeled 1–6 around the phenyl ring, and the substituted portions are numbered C1AOACH3 and C4ACl. Both HF 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. Both CIS and TD-B3LYP methods were performed to optimize the molecular structures and to calculate the frequencies in the first excited state. To search for stable molecular structures, various basis sets, namely, 6-31G+(d,p), 6-311G++(d,p) and cc-PVDZ (correlation consistent-polarized valence double zeta), and various starting geometric structures were used to optimize the geometry. The calculated vibrational frequencies quoted in this paper were scaled by the proper factor to approximately correct the combined errors stemming from the basis-set incompleteness and vibrational anharmonicity.
1 0
16b
37
00
Cl
7a10
-100
0
100
200
300
6a10
1 0
16b
400
500
600
700
800
900
-1
Wavenumber / cm
(b) 35
Cl
1 0
3 1310
35
Ion Intensity
Ion Intensity
Cl
37
Cl
6a
Cl 1310 900
mass Fig. 1. TOF spectrum of p-chloroanisole recorded at a laser wavelength of 280.59 nm.
6a
2 0
37
115 120 125 130 135 140 145 150 155 160 165 170
2 0
310
1000 1100 1200 1300 1400 1500 1600 1700
Wavenumber / cm-1 Fig. 2. (a) 1C-R2PI spectra of 35Cl and 37Cl isotopomers of p-chloroanisole in the energy range of 100 to 900 cm1 from the band origin. (b) 1C-R2PI spectra of 35Cl and 37Cl isotopomers of p-chloroanisole in the energy range of 850–1800 cm1 from the band origin.
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D. Yu et al. / Journal of Molecular Spectroscopy 265 (2011) 86–91
origin of the S1 S0 electronic transition. Similarly, the intense peak located at 34 859 cm1 in the 1C-R2PI spectrum of the 37Cl isotopomer can be assigned to the band origin of the S1 S0 transition. The similarity of the first electronic transition energies for 35 Cl and 37Cl isotopomers indicates no chlorine isotope effect on the electronic transition energy. The same phenomenon was observed for the 35Cl and 37Cl isotopomers of p-chlorophenol [19] and p-chloroaniline [20]. The observed bands, except the 00 band in the REMPI spectra of p-chloroanisole, should originate from the excitation of the vibrations in the S1 state. p-chloroanisole has 42 normal mode vibrations, including 30 benzene-like vibrations and 12 OCH3 vibrations. Because the intensity of the vibronic band is proportional to the Franck–Condon factor, not all vibrations are active for the 1C-R2PI process. Table 1 lists the frequencies of the observed REMPI bands of both the 35Cl and 37Cl isotopomers of pchloroanisole, the vibrational frequencies of p-chloroanisole in the S0 state, the calculated vibrational frequencies of the S1 state and the possible assignments. The spectral assignments were made according to the available vibrational data of p-chloroanisole in the S0 state [21], the vibrational frequencies of the similar molecules [19,20,22,23] and our theoretical calculations. Most of the vibrational bands in the REMPI spectra of 35Cl- and 37 Cl-p-chloroanisole are related to the in-plane deformation of the aromatic ring. The intense bands at 344 and 780 cm1 for 35Cl-pchloroanisole and 341 and 780 cm1 for 37Cl-p-chloroanisole are related to the in-plane ring deformation of 7a10 and 6a10 , respectively. The relatively weak bands at 623, 1004, 1055, 1123, 1169, 1276 and 1396 cm1 for 35Cl-p-chloroanisole and 623, 1003, 1053, 1120, 1169, 1276 and 1394 cm1 for 37Cl-p-chloroanisole are then assigned to the transitions of 1210 , 18a10 , 110 , 1310 , 1410 , 1 310 and 19b0 , respectively. These vibrations are also related to the in-plane vibrations of the aromatic ring. The bands at 480 cm1 for both 35Cl and 37Cl isotopomers result from the transition of 1 16b0 , involving the out-of-plane vibration mode of the aromatic ring. The two bands at 1260 and 1558 cm1 for both 35Cl and 37 Cl isotopomers are tentatively assigned to the combination bands. The weak peaks at 188 and 207 cm1 for 35Cl-p-chloroanisole and 166 and 183 cm1 for 37Cl-p-chloroanisole are then assigned to the bending vibrations of the CAOCH3 and the CACl bonds.
4. Discussion 4.1. Molecular structure According to the Franck–Condon principle, the probability of a vibrational transition and, thus, the relative intensity of a vibrational band is proportional to the Franck–Condon factor, which is governed by the structural difference between two electronic states. Therefore, the structures of p-chloroanisole in the S0 and S1 state are useful for the analysis of the 1C-R2PI spectra. Because there are no experimental data available in the literature, ab initio and DFT calculations were performed to optimize the structures of p-chloroanisole in the different states. HF and B3LYP calculations with various basis sets were performed to predict the structures of p-chloroanisole in the S0 and D0 states, whereas CIS and TDB3LYP calculations with various basis sets were performed to predict the structure of p-chloroanisole in the S1 state. Table 2 lists the calculated parameters with the basis set of cc-PVDZ. Because there are no differences between the structures of the 35Cl isotopomer in the S0, S1 and D0 states and those of the 37Cl isotopomer in the respective states calculated with the same methods and basis sets, only the geometric parameters of the 35Cl isotopomer in the S0, S1 and D0 states are listed in Table 2. In the S0 state, p-chloroanisole is non-planar with the hydrogen atoms of the methyl group bent out of the plane of the ring by approximately 61° and possesses a symmetry point group of Cs. In the S1 state, p-chloroanisole also possesses a symmetry point group of Cs with the two hydrogen atoms of the methyl group bending out of the ring by 61°. The bond lengths of the CAC bonds of the aromatic ring in the S1 state are increased when compared to those in the S0 state. This indicates that the aromatic ring expands during the S1 S0 transition. The ring expansion is caused by the electron transfer from the highest occupied molecular orbital (HOMO, p) to the lowest unoccupied molecular orbital (LUMO, p⁄) during the S1 S0 transition. The C1AO bond is shortened during the S1 S0 transition, which exhibits partial double bond character in the excited S1 state. This may result from the enhanced p– p conjugation in the S1 state. The C4ACl bond is also shortened during the S1 S0 transition. This indicates that the interaction between the chlorine atom and aromatic ring in the S1 state is stronger than that in the S0 state. The expansion of the ring and the
Table 1 Observed bands (cm1) in the REMPI spectrum of p-chloroanisole and their possible assignments. S0a
S1 Cl35 Exp.
S1 Cl37 Calc.
b
Exp.
50
16a10
166
123
207 344
200 334
183 341
200 331
506
480
481
480
481
16b0
636
623
609
623
609
1210 6a10
16a10 , c(O–CH3) + c(C–Cl) b(O–CH3) + b(C–Cl) 7a10 1
798
780
771
780
771
1005
1004
955
1003
955
1094
1055
1018
1053
1018
1248
1123
1054
1120
1054
1310
1280
1169
1127
1169
1127
1410
1293
1276
1303
1276
1303
310
1442
1396
1406
1394
1406
19b0
1558
c
50
123
1260
a
Calc.
188 365
b
Assignment and approximate descriptionc b
1
1260
1558
18a10 110
6a10 6b0 1
6a20
Ref. [22]. Obtained from the CIS/cc-PVDZ calculation, scaled by 0.90. Varsanyi and Szoke’s notations are applied for the ring vibrational modes; c denotes out-of-plane bending; b denotes in-plane bending.
89
D. Yu et al. / Journal of Molecular Spectroscopy 265 (2011) 86–91 Table 2 The calculated geometric parameters of p-chloroanisole in the S0, S1 and D0 states using the basis set of cc-PVDZ. S0 RHF
S1 RB3LYP
CIS
D0 TD-B3LYP
UHF
Table 3 The deviation of the calculated (a) E1 and (b) IE with respect to the experimental result. (a)
UB3LYP
Bond length (Å) OACH3 1.400 OAC1 1.346 C4ACl 1.750 C1AC2 1.388 C2AC3 1.393 C3AC4 1.378 C4AC5 1.390 C5AC6 1.379 C6AC1 1.396
1.419 1.363 1.762 1.402 1.400 1.392 1.399 1.391 1.405
1.409 1.320 1.725 1.417 1.412 1.415 1.403 1.411 1.424
1.426 1.352 1.739 1.421 1.430 1.418 1.414 1.429 1.424
1.450 1.267 1.706 1.438 1.365 1.422 1.418 1.368 1.442
1.454 1.309 1.704 1.431 1.374 1.426 1.422 1.371 1.437
Bond angle (°) \C1OCH3 119.8 \C2C1O 124.8 \C6C1O 115.8 \C3C4Cl 120.0 \C5C4Cl 119.7 \C6C1C2 119.3 \C1C2C3 120.0 \C2C3C4 120.1 \C3C4C5 120.3 \C4C5C6 119.7 \C5C6C1 120.6
118.2 124.9 115.7 119.8 119.6 119.4 120.0 120.0 120.5 119.5 120.6
121.4 123.6 114.5 118.0 118.8 121.9 118.6 118.8 123.2 118.2 119.3
120.2 123.0 113.7 117.6 118.0 123.3 118.1 118.0 124.4 117.6 118.6
124.7 124.4 115.4 119.3 119.8 120.2 118.9 120.6 120.9 119.6 119.8
122.3 124.8 114.8 119.2 119.6 120.4 119.2 120.0 121.2 119.2 120.1
35
E1
shrinkage of C1AO and C4ACl bonds are consistent with the changes in p-chlorophenol [24–26]. This result indicates that pchlorophenol can be used for the comparative analysis of pchloroanisole. As shown in Table 1, the frequency of each vibrational mode of the ring deformation in the S1 state is less than that in the S0 state. This indicates that the aromatic ring of p-chloroanisole in the S1 state is not as rigid as that in the S0 state. This argument is supported by the calculated results, in which the perimeter of the aromatic ring in the S1 state is slightly greater than that in the S0 state. Compared to p-chloroanisole in the S1 state, the bond lengths of the CAC bonds of the aromatic ring in the D0 state are shortened. This indicates that the aromatic ring shrinks during the D0 S1 transition. This results from the excited electron being removed from the anti-bond orbital (HOMO, p⁄), so that the aromatic ring reverts. The bond lengths of the C1AO and C4ACl are shortened during the D0 S1 transition. This indicates that the interaction between the aromatic ring and substituted groups in the D0 state is stronger than that in the S1 state. 4.2. Transition energy Table 3 lists the transition energy of 35Cl- and 37Cl-p-chloroanisole determined by experiments and calculations. The deviation in the calculated first electronically excited transition energy (E1) with respect to the experimental result is more than 20% for the CIS method, while the deviation is less than 5% for the TD-B3LYP method. It seems that the TD-B3LYP method is more accurate for the prediction of E1, compared to the CIS method. For the ionization energy (IE), the deviation in the calculated results is approximately 15% for the HF method and approximately 4% for the DFT method. It seems that the DFT method is more accurate for the prediction of the IE when compared to the HF method. As shown in Table 3, the calculated E1 of 35Cl-p-chloroanisole is the same as that of the 37Cl-p-chloroanisole for the same method with the same basis set. The similarity of the calculated E1s is consistent with that of the experimental results. Table 4 lists the origins of the S1 S0 electronic transition energies (E01 s) of phenol [27–29], anisole [30–32], aniline [33–35] and
IE
E1 (cm1)
Deviation (%)
34 859 42 436 42 287 45 349 34 704 34 787
– 21.7 21.3 30.1 0.4 0.2
34 859 42 436 42 287 45 349 34 704 34 787
– 21.7 21.3 30.1 0.4 0.2
36 520
4.8
36 520
4.8
)
35
37
Cl isotopomer
IE (cm1) Experimental result HF/631+dp HF/6311++dp HF/cc-PVDZ B3LYP/631+dp B3LYP/6311 ++dp B3LYP/ccPVDZ
1
Cl isotopomer
Deviation (%)
E1 (cm Experimental result CIS/631+dp CIS/6311++dp CIS/cc-PVDZ TD-B3LYP/631+dp TD-B3LYP/ 6311++dp TD-B3LYP/cc-PVDZ (b)
37
Cl isotopomer
IE (eV)
Cl isotopomer
Deviation (%)
8.25
–
56 187 56 226 55 662 64 077 64 427
6.97 6.97 6.90 7.94 7.99
15.1 15.1 16.4 3.8 3.1
63 135
7.83
5.1
IE (cm1)
IE (eV)
Deviation (%)
8.25
–
56 187 56 226 55 662 64 077 64 427
6.97 6.97 6.90 7.94 7.99
15.1 15.1 16.4 3.8 3.1
63 135
7.83
5.1
Table 4 Measured transition energies (in cm1) of phenol, anisole and some of its parahalogen derivativesa. Molecule
E1
4E1
Phenolb p-Fluorophenolc p-Chlorophenold Anisolee p-Fluoroanisolef p-Chloroanisoleg Anilineh p-Fluoroanilineh p-Chloroanilinei
36 349 35 117 34 813 36 383 35 149 34 859 34 029 32 652 32 572
0 1232 1536 0 1234 1524 0 1377 1457
a E1 is the first electronically excited transition energy; DE1 is the shift in E1 with respect to the corresponding original molecule. b Ref. [27]. c Ref. [36]. d Ref. [19]. e Ref. [30]. f Ref. [16]. g This experiment. h Ref. [33]. i Ref. [20].
their para-halogen substituted derivatives [16,20,22,33,36]. The band origins of the S1 S0 vibrationless transitions of p-fluoroanisole and p-chloroanisole are 35 146 and 34 859 cm1, which are red-shifted by 1234 and 1524 cm1, respectively, relative to that of anisole. Similarly, the E1s of p-fluorophenol and p-chlorophenol are red-shifted by 1232 and 1536 cm1 compared to that of phenol. The E1s of p-fluoroaniline and p-chloroaniline are red-shifted by 1377 and 1456 cm1 relative to that of aniline. This indicates that the para-substitution of the halogen atom on the aromatic ring causes the E1 red-shift. The degree of the red-shift induced by the chlorine substitution is larger than that induced by the fluorine substitution. As mentioned in the previous sections, the S1 S0 excitation is mainly subject to a p⁄ p transition [19,20,30,33,36]. Therefore, the change in the p-electron density around the ring influences the transition energy of the molecule. It was found that the substitution of a functional group on a phenyl
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Table 5 Calculated energies in (Eh) of the S0, S1 and D0 states.a 35
35
Cl and
37
Cl isotopomers of p-chloroanisole in the
Cl
37
Cl
S0 ERHF ZPC E00
803.530048 0.131734 803.398314
803.530048 0.131703 803.398345
S1 ECIS ZPC E0
803.316703 0.126572 803.190131
803.316703 0.126541 803.190162
D0 EUHF ZPC E+
803.276434 0.130450 803.145984
803.276434 0.130418 803.146017
the two isotopomers of p-chloroanisole are identical, the energies at various states are expected to be different. Table 5 shows that 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 states have the same value (7 cm1). Thus, the electronic transition energy and the ionization energy are the same for these two isotopomers. Because the chlorine atom is relatively far away from the methoxy group in p-chloroanisole, there is no sterically hindered effect between the chlorine atom and the methoxy group, and the isotope effect on the electronic transition and ionization energy is insignificant. These calculated results provide a reasonable explanation of the experimental findings. 4.3. Isotope effect on molecular vibrations
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 = 219 474 cm1.
ring can cause a significant change in the nearby p-electron density and the molecular geometry, which led to a decrease in the ZPL (zero-point level) energy of an electronic state. The extent of the ZPL decrease reflects the strength of the interaction of the substituent and the ring. If the ZPL energy decrease of the upper electronic state is greater than that of the lower electronic state, it gives rise to a red shift in the transition energy. In the opposite case, it causes a blue shift. The interaction between the halogen atom and the aromatic ring may result from the inductive effect through the r bond and the conjugation effect through the p orbitals. The former depends on the electronegativity of the main atom in the substituted group, while the latter reflects the overtone of the p system of the aromatic ring and the p orbital of the main atom in the substituted group. The observed red-shift in the first electronic transition energy suggests that the interaction between the halogen atom and the ring is stronger in the S1 state than in the S0 state. The larger chlorine atom has a greater p-electron overlap with the ring than the fluorine atom. Thus, the red-shift in the E1 induced by chlorine substitution is greater than that induced by fluorine substitution. Due to the deficiency of the theoretical calculations, the ab initio and DFT calculations may lead to a deviation in the prediction of the total energy. However, the general trend obtained from the calculated results may be useful for comparison with the experimental results in terms of the isotope effect. Although the structures of
For the ring deformation vibrations, the frequency difference between the 35Cl and 37Cl isotopomers of p-chloroanisole is less than 3 cm1. Because the error limit of the measured vibrational frequency in our REMPI spectrum is ± 3 cm1, it is difficult to confirm the existence of the frequency differences between the 35Cl and 37Cl isotopomers. The conclusion may be that the isotope effect on the normal vibrations of the aromatic ring is not strong enough to be measured by our REMPI spectrum. For the substituted group vibrations, the frequencies of the 16a10 vibration, c(OACH3) + c(CACl) vibration and b(OACH3) + b(CACl) vibration are 188 and 207 cm1 for the 35Cl isotopomer and 166 and 183 cm1 for the 37Cl isotopomer, respectively. The frequencies of the 16a10 vibration, c(OACH3) + c(CACl) vibration and b(OACH3) + b(CACl) vibration of the 37Cl-p-chloroanisole are redshifted by 22 and 24 cm1 compared to those of the 35Cl-p-chloroanisole. It can be deduced that the red-shift of the two vibrational frequencies is caused by the isotope effect of the chorine atom. The analysis indicates that the isotope effect on the c(OACH3) + c(CACl) and b(OACH3) + b(CACl) vibrations is stronger than that on the normal ring deformation vibrations. This might be related to the higher degree of the involvement of the chlorine atom in the two vibrations than that in the normal ring deformation vibrations, as shown in Fig. 3. 5. Conclusion The REMPI spectroscopic method was applied to record the vibronic spectra of the 35Cl and 37Cl isotopomers of p-chloroanisole in
Fig. 3. Vibrational structures of 35Cl and 37Cl isotopomers of p-chloroanisole in the S1 state. The open circles designate the original locations of the carbon atoms, and the solid dots mark the displacements.
D. Yu et al. / Journal of Molecular Spectroscopy 265 (2011) 86–91
the S1 state. The transition energies of both isotopic species were found to be equivalent, at 34 859 cm1. Comparing the E1 and IE of p-chloroanisole with those of p-fluoroanisole led to the following insights: (i) the p-electron overlap between the halogen atom and the aromatic ring is responsible for the observed red-shift in the origin of the S1 S0 excitation; and (ii) because the chlorine atom is larger than the fluorine atom, the red-shift in the E1 of pchloroanisole is greater than that of p-fluoroanisole. The analysis of the frequencies of the vibrations for 35Cl and 37Cl p-chloroanisole in the S1 state shows that the frequency shifts of the vibrational modes of isotopomers partially depends on the degree of the involvement of chlorine atom in the molecular motion. These experimental findings are supported by ab initio and DFT calculations. Acknowledgments We gratefully acknowledge the financial support from the Center of Molecular Sciences of ICCAS under Contract No. CMSLX200915 and the National Natural Science Foundation of China under Grant 20973180. References [1] U. Fuhrer, K. Ballschmiter, Environ. Sci. Technol. 32 (1998) 2208–2215. [2] C. Flodin, F.B. Whitfield, Phytochemistry 53 (2000) 77–80. [3] P. Chatonnet, S. Bonnet, S. Boutou, M.D. Labadie, J. Agric. Food Chem. 52 (2004) 1255–1262. [4] S. Insa, V. Salvado, E. Antico, J. Chromatogr. A 1122 (2006) 215–221. [5] L.H. Aung, J.L. Smilanick, P.V. Vail, P.L. Hartsell, E. Gomez, J. Agric. Food Chem. 44 (1996) 3294–3296. [6] R.M. Callejon, A.M. Troncoso, M.L. Morales, Talanta 71 (2007) 2092–2097. [7] A. Diaz, F. Ventura, T. Galceran, J. Chromatogr. A 1064 (2005) 97–106. [8] Y. Ukisu, Appl. Catal. A 349 (2008) 229–232. [9] J.P. Da Silva, L. Ferreira, I. Osipov, I. Machado, J. Hazard. Mater. 179 (2010) 187– 191. [10] J.P. Da Silva, S. Jockkusch, J.M.G. Martinho, M.F. Ottaviani, N.J. Turo, Org. Lett. 12 (2010) 3062–3065. [11] J.N. Rai, K.N. Upadhya, Spectrochim. Acta 22 (1966) 1427–1430. [12] C.E.H. Dessent, K. Müller-Dethlefs, Chem. Rev. 100 (2000) 3999–4021. [13] B. Brutschy, Chem. Rev. 100 (2000) 3891–3920. [14] R. Tembreull, D.M. Lubman, Anal. Chem. 56 (1984) 1962–1967.
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