Two-color resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy of o-chloroanisole

Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 73–79

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Two-color resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy of o-chloroanisole Hsin Chang Huang a,b , Bih Yaw Jin b , Wen Bih Tzeng a,c,∗ a b c

Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, 1, Roosevelt Road, Taipei 10617, Taiwan Department of Chemistry, National Taiwan University, 1, Roosevelt Road, Taipei 10617, Taiwan Department of Chemistry, National Taiwan Normal University, 88, Tingzhou Road, Taipei 11677, Taiwan

a r t i c l e

i n f o

Article history: Received 1 May 2012 Received in revised form 13 June 2012 Accepted 19 June 2012 Available online 27 June 2012 Keywords: Resonant two-photon ionization Mass-analyzed threshold ionization o-Chloroanisole Isotopomer Vibronic spectra Cation spectra

a b s t r a c t We applied the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibrationally resolved spectra of the selected isotopomers of o-chloroanisole in the electronically excited S1 and cationic ground D0 states. As supported by our theoretical calculations, only the trans form of o-chloroanisole involves in the two-photon photoexcitation and ionization processes. The band origins of the S1 ← S0 electronic transition and adiabatic ionization energies for the 35 Cl and 37 Cl isotopomers of o-chloroanisole are found to be the same within our detection limit, with the values of 35 745 ± 2 and 66 982 ± 5 cm−1 , respectively. The general spectral features of the two isotopomers are nearly identical and result from in-plane ring deformation and substituent-sensitive bending and stretching as well as the CH3 torsional motions. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Because chloroanisoles are widely found in our daily lives, they become targets of various academic and practical studies [1]. However, most of the investigations focus on the isolation and determination processes by using the analytical approaches such as gas chromatography–mass spectrometry and solid-phase microextraction [2]. Spectroscopic studies regarding the molecular properties in the electronically excited S1 and cationic ground D0 states are still rare in the literature. The deduced information about the molecular geometry, electronic transition, and normal vibration in different electronic states may be useful to understand the photochemical and photophysical processes with UV light sources. Many biological systems consist of rotational conformers (rotamers) which can be interconverted by rotation about a single bond. Studies on molecular rotamers can provide insights into some biological phenomena and reaction processes [3]. Previous experimental and theoretical studies show that the trans and ortho forms of o-chloroanisole are stable, whereas the cis form is

∗ Corresponding author at: Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, 1, Roosevelt Road, Taipei 10617, Taiwan. Tel.: +886 2 23668236; fax: +886 2 23620200. E-mail address: [email protected] (W.B. Tzeng). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2012.06.015

unstable in the ground S0 state [4,5]. The ionization energy (IE) of this molecule has been reported on the basis of the electron impact and charge transfer experiments [6,7]. Detailed spectroscopic data of o-chloroanisole in the S1 and D0 states are not yet available in the literature. Laser-induced fluorescence [8] and zero-kinetic energy (ZEKE) photoelectron spectroscopy [9,10] can be used to record the vibronic and cation spectra of molecules. These methods involve detection of either photons or electrons with high sensitivity and spectral resolution. In contrast, the resonance-enhanced multiphoton ionization (REMPI) [11,12] and mass-analyzed threshold ionization (MATI) [13–15] techniques are subject to detection of ions and can also be used to obtain the same experimental data. We adopted the latter methods which provide mass information and are suitable for studies of molecular isotopomers, free radicals, complexes, and clusters. In this paper, we report the vibronic and cation spectra of the selected isotopomers of o-chloroanisole by using the one-color and two-color resonant two-photon ionization (1C, 2C-R2PI) and MATI techniques. These new data provide information about the excitation energy of the S1 ← S0 electronic transition (E1 ) and IE as well as the active vibrations of this molecule in the S1 and D0 states. Comparing these data with those of halogen substituted benzene derivatives [16–18] allows us to gain knowledge about the isotope and chlorine substitution effects on transition energy and molecular vibration.

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2. Experimental and computational details 2.1. Experimental method Our R2PI and MATI experiments were performed with a laserbased time-of-flight (TOF) mass spectrometer, as described in our previous publication [19]. o-Chloroanisole (98% purity) was purchased from Sigma–Aldrich and used without further purification. The vapors of the liquid sample kept at room temperature were seeded into 2–3 bar of helium gas and expanded into the vacuum through a pulsed valve with a 0.15 mm diameter orifice. The 2CR2PI process was accomplished by using two independent tunable UV lasers controlled by a delay/pulse generator (Stanford Research Systems DG535). A laser wavelength meter (Coherent, WaveMaster) was used here to calibrate the wavelengths of both lasers. These two counter-propagating laser beams were focused and intersected perpendicularly with the molecular beam at 50 mm downstream from the nozzle orifice. The 2C-R2PI experiments involve detection of non-energyselected prompt ion signal. Analysis on the rising step in the photoionization efficiency (PIE) curve can give an IE with an uncertainty of about 10–20 cm−1 . In the MATI experiments, the pump laser is used to excite the selected molecular species to a specific vibronic level in the S1 state. The probe laser is scanned to bring the electronically excited molecule to high n Rydberg states lying a few wavenumbers below the ionization limit. Under this condition, both prompt ions and Rydberg neutrals were formed simultaneously in the laser and molecular beam interaction zone. A pulsed electric field of −1 V/cm (duration = 10 ␮s) was switched on about 20 ns after the occurrence of the pump and probe laser pulses to reject prompt ions. After a time delay of about 11.8 ␮s, a second pulsed electric field of +200 V/cm (duration = 10 ␮s) was applied to field-ionize the Rydberg neutrals. These threshold ions were then accelerated and passed through a 1.2 m field-free region before being detected by a dual-stack microchannel plate detector. The ion signal from the detector was collected and analyzed by a multichannel scaler (MCS, Stanford Research Systems, SR430). Each TOF mass spectrum at a particular laser wavelength was accumulated for 300 laser shots and shown on the screen of the MCS. The MCS and the transient digitizer were interfaced to a personal computer (PC). All TOF mass spectra were saved in the PC at a 0.020 nm (equivalent to 1.2 cm−1 ) interval over the entire scanning range of laser wavelength. Composite optical spectra of intensity versus wavelength were then constructed from the individual mass spectra. As the ion signal is proportional to the photon intensities of the excitation and ionization lasers in a two-color two-photon process, the obtained optical spectra were normalized to the laser power in order to avoid spurious signals due to shot-to-shot laser fluctuation. 2.2. Computational method We used the GAUSSIAN 09 program package [20] to perform all ab initio and density functional (DFT) calculations with the aug-ccpVDZ basis set. The IE was deduced by taking the difference of the total energies at the zero-point energy (ZPE) levels of the cation and corresponding neutral in ground state. Because the frequency calculations are based on the harmonic oscillator model, the predicted values may be slightly greater that the measured ones. To compare with the experimental values, the calculated frequencies are scaled by multiplying an appropriate factor to correct approximately for the combined errors stemming from basis set incompleteness and neglect of electron correlation and vibrational anharmonicity. The magnitude of scaling factor depends on the molecular system, electronic state, computational method, and basis set. The calculated frequencies and scaling factors are listed in the tables along with measured values.

Fig. 1. Vibronic spectra of the (a) 35 Cl and (b) 37 Cl isotopomers of o-chloroanisole.

3. Results 3.1. Vibronic spectra Due to the natural abundance of chlorine, the TOF mass spectrum of o-chloroanisole shows two peaks resulting from the 35 Cl and 37 Cl isotopomers of o-chloroanisole, as those found in pchlorophenol [17], p-chloroaniline [21], and m-chloroaniline [22]. Fig. 1 shows the vibronic spectra of the 35 Cl and 37 Cl isotopomers of o-chloroanisole in the energy range near their S1 ← S0 electronic transitions. The general features of these two spectra are nearly identical with the same E1 of 35 745 ± 2 cm−1 . Similar to the cases of o-dimethoxybenzene [23] and o-fluoroanisole [24,25], only one stable isomer of o-chloroanisole is found to involve in the present R2PI and MATI experiments. The configuration interaction singles (CIS), time-dependent Becke three-parameter functional with the Lee–Yang–Parr functional (TD-B3LYP), and TD-B3PW91 methods with the aug-cc-pVDZ basis set predict the E1 ’s of 43 723, 36 850, and 37 176 cm−1 , corresponding to overestimations of 22.3, 3.1, and 4.0%, respectively. All of these calculations show that the E1 ’s of the 35 Cl and 37 Cl isotopomers of o-chloroanisole are the same, as that found in our experiments. Table 1 lists the measured frequencies of the observed bands of the 35 Cl and 37 Cl isotopomers of o-chloroanisole, along with the calculated frequencies and possible assignments. The spectral assignment was done by comparing the measured frequencies with those of anisole [26], o-fluorophenol [27], and o-fluoroanisole [25], and the predicted values from the CIS/aug-cc-pVDZ calculations. The numbering system for the normal vibrations of benzene derivatives follows that used by Varsanyi [28] and is based on Wilson’s notations [29]. The pronounced vibronic bands at 653, 773, 960, and 1063 cm−1 are assigned to the 6a1 0 , 121 0 , 11 0 , and 18a1 0 transitions, of 35 Cl o-chloroanisole, respectively. Normal vibrations 6a, 12, and 1 mainly involve in-plane ring deformations, whereas mode 18a mostly involves in-plane C H bending vibration, as seen in Fig. 2. The normal vibrations can be viewed by following the GAUSSVIEW procedure of the Gaussian 03 program [20]. The 6a1 0 , 11 0 , and 18a1 0 vibronic transitions have also been observed for o-fluoroanisole at 566, 747, and 995 cm−1 , respectively [25].

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Fig. 2. Some active normal vibrations of o-chloroanisole, with measured frequencies for 35 Cl and 37 Cl isotopomer (in the parentheses) in the S1 and D0 states. The open circles designate the original locations of the atoms, whereas the solid dots mark the displacements. The calculated frequencies of each mode are listed in Tables 1–3.

According to the Varsanyi numbering system [28], o-chloroanisole is classified as one-heavy one-light disubstituted benzene whereas o-fluoroanisole belong to di-light substituted benzenes. Therefore, care must be used when one compares the frequencies of normal vibrations of these two molecules. The band at 369 cm−1 is assigned to the 7a1 0 vibronic transition, related to the in-plane C Cl stretching vibration of 35 Cl o-chloroanisole. The corresponding vibronic transition of p-chloroaniline has been reported to be 360 cm−1 [21]. The low-frequency features at 103, 193, and 521 cm−1 in Fig. 1 result from the transitions related to the substituent-sensitive out-of-plane O CH3 bending, CH3 torsion, and in-plane C OCH3 Table 1 Observed bands (in cm−1 ) in the vibronic spectra of with possible assignments.a 35

37

Cl

Cl

35

Cl and

37

Cl o-chloroanisole

Assignment and approximate descriptionb

Expt.

Calc.

Expt.

Calc.

103 193 369 521 653 773 960 1063

87 189 355 538 635 771 973 1059

103 193 369 521 650 773 960 1063

87 189 351 538 633 770 973 1059

(O CH3 ) (CH3 ) 7a1 0 , (CCl) ˇ(C OCH3 ) 6a1 0 , ˇ(CCC) 121 0 , ˇ(CCC) 11 0 , breathing 18a1 0 , ˇ(CH)

a The experimental values are shifts from 35 745 cm−1 , whereas the calculated ones are obtained from the CIS/aug-cc-pVDZ calculations, scaled by 0.91. b ˇ, in-plane bending; , out-of-plane bending; , torsion motion; , stretching motion.

bending vibrations, respectively. Table 1 shows that the frequency of each vibrational mode of the 37 Cl is less than that of the 35 Cl isotopomer by no more than 3 cm−1 . This frequency difference may reflect the degree of the chlorine atom involved in the overall molecular vibration.

3.2. Cation spectra Fig. 3 shows the PIE curves of the 35 Cl and 37 Cl isotopomers of o-chloroanisole, recorded by ionizing via the S1 00 level at 35 745 cm−1 . The abruptly rising steps indicate that the IEs of the two isotopic species are the same, with the value of 66 982 ± 10 cm−1 . Because the MATI experiment only detects the threshold ions with zero kinetic energy, and it leads to a sharp peak at the ionization limit. Figs. 4 and 5 show the MATI spectra of the 35 Cl and 37 Cl isotopomers of o-chloroanisole, recorded by ionizing via the 00 , (CH3 ), 7a1 , ˇ(C–OCH3 ), and 121 levels in the S1 state, respectively. Analysis on the 0+ bands yields a field-corrected adiabatic IE of 66 982 ± 5 cm−1 (8.3042 ± 0.0006 eV) for both isotopomers, which is in excellent agreement with that obtained by our PIE experiments. The previously reported IE is 8.15–8.42 eV on the basis of the charge transfer spectroscopy and electron impact ionization experiments [6,7,30]. The Hartree–Fock (HF), B3LYP, and B3PW91 calculations with the aug-cc-pVDZ basis set predict this value to be 55 868, 65 045, and 65 181 cm−1 , corresponding to underestimations of 16.9, 3.1, and 2.6%, respectively. All of the calculated results show that the IEs of the two isotopomers are the same, as found in our experiments.

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Fig. 3. Photoionization efficiency curves for the (a) 35 Cl and (b) 37 Cl isotopomers of o-chloroanisole recorded by ionizing via S1 00 intermediate level.

One of the major advantages of the MATI over the PIE experiment is that it provides information about the active vibrations of the cation. Tables 2 and 3 list the observed MATI bands resulting from the active vibrations of the cations of the 35 Cl and 37 Cl isotopomers of o-chloroanisole. In Fig. 4, the distinct bands shifted Fig. 5. MATI spectra for the 37 Cl isotopomer of o-chloroanisole, which is recorded by ionizing via the (a) 00 , (b) , (c) 7a1 , (d) ˇ(C OCH3 ), and (e) 121 levels in the S1 state, respectively.

from the 0+ band by 289, 379, 496, 682, 767, 790, 870, 946, 963, 996, 1058, and 1156 cm−1 result from the 9b1 , 7a1 , 6b1 , 6a1 , 111 , 121 , 17a1 , (O CH3 ), 17b1 , 11 0 , 18b1 , and 9a1 vibrations of the ochloroanisole, respectively. The CH3 torsion and in-plane C OCH3 bending vibrations of the cation are found to have frequencies of 191 and 568 cm−1 , respectively. Similar to that found in the vibronic spectra, the measured frequency of each vibrational mode of the 37 Cl is slightly less than that of the 35 Cl isotopomer by no more than 3 cm−1 . Table 2 Observed bands (in cm−1 ) in the MATI spectra of 35 Cl o-chloroanisole and possible assignments.a Intermediate level in the S1 state 00



7a1

192 289

191

496 566 682

496

ˇ(C OCH3 )

121

379 568 767 790 870 946 963 996 1058 1156 Fig. 4. MATI spectra for the 35 Cl isotopomer of o-chloroanisole, which is recorded by ionizing via the (a) 00 , (b) , (c) 7a1 , (d) ˇ(C OCH3 ), and (e) 121 levels in the S1 state, respectively.

Cal.

Assignment and approximate descriptionb

187 285 379 480 566 674 768 776 849 953 962 998 1060 1170

(CH3 ) 9b1 , ˇ(CCl), ˇ(OCH3 ) 7a1 , (CCl) 6b1 , ˇ(CCC) ˇ(C OCH3 ) 6a1 , ˇ(CCC) 111 , (CH) 121 , ˇ(CCC) 17a1 , (CH) (O CH3 ) 17b1 , (CH) 11 , breathing 18b1 , ˇ(CCC) 9a1 , ˇ(CH)

a The experimental values are shifts from 66 982 cm−1 , whereas the calculated ones are obtained from the B3LYP/aug-cc-pVDZ calculations, scaled by 0.98. b ˇ, in-plane bending; ␥, out-of-plane bending; , torsion motion; , stretching motion.

H.C. Huang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 73–79 Table 3 Observed bands (in cm−1 ) in the MATI spectra of 37 Cl o-chloroanisole and possible assignments.a Intermediate level in the S1 state 0

0



1

7a

192 289

189

496 568 678

494

ˇ(C OCH3 )

1

12

379 568 767 790 870 963 996 1156

Cal.

Assignment and approximate descriptionb

187 283 374 480 566 672 768 775 849 962 998 1170

(CH3 ) 9b1 , ˇ(CCl), ˇ(OCH3 ) 7a1 , (CCl) 6b1 , ˇ(CCC) ˇ(C OCH3 ) 6a1 , ˇ(CCC) 111 , (CH) 121 , ˇ(CCC) 17a1 , (CH) 17b1 , (CH) 11 , breathing 9a1 , ˇ(CH)

The experimental values are shifts from 66 982 cm−1 , whereas the calculated ones are obtained from the B3LYP/aug-cc-pVDZ calculations, scaled by 0.98. b ˇ, in-plane bending; , out-of-plane bending; , torsion motion; , stretching motion. a

When the (CH3 ), 7a1 , ˇ(C OCH3 ), and 121 levels in the S1 state are used as the intermediate levels, the strongest band in each corresponding MATI spectrum results from the same vibration of the cation, as seen in Figs. 4 and 5. These indicate that the molecular geometry and vibrational coordinates of the o-chloroanisole cation in the D0 state resemble those of the neutral in the S1 state. Similar observations have been reported for o-dimethoxybenzene [23] and o-fluoroanisole [25]. When the S1 7a1 state is used as the intermediate level, two weak MATI bands resulting from the (O CH3 ) and 18b1 vibrations of the 35 Cl o-chloroanisole cation appear in Fig. 4(c). However, the corresponding bands of the 37 Cl o-chloroanisole cation are very weak and nearly not seen in Fig. 5(c). In a similar fashion, when the S1 ˇ(C OCH3 ) state is used as the intermediate level, a weak 0+ band is seen in Fig. 4(d) and is not seen in Fig. 5(d). These findings

Fig. 6. One-dimensional potential energy surfaces of o-chloroanisole in the (a) D0 , and (b) S0 states, obtained from the RHF and UHF calculations with the aug-cc-pVDZ basis set.

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can be easily understood by the fact that the natural abundance of the 37 Cl is about 30% of the 35 Cl isotopomer. 4. Discussion 4.1. Isotope effect on the transition energy and molecular vibration Our restricted Hartree–Fock (RHF), CIS, and unrestricted Hartree–Fock (UHF) calculations with the aug-cc-pVDZ basis set predict that the ZPE level of the 37 Cl isotopomer is lower than that of the 35 Cl by about 7 cm−1 for each of the S0 , S1 , and D0 states. In other words, these ab initio calculations predict that the E1 ’s and IEs of the two isotopic species are the same, as those found in our R2PI and MATI experiments. Careful examination on the R2PI and MATI spectra shows that the frequency of each active normal vibration of the 37 Cl isotopomer is less than that of the 35 Cl isotopomer by no more than 3 cm−1 . The magnitude of this frequency difference may depend on the vibrational pattern and the involvement of the chlorine atom in the overall vibration. As supported by the theoretical calculations, our experimental results show that the isotope effect is insignificant on the transition energy and the observed active vibrations. Similar findings have been reported for p-chlorophenol [17], p-chloroaniline [21], and m-chloroaniline [22]. 4.2. Stable conformation of o-chloroanisole in the S0 , S1 , and D0 states Due to the relative orientation of the OCH3 and Cl groups, different rotamers of o-chloroanisole may coexist in the chemical sample. Popik et al. applied the gas-phase electron diffraction technique and ab initio calculations with the 6-31G basis set to investigate the stable rotamers of o-chloroanisole in the S0 state [4,5]. They found that the trans and ortho forms are stable, whereas the cis form is not. They reported that the ortho form lies in an energy level higher than the trans form by 0.9–1.0 kcal/mol with a barrier height of 1.4–1.6 kcal/mol. In the present studies, we performed the RHF, CIS, and UHF calculations with the aug-cc-pVDZ basis set to investigate the stable rotamers of o-chloroanisole in the S0 , S1 , and D0 states, respectively. These calculations were done by scanning the one-dimensional potential energy surface on the rotation of C OCH3 bond. As seen in Fig. 6(b), in the S0 state, the trans species has the lowest energy whereas the ortho situates in a local minimum with an energy of 247 cm−1 . Due to the repulsive force between the CH3 of the methyl group and the Cl substituent, the cis form is not stable with an energy of 2042 cm−1 . These results are in accordance with those reported by Popik et al. [4,5]. The CIS/aug-cc-pVDZ calculations show that the trans rotamer is the only stable species for o-chloroanisole in the S1 state. The one-dimensional potential energy surface calculations show that the C OCH3 bond rotation may couple to a repulsive state, leading to the C Cl bond dissociation. In the case of the D0 state, the trans form has the lowest total energy, whereas the cis form lies in a local minimum with an energy of 1543 cm−1 , as seen in Fig. 6(a). In contrast to that in S0 state, the ortho form is not stable with an energy of 4402 cm−1 . The present calculated results suggest that only the trans form of o-chloroanisole involves in the S1 ← S0 electronic excitation and the D0 ← S1 ionization processes. Concerning the planarity of this molecule, our calculations show that o-chloroanisole is planar in both the S0 and D0 states. However, it becomes non-planar with a small angle of less than 2◦ between the ring and the methoxyl group in the S1 state. These calculated results may account for the observed low-frequency CH3 torsion

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H.C. Huang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 73–79

Table 4 Measured transition energies (in cm−1 ) of anisole and its derivatives.a Molecule

S1 ← S0

ıE1

D0 ← S1

ıE2

IE

ıIE

Anisoleb o-Fluoroanisolec 35 Cl o-chloroanisoled 37 Cl o-chloroanisoled

36 383 36 609 35 745 35 745

0 226 −638 −638

30 016 30 745 31 237 31 237

0 729 1221 1221

66 399 67 354 66 982 66 982

0 955 583 583

a b c d

ıE1 , ıE2 , and ıIE are shifts of S1 ← S0 , D0 ← S1 , and IE with respect to those of anisole. Ref. [25]. Ref. [24]. This work.

and out-of-plane O CH3 bending vibration in the vibronic spectra, as seen in Fig. 1. Previous studies suggest that both cis and trans rotamers of ochlorophenol [31,32] involve in the S1 ← S0 electronic transition, whereas only the cis rotamer of o-fluorophenol [27] involves in the transition. As for o-fluoroanisole [24,25] and o-chloroanisole, only the trans rotamers involve in the photoexcitation and photoionization processes of the R2PI and MATI experiments. These results indicate that the nature of the substituent can affect the through-space interaction between the two substituents at the ortho position.

underestimation of about 4.5% with respect to our measurement IEs. Similar to the results of the ab initio calculations described in Section 4.1, these DFT calculations show that the IEs of 35 Clo-chloroanisole and 37 Cl-o-chloroanisole are the same, as those found in our PIE and MATI experiments. However, these calculations predict that the IEs of o-chloroanisole and o-fluoroanisole are greater than that of anisole by 478 and 554 cm−1 , respectively. Therefore, the calculated results support our experimental findings in the chloro or fluoro substitution effect on the IE of anisole.

4.3. Substitution effect on electronic excitation and ionization energies

The present R2PI and MATI experiments show the in-plane ring deformation vibrations 6a, 12, and 1 have frequencies of 653, 773, and 960 cm−1 in the S1 state and 682, 790, and 996 cm−1 in the D0 state. Our RHF, CIS, and UHF calculations with the aug-cc-pVDZ basis set show that the C1 · · · C2 distances of the aromatic ring ˚ respectively. These calculated results are 2.803, 2.806, and 2.799 A, support a previous statement that the S1 ← S0 electronic excitation of benzene derivative is subject to a ␲* ← ␲ transition, leading to an expansion of the aromatic ring [33]. The second step (D0 ← S1 transition) in the R2PI and MATI processes involves the removal of one electron from the neutral o-chloroanisole in the S1 state and causes a shrinking of the aromatic ring. In other words, the molecular geometry becomes slightly more rigid in cationic state as a result of the D0 ← S1 transition. Therefore, the frequencies of these in-plane ring vibrations of this molecule in the D0 state are slightly greater than those of the corresponding modes in the S1 state. It is interesting to note that substituent-sensitive C OCH3 bending and C Cl stretching (mode 7a) vibrations were observed with frequencies of 521 and 369 cm−1 for neutral o-chloroanisole in the S1 state and 566 and 379 cm−1 for the o-chloroanisole cation in the D0 state. Our CIS and UHF calculations with the aug-cc-pVDZ basis set show that the C OCH3 bond shortens from 1.312 A˚ to 1.264 A˚ and the C Cl bond shrinks from 1.722 A˚ to 1.709 A˚ upon the D0 ← S1 transition. A shorter bond length is generally an indication of a stronger bond and a larger vibrational frequency. Therefore, the calculated results are in consistence with our experimental observations.

Table 4 lists the measured transition energies of anisole, ofluoroanisole, and o-chloroanisole. The E1 of o-chloroanisole is less than that of anisole by 638 cm−1 . It is known that substitution of a functional group on the aromatic ring can lead to a lowering of the ZPE level. The degree of this lowering reflects the interaction between the substituent and the ring. The red shift indicates that the lowering of ZPE level of the upper state is greater than that of the lower state. The halogen substituent can interact with the ring by resonance effect through the overlap of ␲ electrons or by inductive effect through the ␴ bond. The present data indicate that the interaction of the Cl atom and the ring is stronger in the S1 state than that in the S0 state. Furthermore, the IE of o-chloroanisole is greater than that of anisole by 583 cm−1 . This indicates that the interaction of the Cl atom and the ring is weaker in the D0 state than that in the S0 state. In contrast, both the E1 and IE of o-fluoroanisole are greater than those of anisole by 226 and 955 cm−1 , respectively. These experimental data suggest that the interaction of the F atom and the aromatic ring is weaker in either the S1 or the D0 state than that in the S0 state. Therefore, the nature of the halogen atom can affect the electronic excitation and ionization energies. Both ab initio and DFT calculations can be used to support the above statements. For example, our B3LYP/cc-pVDZ calculations predict that the ZPEs of 35 Cl-o-chloroanisole, 37 Clo-chloroanisole, o-fluoroanisole, and anisole are −806.282255, −806.282285, −445.902438, and −346.658419 hartree in the S0 state, and −805.990791, −805.990823, −445.610634, and −346.369134 hartree in the D0 state. These show that in the S0 state the 35 Cl and 37 Cl substitutions on the aromatic ring lower the ZPE level by 459.623836 and 459.623866 hartree, whereas the F substitution lowers the ZPE level by 99.244019 hartree. In the D0 state, the 35 Cl and 37 Cl substitutions lower the ZPE level by 459.621657 and 459.621689 hartree, whereas the F substitution lowers by 99.241500 hartree. For each species (35 Cl-o-chloroanisole, 37 Cl-ochloroanisole, or o-fluoroanisole), the degree of the lowering of the ZPE level in the S0 state is slightly greater than that in the D0 state. With these ZPE values, the IEs of 35 Cl-o-chloroanisole, 37 Clo-chloroanisole, o-fluoroanisole, and anisole are deduced to be 63 969, 63 969, 64 044, 63 491 cm−1 , respectively, corresponding to an

4.4. Active vibrations in the S1 and D0 states

5. Conclusion We applied the R2PI and MATI techniques to record the vibronic and cation spectra of the selected isotopomers of ochloroanisole. As supported by our theoretical calculations, only trans o-chloroanisole involves in the two-photon photoexcitation and ionization processes. The E1 ’s and IEs of the 35 Cl and 37 Cl isotopomers of o-chloroanisole are found to be the same within our detection limit, with the values of 35 745 and 66 982 cm−1 , respectively. The general spectral features of two isotopomers are nearly identical. Detailed analysis shows that frequencies of a few vibrations are different by no more than 3 wavenumbers. This frequency difference may somewhat depend on the degree of Cl atom involved in overall vibration. The observed vibronic and MATI bands result

H.C. Huang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 73–79

from the in-plane ring deformation and substituent-sensitive bending and stretching as well as the CH3 torsional motions. Concerning the chlorine atom substitution effect, we compare the experimental data of anisole and o-chloroanisole. The E1 of o-chloroanisole is less than that of anisole by 638 cm−1 . This indicates that the interaction of the Cl atom and the ring is stronger in the S1 state than that in the S0 state. On the contrary, the IE of o-chloroanisole is greater than that of anisole by 583 cm−1 . This suggests that the interaction of the Cl atom and the ring is weaker in the D0 state than that in the S0 state. When some vibronic states are used as the intermediate levels, the strongest bands in the corresponding cation spectra result from the same vibrations. These indicate that the molecular geometry and vibrational coordinates of the o-chloroanisole cation in the D0 state resemble those of neutrals in the S1 state. The in-plane ring deformation vibrations 6a, 12, and 1 are found to have frequencies of 653, 773, and 960 cm−1 for o-chloroanisole in the S1 state and 682, 790, and 996 cm−1 in the D0 state. The observed greater frequencies of these ring vibrations imply that the molecular geometry of the cation in the D0 state is slightly more rigid than that of the neutral in the electronically excited S1 state. Acknowledgments We gratefully thank the National Science Council of the Republic of China for financial support of this work under Grant Number NSC-98-2113-M-001-023-MY3. References [1] L.H. Aung, J.L. Smilanick, P.V. Vail, P.L. Hartsell, E. Gomez, Journal of Agricultural and Food Chemistry 44 (1996) 3294–3296. ˜ Journal of Chromatogra[2] E. Lizarraga, Á. Irigoyen, V. Belsue, E. González-Penas, phy A 1052 (2004) 145–149. [3] B.C. Dian, A. Longarte, P.R. Winter, T.S. Zwier, Journal of Chemical Physics 120 (2004) 133–147. [4] M.V. Popik, V.P. Novikov, L.V. Vilkov, S. Samdal, M.A. Tafipolsky, Journal of Molecular Structure 376 (1996) 173–181. [5] M.V. Popik, V.P. Novikov, L.V. Vilkov, S. Samdal, Journal of Structural Chemistry 36 (1995) 784–792.

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[6] E.M. Voigt, C. Reid, Journal of the American Chemical Society 86 (1964) 3930–3934. [7] D.H. Russell, B.S. Freiser, E.H. McBay, D.C. Canada, Organic Mass Spectrometry 18 (1983) 474–485. [8] S. Wateganonkar, S. Doraiswamy, Journal of Chemical Physics 106 (1997) 4894–4901. [9] M.S. Ford, K. Müller-Dethlefs, Physical Chemistry Chemical Physics 6 (2004) 23–31. [10] M.C.R. Cockett, Chemical Society Reviews 34 (2005) 935–948. [11] C. Schon, W. Roth, I. Fischer, J. Pfister, R.F. Fink, B. Engels, Physical Chemistry Chemical Physics 13 (2011) 11076–11082. [12] C. Mukarakate, A.M. Scheer, D.J. Robichaud, M.W. Jarvis, D.E. David, Review of Scientific Instruments 82 (2011) 033104. [13] L. Zhu, P. Johnson, Journal of Chemical Physics 94 (1991) 5769–5771. [14] A. Armentano, X. Tong, M. Riese, S.M. Pimblott, K. Müller-Dethlefs, M. Fujii, O. Dopfer, Physical Chemistry Chemical Physics 13 (2011) 6071–6076. [15] R. Karaminkov, S. Chervenkov, H.J. Neusser, Journal of Physical Chemistry A 114 (2010) 11263–11268. [16] G. Lembach, B. Brutschy, Chemical Physics Letters 273 (1997) 421–428. [17] J. Huang, J.L. Lin, W.B. Tzeng, Chemical Physics Letters 422 (2006) 271–275. [18] F. Witte, M. Riese, F. Gunzer, J. Grotemeyer, International Journal of Mass Spectrometry 306 (2011) 129–137. [19] W.B. Tzeng, J.L. Lin, Journal of Physical Chemistry A 103 (1999) 8612–8619. [20] M.J. Frisch, et al., Gaussian 09, Revision A. 02, Gaussian, Inc., Wallingford, CT, USA, 2009. [21] J.L. Lin, W.B. Tzeng, Journal of Chemical Physics 113 (2000) 4109–4115. [22] W.C. Huang, W.L. Yeh, W.B. Tzeng, Journal of Molecular Spectroscopy 169 (2011) 248–253. [23] S.C. Yang, S.W. Huang, W.B. Tzeng, Journal of Physical Chemistry A 114 (2010) 11144–11152. [24] T. Isozaki, K. Sakeda, T. Suzuki, T. Ichimura, Journal of Chemical Physics 132 (2010) 214308. [25] K.S. Shiung, D. Yu, S.Y. Tzeng, W.B. Tzeng, Chemical Physics Letters 524 (2012) 38–41. [26] M. Pradhan, C. Li, J.L. Lin, W.B. Tzeng, Chemical Physics Letters 407 (2005) 100–104. [27] L. Yuan, C. Li, J.L. Lin, S.C. Yang, W.B. Tzeng, Chemical Physics 323 (2006) 429–438. [28] G. Varsanyi, Assignments of Vibrational Spectra of Seven Hundred Benzene Derivatives, Wiley, New York, 1974. [29] E.B. Wilson, Physical Review 45 (1934) 706–714. [30] The NIST Chemistry Webbook. http://webbook.nist.gov/ and references therein. [31] S. Yamamoto, T. Ebata, M. Ito, Journal of Physical Chemistry 93 (1989) 6340–6345. [32] K. Remmers, W.L. Meerts, A. Zehnacker-Rentien, K.L. Barbu, F. Lahmani, Journal of Chemical Physics 112 (2000) 6237–6244. [33] A.G. Csaszar, F. Fogarasi, Spectrochimica Acta Part A 45 (1989) 845.