- Email: [email protected]
Vibrational spectrum and ab initio calculations of m-xylene
Chemical Physics Letters 397 (2004) 495–499 www.elsevier.com/locate/cplett
Vibrational spectrum and ab initio calculations of m-xylene Song Zhang, Bifeng Tang, Yanmei Wang, Bing Zhang
*
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, PR China Graduate School of the Chinese Academy of Sciences, Beijing, PR China Received 21 June 2004; in final form 3 September 2004
Abstract We present an assigned spectrum of the first electronic transition of m-xylene observed by resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. The band origin of the S1 S0 electronic transition is found to be 36 951 cm1. These vibrationally resolved spectral features have been successfully assigned on the basis of comparison with the results from ab initio and density functional theory calculations. These suggest that the substituents on benzene ring can influence the electronic transition and molecular vibrational modes of m-xylene. The spectrum shows a rich structure and some vibrational frequencies of S1 state are determined. 2004 Elsevier B.V. All rights reserved.
1. Introduction Over the past few decades, the electronically excited states of benzene and its derivatives have attracted considerable interest in the context of understanding the spectra. It is known that supersonic molecular beam REMPI in conjunction with time-of-flight mass spectrometry (TOFMS) is a powerful tool to probe photophysical and photochemical properties of polyatomic molecules, complexes and clusters [1,2]. For many substituted benzenes, like toluene [3] and p-xylene [4], the first excited state S1 has been investigated and assigned vibrationally resolved spectra are available. There exists a fluorescence excitation spectrum [5] over 1000 cm1 and some theoretical work has been done [6,7]. T.G. Blease and co-workers have studied photoionization of m-, o-, and p-xylene by (2 + 2) photoionization [8]. Cooper and co-workers [9,10] reported vibrationally resolved structure in the absorption spectra for three xylene isomers, but no vibrational modes were assigned. Both Breen and Held [11,12] studied the internal rotational levels of the methyl rotors *
Corresponding author. Fax: +86 27 87199291. E-mail address: [email protected] (B. Zhang).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.025
in o-, m- and p-xylene . Since then, numerous spectroscopic studies for the internal rotation of methyl groups were reported [4,12–14]. For the S1 state of m-xylene, however, less vibrational information is available. In this work, we present a spectrum of the electronic transition S1 S0 of m-xylene over a region of 2000 cm1 and an assignment of the vibronic structure. The obtained spectra yield new information about the origin of electronic transition and the active vibrations in the S1 excited state. Comparison of the present results with those of toluene [3] and p-xylene [4] leads to a good understanding of the methyl group effects on the electronic transition and molecular vibrations. We have also performed ab initio and density functional theory (DFT) calculations to provide satisfactory explanation for the experimental findings.
2. Experimental and computational details 2.1. Experimental method The experimental apparatus with a laser-based TOF mass spectrometer used here has been described in our
496
S. Zhang et al. / Chemical Physics Letters 397 (2004) 495–499
previous publication [15]. The molecular beam was produced by argon flowing through a reservoir of liquid m-xylene (99% purity), which was kept at room temperature. A total pressure of 1.5 bar was applied behind a pulsed valve (General Valve Corp. Series 9) with 0.2 mm diameter orifice. The vacuum system maintains a background pressure of <106 Torr. The pulsed valve was operated at 10 Hz with pulse duration of 180 ls and the resulting average chamber pressure was 1.0 · 105 Torr. The one color (1 + 1) resonant two-photon excitation process was initiated by a tunable UV laser system controlled by a pulse delay generator (Stanford Research System, DG 535), which the nozzle was synchronized by with each laser shot (10 Hz). The dye laser (Lambda Physik Scanmate 2E OG) was pumped by the third harmonic (355 nm) of a nanosecond Nd:YAG laser (YG 981 E10). The visible radiation was frequency doubled using BBO to produce the UV radiation. The UV laser output was not focused and interacted perpendicularly with the molecular beam. The wavelengths were calibrated with an optogalvanic (OG) signal, which was produced by a neon hollow cathode lamp, and represented the absorption spectrum of neon. The ions were then accelerated by double fields [16] and detected by a dual-stacked microchannel plate (MCP) detector after passing a 60 cm field-free drift tube. The ion signal from the detector was collected and recorded by a Boxcar (Stanford Research System, SR254). The Boxcar and digital oscilloscope (Tektronix TDS 2012) were interfaced to a personal computer that controlled the experiment. Spectra were accumulated at 0.02 nm spacing for 30 laser shots. The recorded wavelength is over 2000 cm1. 2.2. Computational method Ab initio and DFT calculations were performed to predict molecular structure parameters, excited energies
(EEs), vibrational frequencies and other properties using the GAUSSIAN 03 program package [17]. The restricted Hartree–Fock (HF), configuration interaction singles (CIS) and DFT methods using 6-311G** basis set were applied to predict the vibrational frequencies and EE of m-xylene in the S0 and S1 states. When a scaling factor of 0.925 is applied, the calculated vibrational frequencies match well the measured values from the one-color R2PI experiments. The calculated vibrational frequencies and scaling factors are listed in Table 1 along with the measured values. The EE was obtained as the difference in the zero-point energies of the first excited state and the corresponding ground state.
3. Results and discussion 3.1. R2PI spectra of m-xylene m-Xylene has 48 normal vibrations, which include 30 benzene-like and 18 methyl vibrational modes. Concerning the active vibrations in the S1 state, only those vibronic transitions with large Franck–Condon overlaps can be observed in the R2PI spectrum. Fig. 1. displays the 1C-R2PI spectrum of m-xylene recorded in the energy range near its S1 S0 transition. The band origin of the S1 S0 electronic transition appears at 36 951 cm1. It is in very good agreement with that reported in the literature [5,11,12]. The intense bands related to the vibrational bands are found at 473, 685, 966, and 1609 cm1, which we shall return to below. Although previous researchers have observed the internal rotation of the CH3 group and assigned these bands, but they did not attempt to research the vibrational bands home. In the present study, we have successfully assigned these vibrational bands as well as the results from the ab initio calculations, as shown in Table 1. When the frequencies obtained from the CIS/6-311G** calculations are scaled by 0.925, they become very close to the measured values
Table 1 Assignment of the observed bands (cm1) in the 1C-R2PI spectrum of m-xylene This worka
Previous work Ref. [5]
455 677 964
Ref. [3]
Exp.
86.5 97.7
80 97 331 473 546 685 738 966 1253 1427 1644
Assignment and approximate descriptionb Cal.
475 597 691 741 982 1260 1414 1609
CH3 torsion CH3 torsion m (C–CH3) 6a b(CCC) 11 c (CC) 1 breathing 17b c(CH) 12 b(CCC) 13 b(CCC) 19a b(CCC) 8b m(CC)
The experimental values are shift from 36 951 cm1, whereas the computer ones (scaled by 0.925) are obtained from the CIS/6-311G** calculations. b b, in-plane bending; m, in-plane stretch; c, out-of-plane bending. a
S. Zhang et al. / Chemical Physics Letters 397 (2004) 495–499
497
-1
36951 cm
12
torsion
8b 6a 13 19a
1
0
500
1000
1500
ring deformation 6a β (CCC)
ring deformation 1breathing
2000
-1
wavenumber (cm ) Fig. 1. 1C-R2PI spectrum of m-xylene.
from the R2PI experiments. The spectral assignment was made mainly on the conformity with the available data in the S0 and S1 states [18]. The Wilson notation [19] was adopted to approximately describe the benzene-like vibrational modes. The noticeable bands at 473, 966, 1253 and 1427 cm1 are assigned, respectively, to the 6a1, 121, 131 and 19a1 transitions, which are mainly involved in inplane ring deformations, as shown in Fig. 2. The normal vibrations can be viewed by using the GAUSSVIEW program, which is a complementary program for being used with the GAUSSAIN 03W package [17]. Modes 11 and 17b, which result from the out-of-plane CC and CH bending vibrations, appear at 546 and 738 cm1, respectively. The bands at 685 and 1644 cm1 are assigned to transitions 1 and 8b, which are related to the breathing and in-plane C–C stretching vibrations, respectively, as seen in Fig. 2. The CH3 torsions are observed at 80 and 97 cm1.
ring deformation 12 β (CCC)
ring deformation 13 β (CCC)
ring deformation 19a β (CCC)
C-C stretching 8b ν (CC)
3.2. Methyl substitution effect on the structure and electronic transition of the m-xylene
Fig. 2. Some large amplitude vibrations of m-xylene.
The interaction between the methyl groups and the aromatic ring may result from the inductive effect through the r bond and the hyperconjugation through the p orbits, leading to a change in the zero-point level (ZPL). The inductive effect is related to the ability of withdrawing or donating electrons, whereas the resonance effect reflects the extent of the p overlaps between the CH3 groups and the ring. As the methyl substitution takes place at the hydrogen atoms of phenyl ring, the CH3 groups may donate electrons through the r bond. The methyl-ring interaction can cause a change in the neighboring electron density as well as the molecular geometry. It is generally known that the S1 S0 transition of benzene derivatives corresponds to p–p* excitation,
leading to an expansion in the ring and a significant change in overall molecular geometry. The CIS calculations with 6-311G** predict the CH3 group rotation for m-xylene. The calculated results also suggest that the small rotational angle occurs on the CH3 groups in the S1 state. The respective band origins of the S1 S0 electronic transition of toluene and m-xylene are measured to be 37 477.5 [11] and 36 951 cm1. In other words, the methyl substitution results in a red shift of 526.5 cm1. The magnitude of this energy shift is somewhat dependent on the location of CH3 groups on the ring. The observed shift in the EE indicates that the magnitude of the lowering of the ZPL in the S1 state is greater than that in the S0 state. In other words, the interaction between the substituent and the ring is
498
S. Zhang et al. / Chemical Physics Letters 397 (2004) 495–499
Table 2 Experimental and calculated excited state energy of m-xylene Method
The first excited state energy (cm1)
Dev (%)
Experimental 36 951a 36 949b 36 956c 36 951d Computational CIS/6-311G(d,p) B3LYP/6-31G(d,p) B3PW91/6-31G(d,p) a b c d
48 102 41 656 41 827
these modes are 331, 738, 1253, 1427 and 1644 cm1, respectively. The noticed low-frequency bands at 80 and 97 cm1 stand for CH3 torsion. As shown in Fig. 2, active modes in the S1 may be approximately described as the in-plane ring deformation.
4. Conclusion 30 12.7 13.1
This work. Ref. [5]. Ref. [3]. Ref. [12].
greater in the S1 state than in the S0 state. The present ab initio and DFT calculations show that the methyl substitution causes a lowering of the ZPL both in the S1 state and in the S0 states. In order to provide a comparison with the present experimental results, we have performed the HF, CIS and DFT calculations. The computed EEs were obtained as the difference in total energies including ZPL between the first excited state S1 and the corresponding ground state S0 using different methods and basis sets. As shown in Table 2, the CIS/6311G** method predicts an EE of about 30% higher than the measured value. It was found that both the Becke three-parameter functional with the PW91 correlation functional (B3LYP) and the hybrid B3PW91 density functional methods improved the deviation to 12.7% and 13.1%.
The vibrationally resolved spectra of m-xylene in the first excited state have been recorded by using R2PI spectroscopic technique. The band origin of the S1 S0 electronic transition is determined to be 36 951 cm1. The S1 S0 excitation causes the CH3 groups of the m-xylene to rotate a small angle. This significant geometry change leads to rich vibrational spectral features, which represent the motions related to the localized benzene ring. The ab initio and DFT calculations have been performed to predict the structures, total energies, and vibrational frequencies of m-xylene in the S0, S1 states. The vibrational features of m-xylene are successfully assigned by comparing them with the results from the theoretical calculations. Analysis on these R2PI spectra shows that most of the observed active vibrations are related to the CH3 torsion and in-plane bending vibrations.
Acknowledgement All the authors gratefully acknowledge support from National Natural Science Foundation of China (299730397).
3.3. Active vibrations of the m-xylene in the S1 state
References
Up to now, only the internal rotations of the CH3 groups of the m-xylene have been studied [11–14]. The present experiments provide new experimental information on the active vibrations of m-xylene, as shown in Fig. 1. Most of the moderately intense bands are related to the in-plane ring vibrations, as listed in Table 2. The band at 473 cm1 results from the phenyl ring deformation pattern 6a, whereas that at 685 cm1 represents the breathing vibration of m-xylene in the S1 state. Similar observation is found for normal vibration 12, with the frequency of 966 cm1 for m-xylene. However, mode 12 mainly involves an in-plane trigonal CCC bending vibration, where the ring carbon C1, C3, C 5 atoms move as a group in one direction and the C2, C4, C6 atoms in an opposite way. It is interesting to note that other vibrations, 11, 17b, 13, 19a and 8b, are active. The frequencies of
[1] K. Kimura, J. Electron. spectrosc. Relat. Phenom. 100 (1999) 273. [2] C.E.H. Dessent, K. Muller-Dethlefs, Chem. Rev. 100 (2000) 3999. [3] P.J. Breen, J.A. Warren, E.R. Bernstein, J. Chem. Phys. 87 (1987) 1917. [4] K. Walter, K. Scherm, U. Boesl, Chem. Phys. Lett. 161 (1989) 473. [5] T. Ebata, Y. Suzuki, N. Mikami, T. Miyashi, M. Ito, Chem. Phys. Lett. 110 (1984) 597. [6] T. Suzuki, T. Ikegami, M. Fujii, S. Iwata, J. Mol. Struct. 461–462 (1999) 79. [7] O. Kirstein, M. Prager, Appl. Phys. A 74 (2002) S1326. [8] R.P. Rava, L. Goodman, J.G. Philis, J. Chem. Phys. 77 (1982) 4912. [9] C.D. Cooper, M.L.N. Sastri, J. Chem. Phys. 20 (1952) 607. [10] C.D. Cooper, H. Sponer, J. Chem. Phys. 20 (1952) 1248. [11] P.J. Breen, J.A. Watten, E.R. Bernstein, J. Chem. Phys. 87 (1987) 1917. [12] A. Held, H.L. Selzle, E.W. Schlag, J. Phys. Chem. A 102 (1998) 9625. [13] K.C. Ingham, S.J. Strickler, J. Chem. Phys. 53 (1970) 4313.
S. Zhang et al. / Chemical Physics Letters 397 (2004) 495–499 [14] T.Y.D. Lin, T.A. Miller, J. Phys. Chem. 94 (1990) 3554. [15] B. Tang, R. Rong, Y. Tang, L. Ji, B. Zhang, Chem. Phys. Lett. 381 (2003) 617. [16] W.C. Wiley, I.H. Mclaren, Rev. Sci. Instrum. 26 (1995) 1150.
499
[17] M.J. Frisch, et al., GAUSSAIN 03, Revision B.01, Gaussian, Inc, Pittsburgh, PA, 2003. [18] G. Varsanyi, Assignments of Vibrational Spectra of Seven Hundred Benzene Derivative Wiley, Wiley, New York, 1974. [19] E.B. Wilson, Phys. Rev. 45 (1934) 706.
Vibrational spectrum and ab initio calculations of m-xylene Song Zhang, Bifeng Tang, Yanmei Wang, Bing Zhang
*
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, PR China Graduate School of the Chinese Academy of Sciences, Beijing, PR China Received 21 June 2004; in final form 3 September 2004
Abstract We present an assigned spectrum of the first electronic transition of m-xylene observed by resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. The band origin of the S1 S0 electronic transition is found to be 36 951 cm1. These vibrationally resolved spectral features have been successfully assigned on the basis of comparison with the results from ab initio and density functional theory calculations. These suggest that the substituents on benzene ring can influence the electronic transition and molecular vibrational modes of m-xylene. The spectrum shows a rich structure and some vibrational frequencies of S1 state are determined. 2004 Elsevier B.V. All rights reserved.
1. Introduction Over the past few decades, the electronically excited states of benzene and its derivatives have attracted considerable interest in the context of understanding the spectra. It is known that supersonic molecular beam REMPI in conjunction with time-of-flight mass spectrometry (TOFMS) is a powerful tool to probe photophysical and photochemical properties of polyatomic molecules, complexes and clusters [1,2]. For many substituted benzenes, like toluene [3] and p-xylene [4], the first excited state S1 has been investigated and assigned vibrationally resolved spectra are available. There exists a fluorescence excitation spectrum [5] over 1000 cm1 and some theoretical work has been done [6,7]. T.G. Blease and co-workers have studied photoionization of m-, o-, and p-xylene by (2 + 2) photoionization [8]. Cooper and co-workers [9,10] reported vibrationally resolved structure in the absorption spectra for three xylene isomers, but no vibrational modes were assigned. Both Breen and Held [11,12] studied the internal rotational levels of the methyl rotors *
Corresponding author. Fax: +86 27 87199291. E-mail address: [email protected] (B. Zhang).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.025
in o-, m- and p-xylene . Since then, numerous spectroscopic studies for the internal rotation of methyl groups were reported [4,12–14]. For the S1 state of m-xylene, however, less vibrational information is available. In this work, we present a spectrum of the electronic transition S1 S0 of m-xylene over a region of 2000 cm1 and an assignment of the vibronic structure. The obtained spectra yield new information about the origin of electronic transition and the active vibrations in the S1 excited state. Comparison of the present results with those of toluene [3] and p-xylene [4] leads to a good understanding of the methyl group effects on the electronic transition and molecular vibrations. We have also performed ab initio and density functional theory (DFT) calculations to provide satisfactory explanation for the experimental findings.
2. Experimental and computational details 2.1. Experimental method The experimental apparatus with a laser-based TOF mass spectrometer used here has been described in our
496
S. Zhang et al. / Chemical Physics Letters 397 (2004) 495–499
previous publication [15]. The molecular beam was produced by argon flowing through a reservoir of liquid m-xylene (99% purity), which was kept at room temperature. A total pressure of 1.5 bar was applied behind a pulsed valve (General Valve Corp. Series 9) with 0.2 mm diameter orifice. The vacuum system maintains a background pressure of <106 Torr. The pulsed valve was operated at 10 Hz with pulse duration of 180 ls and the resulting average chamber pressure was 1.0 · 105 Torr. The one color (1 + 1) resonant two-photon excitation process was initiated by a tunable UV laser system controlled by a pulse delay generator (Stanford Research System, DG 535), which the nozzle was synchronized by with each laser shot (10 Hz). The dye laser (Lambda Physik Scanmate 2E OG) was pumped by the third harmonic (355 nm) of a nanosecond Nd:YAG laser (YG 981 E10). The visible radiation was frequency doubled using BBO to produce the UV radiation. The UV laser output was not focused and interacted perpendicularly with the molecular beam. The wavelengths were calibrated with an optogalvanic (OG) signal, which was produced by a neon hollow cathode lamp, and represented the absorption spectrum of neon. The ions were then accelerated by double fields [16] and detected by a dual-stacked microchannel plate (MCP) detector after passing a 60 cm field-free drift tube. The ion signal from the detector was collected and recorded by a Boxcar (Stanford Research System, SR254). The Boxcar and digital oscilloscope (Tektronix TDS 2012) were interfaced to a personal computer that controlled the experiment. Spectra were accumulated at 0.02 nm spacing for 30 laser shots. The recorded wavelength is over 2000 cm1. 2.2. Computational method Ab initio and DFT calculations were performed to predict molecular structure parameters, excited energies
(EEs), vibrational frequencies and other properties using the GAUSSIAN 03 program package [17]. The restricted Hartree–Fock (HF), configuration interaction singles (CIS) and DFT methods using 6-311G** basis set were applied to predict the vibrational frequencies and EE of m-xylene in the S0 and S1 states. When a scaling factor of 0.925 is applied, the calculated vibrational frequencies match well the measured values from the one-color R2PI experiments. The calculated vibrational frequencies and scaling factors are listed in Table 1 along with the measured values. The EE was obtained as the difference in the zero-point energies of the first excited state and the corresponding ground state.
3. Results and discussion 3.1. R2PI spectra of m-xylene m-Xylene has 48 normal vibrations, which include 30 benzene-like and 18 methyl vibrational modes. Concerning the active vibrations in the S1 state, only those vibronic transitions with large Franck–Condon overlaps can be observed in the R2PI spectrum. Fig. 1. displays the 1C-R2PI spectrum of m-xylene recorded in the energy range near its S1 S0 transition. The band origin of the S1 S0 electronic transition appears at 36 951 cm1. It is in very good agreement with that reported in the literature [5,11,12]. The intense bands related to the vibrational bands are found at 473, 685, 966, and 1609 cm1, which we shall return to below. Although previous researchers have observed the internal rotation of the CH3 group and assigned these bands, but they did not attempt to research the vibrational bands home. In the present study, we have successfully assigned these vibrational bands as well as the results from the ab initio calculations, as shown in Table 1. When the frequencies obtained from the CIS/6-311G** calculations are scaled by 0.925, they become very close to the measured values
Table 1 Assignment of the observed bands (cm1) in the 1C-R2PI spectrum of m-xylene This worka
Previous work Ref. [5]
455 677 964
Ref. [3]
Exp.
86.5 97.7
80 97 331 473 546 685 738 966 1253 1427 1644
Assignment and approximate descriptionb Cal.
475 597 691 741 982 1260 1414 1609
CH3 torsion CH3 torsion m (C–CH3) 6a b(CCC) 11 c (CC) 1 breathing 17b c(CH) 12 b(CCC) 13 b(CCC) 19a b(CCC) 8b m(CC)
The experimental values are shift from 36 951 cm1, whereas the computer ones (scaled by 0.925) are obtained from the CIS/6-311G** calculations. b b, in-plane bending; m, in-plane stretch; c, out-of-plane bending. a
S. Zhang et al. / Chemical Physics Letters 397 (2004) 495–499
497
-1
36951 cm
12
torsion
8b 6a 13 19a
1
0
500
1000
1500
ring deformation 6a β (CCC)
ring deformation 1breathing
2000
-1
wavenumber (cm ) Fig. 1. 1C-R2PI spectrum of m-xylene.
from the R2PI experiments. The spectral assignment was made mainly on the conformity with the available data in the S0 and S1 states [18]. The Wilson notation [19] was adopted to approximately describe the benzene-like vibrational modes. The noticeable bands at 473, 966, 1253 and 1427 cm1 are assigned, respectively, to the 6a1, 121, 131 and 19a1 transitions, which are mainly involved in inplane ring deformations, as shown in Fig. 2. The normal vibrations can be viewed by using the GAUSSVIEW program, which is a complementary program for being used with the GAUSSAIN 03W package [17]. Modes 11 and 17b, which result from the out-of-plane CC and CH bending vibrations, appear at 546 and 738 cm1, respectively. The bands at 685 and 1644 cm1 are assigned to transitions 1 and 8b, which are related to the breathing and in-plane C–C stretching vibrations, respectively, as seen in Fig. 2. The CH3 torsions are observed at 80 and 97 cm1.
ring deformation 12 β (CCC)
ring deformation 13 β (CCC)
ring deformation 19a β (CCC)
C-C stretching 8b ν (CC)
3.2. Methyl substitution effect on the structure and electronic transition of the m-xylene
Fig. 2. Some large amplitude vibrations of m-xylene.
The interaction between the methyl groups and the aromatic ring may result from the inductive effect through the r bond and the hyperconjugation through the p orbits, leading to a change in the zero-point level (ZPL). The inductive effect is related to the ability of withdrawing or donating electrons, whereas the resonance effect reflects the extent of the p overlaps between the CH3 groups and the ring. As the methyl substitution takes place at the hydrogen atoms of phenyl ring, the CH3 groups may donate electrons through the r bond. The methyl-ring interaction can cause a change in the neighboring electron density as well as the molecular geometry. It is generally known that the S1 S0 transition of benzene derivatives corresponds to p–p* excitation,
leading to an expansion in the ring and a significant change in overall molecular geometry. The CIS calculations with 6-311G** predict the CH3 group rotation for m-xylene. The calculated results also suggest that the small rotational angle occurs on the CH3 groups in the S1 state. The respective band origins of the S1 S0 electronic transition of toluene and m-xylene are measured to be 37 477.5 [11] and 36 951 cm1. In other words, the methyl substitution results in a red shift of 526.5 cm1. The magnitude of this energy shift is somewhat dependent on the location of CH3 groups on the ring. The observed shift in the EE indicates that the magnitude of the lowering of the ZPL in the S1 state is greater than that in the S0 state. In other words, the interaction between the substituent and the ring is
498
S. Zhang et al. / Chemical Physics Letters 397 (2004) 495–499
Table 2 Experimental and calculated excited state energy of m-xylene Method
The first excited state energy (cm1)
Dev (%)
Experimental 36 951a 36 949b 36 956c 36 951d Computational CIS/6-311G(d,p) B3LYP/6-31G(d,p) B3PW91/6-31G(d,p) a b c d
48 102 41 656 41 827
these modes are 331, 738, 1253, 1427 and 1644 cm1, respectively. The noticed low-frequency bands at 80 and 97 cm1 stand for CH3 torsion. As shown in Fig. 2, active modes in the S1 may be approximately described as the in-plane ring deformation.
4. Conclusion 30 12.7 13.1
This work. Ref. [5]. Ref. [3]. Ref. [12].
greater in the S1 state than in the S0 state. The present ab initio and DFT calculations show that the methyl substitution causes a lowering of the ZPL both in the S1 state and in the S0 states. In order to provide a comparison with the present experimental results, we have performed the HF, CIS and DFT calculations. The computed EEs were obtained as the difference in total energies including ZPL between the first excited state S1 and the corresponding ground state S0 using different methods and basis sets. As shown in Table 2, the CIS/6311G** method predicts an EE of about 30% higher than the measured value. It was found that both the Becke three-parameter functional with the PW91 correlation functional (B3LYP) and the hybrid B3PW91 density functional methods improved the deviation to 12.7% and 13.1%.
The vibrationally resolved spectra of m-xylene in the first excited state have been recorded by using R2PI spectroscopic technique. The band origin of the S1 S0 electronic transition is determined to be 36 951 cm1. The S1 S0 excitation causes the CH3 groups of the m-xylene to rotate a small angle. This significant geometry change leads to rich vibrational spectral features, which represent the motions related to the localized benzene ring. The ab initio and DFT calculations have been performed to predict the structures, total energies, and vibrational frequencies of m-xylene in the S0, S1 states. The vibrational features of m-xylene are successfully assigned by comparing them with the results from the theoretical calculations. Analysis on these R2PI spectra shows that most of the observed active vibrations are related to the CH3 torsion and in-plane bending vibrations.
Acknowledgement All the authors gratefully acknowledge support from National Natural Science Foundation of China (299730397).
3.3. Active vibrations of the m-xylene in the S1 state
References
Up to now, only the internal rotations of the CH3 groups of the m-xylene have been studied [11–14]. The present experiments provide new experimental information on the active vibrations of m-xylene, as shown in Fig. 1. Most of the moderately intense bands are related to the in-plane ring vibrations, as listed in Table 2. The band at 473 cm1 results from the phenyl ring deformation pattern 6a, whereas that at 685 cm1 represents the breathing vibration of m-xylene in the S1 state. Similar observation is found for normal vibration 12, with the frequency of 966 cm1 for m-xylene. However, mode 12 mainly involves an in-plane trigonal CCC bending vibration, where the ring carbon C1, C3, C 5 atoms move as a group in one direction and the C2, C4, C6 atoms in an opposite way. It is interesting to note that other vibrations, 11, 17b, 13, 19a and 8b, are active. The frequencies of
[1] K. Kimura, J. Electron. spectrosc. Relat. Phenom. 100 (1999) 273. [2] C.E.H. Dessent, K. Muller-Dethlefs, Chem. Rev. 100 (2000) 3999. [3] P.J. Breen, J.A. Warren, E.R. Bernstein, J. Chem. Phys. 87 (1987) 1917. [4] K. Walter, K. Scherm, U. Boesl, Chem. Phys. Lett. 161 (1989) 473. [5] T. Ebata, Y. Suzuki, N. Mikami, T. Miyashi, M. Ito, Chem. Phys. Lett. 110 (1984) 597. [6] T. Suzuki, T. Ikegami, M. Fujii, S. Iwata, J. Mol. Struct. 461–462 (1999) 79. [7] O. Kirstein, M. Prager, Appl. Phys. A 74 (2002) S1326. [8] R.P. Rava, L. Goodman, J.G. Philis, J. Chem. Phys. 77 (1982) 4912. [9] C.D. Cooper, M.L.N. Sastri, J. Chem. Phys. 20 (1952) 607. [10] C.D. Cooper, H. Sponer, J. Chem. Phys. 20 (1952) 1248. [11] P.J. Breen, J.A. Watten, E.R. Bernstein, J. Chem. Phys. 87 (1987) 1917. [12] A. Held, H.L. Selzle, E.W. Schlag, J. Phys. Chem. A 102 (1998) 9625. [13] K.C. Ingham, S.J. Strickler, J. Chem. Phys. 53 (1970) 4313.
S. Zhang et al. / Chemical Physics Letters 397 (2004) 495–499 [14] T.Y.D. Lin, T.A. Miller, J. Phys. Chem. 94 (1990) 3554. [15] B. Tang, R. Rong, Y. Tang, L. Ji, B. Zhang, Chem. Phys. Lett. 381 (2003) 617. [16] W.C. Wiley, I.H. Mclaren, Rev. Sci. Instrum. 26 (1995) 1150.
499
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