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Diode–Laser Spectroscopy of the ν19a Band of Chlorobenzene in a Supersonic Jet
Journal of Molecular Spectroscopy 202, 262–271 (2000) doi:10.1006/jmsp.2000.8132, available online at http://www.idealibrary.com on
Diode–Laser Spectroscopy of the 19a Band of Chlorobenzene in a Supersonic Jet Araitz Uskola, Francisco J. Basterretxea, 1 and Fernando Castan˜o Departamento de Quı´mica Fı´sica, Facultad de Ciencias, Universidad del Paı´s Vasco/Euskal Herriko Unibertsitatea, Apdo. 644, 48080 Bilbao, Spain Received February 29, 2000; in revised form April 6, 2000
The diode–laser absorption spectrum of the 19a band of the chlorobenzene molecule cooled in a supersonic jet is presented and transitions from low J, K a , and K c values (0 –14) are reported. The C 6H 5 35Cl band center has been found at 0 ⫽ 1483.894 cm ⫺1 and rotational constants for the upper state have been determined from a least-squares fitting to experimental data. The band center for the C 6H 5 37Cl isotope has also been obtained. © 2000 Academic Press Key Words: diode–laser spectroscopy; supersonic jet; chlorobenzene 19a band. 1. INTRODUCTION
Benzene and, to a lesser extent, its haloderivatives have been benchmarks of the recent developments in high-resolution vibrational spectroscopy of large molecules (1, 2). Assignment of the fundamental vibrational modes of monohalogenated benzenes was first reported by Whiffen (3), while a set of chlorobenzene rotational constants was obtained later from electronic spectra with partially resolved rotational structure (4, 5). By combining vapor-phase electronic and infrared absorption spectra, Bist et al. provided vibrational frequencies for the in-plane vibrations of the ground (6) and excited state (7), and later also the out-of-plane vibrational modes of both states (8). The geometry of chlorobenzene has also been investigated by microwave spectroscopy in conjunction with isotopic substitution (9, 10), leading to the determination of the r s coordinates of the vibrational ground states and pointing out that the aromatic ring structure departs considerably from the C 6 symmetry. Despite of the spectroscopic interest of benzene derivatives, few high-resolution infrared spectra have been reported (11). In particular, neither rotationally resolved infrared spectrum of chlorobenzene has been presented up to date, nor are precise band origins or excited state rotational constants available. In this work, we report the diode–laser spectra of the 19a vibrational mode of chlorobenzene cooled in a supersonic jet. Tunable diode–laser spectroscopy of jet-cooled molecules yields simplified rotationally resolved spectra with respect to those at room temperature; in particular, the removal of hot bands permits the accurate location of band centers, and the rotational analysis supplies structural parameters for both vibrational levels. The advantage of combining supersonic cooling and tunable diode–laser spectroscopy is demonstrated for this 1
To whom correspondence should be addressed. E-mail: qfpbaelf@ lg.ehu.es.
rather heavy molecule, for which room-temperature spectrum is very congested and difficult to assign, even at high resolution. We report values for the band origin and rotational constants of the excited state that are valuable as a starting point for a detailed rotational analysis. 2. EXPERIMENTAL
The experimental system has been described elsewhere (12), and only the relevant details will be given here. Natural abundance chlorobenzene liquid samples (Merck, ⬎99%) were placed in a reservoir connected to a pulsed valve with a 0.5-mm-diameter nozzle (General Valve). Both valve and reservoir were heated at ca. 60°C by wrapped resistive wires to ensure a sufficient vapor pressure of the halobenzene. Heated chlorobenzene was seeded in He (Praxair, 99.999%) at 1.5-bar stagnation pressure and expanded through the pulsed nozzle (20 Hz, 200-s pulse duration) into a vacuum chamber pumped down by a turbomolecular-rotary pump coupled system. An infrared tunable laser beam was provided by a lead salt diode working at liquid-nitrogen temperatures (Laser Photonics, L5000) and centered around 1480 cm ⫺1. To guarantee complete spectral coverage, two diode lasers emitting in the same range were used. Multimode laser emission was passed through a monochromator to obtain single-mode radiation, which crossed the supersonic beam several times aided by a multipass optical system, and finally reached a HgCdTe detector. The signal was sent to a lock-in amplifier tuned at 2.5 kHz and further routed to a digital oscilloscope, which averaged up to 256 signals between user-selected time intervals. Laser absorption spectra were recorded by averaging the detector signal while slowly scanning the laser frequency. Chloromethane was used as a calibration standard (13) and frequency scale was provided by a 1-in. Ge solid e´talon, with a free spectral range of 0.0471 cm ⫺1.
262 0022-2852/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved.
19a BAND OF C 6H 5Cl
263 3. RESULTS AND DISCUSSION
3.1. The 19a and 19b Normal Modes of Chlorobenzene
FIG. 1. Ab initio calculated geometry of the chlorobenzene molecule: (a) equilibrium geometry, showing the axis system chosen; (b) geometry with the 19a normal mode at its maximum amplitude; (c) same as (b) for the 19b normal mode. Below the structures shown in (b) and (c), the calculated and experimental vibrational wavenumbers are indicated.
Figure 1a shows the chlorobenzene geometry and the axis system used in this study. Benzene substitution of one hydrogen atom by chlorine reduces the group symmetry from D 6h to C 2v , splitting its degenerate vibrational modes (e 1g , e 2g , e 1u , and e 2u ) into two components, each of a 1 and b 2 symmetry. Nondegenerate irreducible representations a 1g and b 1u transform to a 1 symmetry, a 2u and b 2g to b 1 , and finally a 2g and b 2u to b 2 . In summary, chlorobenzene normal modes are 11 a 1 species, giving rise to A-type infrared bands, 6 b 1 species with C-type bands, and 10 b 2 species with B-type bands. The remaining three species, of a 2 symmetry, are infrared inactive. According to the vibrational mode numbering proposed by Wilson (14), chlorobenzene 19a and 19b normal modes derive from benzene 19(e 1g ) mode, i.e., chlorobenzene bands 19a (a 1 ) at 1482 cm ⫺1 and 19b (b 2 ) at 1447 cm ⫺1 (6) arise from benzene 19 (e 1g ) at 1484 cm ⫺1 (15). The 19a, b vibrational frequencies, together with atom displacements, have been accurately computed with the GAUSSIAN 94 package (16) at the B3LYP/ AUG-cc-pVDZ level, in order to better confirm the identification of normal modes. The B3LYP density functional is known to give accurate harmonic vibrational frequencies of small molecules and also of benzene (17) at relatively low computational cost. The results of the calculation are depicted in Fig. 1; in Fig. 1a the equilibrium configuration is shown, whereas Figs. 1b and 1c display a picture of the maximum amplitude vibrational positions of the 19a and 19b modes. The calculated vibrational wavenumbers, along with the experimental values
FIG. 2. Diode–laser absorption spectrum of the chlorobenzene 19a band center as obtained in a supersonic jet, showing the prominent Q branch and the lowest wavenumber transitions of the P and R branches.
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available (6), are also given. It can be seen that the computed wavenumbers are very close to the experimental ones, in particular for the 19a mode. The calculated ground state rotational constants are A⬙ ⫽ 0.188286, B⬙ ⫽ 0.051744, and C⬙ ⫽ 0.040590 cm ⫺1, in good agreement with the microwave values (9). Mode character in the 19a, b pair can be described as in-plane vibration, combining C–H bending with ring deformation. The C–Cl bond stretches in the 19a vibration, whereas it basically bends in 19b. In the 19a mode, the transition dipole moment is oriented along the a axis, giving rise to an A-type infrared band, while in the 19b mode lies on the b axis, yielding a B-type band. 3.2. Diode–Laser Spectrum of the 19a Band Diode spectra of chlorobenzene in a supersonic jet were recorded between 1481.6 and 1485.5 cm ⫺1. Chlorobenzene, with an asymmetry parameter ⫽ ⫺0.84, is very near the prolate symmetric limit, where an A-type band corresponds to a parallel-type band. Indeed, the spectrum has a marked resemblance to a parallel band of a symmetric rotor, with a prominent central Q branch and two P, R sidebands at lower and higher wavenumbers, respectively. Although the spectrum obtained still shows widespread overlapping, hot bands have been eliminated, and rotational transitions for values of J, K a , K c ⱕ 14 can be observed. Figure 2 shows the central part of the band, including the strong Q branch and low J, K a , K c values for the P and R branches, of much weaker intensity. Figures 3 and 4 show portions of the P and R branches, respectively. In the first case the calculated spectrum is also included for comparison purposes (see later). The spectra have been scaled linearly in wavenumbers. Intensity normalization was not considered necessary, since laser power is roughly constant in the whole mode, decreasing only slightly at the sides. The structure of the Q branch is not resolvable with our experimental setup. In both P and R branches, for low J, K a , K c values it is useful to consider groups of lines with the same J. The asymmetry produces a splitting of the K a levels, being largest for K a ⫽ ⫹1. In Figs. 3 and 4, it is easily appreciated that, for each group, the external two transitions correspond to K a ⫽ 1. The splitting magnitude increases with J, and eventually the patterns of consecutive J values overlap. In this case it is more useful to consider the spectrum as a superposition of subbands, each characterized by a constant value of K a and the parity of the ground state levels (e, o). For clarity purposes, Figs. 3 and 4 only show the assignment of the most intense subbands. The nomenclature used is as follows: e,o (⌬J) K a ( J), where ⌬J means P (⌬J ⫽ ⫺1) or R (⌬J ⫽ ⫹1) branch, the subscript is the K a value of a given subband, the running J value appears in parentheses, and the e or o superscript stands for the parity of the rotational level. According to selection rules, ⌬K a ⫽ 0, and ⌬K c ⫽ ⌬J in both R and P branches. In
FIG. 3. (Upper trace) Diode–laser absorption spectrum of the chlorobenzene 19a band as obtained in a supersonic jet, including part of the P branch. Note the high intensity around the J ⫽ 4 line group. (Lower trace) Calculated spectrum, assuming a rotational temperature of 10 K.
addition, for e levels, K a ⫹ K c ⫽ J, whereas for o levels, K a ⫹ K c ⫽ J ⫹ 1. For K a ⫽ 0, only e levels exist (18). Most rotational transitions could be assigned with the initial help of a simulation program that models the infrared spectrum of a rigid rotor, given the rotational constants of both vibrational states, the band center, selection rules, nuclear spin statistics, and rotational temperature of the expansion (19). Although the lower J transitions of the P and R branches (Fig. 2) have a low signal-to-noise ratio, these transitions have been reasonably assigned by matching the wavenumbers of a series of recorded spectra with those computed using the rotational constants obtained by fitting only the most intense lines. The same can be said of the weak lines in Figs. 3 and 4. A rotational temperature value of T rot ⫽ 10 K satisfactorily reproduces the intensity distribution of the experimental spectrum and was selected from the jet expansion conditions observed in our previous work with benzene (12). Regarding nuclear spin statistics, the hydrogens in positions 2, 3 exchange by rotation with those in 6 and 5, respectively. The number of symmetric
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FIG. 4. Diode–laser absorption spectrum of the chlorobenzene 19a band in a supersonic jet, showing a section of the R branch. Only the assignments of the most intense transitions are depicted.
and antisymmetric nuclear spin wavefunctions for two equivalent nuclei is n symm ⫽ 10 and n anti ⫽ 6 (20). For C 2v molecules with totally symmetric electronic and vibrational wavefunctions, if the C 2 axis coincides with a axis, ee and eo rotational levels are symmetric, whereas oe, oo levels are antisymmetric (21). As a result, the statistical weight for ee and eo levels is g ⫽ 10, whereas for oe and oo levels it is g ⫽ 6. Different absolute values have been reported by Riedle et al. (22), with g(ee, eo) ⫽ 60 and g(oo, oe) ⫽ 36, which yield identical relative absorption intensities. As an illustration of the overall satisfactory agreement between calculated and experimental spectra, Fig. 3 shows both spectra in the 1482.5–1483.6 cm ⫺1 region. Observed transitions were dominated by the C 6H 5 35Cl species, which is three times more abundant than C 6H 5 37Cl on account of the relative natural 35Cl/ 37Cl ratio. A notable exception is the unexpectedly high intensity feature found in the group of lines corresponding to P(4) near 1483.5 cm ⫺1 (Fig. 3), which can be assigned to the Q branch of the C 6H 5 37Cl isotope with reasonable confidence. Indeed, the intensity ratio between this group and the strong Q branch is ca. 1:3, implying a ⌬ 0i ⫽ 0 (37)– 0 (35) ⫽ ⫺0.4 cm ⫺1 small red shift, as expected on account of the slightly heavier mass of the 37Cl isotope. This hypothesis was further confirmed by calculating the rotational spectrum of C 6H 5 37Cl, taking ⌬ 0i ⫽ ⫺0.4 cm ⫺1, A⬙, B⬙, and C⬙ values from the literature (9) and varying the excited state constants in the same proportion as found for C 6H 5 35Cl (see below). Although, in general, few lines of the heavier isotope are expected to be observed in the recorded
spectrum, it was noted that the rotational structure of the calculated spectrum including both isotopic species was in good agreement with the experimental one. Other alternatives, especially around ⌬ 0i ⫽ 0.0 cm ⫺1 were also tested, but yielded worse agreement. Therefore, it is proposed that the band origin of the 19a vibrational mode of the C 6H 5 37Cl molecule is red shifted by ⬃0.4 cm ⫺1 with respect to that of C 6H 5 35Cl. Although the vibration is not substantially changed and the shift is very small, this value is of interest to improve the ab initio frequencies and force-field calculations. To check the consistency of the assignment, ground state rotational constants were obtained by least-squares fitting of the combination differences ⌬ 2 F⬙ ⫽ R共 J ⫺ 1, K a , K c ⫺ 2兲 ⫺ P共 J ⫹ 1, K a , K c 兲
by means of energy derivatives calculated numerically (18). The dependence of the ⌬ 2 F⬙ values on A⬙ is small, as expected for ⌬K a ⫽ 0 transitions in the prolate symmetric rotor limit of an A-type band, especially for low K a values (18). As a result, the fitting did not converge satisfactorily for A⬙. However, ⌬ 2 F⬙ values show a stronger dependence on B⬙ and C⬙, and improved values of these parameters have been obtained, namely B⬙ ⫽ 0.05207 and C⬙ ⫽ 0.04135 cm ⫺1, in reasonable accordance with the more accurate microwave values (9). All experimental wavenumbers (297 lines) were included in a linear least-squares fitting to determine 0, A⬘, B⬘, and C⬘,
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TABLE 1 Experimental Wavenumbers for the C 6H 5 35Cl 19a Band
Note. The calculated wavenumbers using upper state spectroscopic constants obtained from a least-squares fitting are also presented, together with the difference between both sets of data.
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19a BAND OF C 6H 5Cl
267
TABLE 1—Continued
keeping ground state constants fixed to their microwave values. The line wavenumbers were fitted to the expression
obs ⫺ E g ⫽ 0 ⫹ ␣ ⬘A⬘ ⫹  ⬘B⬘ ⫹ ␥ ⬘C⬘,
where E g are the rotational energies of the ground vibrational state, and ␣ ⫽ ⭸E/⭸A,  ⫽ ⭸E/⭸B, ␥ ⫽ ⭸E/⭸C are the rigid rotor energy derivatives with respect to the rotational constants. Weak lines with low signal-to-noise ratio and broad overlapped lines were given a low-statistical weight in the
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TABLE 1—Continued
fitting. All experimental frequencies, those calculated using the constants obtained in the fit together with the difference between both sets, are collected in Table 1. In general, the deviations are on the order of magnitude of the precision of experimental frequencies. In a few cases there are significant
differences between both sets of data. The differences observed in the R Ka (13) and R Ka (14) group of lines can be largely attributed to the effects of centrifugal distortion. Other cases, such as o P 2 (5) and e R 3 (7), appear at the wings of wide overlapped lines, and their frequency has not been accurately
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19a BAND OF C 6H 5Cl
269
TABLE 1—Continued
determined. A lower statistical weight was assigned to these lines. Table 2 summarizes the values of the spectroscopic constants of the 19a band of chlorobenzene. The values in parentheses stand for the uncertainty in the last digit. Owing
to the limited accuracy of our experimental frequencies and the low values of the rotational levels studied ( J, K a , K c ⫽ 0–14), no centrifugal distortion parameters have been obtained. From Table 2, the band center value of C 6 H 5 35 Cl species is ca. 2 cm ⫺1 higher than previously reported lower
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TABLE 1—Continued
resolution values. The variation in the rotational constants is small in all cases, being larger for A (⫺0.18%) than for B and C (⫺0.06 and ⫺0.05%, respectively). These relative variations can be explained with the help of Fig. 1. The smaller variation in B and C comes from the fact that I b and I c depend both on chlorine mass and r(C–Cl) distance. As the distance does not change substantially during the vibration of the 19a normal mode, low changes in I b and I c are expected and both should be of similar magnitude. The larger variation in A can be explained taking into account
that I a does not depend on chlorine mass and is thus more sensitive to other changes, such as hydrogen displacements. In the transitions studied, no rotational perturbations have been detected. According to Jahn’s rules (21), chlorobenzene is expected to exhibit a Coriolis z-type perturbation between the 19a and 19b vibrational modes (a 1 –b 2 Coriolis-type). The frequency difference between these two modes is ⬃35 cm ⫺1 and is sufficiently close as to be perturbed. However, few rotational levels of the 19b mode
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19a BAND OF C 6H 5Cl
TABLE 2 Band Origin and Rotational Constants for the 19a Band of Chlorobenzene Molecule
overlap with those of the 19a studied, since they correspond to high rotational energies. ACKNOWLEDGMENTS We thank MEC (Madrid) for a research grant (PB95-0510), Gobierno Vasco/Eusko Jaurlaritza (GV/EJ) for a complementary grant (PI-1997-89), and UPV/EHU for general financial support. A.U. thanks GV/EJ for the award of a graduate fellowship.
REFERENCES 1. 2. 3. 4. 5.
D. W. Pratt, Annu. Rev. Phys. Chem. 49, 481–530 (1998). M. Quack, Annu. Rev. Phys. Chem. 41, 839 – 874 (1990). D. H. Whiffen, J. Chem. Soc. 1350 –1356 (1956). T. Cvitas and J. M. Hollas, Mol. Phys. 18, 101–111 (1970). T. Cvitas, J. M. Hollas, and G. H. Kirby, Mol. Phys. 19, 305–316 (1970).
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6. H. D. Bist, V. N. Sarin, A. Ojha, and Y. S. Jain, Spectrochim. Acta, Part A 26, 841– 847 (1970). 7. H. D. Bist, V. N. Sarin, A. Ojha, and Y. S. Jain, Appl. Spectrosc. 24, 292–294 (1970). 8. Y. S. Jain and H. D. Bist, J. Mol. Spectrosc. 47, 126 –133 (1973). 9. G. Roussy and F. Michel, J. Mol. Struct. 30, 399 – 407 (1976). 10. F. Michel, H. Nery, P. Nosberger, and G. Roussy, J. Mol. Struct. 30, 409 – 415 (1976). 11. M. Snels, A. Beil, H. Hollenstein, and M. Quack, Chem. Phys. 225, 107–130 (1997). 12. A. Uskola, F. J. Basterretxea, and F. Castan˜o, J. Mol. Spectrosc. 198, 429 – 434 (1999). 13. G. Gelachvili and R. N. Rao, “Handbook of Infrared Standards. II,” Academic Press, San Diego, 1993. 14. E. B. Wilson, Phys. Rev. 45, 706 –714 (1934). 15. J. Plı´va and J. W. C. Johns, Can. J. Phys. 61, 269 –277 (1983). 16. M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, GAUSSIAN 94, Revision C.3, Gaussian, Inc., Pittsburgh, PA, 1995. 17. N. C. Handy, P. E. Maslen, R. D. Amos, Jamie S. Andrews, C. W. Murray, and G. J. Laming, Chem. Phys. Lett. 197, 506 –515 (1992). 18. H. C. Allen and P. C. Cross, “Molecular Vib-Rotors,” Wiley, New York, 1963. 19. B. D. Howells, J. McCombie, T. F. Palmer, J. P. Simons, and A. Walters, J. Chem. Soc., Faraday Trans. 88, 2587–2594; J. Chem. Soc., Faraday Trans. 2595–2601 (1992); J. Chem. Soc., Faraday Trans. 2603–2610 (1992). 20. C. H. Townes and A. L. Schawlow, “Microwave Spectroscopy,” Dover, N.Y., 1975. 21. G. Herzberg, “Infrared and Raman Spectra,” Van Nostrand–Reinhold, 1945. 22. E. Riedle, A. Beil, D. Luckhaus, and M. Quack, Mol. Phys. 81, 1–15 (1994).
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Diode–Laser Spectroscopy of the 19a Band of Chlorobenzene in a Supersonic Jet Araitz Uskola, Francisco J. Basterretxea, 1 and Fernando Castan˜o Departamento de Quı´mica Fı´sica, Facultad de Ciencias, Universidad del Paı´s Vasco/Euskal Herriko Unibertsitatea, Apdo. 644, 48080 Bilbao, Spain Received February 29, 2000; in revised form April 6, 2000
The diode–laser absorption spectrum of the 19a band of the chlorobenzene molecule cooled in a supersonic jet is presented and transitions from low J, K a , and K c values (0 –14) are reported. The C 6H 5 35Cl band center has been found at 0 ⫽ 1483.894 cm ⫺1 and rotational constants for the upper state have been determined from a least-squares fitting to experimental data. The band center for the C 6H 5 37Cl isotope has also been obtained. © 2000 Academic Press Key Words: diode–laser spectroscopy; supersonic jet; chlorobenzene 19a band. 1. INTRODUCTION
Benzene and, to a lesser extent, its haloderivatives have been benchmarks of the recent developments in high-resolution vibrational spectroscopy of large molecules (1, 2). Assignment of the fundamental vibrational modes of monohalogenated benzenes was first reported by Whiffen (3), while a set of chlorobenzene rotational constants was obtained later from electronic spectra with partially resolved rotational structure (4, 5). By combining vapor-phase electronic and infrared absorption spectra, Bist et al. provided vibrational frequencies for the in-plane vibrations of the ground (6) and excited state (7), and later also the out-of-plane vibrational modes of both states (8). The geometry of chlorobenzene has also been investigated by microwave spectroscopy in conjunction with isotopic substitution (9, 10), leading to the determination of the r s coordinates of the vibrational ground states and pointing out that the aromatic ring structure departs considerably from the C 6 symmetry. Despite of the spectroscopic interest of benzene derivatives, few high-resolution infrared spectra have been reported (11). In particular, neither rotationally resolved infrared spectrum of chlorobenzene has been presented up to date, nor are precise band origins or excited state rotational constants available. In this work, we report the diode–laser spectra of the 19a vibrational mode of chlorobenzene cooled in a supersonic jet. Tunable diode–laser spectroscopy of jet-cooled molecules yields simplified rotationally resolved spectra with respect to those at room temperature; in particular, the removal of hot bands permits the accurate location of band centers, and the rotational analysis supplies structural parameters for both vibrational levels. The advantage of combining supersonic cooling and tunable diode–laser spectroscopy is demonstrated for this 1
To whom correspondence should be addressed. E-mail: qfpbaelf@ lg.ehu.es.
rather heavy molecule, for which room-temperature spectrum is very congested and difficult to assign, even at high resolution. We report values for the band origin and rotational constants of the excited state that are valuable as a starting point for a detailed rotational analysis. 2. EXPERIMENTAL
The experimental system has been described elsewhere (12), and only the relevant details will be given here. Natural abundance chlorobenzene liquid samples (Merck, ⬎99%) were placed in a reservoir connected to a pulsed valve with a 0.5-mm-diameter nozzle (General Valve). Both valve and reservoir were heated at ca. 60°C by wrapped resistive wires to ensure a sufficient vapor pressure of the halobenzene. Heated chlorobenzene was seeded in He (Praxair, 99.999%) at 1.5-bar stagnation pressure and expanded through the pulsed nozzle (20 Hz, 200-s pulse duration) into a vacuum chamber pumped down by a turbomolecular-rotary pump coupled system. An infrared tunable laser beam was provided by a lead salt diode working at liquid-nitrogen temperatures (Laser Photonics, L5000) and centered around 1480 cm ⫺1. To guarantee complete spectral coverage, two diode lasers emitting in the same range were used. Multimode laser emission was passed through a monochromator to obtain single-mode radiation, which crossed the supersonic beam several times aided by a multipass optical system, and finally reached a HgCdTe detector. The signal was sent to a lock-in amplifier tuned at 2.5 kHz and further routed to a digital oscilloscope, which averaged up to 256 signals between user-selected time intervals. Laser absorption spectra were recorded by averaging the detector signal while slowly scanning the laser frequency. Chloromethane was used as a calibration standard (13) and frequency scale was provided by a 1-in. Ge solid e´talon, with a free spectral range of 0.0471 cm ⫺1.
262 0022-2852/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved.
19a BAND OF C 6H 5Cl
263 3. RESULTS AND DISCUSSION
3.1. The 19a and 19b Normal Modes of Chlorobenzene
FIG. 1. Ab initio calculated geometry of the chlorobenzene molecule: (a) equilibrium geometry, showing the axis system chosen; (b) geometry with the 19a normal mode at its maximum amplitude; (c) same as (b) for the 19b normal mode. Below the structures shown in (b) and (c), the calculated and experimental vibrational wavenumbers are indicated.
Figure 1a shows the chlorobenzene geometry and the axis system used in this study. Benzene substitution of one hydrogen atom by chlorine reduces the group symmetry from D 6h to C 2v , splitting its degenerate vibrational modes (e 1g , e 2g , e 1u , and e 2u ) into two components, each of a 1 and b 2 symmetry. Nondegenerate irreducible representations a 1g and b 1u transform to a 1 symmetry, a 2u and b 2g to b 1 , and finally a 2g and b 2u to b 2 . In summary, chlorobenzene normal modes are 11 a 1 species, giving rise to A-type infrared bands, 6 b 1 species with C-type bands, and 10 b 2 species with B-type bands. The remaining three species, of a 2 symmetry, are infrared inactive. According to the vibrational mode numbering proposed by Wilson (14), chlorobenzene 19a and 19b normal modes derive from benzene 19(e 1g ) mode, i.e., chlorobenzene bands 19a (a 1 ) at 1482 cm ⫺1 and 19b (b 2 ) at 1447 cm ⫺1 (6) arise from benzene 19 (e 1g ) at 1484 cm ⫺1 (15). The 19a, b vibrational frequencies, together with atom displacements, have been accurately computed with the GAUSSIAN 94 package (16) at the B3LYP/ AUG-cc-pVDZ level, in order to better confirm the identification of normal modes. The B3LYP density functional is known to give accurate harmonic vibrational frequencies of small molecules and also of benzene (17) at relatively low computational cost. The results of the calculation are depicted in Fig. 1; in Fig. 1a the equilibrium configuration is shown, whereas Figs. 1b and 1c display a picture of the maximum amplitude vibrational positions of the 19a and 19b modes. The calculated vibrational wavenumbers, along with the experimental values
FIG. 2. Diode–laser absorption spectrum of the chlorobenzene 19a band center as obtained in a supersonic jet, showing the prominent Q branch and the lowest wavenumber transitions of the P and R branches.
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available (6), are also given. It can be seen that the computed wavenumbers are very close to the experimental ones, in particular for the 19a mode. The calculated ground state rotational constants are A⬙ ⫽ 0.188286, B⬙ ⫽ 0.051744, and C⬙ ⫽ 0.040590 cm ⫺1, in good agreement with the microwave values (9). Mode character in the 19a, b pair can be described as in-plane vibration, combining C–H bending with ring deformation. The C–Cl bond stretches in the 19a vibration, whereas it basically bends in 19b. In the 19a mode, the transition dipole moment is oriented along the a axis, giving rise to an A-type infrared band, while in the 19b mode lies on the b axis, yielding a B-type band. 3.2. Diode–Laser Spectrum of the 19a Band Diode spectra of chlorobenzene in a supersonic jet were recorded between 1481.6 and 1485.5 cm ⫺1. Chlorobenzene, with an asymmetry parameter ⫽ ⫺0.84, is very near the prolate symmetric limit, where an A-type band corresponds to a parallel-type band. Indeed, the spectrum has a marked resemblance to a parallel band of a symmetric rotor, with a prominent central Q branch and two P, R sidebands at lower and higher wavenumbers, respectively. Although the spectrum obtained still shows widespread overlapping, hot bands have been eliminated, and rotational transitions for values of J, K a , K c ⱕ 14 can be observed. Figure 2 shows the central part of the band, including the strong Q branch and low J, K a , K c values for the P and R branches, of much weaker intensity. Figures 3 and 4 show portions of the P and R branches, respectively. In the first case the calculated spectrum is also included for comparison purposes (see later). The spectra have been scaled linearly in wavenumbers. Intensity normalization was not considered necessary, since laser power is roughly constant in the whole mode, decreasing only slightly at the sides. The structure of the Q branch is not resolvable with our experimental setup. In both P and R branches, for low J, K a , K c values it is useful to consider groups of lines with the same J. The asymmetry produces a splitting of the K a levels, being largest for K a ⫽ ⫹1. In Figs. 3 and 4, it is easily appreciated that, for each group, the external two transitions correspond to K a ⫽ 1. The splitting magnitude increases with J, and eventually the patterns of consecutive J values overlap. In this case it is more useful to consider the spectrum as a superposition of subbands, each characterized by a constant value of K a and the parity of the ground state levels (e, o). For clarity purposes, Figs. 3 and 4 only show the assignment of the most intense subbands. The nomenclature used is as follows: e,o (⌬J) K a ( J), where ⌬J means P (⌬J ⫽ ⫺1) or R (⌬J ⫽ ⫹1) branch, the subscript is the K a value of a given subband, the running J value appears in parentheses, and the e or o superscript stands for the parity of the rotational level. According to selection rules, ⌬K a ⫽ 0, and ⌬K c ⫽ ⌬J in both R and P branches. In
FIG. 3. (Upper trace) Diode–laser absorption spectrum of the chlorobenzene 19a band as obtained in a supersonic jet, including part of the P branch. Note the high intensity around the J ⫽ 4 line group. (Lower trace) Calculated spectrum, assuming a rotational temperature of 10 K.
addition, for e levels, K a ⫹ K c ⫽ J, whereas for o levels, K a ⫹ K c ⫽ J ⫹ 1. For K a ⫽ 0, only e levels exist (18). Most rotational transitions could be assigned with the initial help of a simulation program that models the infrared spectrum of a rigid rotor, given the rotational constants of both vibrational states, the band center, selection rules, nuclear spin statistics, and rotational temperature of the expansion (19). Although the lower J transitions of the P and R branches (Fig. 2) have a low signal-to-noise ratio, these transitions have been reasonably assigned by matching the wavenumbers of a series of recorded spectra with those computed using the rotational constants obtained by fitting only the most intense lines. The same can be said of the weak lines in Figs. 3 and 4. A rotational temperature value of T rot ⫽ 10 K satisfactorily reproduces the intensity distribution of the experimental spectrum and was selected from the jet expansion conditions observed in our previous work with benzene (12). Regarding nuclear spin statistics, the hydrogens in positions 2, 3 exchange by rotation with those in 6 and 5, respectively. The number of symmetric
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FIG. 4. Diode–laser absorption spectrum of the chlorobenzene 19a band in a supersonic jet, showing a section of the R branch. Only the assignments of the most intense transitions are depicted.
and antisymmetric nuclear spin wavefunctions for two equivalent nuclei is n symm ⫽ 10 and n anti ⫽ 6 (20). For C 2v molecules with totally symmetric electronic and vibrational wavefunctions, if the C 2 axis coincides with a axis, ee and eo rotational levels are symmetric, whereas oe, oo levels are antisymmetric (21). As a result, the statistical weight for ee and eo levels is g ⫽ 10, whereas for oe and oo levels it is g ⫽ 6. Different absolute values have been reported by Riedle et al. (22), with g(ee, eo) ⫽ 60 and g(oo, oe) ⫽ 36, which yield identical relative absorption intensities. As an illustration of the overall satisfactory agreement between calculated and experimental spectra, Fig. 3 shows both spectra in the 1482.5–1483.6 cm ⫺1 region. Observed transitions were dominated by the C 6H 5 35Cl species, which is three times more abundant than C 6H 5 37Cl on account of the relative natural 35Cl/ 37Cl ratio. A notable exception is the unexpectedly high intensity feature found in the group of lines corresponding to P(4) near 1483.5 cm ⫺1 (Fig. 3), which can be assigned to the Q branch of the C 6H 5 37Cl isotope with reasonable confidence. Indeed, the intensity ratio between this group and the strong Q branch is ca. 1:3, implying a ⌬ 0i ⫽ 0 (37)– 0 (35) ⫽ ⫺0.4 cm ⫺1 small red shift, as expected on account of the slightly heavier mass of the 37Cl isotope. This hypothesis was further confirmed by calculating the rotational spectrum of C 6H 5 37Cl, taking ⌬ 0i ⫽ ⫺0.4 cm ⫺1, A⬙, B⬙, and C⬙ values from the literature (9) and varying the excited state constants in the same proportion as found for C 6H 5 35Cl (see below). Although, in general, few lines of the heavier isotope are expected to be observed in the recorded
spectrum, it was noted that the rotational structure of the calculated spectrum including both isotopic species was in good agreement with the experimental one. Other alternatives, especially around ⌬ 0i ⫽ 0.0 cm ⫺1 were also tested, but yielded worse agreement. Therefore, it is proposed that the band origin of the 19a vibrational mode of the C 6H 5 37Cl molecule is red shifted by ⬃0.4 cm ⫺1 with respect to that of C 6H 5 35Cl. Although the vibration is not substantially changed and the shift is very small, this value is of interest to improve the ab initio frequencies and force-field calculations. To check the consistency of the assignment, ground state rotational constants were obtained by least-squares fitting of the combination differences ⌬ 2 F⬙ ⫽ R共 J ⫺ 1, K a , K c ⫺ 2兲 ⫺ P共 J ⫹ 1, K a , K c 兲
by means of energy derivatives calculated numerically (18). The dependence of the ⌬ 2 F⬙ values on A⬙ is small, as expected for ⌬K a ⫽ 0 transitions in the prolate symmetric rotor limit of an A-type band, especially for low K a values (18). As a result, the fitting did not converge satisfactorily for A⬙. However, ⌬ 2 F⬙ values show a stronger dependence on B⬙ and C⬙, and improved values of these parameters have been obtained, namely B⬙ ⫽ 0.05207 and C⬙ ⫽ 0.04135 cm ⫺1, in reasonable accordance with the more accurate microwave values (9). All experimental wavenumbers (297 lines) were included in a linear least-squares fitting to determine 0, A⬘, B⬘, and C⬘,
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TABLE 1 Experimental Wavenumbers for the C 6H 5 35Cl 19a Band
Note. The calculated wavenumbers using upper state spectroscopic constants obtained from a least-squares fitting are also presented, together with the difference between both sets of data.
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TABLE 1—Continued
keeping ground state constants fixed to their microwave values. The line wavenumbers were fitted to the expression
obs ⫺ E g ⫽ 0 ⫹ ␣ ⬘A⬘ ⫹  ⬘B⬘ ⫹ ␥ ⬘C⬘,
where E g are the rotational energies of the ground vibrational state, and ␣ ⫽ ⭸E/⭸A,  ⫽ ⭸E/⭸B, ␥ ⫽ ⭸E/⭸C are the rigid rotor energy derivatives with respect to the rotational constants. Weak lines with low signal-to-noise ratio and broad overlapped lines were given a low-statistical weight in the
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TABLE 1—Continued
fitting. All experimental frequencies, those calculated using the constants obtained in the fit together with the difference between both sets, are collected in Table 1. In general, the deviations are on the order of magnitude of the precision of experimental frequencies. In a few cases there are significant
differences between both sets of data. The differences observed in the R Ka (13) and R Ka (14) group of lines can be largely attributed to the effects of centrifugal distortion. Other cases, such as o P 2 (5) and e R 3 (7), appear at the wings of wide overlapped lines, and their frequency has not been accurately
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TABLE 1—Continued
determined. A lower statistical weight was assigned to these lines. Table 2 summarizes the values of the spectroscopic constants of the 19a band of chlorobenzene. The values in parentheses stand for the uncertainty in the last digit. Owing
to the limited accuracy of our experimental frequencies and the low values of the rotational levels studied ( J, K a , K c ⫽ 0–14), no centrifugal distortion parameters have been obtained. From Table 2, the band center value of C 6 H 5 35 Cl species is ca. 2 cm ⫺1 higher than previously reported lower
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TABLE 1—Continued
resolution values. The variation in the rotational constants is small in all cases, being larger for A (⫺0.18%) than for B and C (⫺0.06 and ⫺0.05%, respectively). These relative variations can be explained with the help of Fig. 1. The smaller variation in B and C comes from the fact that I b and I c depend both on chlorine mass and r(C–Cl) distance. As the distance does not change substantially during the vibration of the 19a normal mode, low changes in I b and I c are expected and both should be of similar magnitude. The larger variation in A can be explained taking into account
that I a does not depend on chlorine mass and is thus more sensitive to other changes, such as hydrogen displacements. In the transitions studied, no rotational perturbations have been detected. According to Jahn’s rules (21), chlorobenzene is expected to exhibit a Coriolis z-type perturbation between the 19a and 19b vibrational modes (a 1 –b 2 Coriolis-type). The frequency difference between these two modes is ⬃35 cm ⫺1 and is sufficiently close as to be perturbed. However, few rotational levels of the 19b mode
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19a BAND OF C 6H 5Cl
TABLE 2 Band Origin and Rotational Constants for the 19a Band of Chlorobenzene Molecule
overlap with those of the 19a studied, since they correspond to high rotational energies. ACKNOWLEDGMENTS We thank MEC (Madrid) for a research grant (PB95-0510), Gobierno Vasco/Eusko Jaurlaritza (GV/EJ) for a complementary grant (PI-1997-89), and UPV/EHU for general financial support. A.U. thanks GV/EJ for the award of a graduate fellowship.
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