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Diode laser spectroscopy of the ν4 band of benzene
JOURNAL OF MOLECULAR SPECTROSCOPY 128, 112- I75
(I 988)
Diode Laser Spectroscopy of the JOSEF LINDENMAYER,
ULRICH
u4
Band of Benzene
MAGG, AND HAROLD JONES
Abteilung Physikalische Chemie, Universitii[ Urn, D-7900 U/m, Federal Republic
ofGermany
Sections of the Yeband of benzene near 674 cm -’ have been recorded under conditions of Doppler-limited resolution using a diode laser spectrometer. In contrast to earlier measurements on this parallel band both the J and the K structures were resolved and this has enabled a number ofband parameters to be determined accurately for the first time. The set ofground state constants determined from these data are compared with those determined by J. Pliva and A. S. Pine (J MO/. Spectrosc. 93, 209-236 (198 1)) using a difference frequency laser system. o 1988Academic Press, Inc.
INTRODUCTION
For chemists, the benzene molecule occupies a position of almost unique importance and this has prompted a large number of infrared spectroscopic studies. The main aim of this work has usually been the accurate determination of the ground state rotational constants, but it is only in relatively recent times that the resolution of infrared spectrometers has been a match for this problem. The most accurate work to date on the v4 band of benzene was carried out by Kauppinen et al. (I) with a resolution of 0.010 cm-‘. Doppler-limited work on Y,*, using a difference frequency laser system, has been published by Pliva and Pine (2) and on v13by Pliva and Johns (3) using a BOMEM Fourier transform spectrometer. The ultimate in accuracy has been recently obtained for monodeutero-benzene by Oldani and Bauder (4, who observed the pure rotational spectrum of this molecule using a Fourier transform microwave spectrometer. We report here the results of the analysis of a Doppler-limited diode laser spectrum of the u4 band. EXPERIMENTAL
DETAILS
The diode laser spectrometer used was based on the cold-head assembly of Spectra Physics and measurements were carried out using a single diode from the same firm, An absorption path length of 20 m was used with gas pressures in the region of 0. I mbar. Frequency measurements were carried out using a confocal &talon with FSR = 0.009811 cm-’ and accurately measured CO2 absorption lines (5). OBSERVATIONS AND DISCUSSION
A parallel band spectrum (c type) of an oblate symmetric rotor consists of groups of lines separated by approximately 28. The spacing of the K structure within each group is determined by (C’ - C”), (B’ - B”), and DJK. As is well known, such a spectrum contains no information over c” or DK. 0022-2852/88 $3.00 Copyri@
0
1988 by Academic Press, Inc.
All rights of reproduction in any form reserved.
172
vq BAND
OF BENZENE
173
In the work of Kauppinen et al. (I) only the coarse J structure of the u4 band (which is a parallel band) was resolved and the K structure remained completely blended. A trace of the diode laser spectrum of the R(32) transition of this band near 686.4 cm-’ is shown in Fig. 1. As can be seen all transitions with K 3 5 are clearly resolved. Moreover, the variation in intensity produced by the sixfold symmetry axis of the benzene molecule is clearly seen. The expected intensity variation was first calculated by Wilson (6) to follow the cyclic pattern 10, 11, 9, 14, 9, 11, . . . , with the most intense lines corresponding to K = (6n + 3). This intensity pattern provides a very useful assignment aid (Fig. 1). The experimental linewidth of the transitions in Fig. 1 is -0.00 1 cm-‘, which is very close to the theoretical Doppler width of benzene (25 MHz) under these conditions. The v4band parameters were determined using the usual expressions for the energy levels of a symmetric rotor with terms up to the quartic level. Only the frequencies of completely resolved transitions were used in the fit. The data set of 183 transitions with J = 20 to 53 and K = 5 to 33 and the parameters determined from them are shown in Tables I and II, respectively. The band parameters determined by Kauppinen et al. (I) and the ground state constants of Pliva and Pine (2) are also included in Table II for comparison. Our u4data are a considerable improvement on the earlier work (I) but the situation is not so clear with regard to the ground state data of Pliva and Pine (2). The latter authors determined the values shown in Table II from 1500 ground state combination differences and by virtue of this much larger data set it would seem that their results should be significantly more accurate than ours. However, as can be seen from Table II the main discrepancy seems to be with the ground state rotational constant which
FIG. 1. nominally transitions seen, with
The R(32) transition of the yq band of benzene at Doppler-limited resolution. 0.001 cm-‘, in good agreement with the calculated Doppler width at room with K 2 5 are resolved. The expected 10, 11, 9, 14, 9, 11 intensity variation the most intense lines occurring at K = (6n + 3).
The linewidth is temperature. All with K is clearly
174
LINDENMAYER,
MAGG, AND JONES
TABLE I Observed Transitions of the vq Band of Benzene (cm-‘)
12 49 49 49 49 49 :; 09 49 49 49 49 49 19 49 49
:z 49 49 53 :: 53 53 53 53 53 53 53 53 :: :: :: :i 39 :; 39 39 39 39 39 39 :; 39 39 39 39 39 39 :; 39 19 :; 39 :; 39 32 32 32 32 32 32 32 32 32 32 32 32 32 32 :: :: :: 32 ::
175
vq BAND OF BENZENE TABLE II The yg Band Parameters of Benzene and the Ground State Parameters from Ref. (2) (in cm-‘) reference
1
reference 2
y4
y12
v.
673.9773131
8"
0.1897543(61)
0.1897618(14)
3.66(12)
3.934(76)
DJ
".108
DJK ".108 DK -.108
0.1897729(15) 3.973(36) -6.81(16)
3.21(50) 0.1896199(78)
CC'-C"j.104
0.1896403(34)
0.7(2)
0.3789(34)
I.108
4.040(32)
DJK *.108 Numbers
y4
673.973212)
-6.90(24)
B'
DJ
this work
-6.80(15) in parentheses
are 1 std. dev.
in units
of last digit.
differs by eight times the standard deviation. Moreover, when we attempted to fit our data with the ground state constants constrained at values determined by Pliva and Pine (2) the standard deviation of the fit increased from 0.0003 to 0.001 cm-’ with quite obvious systematic residuals. Oldani and Bauder (4) have obtained very accurate rotational constants for C6H5D. To a first approximation, the A rotational constant of this asymmetric rotor should be very close to the B constant of C6H6. Their value of 0.1897694(2) cm-’ is considerably closer to our value (Table II) than that of Pliva and Pine (2). This may however be accidental since it is difficult to estimate the effects of the changes in the zero-point motion. At this point in time we are unable to draw a definitive conclusion. Note added in proof Afier this paper was submitted for publication it was brought to our attention that a value for the ground state rotational constant of benzene was determined from an analysis of the Dopplerfree two-photon electronic spectrum of the 14; band (7). Their value, B” = 0.189 775(3) cm-’ is in better agreement with our value (Table II) than with that of Ref. (2).
RECEIVED:
July 13, 1987 REFERENCES
J. KAUPPINEN, P. JENSEN,AND S. BRODERSEN, J.Mol. Spectmsc. 83, 16 l- 174 (1980). J. PLIVA AND A. S. PINE, J. Mol. Spectrosc. 93, 209-236 (1981). J. PLIVA AND J.W. C. JOHNS, Cunad. J. Phys. 61,269-277 (1983). N. OLDANI AND A. BAUDER, Chem. Phys. Lett. 108,7-10 (1984). J. KAUPPINEN, K.JOLMA, ANDV. W.HORNEMANN, Appl. Opt. 21,3332-3336(1982). E. B. WILSON, J. Chem. Phys. 3, 276-285 (1935). 7. E. RIEDLE, H. J. NEUSSER, AND E. W. SCHLAG, private communication.
I. 2. 3. 4. 5. 6.
(I 988)
Diode Laser Spectroscopy of the JOSEF LINDENMAYER,
ULRICH
u4
Band of Benzene
MAGG, AND HAROLD JONES
Abteilung Physikalische Chemie, Universitii[ Urn, D-7900 U/m, Federal Republic
ofGermany
Sections of the Yeband of benzene near 674 cm -’ have been recorded under conditions of Doppler-limited resolution using a diode laser spectrometer. In contrast to earlier measurements on this parallel band both the J and the K structures were resolved and this has enabled a number ofband parameters to be determined accurately for the first time. The set ofground state constants determined from these data are compared with those determined by J. Pliva and A. S. Pine (J MO/. Spectrosc. 93, 209-236 (198 1)) using a difference frequency laser system. o 1988Academic Press, Inc.
INTRODUCTION
For chemists, the benzene molecule occupies a position of almost unique importance and this has prompted a large number of infrared spectroscopic studies. The main aim of this work has usually been the accurate determination of the ground state rotational constants, but it is only in relatively recent times that the resolution of infrared spectrometers has been a match for this problem. The most accurate work to date on the v4 band of benzene was carried out by Kauppinen et al. (I) with a resolution of 0.010 cm-‘. Doppler-limited work on Y,*, using a difference frequency laser system, has been published by Pliva and Pine (2) and on v13by Pliva and Johns (3) using a BOMEM Fourier transform spectrometer. The ultimate in accuracy has been recently obtained for monodeutero-benzene by Oldani and Bauder (4, who observed the pure rotational spectrum of this molecule using a Fourier transform microwave spectrometer. We report here the results of the analysis of a Doppler-limited diode laser spectrum of the u4 band. EXPERIMENTAL
DETAILS
The diode laser spectrometer used was based on the cold-head assembly of Spectra Physics and measurements were carried out using a single diode from the same firm, An absorption path length of 20 m was used with gas pressures in the region of 0. I mbar. Frequency measurements were carried out using a confocal &talon with FSR = 0.009811 cm-’ and accurately measured CO2 absorption lines (5). OBSERVATIONS AND DISCUSSION
A parallel band spectrum (c type) of an oblate symmetric rotor consists of groups of lines separated by approximately 28. The spacing of the K structure within each group is determined by (C’ - C”), (B’ - B”), and DJK. As is well known, such a spectrum contains no information over c” or DK. 0022-2852/88 $3.00 Copyri@
0
1988 by Academic Press, Inc.
All rights of reproduction in any form reserved.
172
vq BAND
OF BENZENE
173
In the work of Kauppinen et al. (I) only the coarse J structure of the u4 band (which is a parallel band) was resolved and the K structure remained completely blended. A trace of the diode laser spectrum of the R(32) transition of this band near 686.4 cm-’ is shown in Fig. 1. As can be seen all transitions with K 3 5 are clearly resolved. Moreover, the variation in intensity produced by the sixfold symmetry axis of the benzene molecule is clearly seen. The expected intensity variation was first calculated by Wilson (6) to follow the cyclic pattern 10, 11, 9, 14, 9, 11, . . . , with the most intense lines corresponding to K = (6n + 3). This intensity pattern provides a very useful assignment aid (Fig. 1). The experimental linewidth of the transitions in Fig. 1 is -0.00 1 cm-‘, which is very close to the theoretical Doppler width of benzene (25 MHz) under these conditions. The v4band parameters were determined using the usual expressions for the energy levels of a symmetric rotor with terms up to the quartic level. Only the frequencies of completely resolved transitions were used in the fit. The data set of 183 transitions with J = 20 to 53 and K = 5 to 33 and the parameters determined from them are shown in Tables I and II, respectively. The band parameters determined by Kauppinen et al. (I) and the ground state constants of Pliva and Pine (2) are also included in Table II for comparison. Our u4data are a considerable improvement on the earlier work (I) but the situation is not so clear with regard to the ground state data of Pliva and Pine (2). The latter authors determined the values shown in Table II from 1500 ground state combination differences and by virtue of this much larger data set it would seem that their results should be significantly more accurate than ours. However, as can be seen from Table II the main discrepancy seems to be with the ground state rotational constant which
FIG. 1. nominally transitions seen, with
The R(32) transition of the yq band of benzene at Doppler-limited resolution. 0.001 cm-‘, in good agreement with the calculated Doppler width at room with K 2 5 are resolved. The expected 10, 11, 9, 14, 9, 11 intensity variation the most intense lines occurring at K = (6n + 3).
The linewidth is temperature. All with K is clearly
174
LINDENMAYER,
MAGG, AND JONES
TABLE I Observed Transitions of the vq Band of Benzene (cm-‘)
12 49 49 49 49 49 :; 09 49 49 49 49 49 19 49 49
:z 49 49 53 :: 53 53 53 53 53 53 53 53 :: :: :: :i 39 :; 39 39 39 39 39 39 :; 39 39 39 39 39 39 :; 39 19 :; 39 :; 39 32 32 32 32 32 32 32 32 32 32 32 32 32 32 :: :: :: 32 ::
175
vq BAND OF BENZENE TABLE II The yg Band Parameters of Benzene and the Ground State Parameters from Ref. (2) (in cm-‘) reference
1
reference 2
y4
y12
v.
673.9773131
8"
0.1897543(61)
0.1897618(14)
3.66(12)
3.934(76)
DJ
".108
DJK ".108 DK -.108
0.1897729(15) 3.973(36) -6.81(16)
3.21(50) 0.1896199(78)
CC'-C"j.104
0.1896403(34)
0.7(2)
0.3789(34)
I.108
4.040(32)
DJK *.108 Numbers
y4
673.973212)
-6.90(24)
B'
DJ
this work
-6.80(15) in parentheses
are 1 std. dev.
in units
of last digit.
differs by eight times the standard deviation. Moreover, when we attempted to fit our data with the ground state constants constrained at values determined by Pliva and Pine (2) the standard deviation of the fit increased from 0.0003 to 0.001 cm-’ with quite obvious systematic residuals. Oldani and Bauder (4) have obtained very accurate rotational constants for C6H5D. To a first approximation, the A rotational constant of this asymmetric rotor should be very close to the B constant of C6H6. Their value of 0.1897694(2) cm-’ is considerably closer to our value (Table II) than that of Pliva and Pine (2). This may however be accidental since it is difficult to estimate the effects of the changes in the zero-point motion. At this point in time we are unable to draw a definitive conclusion. Note added in proof Afier this paper was submitted for publication it was brought to our attention that a value for the ground state rotational constant of benzene was determined from an analysis of the Dopplerfree two-photon electronic spectrum of the 14; band (7). Their value, B” = 0.189 775(3) cm-’ is in better agreement with our value (Table II) than with that of Ref. (2).
RECEIVED:
July 13, 1987 REFERENCES
J. KAUPPINEN, P. JENSEN,AND S. BRODERSEN, J.Mol. Spectmsc. 83, 16 l- 174 (1980). J. PLIVA AND A. S. PINE, J. Mol. Spectrosc. 93, 209-236 (1981). J. PLIVA AND J.W. C. JOHNS, Cunad. J. Phys. 61,269-277 (1983). N. OLDANI AND A. BAUDER, Chem. Phys. Lett. 108,7-10 (1984). J. KAUPPINEN, K.JOLMA, ANDV. W.HORNEMANN, Appl. Opt. 21,3332-3336(1982). E. B. WILSON, J. Chem. Phys. 3, 276-285 (1935). 7. E. RIEDLE, H. J. NEUSSER, AND E. W. SCHLAG, private communication.
I. 2. 3. 4. 5. 6.