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The gas phase Raman band contours of 1,3,5-trifluorobenzene and the infrared and Raman band contours of 1,3,5-trifluorobenzene-d3
JOURNAL
OF
MOLECULAR
SPECTROSCOPY
87,
382-392 (1981)
The Gas Phase Raman Band Contours of 1,3,5Trifluorobenzene and the Infrared and Raman Band Contours of 1 ,3,5-Trifluorobenzene-dSi*2 J. KORPPI-TOMMOLA~AND H. F. SHURVELL Department
of Chemistry,
Queen’s
University, Kingston,
Ontario, Canada,
K7L 3N6
S. J. DAUNTS Molecular
Spectroscopy Laboratory, Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37916
AND
D. STEELE Department
of Chemistry, Royal Holloway College, University of London, Egham, Surrey, TW20 OEX, England
The gas phase infraredand Raman spectra of 1,3,5-trifluorobenzene-d3has been recorded. The gas phase Raman spectrum of 1,3,5-trifluorobenzene-h, has also been recorded. Computer simulations of the band contours of the degenerate vibrational modes have been used to determine the first-order Coriolis coupling constants for both molecules. The 5’ values were then compared with those evaluated from force field calculations. 1. INTRODUCTION
The literature concerning the infrared and Raman spectra of 1,3, Wifluorobenzene (s-TFB) can be found in the references of our recent study of the infrared rotation-vibration bands of that molecule (1). The present paper, which is a continuation of the work of Ref. (I ), reports the gas phase infrared and Raman spectra of 1,3,5-trifluoro-2,4,6_trideuterobenzene (s-TFB-d3) as well as the gas phase Raman spectrum of s-TFB. The results detailed below have suggested improved vibrational assignments for these molecules. An additional objective of this work was to compare the values of the Coriolis constants for the E’ degenerate vibrational bands obtained from Raman contours with those obtained from the infrared I Supported in part by the Planetary Atmospheres Program of the National Aeronautics and Space Administration (Grant NGL-43-001-006). 2 A preliminary report of this work was presented at the Ohio State Molecular Spectroscopy Symposium, 1979, Paper WB2. 3 Present address: Department of Chemistry, University of Jyvaskyla, SF-40100 Jyvaskyla 10, Finland. 4 To whom correspondence concerning this paper should be addressed. 0022-2852/81/060382-l 1$02.00/O Copyright All ri&ts
0 1981 by Academic of reproduction
Press,
Inc.
in any form reserved.
382
383
BAND CONTOURS OF s-C6F3H3 AND s-&F,D, TABLE I Infrared and Raman Fundamental
s-
Frequencies 5
Infrared
of s-TFB and s-TFB-d, in the Gas Phaseta’
lRaman
s-TFI Infrared
-r
I? Raman
Product Rule
i.a.
3076
i.a.
2319.2
Calc.
i.a.
1362.6
i.a.
1359.7
1.414
i.a.
1012.4
i.a.
969.2
Exper.
i.a.
579.9
i.a.
577.3
1.395
__
3113.0
3115.5
2314
1629(b)
1622(b)
1617tb)
1618(b)
Calc.
1475.4
1475
1425.0
1428.2
1.977
1127.6
1129.2
1054
1054
Exper.
995.7
996.1
__
792.0
1.975
502.4
504.0
487.0
487.4
324.2
327.5
322.4
322.5
847.0
i.a.
777.2
i.a.
663.1
i.a.
522.0
i.a.
207.0
i.a.
205.9
i.a.
Exper. 1.391
i.a.
792
i.a.
646
Calc. 1.389
i.a.
598
i.a.
537.6
i.a.
245.8
i.a.
231.3
C.SlC. 1.398
Exper. 1.449
(b)
Average frequencies of the two band maxima region.
observed
in the
contours of the same transitions. Finally, a comparison of the experimental 5” values with those obtained from force field calculations of s-TFB (2) was considered to be of interest. 2. EXPERIMENTAL
DETAILS
1,3,5-Trifluorobenzene was purchased from Pierce Chemical Company and was purified by distillation on a vacuum line. The deuterated trifluorobenzene was prepared by refluxing s-TFB over concentrated D,SO, (Merck Sharpe & Dohme, Canada, 96% D2S04). The exchange reaction was monitored by recording the Raman spectrum of the exchange product in the liquid phase. The monitoring was done by observation of the symmetrical ring stretching band (Q), which showed clearly resolved components for s-TFB, s-TFB-d, s-TFB-d,, and s-TFB-dB. More than five exchanges were needed to make the final product, which then contained only about 5% s-TFB-d, plus traces of the lighter isotopic species. Infrared spectra were recorded on a Perkin-Elmer 180 spectrometer using a
384
KORPPI-TOMMOLA
3000
FIG. 1. The infrared spectrum
2000
of gaseous
1600
ET AL.
1200
s-TFB-d3,
800
LOO b-9
P = 0.5mmHg, path length 10 cm; insert: P
= 30 mmHg (4000 Pa).
lo-cm gas cell equipped with KBr or polyethylene windows. Survey Raman spectra were recorded on a Jarrell-Ash l-m double monochromator using a multipass gas cell (3) and about 1 W of laser power at the sample. Detailed contours were recorded in the laboratory of Dr. W. F. Murphy at the National Research Council of Ottawa, Canada using the same multipass cell, a Spex double monochromator and about 7 W of laser power at the sample. In both cases 514.5 nm excitation from an argon ion laser was used. The spectra of the strongest Raman bands of both trifluorobenzenes were obtained with slit widths of about 3 cm-’ and up to 40 kcps of signal. The spectra were calibrated with a Ne pen lamp. The wavenumbers listed in Table I are believed accurate to &OS cm-‘. 3. CALCULATION
OF CONTOURS
The rotational constants of both isotopic molecules were calculated using the structural parameters of s-TFB reported in the rotational Raman study of Schlupf and Weber (4). These calculations yielded for s-TFB values of& = 0.05864 cm-’ and Co = (l/2)& = 0.02932 cm-‘, while fors-TFB-d, the values were& = 0.05680 cm-’ and C, = (l/2)& = 0.02840 cm-‘. To obtain the first-order Coriolis constants, <“, a band contour simulation program, originally written by Masri and Williams (5) and revised by one of the authors (SJD) was used. Initial trial values of 5” were obtained from the 5” versus AVPR plot of our earlier publication (1). The errors in 5” values obtained from undistorted band contours are typically quoted to be -tO.OS, but in practice are usually greater due to various complicating factors discussed below. The Coriolis constants of the deuterated trifluorobenzene
I 2400
/ I
1
2200
FIG. 2. The Raman spectrum
1400
of gaseous
1000
s-TFB-da,
KM
equilibrium
200
vapor pressure
(cm-‘)
at about 40°C.
385
BAND CONTOURS OF s-CGF,H, AND s-C,F,Ds
tl
RAMAN s-TFB-h3(g)
FIG. 3. The Raman spectrum of gaseous s-TFB, equilibrium vapor pressure at
about 40°C.
were calculated from a force field of s-TFB (6, 7) using the method previously described by Steele (2). 4.
1.
RESULTS AND DISCUSSION
Assignments
Infrared and Raman survey spectra of s-TFB-d, are shown in Figs. 1 and 2. A Raman survey spectrum of s-TFB can be seen in Fig. 3. Assignments of the fundamental bands of both molecules are summarized in Table I. Since several vibrational assignments of these molecules have been published (8-1 I ), we comment on our gas phase results only where some new information is now available. Complete
V,+;:;)
V
nPR=lBOcm"
FIG. 4. The infrared band contours of the A;’ modes of s-TFB-d,. = 14.6 cm-‘), v,~ (AvPR = 14.4 cm-‘), and v,? (AvPR = 16.0 cm-‘).
From top to bottom: I+, (AU,+
386
KORPPI-TOMMOLA TABLE
Comparison
ET AL. II
of Coriolis Constants Obtained from Computer Simulated s-TFB and s-TFB-d, with Constants Obtained from Force s-TFB
.Mode E'
Infrared and Raman Field Calculations
iz FF Cc)
r
i= FF Cd)
;' Raman (b)
__
-_
-0.002
-0.05
__
+0.014
FR
FR
to.805
FA
FR
co.717
-0.55
__
-0.682
-0.40
-0.50
-0.770
+0.02(f)
zz infrared (b)
-0.05
+0.10
-0.024
-0.47
__
-0.470
-0.30
-0.30
-0.281
-0.25
-0.20
-0.30
-0.345
-0.25
(ai Obserwd
[l].
(b) Observed
- this work.
of
s-TFB-d,
(a)
5 ' infrared
Bands
L
__
1
(c) Predicted
from force field calculations
[zI.
(d) Predicted
from force field calculations
- this work.
-0.014
-0.40
-0.392
-0.20
-0.340
-0.30
-0.215
(e) FR - Fermi resonance. (f) From combination
bands, see text.
wavenumber listings of the Raman gas phase spectrum of s-TFB and the infrared and Raman spectra of s-TFB-d, are available from the authors on request. The A i bands of both molecules appeared as strong, narrow Raman bands in the gas phase spectra, with the exception of the vI band of s-TFB, which had to be one of the three polarized bands in the CH stretching region. Our assignment of the v1 band was based on the fact that there was no counterpart of the 3076-cm-’ Raman band in the infrared spectrum of s-TFB. The infrared active A! bands of s-TFB-d3 show the expected P&R structure (Fig. 4). The lowest lying A; band at 205.9 cm-‘, assigned to Q,, has been observed for / I
RAMAN VO(E')
s-TFE-h3
(9) VUE')
--N-
llL0
1120
2;;ytx -:Lo-7310~3od/
and Y,~(E’) modes of s-TFB. The upper curves FIG.5.Raman band contours of the v,,(E’), Vet, of each pair are the observed band contours. The lower curves are the computed contours obtained respectively. using <:, = +O.lO, & = -0.30, and & = -0.30,
387
BAND CONTOURS OF s-C~F~H~ AND s-&F,D,
;‘~;\ql._ i,j
I
A,=O.OZUO \\ Bo=0.05660cm4', L
2 =-o-o5
1;
1-u 260
(cm-')
220
_,
’
_._
2 =-0.05
1
260
~~
-_
220
l_ 1
(cm-')
FIG. 6. The observed and computed Raman band contours of the v,,(E”) fundamental of s-TFB and s-TFB-d,. The computed contours were produced using & = -0.05 in each case.
the first time. The PR separation of the v17band which was greater than expected is almost certainly due to underlying hot bands. The PR separations of the two other A: bands agreed well with the value of 14.5 cm-’ obtained from Gerhard and Dennison’s formula (12). The weak peak observed on the low-frequency side of vIj is probably a hot-band Q branch. The P branch of the v16contour seemed to be slightly more intense than the R branch, while for v15 and v17 the R branch was slightly stronger. This intensity enhancement in the P branch of v16may be due to a second order A x E-type Coriolis interaction with the nearby v13fundamental. The Raman bands of s-TFB at 1622, 1475, 996.1, and 504 cm-’ and the Raman bands of s-TFB-d, at 1618, 1428.2, and 1054 cm-’ have distorted contours in the gas phase. However, these contours were typically E’ type in nature, thus confirming the earlier assignments (d-11) from the liquid phase spectra. The counterparts of all these fundamental bands could also be assigned in the infrared spectra. The v,,(E’) Raman band of s-TFB at 1475 cm-’ was an extremely weak, broad feature but could be recorded under conditions of high sensitivity. The infrared counterpart of the strong v&E’) Raman band of s-TFB-d, near 792 cm-l seemed completely obscured by the R branch of the vJA;I) fundamental. However, a weak infrared band observed at 791 cm-’ in the liquid state, made an assignment to the E’ species plausible. The two lowest lying E” bands v19 and vZOof s-TFB and s-TFB-d, showed pronounced double Q structures in the gas phase Raman spectra (Figs. 6 and 8). A detailed contour was not obtained for s-TFB for the third E” mode (vJ, but a broad feature centered at 792 cm-’ was assigned to this mode. Normal coordinate calculations suggested a frequency of less than 900 cm-l for v18 in s-TFB (6, 7), and the diffuse Raman band at 847 cm-’ in liquid s-TFB had been suggested in an earlier assignment of this mode (II ). However, in the gas phase Raman spectrum this band was found at 844 cm-’ and was very sharp and polarized. Accordingly, this band could not readily be assigned to a degenerate species. The broad, featureless Raman band of s-TFB at 792 cm-‘, which was more intense than the extremely
388
KORPPI-TOMMOLA
I 1405
I
ET AL.
I 1425
I (cm-l)
1445
FIG. 7. The experimental and computer simulated infrared band contours of the v,,,(E’) fundamental of s-TFB-d3. The computed contour was obtained using k$, = -0.40.
weak JQ,,band, was a more suitable candidate for v18. A previously unreported and very weak Raman band was observed at about 646 cm-’ in both the gas and liquid phase spectra of s-TFB-d3. This band had no counterpart in the infrared and could be due to y18of s-TFB-d,. Assignment of this band to r+, of s-TFB-d, resulted in a rather high product ratio (1.449) for the frequencies of the E” modes. However, the experimental product ratio may be affected by uncertainties in the observed frequencies of the u19band for both s-TFB and s-TFB-d, due to overlapping with their respective v4 bands. This can be seen in Fig. 2, where vIg of s-TFB-d, appears as a small doublet on the low wavenumber side of y4, and in Fig. 3, where v19of s-TFB is seen on the high wavenumber side of y4. 2. Coriolis Constants Degenerate E’ and E” fundamentals are expected to have widely differing band contours in the gas phase spectra due to first-order Coriolis interactions.’ As discussed above, first-order Coriolis constants (5”) of the degenerate vibrational modes were calculated by computer simulation of the corresponding observed band contours in the infrared and Raman spectra. The method for obtaining the molecular constant 5” by matching a computed contour with an experimentally observed band has several difficulties associated with it. Experimental contours may be distorted by overlap with combination or overtone bands, which may or may not be in Fermi resonance with the fundamental. Second order Coriolis interactions (A x E and E x E type) and I-resonance ’ The simplest criterion for a non zero Coriolis constant (Jahn’s rule) that the representation of the product of the vibrational wavefunctions should include the representation of a class to which a rotation belongs is a necessary but not necessarily a sufficient condition for the existence of a Coriolis interaction. The identification of the Coriolis force with vibrational and rotational coupling through terms of the type w x p, where o is rotational velocity and p represents linear momentum, shows that one must have terms of the type cl,, 48, (u, 6 = x, y or z and (T # 6). Vibrations of the A;,A; and E” species of Da,, can only have components of displacement or velocity perpendicular to the cr,,-that is along the z axis. As such the Coriolis forces arizing from /Z” x E” or A; x E” interactions must be identically zero, even though allowed by Jahn’s rule. This is in accord with the zero or very small 5 values determined by contour matching.
BAND CONTOURS OF s-C,F,H,
AND s-C,~F~D~
389
TABLE III Comparison of Coriolis Constants Obtained from Simulated Contours of Combination Bands of s-TFB-d, with Calculated Effective Coriolis Constants
(3)
Obtained from simulated contours of combination
(b)
Calculated from iz values obtained from simulated contours of fundamentals using the appropriate effective zeta combination rules given by Mills [1,141.
bands.
(cl
Calculated from normal coordinate values of the $'s of the fundamentals and using the effective zeta combination rules.
effects (13) may also be present in the spectrum. These interactions as well as the effects of hot bands were not included in the present calculations. The most serious problem expected in contour fitting of heavier molecules such as the ones under study in this paper will almost certainly be the presence of underlying hotbands. A spectral slit width of 3.0 cm-’ was used throughout the calculations. The maximum values of the rotational quantum numbers J and K were set at 100. The computation was repeated using varying 5” values until the experimental contours were reproduced as closely as possible. All values of (Ye = (B” - B’) and c& = (C” - C’) were assumed equal to zero. Values for the Coriolis constants were also obtained from a molecular force field of s-TFB (7). Both the experimental and force field 5” values of s-TFB and s-TFB-d, are given in Table II. 3. Rarnan Gas Phase Band Contours of s-TFB The Raman contours of the vll, v13, and v14bands of s-TFB can be seen in Fig. 5. The 5” values of these bands (Table II) compared fairly closely to those obtained from the earlier infrared study (1 I ). The Raman contour of the vll band had a double Q structure, which was very sensitive to changes in the input parameters of the band contour simulation program. Since the corresponding infrared band was distorted (1 ), the Raman L&from the Raman band should be considered more reliable. The asymmetry on the lower frequency side of the v13 Raman band of s-TFB is most probably due to the presence of the overtone 2v2,,. The Raman contour of the lowest lying E’ fundamental, ul.,, gave
KORPPI-TOMMOLA
390
610
790
770
ET AL.
560
540
(cm-5 520
FIG. 8. Raman band contours of the Y,~(E’) and vIS(E”) fundamentals contours were obtained using 1;i2 = -0.40 and & = 0.00.
of s-TFB-d,.
The computed
most probably due to Fermi resonances between yg and vg + q0 in the CH stretching region, and between vg and q5 + q8 and/or q2 + vi9 in the region around 1620 cm-‘. In our earlier infrared work (1) we used the effective 5” values from some combination bands to estimate the [z’s of two of the three Raman active E” fundamentals of s-TFB. Simulation of the now observed v2,, Raman contour (Fig. 6) gave &‘&, = -0.05, which is exactly the same value obtained from the infrared v17 + r+,, combination band (1). The q9 band centered at 598 cm-’ was distorted in the Raman spectrum by the strong v4 band at 579.9 cm-l (Fig. 3). Simulation was not attempted and the value obtained from the infrared combination band, [fs = 0.0, therefore, remains as the only experimental determination for this fundamental. The vi8 band, as discussed above, was too weak and diffuse for simulation studies. 4. Infrared and Raman Gas Phase Band Contours of s-TFB-d3 Several infrared and Raman bands of gaseous s-TFB-ds were simulated. No characteristic E’ band contours could be observed in the CD stretching region of the Raman spectrum. The shape of the 2314 cm-* infrared band suggested a 5; I
I
I
I
I
I
I 495
495
I
3
&-d-gjm$
I
I
I
505
495
465
I
I 475 (cm-‘)
FIG. 9. Infrared and Raman band contours of the v13(E’) fundamental of s-TFB-&. contours were obtained using & = -0.25 (infrared) and & = -0.20 (Raman).
The computed
391
BAND CONTOURS OF s-CsF,H3 AND S-C,F,DS
k7krk&_J___u
330
(cti’)
320
310
FIG. 10. Infrared and Raman band contours of the v,,(E’) fundamental of s-TFB& contours were obtained using & = -0.25 (infrared) and 5” = -0.30 (Raman).
The computed
value of -0.05 for this fundamental, although only a rough simulation could be carried out. In the vg region a pair of overlapping bands was observed. These are most likely due to a Fermi resonance pair involving the v,(E’) fundamental and the combination vq + Q (A; x E' = E'). Thus (“9could not be reliably determined. The infrared contour of the v10band of s-TFB-d, is shown in Fig. 7. The corresponding Raman band was distorted but simulations suggested a [& value of -0.50. Values of &, from both infrared and Raman contours were large and negative, although both show differences from the force field values (Table II). The vll band was very weak in the Raman spectrum and although one of the strongest in the infrared spectrum, it was badly distorted. This is probably due to the Fermi interaction of vll with the combination band vq + z+ (A; x E').Simulation of the lower frequency component at 1054 cm-’ gave (:I = -0.20. This value does not agree with the values of & of +0.05 and -0.02 obtained directly from combination bands (Table III). It also does not agree with the force field value of <;I = - 0.014. Therefore, the observed value of <& given in Table II is the mean of the two values obtained from the combination bands. The y12Raman contour at 792 cm-’ (Fig. 8) gave & = -0.40. No corresponding infrared band was observed because of the strong overlap of the v15contour (Fig. 4, top contour). For v13 and v14of s-TFB-d3 we made two determinations of the experimental 5’s. The observed (and simulated) infrared and Raman contours of these two fundamentals are shown in Figs. 9 and 10. In both cases the values of 5” determined from the infrared and Raman contours agree within experimental error. The Q(E”) mode was too weak for an estimate of [is to be made. The v&E") band is shown in Fig. 8 where the computer simulation yielded a value of <;$ = 0.00. Simulations of the v*,,(E") contour ofs-TFB-d, (Fig. 6), gave {;,, = -0.05, which is in good agreement with the value obtained indirectly by simulating the v17 + vzo (A'; x E')combination band (Table III). The values of 5”‘s obtained by simulation of some of the infrared combination bands agree reasonably well with both experimental and force field effective 5”‘s. ACKNOWLEDGMENTS Financial support for this work was provided by operating grants from the National Research Council of Canada and the Planetary Atmospheres Program of the National Aeronautics and Space Administra-
KORPPI-TOMMOLA
392
ET AL.
tion (Grant NGL-43-001-006). Grants from Queen’s University to aid in the purchase of equipment for Raman spectroscopy are gratefully acknowledged. One of us (J.K.-T.) is grateful to the Jenny and Antii Wihuri Foundation of Finland for a scholarship in 1979. RECEIVED:
August
25,
1980 REFERENCES
1. H. F. SHURVELL, T. E. CAMERON, D. B. BAKER, AND S. J. DAUNT, Spectrochim.
Ada
A 35,
757-764 (1979). 2. D. STEELE, J. Mol. Spectrosc. 66, 233-236 (1977). 3. W. KIEFER, H. J. BERNSTEIN, H. WIESER, AND M. DANYLUK, J. Mol. Spectrosc. 43, 393-400 (1972). 4. J. SCHLUPF AND A. WEBER, J. Raman Spectrosc. 1, 3-15 (1973). 5. F. N. MASRI AND I. R. WILLIAMS, Comput. Phys. Commun. 1, 349-358 (1970); 2, 87-93, 298299 (1971). 6, V. J. EATON AND D. STEELE, J. Mol. Spectrosc. 48, 446-458 (1973). 7. V. J. EATON AND D. STEELE, J. Mol. Spectrosc. 54, 312-317 (1975). 8. J. R. NIELSEN, C. Y. LIANG, ANDD. C. SMITH, Discuss. FaradaySot. 9-10,177-187(1950-1951). 9. J. R. SCHERER, J. C. EVANS, AND W. W. MUELDER, Spectrochim. Acta 18, 1579-1592 (1962). 10. V. J. EATON, R. A. R. PEARCE, D. STEELE, AND J. W. TINDLE, Spectrochim. Acta A 32, 663672 (1976).
E. FERGUSON, J. Chem. Phys. 21, 886-890 (1953). 12. S. L. GERHARD AND D. M. DENNISON, Phys. Rev. 43, 197-204 (1933). 13. G. J. CARTWRIGHT AND I. M. MILLS, J. Mol. Spectrosc. 34, 415-439 (1970). 14. I. M. MILLS, Mol. Phys. 7, 549-563 (1964). II.
OF
MOLECULAR
SPECTROSCOPY
87,
382-392 (1981)
The Gas Phase Raman Band Contours of 1,3,5Trifluorobenzene and the Infrared and Raman Band Contours of 1 ,3,5-Trifluorobenzene-dSi*2 J. KORPPI-TOMMOLA~AND H. F. SHURVELL Department
of Chemistry,
Queen’s
University, Kingston,
Ontario, Canada,
K7L 3N6
S. J. DAUNTS Molecular
Spectroscopy Laboratory, Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37916
AND
D. STEELE Department
of Chemistry, Royal Holloway College, University of London, Egham, Surrey, TW20 OEX, England
The gas phase infraredand Raman spectra of 1,3,5-trifluorobenzene-d3has been recorded. The gas phase Raman spectrum of 1,3,5-trifluorobenzene-h, has also been recorded. Computer simulations of the band contours of the degenerate vibrational modes have been used to determine the first-order Coriolis coupling constants for both molecules. The 5’ values were then compared with those evaluated from force field calculations. 1. INTRODUCTION
The literature concerning the infrared and Raman spectra of 1,3, Wifluorobenzene (s-TFB) can be found in the references of our recent study of the infrared rotation-vibration bands of that molecule (1). The present paper, which is a continuation of the work of Ref. (I ), reports the gas phase infrared and Raman spectra of 1,3,5-trifluoro-2,4,6_trideuterobenzene (s-TFB-d3) as well as the gas phase Raman spectrum of s-TFB. The results detailed below have suggested improved vibrational assignments for these molecules. An additional objective of this work was to compare the values of the Coriolis constants for the E’ degenerate vibrational bands obtained from Raman contours with those obtained from the infrared I Supported in part by the Planetary Atmospheres Program of the National Aeronautics and Space Administration (Grant NGL-43-001-006). 2 A preliminary report of this work was presented at the Ohio State Molecular Spectroscopy Symposium, 1979, Paper WB2. 3 Present address: Department of Chemistry, University of Jyvaskyla, SF-40100 Jyvaskyla 10, Finland. 4 To whom correspondence concerning this paper should be addressed. 0022-2852/81/060382-l 1$02.00/O Copyright All ri&ts
0 1981 by Academic of reproduction
Press,
Inc.
in any form reserved.
382
383
BAND CONTOURS OF s-C6F3H3 AND s-&F,D, TABLE I Infrared and Raman Fundamental
s-
Frequencies 5
Infrared
of s-TFB and s-TFB-d, in the Gas Phaseta’
lRaman
s-TFI Infrared
-r
I? Raman
Product Rule
i.a.
3076
i.a.
2319.2
Calc.
i.a.
1362.6
i.a.
1359.7
1.414
i.a.
1012.4
i.a.
969.2
Exper.
i.a.
579.9
i.a.
577.3
1.395
__
3113.0
3115.5
2314
1629(b)
1622(b)
1617tb)
1618(b)
Calc.
1475.4
1475
1425.0
1428.2
1.977
1127.6
1129.2
1054
1054
Exper.
995.7
996.1
__
792.0
1.975
502.4
504.0
487.0
487.4
324.2
327.5
322.4
322.5
847.0
i.a.
777.2
i.a.
663.1
i.a.
522.0
i.a.
207.0
i.a.
205.9
i.a.
Exper. 1.391
i.a.
792
i.a.
646
Calc. 1.389
i.a.
598
i.a.
537.6
i.a.
245.8
i.a.
231.3
C.SlC. 1.398
Exper. 1.449
(b)
Average frequencies of the two band maxima region.
observed
in the
contours of the same transitions. Finally, a comparison of the experimental 5” values with those obtained from force field calculations of s-TFB (2) was considered to be of interest. 2. EXPERIMENTAL
DETAILS
1,3,5-Trifluorobenzene was purchased from Pierce Chemical Company and was purified by distillation on a vacuum line. The deuterated trifluorobenzene was prepared by refluxing s-TFB over concentrated D,SO, (Merck Sharpe & Dohme, Canada, 96% D2S04). The exchange reaction was monitored by recording the Raman spectrum of the exchange product in the liquid phase. The monitoring was done by observation of the symmetrical ring stretching band (Q), which showed clearly resolved components for s-TFB, s-TFB-d, s-TFB-d,, and s-TFB-dB. More than five exchanges were needed to make the final product, which then contained only about 5% s-TFB-d, plus traces of the lighter isotopic species. Infrared spectra were recorded on a Perkin-Elmer 180 spectrometer using a
384
KORPPI-TOMMOLA
3000
FIG. 1. The infrared spectrum
2000
of gaseous
1600
ET AL.
1200
s-TFB-d3,
800
LOO b-9
P = 0.5mmHg, path length 10 cm; insert: P
= 30 mmHg (4000 Pa).
lo-cm gas cell equipped with KBr or polyethylene windows. Survey Raman spectra were recorded on a Jarrell-Ash l-m double monochromator using a multipass gas cell (3) and about 1 W of laser power at the sample. Detailed contours were recorded in the laboratory of Dr. W. F. Murphy at the National Research Council of Ottawa, Canada using the same multipass cell, a Spex double monochromator and about 7 W of laser power at the sample. In both cases 514.5 nm excitation from an argon ion laser was used. The spectra of the strongest Raman bands of both trifluorobenzenes were obtained with slit widths of about 3 cm-’ and up to 40 kcps of signal. The spectra were calibrated with a Ne pen lamp. The wavenumbers listed in Table I are believed accurate to &OS cm-‘. 3. CALCULATION
OF CONTOURS
The rotational constants of both isotopic molecules were calculated using the structural parameters of s-TFB reported in the rotational Raman study of Schlupf and Weber (4). These calculations yielded for s-TFB values of& = 0.05864 cm-’ and Co = (l/2)& = 0.02932 cm-‘, while fors-TFB-d, the values were& = 0.05680 cm-’ and C, = (l/2)& = 0.02840 cm-‘. To obtain the first-order Coriolis constants, <“, a band contour simulation program, originally written by Masri and Williams (5) and revised by one of the authors (SJD) was used. Initial trial values of 5” were obtained from the 5” versus AVPR plot of our earlier publication (1). The errors in 5” values obtained from undistorted band contours are typically quoted to be -tO.OS, but in practice are usually greater due to various complicating factors discussed below. The Coriolis constants of the deuterated trifluorobenzene
I 2400
/ I
1
2200
FIG. 2. The Raman spectrum
1400
of gaseous
1000
s-TFB-da,
KM
equilibrium
200
vapor pressure
(cm-‘)
at about 40°C.
385
BAND CONTOURS OF s-CGF,H, AND s-C,F,Ds
tl
RAMAN s-TFB-h3(g)
FIG. 3. The Raman spectrum of gaseous s-TFB, equilibrium vapor pressure at
about 40°C.
were calculated from a force field of s-TFB (6, 7) using the method previously described by Steele (2). 4.
1.
RESULTS AND DISCUSSION
Assignments
Infrared and Raman survey spectra of s-TFB-d, are shown in Figs. 1 and 2. A Raman survey spectrum of s-TFB can be seen in Fig. 3. Assignments of the fundamental bands of both molecules are summarized in Table I. Since several vibrational assignments of these molecules have been published (8-1 I ), we comment on our gas phase results only where some new information is now available. Complete
V,+;:;)
V
nPR=lBOcm"
FIG. 4. The infrared band contours of the A;’ modes of s-TFB-d,. = 14.6 cm-‘), v,~ (AvPR = 14.4 cm-‘), and v,? (AvPR = 16.0 cm-‘).
From top to bottom: I+, (AU,+
386
KORPPI-TOMMOLA TABLE
Comparison
ET AL. II
of Coriolis Constants Obtained from Computer Simulated s-TFB and s-TFB-d, with Constants Obtained from Force s-TFB
.Mode E'
Infrared and Raman Field Calculations
iz FF Cc)
r
i= FF Cd)
;' Raman (b)
__
-_
-0.002
-0.05
__
+0.014
FR
FR
to.805
FA
FR
co.717
-0.55
__
-0.682
-0.40
-0.50
-0.770
+0.02(f)
zz infrared (b)
-0.05
+0.10
-0.024
-0.47
__
-0.470
-0.30
-0.30
-0.281
-0.25
-0.20
-0.30
-0.345
-0.25
(ai Obserwd
[l].
(b) Observed
- this work.
of
s-TFB-d,
(a)
5 ' infrared
Bands
L
__
1
(c) Predicted
from force field calculations
[zI.
(d) Predicted
from force field calculations
- this work.
-0.014
-0.40
-0.392
-0.20
-0.340
-0.30
-0.215
(e) FR - Fermi resonance. (f) From combination
bands, see text.
wavenumber listings of the Raman gas phase spectrum of s-TFB and the infrared and Raman spectra of s-TFB-d, are available from the authors on request. The A i bands of both molecules appeared as strong, narrow Raman bands in the gas phase spectra, with the exception of the vI band of s-TFB, which had to be one of the three polarized bands in the CH stretching region. Our assignment of the v1 band was based on the fact that there was no counterpart of the 3076-cm-’ Raman band in the infrared spectrum of s-TFB. The infrared active A! bands of s-TFB-d3 show the expected P&R structure (Fig. 4). The lowest lying A; band at 205.9 cm-‘, assigned to Q,, has been observed for / I
RAMAN VO(E')
s-TFE-h3
(9) VUE')
--N-
llL0
1120
2;;ytx -:Lo-7310~3od/
and Y,~(E’) modes of s-TFB. The upper curves FIG.5.Raman band contours of the v,,(E’), Vet, of each pair are the observed band contours. The lower curves are the computed contours obtained respectively. using <:, = +O.lO, & = -0.30, and & = -0.30,
387
BAND CONTOURS OF s-C~F~H~ AND s-&F,D,
;‘~;\ql._ i,j
I
A,=O.OZUO \\ Bo=0.05660cm4', L
2 =-o-o5
1;
1-u 260
(cm-')
220
_,
’
_._
2 =-0.05
1
260
~~
-_
220
l_ 1
(cm-')
FIG. 6. The observed and computed Raman band contours of the v,,(E”) fundamental of s-TFB and s-TFB-d,. The computed contours were produced using & = -0.05 in each case.
the first time. The PR separation of the v17band which was greater than expected is almost certainly due to underlying hot bands. The PR separations of the two other A: bands agreed well with the value of 14.5 cm-’ obtained from Gerhard and Dennison’s formula (12). The weak peak observed on the low-frequency side of vIj is probably a hot-band Q branch. The P branch of the v16contour seemed to be slightly more intense than the R branch, while for v15 and v17 the R branch was slightly stronger. This intensity enhancement in the P branch of v16may be due to a second order A x E-type Coriolis interaction with the nearby v13fundamental. The Raman bands of s-TFB at 1622, 1475, 996.1, and 504 cm-’ and the Raman bands of s-TFB-d, at 1618, 1428.2, and 1054 cm-’ have distorted contours in the gas phase. However, these contours were typically E’ type in nature, thus confirming the earlier assignments (d-11) from the liquid phase spectra. The counterparts of all these fundamental bands could also be assigned in the infrared spectra. The v,,(E’) Raman band of s-TFB at 1475 cm-’ was an extremely weak, broad feature but could be recorded under conditions of high sensitivity. The infrared counterpart of the strong v&E’) Raman band of s-TFB-d, near 792 cm-l seemed completely obscured by the R branch of the vJA;I) fundamental. However, a weak infrared band observed at 791 cm-’ in the liquid state, made an assignment to the E’ species plausible. The two lowest lying E” bands v19 and vZOof s-TFB and s-TFB-d, showed pronounced double Q structures in the gas phase Raman spectra (Figs. 6 and 8). A detailed contour was not obtained for s-TFB for the third E” mode (vJ, but a broad feature centered at 792 cm-’ was assigned to this mode. Normal coordinate calculations suggested a frequency of less than 900 cm-l for v18 in s-TFB (6, 7), and the diffuse Raman band at 847 cm-’ in liquid s-TFB had been suggested in an earlier assignment of this mode (II ). However, in the gas phase Raman spectrum this band was found at 844 cm-’ and was very sharp and polarized. Accordingly, this band could not readily be assigned to a degenerate species. The broad, featureless Raman band of s-TFB at 792 cm-‘, which was more intense than the extremely
388
KORPPI-TOMMOLA
I 1405
I
ET AL.
I 1425
I (cm-l)
1445
FIG. 7. The experimental and computer simulated infrared band contours of the v,,,(E’) fundamental of s-TFB-d3. The computed contour was obtained using k$, = -0.40.
weak JQ,,band, was a more suitable candidate for v18. A previously unreported and very weak Raman band was observed at about 646 cm-’ in both the gas and liquid phase spectra of s-TFB-d3. This band had no counterpart in the infrared and could be due to y18of s-TFB-d,. Assignment of this band to r+, of s-TFB-d, resulted in a rather high product ratio (1.449) for the frequencies of the E” modes. However, the experimental product ratio may be affected by uncertainties in the observed frequencies of the u19band for both s-TFB and s-TFB-d, due to overlapping with their respective v4 bands. This can be seen in Fig. 2, where vIg of s-TFB-d, appears as a small doublet on the low wavenumber side of y4, and in Fig. 3, where v19of s-TFB is seen on the high wavenumber side of y4. 2. Coriolis Constants Degenerate E’ and E” fundamentals are expected to have widely differing band contours in the gas phase spectra due to first-order Coriolis interactions.’ As discussed above, first-order Coriolis constants (5”) of the degenerate vibrational modes were calculated by computer simulation of the corresponding observed band contours in the infrared and Raman spectra. The method for obtaining the molecular constant 5” by matching a computed contour with an experimentally observed band has several difficulties associated with it. Experimental contours may be distorted by overlap with combination or overtone bands, which may or may not be in Fermi resonance with the fundamental. Second order Coriolis interactions (A x E and E x E type) and I-resonance ’ The simplest criterion for a non zero Coriolis constant (Jahn’s rule) that the representation of the product of the vibrational wavefunctions should include the representation of a class to which a rotation belongs is a necessary but not necessarily a sufficient condition for the existence of a Coriolis interaction. The identification of the Coriolis force with vibrational and rotational coupling through terms of the type w x p, where o is rotational velocity and p represents linear momentum, shows that one must have terms of the type cl,, 48, (u, 6 = x, y or z and (T # 6). Vibrations of the A;,A; and E” species of Da,, can only have components of displacement or velocity perpendicular to the cr,,-that is along the z axis. As such the Coriolis forces arizing from /Z” x E” or A; x E” interactions must be identically zero, even though allowed by Jahn’s rule. This is in accord with the zero or very small 5 values determined by contour matching.
BAND CONTOURS OF s-C,F,H,
AND s-C,~F~D~
389
TABLE III Comparison of Coriolis Constants Obtained from Simulated Contours of Combination Bands of s-TFB-d, with Calculated Effective Coriolis Constants
(3)
Obtained from simulated contours of combination
(b)
Calculated from iz values obtained from simulated contours of fundamentals using the appropriate effective zeta combination rules given by Mills [1,141.
bands.
(cl
Calculated from normal coordinate values of the $'s of the fundamentals and using the effective zeta combination rules.
effects (13) may also be present in the spectrum. These interactions as well as the effects of hot bands were not included in the present calculations. The most serious problem expected in contour fitting of heavier molecules such as the ones under study in this paper will almost certainly be the presence of underlying hotbands. A spectral slit width of 3.0 cm-’ was used throughout the calculations. The maximum values of the rotational quantum numbers J and K were set at 100. The computation was repeated using varying 5” values until the experimental contours were reproduced as closely as possible. All values of (Ye = (B” - B’) and c& = (C” - C’) were assumed equal to zero. Values for the Coriolis constants were also obtained from a molecular force field of s-TFB (7). Both the experimental and force field 5” values of s-TFB and s-TFB-d, are given in Table II. 3. Rarnan Gas Phase Band Contours of s-TFB The Raman contours of the vll, v13, and v14bands of s-TFB can be seen in Fig. 5. The 5” values of these bands (Table II) compared fairly closely to those obtained from the earlier infrared study (1 I ). The Raman contour of the vll band had a double Q structure, which was very sensitive to changes in the input parameters of the band contour simulation program. Since the corresponding infrared band was distorted (1 ), the Raman L&from the Raman band should be considered more reliable. The asymmetry on the lower frequency side of the v13 Raman band of s-TFB is most probably due to the presence of the overtone 2v2,,. The Raman contour of the lowest lying E’ fundamental, ul.,, gave
KORPPI-TOMMOLA
390
610
790
770
ET AL.
560
540
(cm-5 520
FIG. 8. Raman band contours of the Y,~(E’) and vIS(E”) fundamentals contours were obtained using 1;i2 = -0.40 and & = 0.00.
of s-TFB-d,.
The computed
most probably due to Fermi resonances between yg and vg + q0 in the CH stretching region, and between vg and q5 + q8 and/or q2 + vi9 in the region around 1620 cm-‘. In our earlier infrared work (1) we used the effective 5” values from some combination bands to estimate the [z’s of two of the three Raman active E” fundamentals of s-TFB. Simulation of the now observed v2,, Raman contour (Fig. 6) gave &‘&, = -0.05, which is exactly the same value obtained from the infrared v17 + r+,, combination band (1). The q9 band centered at 598 cm-’ was distorted in the Raman spectrum by the strong v4 band at 579.9 cm-l (Fig. 3). Simulation was not attempted and the value obtained from the infrared combination band, [fs = 0.0, therefore, remains as the only experimental determination for this fundamental. The vi8 band, as discussed above, was too weak and diffuse for simulation studies. 4. Infrared and Raman Gas Phase Band Contours of s-TFB-d3 Several infrared and Raman bands of gaseous s-TFB-ds were simulated. No characteristic E’ band contours could be observed in the CD stretching region of the Raman spectrum. The shape of the 2314 cm-* infrared band suggested a 5; I
I
I
I
I
I
I 495
495
I
3
&-d-gjm$
I
I
I
505
495
465
I
I 475 (cm-‘)
FIG. 9. Infrared and Raman band contours of the v13(E’) fundamental of s-TFB-&. contours were obtained using & = -0.25 (infrared) and & = -0.20 (Raman).
The computed
391
BAND CONTOURS OF s-CsF,H3 AND S-C,F,DS
k7krk&_J___u
330
(cti’)
320
310
FIG. 10. Infrared and Raman band contours of the v,,(E’) fundamental of s-TFB& contours were obtained using & = -0.25 (infrared) and 5” = -0.30 (Raman).
The computed
value of -0.05 for this fundamental, although only a rough simulation could be carried out. In the vg region a pair of overlapping bands was observed. These are most likely due to a Fermi resonance pair involving the v,(E’) fundamental and the combination vq + Q (A; x E' = E'). Thus (“9could not be reliably determined. The infrared contour of the v10band of s-TFB-d, is shown in Fig. 7. The corresponding Raman band was distorted but simulations suggested a [& value of -0.50. Values of &, from both infrared and Raman contours were large and negative, although both show differences from the force field values (Table II). The vll band was very weak in the Raman spectrum and although one of the strongest in the infrared spectrum, it was badly distorted. This is probably due to the Fermi interaction of vll with the combination band vq + z+ (A; x E').Simulation of the lower frequency component at 1054 cm-’ gave (:I = -0.20. This value does not agree with the values of & of +0.05 and -0.02 obtained directly from combination bands (Table III). It also does not agree with the force field value of <;I = - 0.014. Therefore, the observed value of <& given in Table II is the mean of the two values obtained from the combination bands. The y12Raman contour at 792 cm-’ (Fig. 8) gave & = -0.40. No corresponding infrared band was observed because of the strong overlap of the v15contour (Fig. 4, top contour). For v13 and v14of s-TFB-d3 we made two determinations of the experimental 5’s. The observed (and simulated) infrared and Raman contours of these two fundamentals are shown in Figs. 9 and 10. In both cases the values of 5” determined from the infrared and Raman contours agree within experimental error. The Q(E”) mode was too weak for an estimate of [is to be made. The v&E") band is shown in Fig. 8 where the computer simulation yielded a value of <;$ = 0.00. Simulations of the v*,,(E") contour ofs-TFB-d, (Fig. 6), gave {;,, = -0.05, which is in good agreement with the value obtained indirectly by simulating the v17 + vzo (A'; x E')combination band (Table III). The values of 5”‘s obtained by simulation of some of the infrared combination bands agree reasonably well with both experimental and force field effective 5”‘s. ACKNOWLEDGMENTS Financial support for this work was provided by operating grants from the National Research Council of Canada and the Planetary Atmospheres Program of the National Aeronautics and Space Administra-
KORPPI-TOMMOLA
392
ET AL.
tion (Grant NGL-43-001-006). Grants from Queen’s University to aid in the purchase of equipment for Raman spectroscopy are gratefully acknowledged. One of us (J.K.-T.) is grateful to the Jenny and Antii Wihuri Foundation of Finland for a scholarship in 1979. RECEIVED:
August
25,
1980 REFERENCES
1. H. F. SHURVELL, T. E. CAMERON, D. B. BAKER, AND S. J. DAUNT, Spectrochim.
Ada
A 35,
757-764 (1979). 2. D. STEELE, J. Mol. Spectrosc. 66, 233-236 (1977). 3. W. KIEFER, H. J. BERNSTEIN, H. WIESER, AND M. DANYLUK, J. Mol. Spectrosc. 43, 393-400 (1972). 4. J. SCHLUPF AND A. WEBER, J. Raman Spectrosc. 1, 3-15 (1973). 5. F. N. MASRI AND I. R. WILLIAMS, Comput. Phys. Commun. 1, 349-358 (1970); 2, 87-93, 298299 (1971). 6, V. J. EATON AND D. STEELE, J. Mol. Spectrosc. 48, 446-458 (1973). 7. V. J. EATON AND D. STEELE, J. Mol. Spectrosc. 54, 312-317 (1975). 8. J. R. NIELSEN, C. Y. LIANG, ANDD. C. SMITH, Discuss. FaradaySot. 9-10,177-187(1950-1951). 9. J. R. SCHERER, J. C. EVANS, AND W. W. MUELDER, Spectrochim. Acta 18, 1579-1592 (1962). 10. V. J. EATON, R. A. R. PEARCE, D. STEELE, AND J. W. TINDLE, Spectrochim. Acta A 32, 663672 (1976).
E. FERGUSON, J. Chem. Phys. 21, 886-890 (1953). 12. S. L. GERHARD AND D. M. DENNISON, Phys. Rev. 43, 197-204 (1933). 13. G. J. CARTWRIGHT AND I. M. MILLS, J. Mol. Spectrosc. 34, 415-439 (1970). 14. I. M. MILLS, Mol. Phys. 7, 549-563 (1964). II.