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Vibrational spectrum and barrier to internal rotation for CF3CFO
Spectrochimica Ach, 1965,Vol.21,pp.119to 125.Pergamon PressLtd. Printed in Northern Ireland
Vibrational spectrum and barrier to internal rotation for CF,CFO* KARL R. Loos? and R. C. LORD Spectroscopy Laboratory, Massachusetts Institute of Technology, Cambridge, Mass. 02139 (Received 10 April
1964)
Abstract-The
vibrational spectrum of CF,CFO has been investigated over the range 3900-35 cm-l with small grating spectrometers. All fundamental frequencies have been assigned to approximate vibrational modes and the torsional vibration haa been located at 50 cm-l. The principal moments of inertia of the molecule have been estimated from assumed bond angles and distances to be 130, 200, and 248 a.m.u.Aa, and the reduced moment of inertia,for the CF, group to be 29.5 a.m.u.A2. From these values the threefold barrier to internal rotation is calculated to be 1390 f 210 Cal/mole or 486 f 75 cm-l on the basis of a harmonic potential function. The harmonic force constant for torsion has been calculated &80.0275 x IO5dyn/cm. ALTHOUGH much work has been reported
in the literature on barriers to internal rotation in molecules, most of it has been concerned with methyl groups [l]. The present study of the far infrared spectrum of perfluoroacetyl fluoride, CF,CFO, was carried out as part of a program of investigation of barriers to rotation of the txifluoromethyl group. Since no previous vibrational study has been reported for perfluoroacetyl fluoride, its spectrum has also been investigated throughout the mid infrared region. EXPERIMENTAL
Perfluoroacetyl fluoride, a colorless gas (b.p. -60°C) was obtained from the Central Research Department, E. I. du Pont de Nemours and Company, through the courtesy of Dr. S. ANDREADES. The sample was used without further purification. The infrared spectrum of the gas was recorded from 3900 cm-l to 600 cm-l with a Perkin-Elmer Model 421 Dual Grating Spectrometer in the M.I.T. Spectroscopy Laboratory, and from 600 cm-l to 400 cm-l on a Perkin-Elmer Model 421 spectrometer equipped with a cesium bromide interchange at Tufts University through the courtesy of Professor M. K. WILSON. The far infrared spectrum from 400 cm-l to 35 cm-l was obtained with the far infrared spectrometer of LORD and MCCUBBIN [2], modified for vacuum operation as described by C. H. PERRY [3]. Absorption cells of 10 cm and 1 meter path length, equipped with KBr and polyethylene windows respectively, were used, with sample pressures up to 600 mm Hg.
* Based on the S.M. thesis of KARL R. Loos, Department of Chemistry, Massachusetts Institute of Technology, September, 1963. t Corning Glass Works Foundation Fellow, 1962-63. [l] A bibliography for hindered internal rotation covering work reported in the literature up to September, 1962, has been prepared by D. R. HERSCHBACH, University of California Radiation Laboratory report UCRL-10404. [2] R. C. LORD and T. K. MCCUBBIN, JR., J. Opt. Sot. Am. 47, 689 (1957). [3] C. H. PERRY, Quarterly Progress Report No. 70, Research Laboratory of Electronics, M.I.T., July 15 (1963). 119
120
KARL
R. Loos and R. C. LORD
Perfluoroacetyl fluoride is relatively inert and no special sample-handling techniques were required. All fundamentals reported are believed accurate to f 1 cm-l. Recordings of the spectra are shown in Figs. 1 and 2 and numerical results summarized in Tables 1 and 2.
%
Fig. 1. Mid infrared spectrum of CF,CFO as recorded by a Perkin-Elmer 421 Dual Grating Spectrophotometer (spectral slit width -2 cm-l).
Model
ASSIGNMENT OF FUNDAMENTALS Since no microwave or electron-diffraction studies of perfluoroacetyl fluoride have yet been published, the molecular parameters must be estimated. The CF, group has been assumed to orient with one CF bond coplanar with the CFO group and opposed to the CO bond. The assumed bond angles and bond distances in Angstroms for the resultant structure of C, symmetry were: CF distances in CF, group, 1.34; CF distance in CFO group, l-35 ; CC distance, 1.50 ; CO distance, 1.18; FCC angles (CF, group), 107”; CC0 angle, 128’; OCF angle, 122”. These estimates are based on the results obtained by PIERCE and KRISHER [4] from their microwave study of acetyl fluoride, together with data from other studies of molecules containing CF, groups [5]. The resultant moments of inertia in a.m.u.A2 are: I, (axis roughly parallel to CC bond in C, plane), 130; II, (axis roughly perpendicular to CC bond in C, plane), 206; I, (axis perpendicular to C, plane), 248. The vibrations of CF,CFO of the above structure divide into 10A’ and 5A”, A’ being the species of vibrations symmetric to the plane of symmetry and A” that antisymmetric to the plane. Since by symmetry the dipole moment of the A” modes vibrates parallel to the axis of greatest moment of inertia I,, these modes should produce infrared bands with type-C contours. On the other hand, the directions of the vibrating dipole moments in the A’ vibrations are not fixed by symmetry with respect to the principal axes of inertia and therefore the contours of the A’ L. PIERCE and L. C. KRISHER, J. Chem. Phys. 31, 875 (1959). [5] Tables of Interatomic Distances, Chemical Society (London) (1958).
‘41
I
600
I
550
I
500
I
450
I
400
I
350 Wavenumber
I
I
250 300 in cm-’ *
I
200
I
150
I
100
I 50
_
Fig. 2. Mid and far infrared spectrum of CF,CFO observed with the Perkin-Elmer Model 421 Dual Grating Spectrophotometer and a vacuum far infrared spectrometer. Bands below 450 cm-l are computed from single-beam data of the vacuum spectrometer. Spectral slit widths are indicated on the Figure.
0
0
122
R. Loos
KARL Table 1.
and R.
C. LORD
Infrared spectrum of perfluoroacetyl
fluoride
a in 1O-8 cm~/mole
Frequency
RT PI.
a=-log-i-
(cm-‘)
0.09
0.28 0.18 0.42 0.18 0.28 1.1 0.45 1.7 0.35 0.50 108.0 1.2 1.3 0.37 75.0 180.0 192.0 239.0 33.0 67.0 45.0 37.0 37.0 78.0 56.0 48.0 109.0 3.7 12.0 37.0 2.5 4.1 0.25
2980
2645 2575 2530 2480 2410 2280 2180 2120 2040 1895 1792 1780 1560 1339 1267 1206 1096 808 807 804 799 796 760 747 743 690 595 531 519 387 227 50
Species
A'
Aff
Fundamental
Assignment
type
2.8
3760 3220
Table 2.
Band
I,
A
A C? B
A? A?
A?
A
A?
c
2 x 1 = 3790 1 + 2 = 3234 1+ 4 = 2991 1 + 6 = 2655 1 + 7 = 2685 2 + 3 = 2546 1+ 8 = 2490 2 x 3 = 2412 1 + 13 = 2287 2 x 4 = 2192 1+ 14 = 2122 2 + 7 = 2029 1 11 + 12 = 1788 11 + 9 = 1776 6 + 6 = 1567 2 11 3 4 5 + 15 - 15 5 5+2x15-2216 5+3x15-3315 5+4x15-4415 6 7 + 15 - 15 7+2x15-2x15 7 8 12 9 13 10,14 15
vibrations of perfluoroacetyl
fluoride
Vibration number
Approximate description
Frequency (cm-‘)
1 2 3 4 5 6 7 8 9 10
CO stretching CF stretching CF, “degenerate” stretching CF, symmetric stretching CC stretching CF, symmetric deformation FCO deformation FCO rocking CF, “degenerate” deformation CF, wagging
1895 1339 1206 1096 807 760 690 595 519 227
11 12 13 14 15
CF, “degenert~te” stretching CF, “degenerate” deformation FCO wagging
1257 531 387 227 50
CF, wagging CF, torsion
Vibrational
spectrum and barrier to internal rotation for CF,CFO
123
bands will‘vary in type from one band to the next. One can expect band contours of type A, type B and intermediate shapes. Because the masses of the various atoms are comparable, considerable interaction of the atomic displacements is expected in all modes and the descriptions of the modes assigned below are only rough approximations. The carbonyl stretching frequency occurs at 1895 cm-l, a reasonable location in view of the highly electronegative fluorine attached to the carbonyl group. The frequency is found in acetyl fluoride at 1865 cm-l [6]. Of the four CF stretching frequencies, three are associated with the CF, group, an A’ stretching vibration symmetric to the local symmetry of the CF, group, and an A’ stretching and an A” stretching derived from the doubly degenerate stretching vibration of the CF, group. The fourth is the stretching of the CF adjacent to the carbonyl group and is of species A’. The band at 1339 cm-l is assigned to the latter mode as the value for this might be expected to be higher than the other CF The three very intense bands in the region stretchings because of its location. 1300 cm-l-1000 cm-l are associated with the CF, group. The band at 1206 cm-r has a type-B contour and is thus of species A’, while the bands at 1257 cm-l and 1096 cm-l have either type-A or type-C contours. These have been assigned respectively to the A’ “degenerate”, A” “degenerate” and A’ symmetric stretchings by analogy to the spectrum of CF,NO, [7] which is isoelectronic with CF,CFO. The band at 807 cm-l is assigned to the CC stretching mode. This vibration is probably coupled with the surrounding CF stretchings, and subject to considerable interaction with them. There are three CF, deformation vibrations, classified as for the CF stretchings with respect to the local symmetry of the CF, group: a symmetric deformation of species A’, and two modes derived from the CF, degenerate deformation which The band at 760 cm-l is assigned to the belong to species A’ and A” respectively. symmetric A’ deformation and the bands at 519 cm-l and 531 cm-l to the A’ and A” “degenerate” modes. This is in accord with the assignment in CF,NO, where the A, deformation occurs at 751 cm-l and the other two deformations are assumed to occur together at 529 cm-l. The two “degenerate” deformation frequencies in CF,CFO would also be expected to lie close to one another, but the distinction between the A’ and A” modes is not obvious and our assignment might well be interchanged. The FCO group undergoes three bending vibrations: an A’ deformation, an A’ rocking and an A” wagging mode. The FCO deformation is expected to occur at a higher frequency than the others and is assigned to the band at 690 cm-l. The rocking mode is assigned to the band at 595 cm-l, and the wagging mode to the band at 387 cm?. The CF, A’ and A” wagging vibrations are taken to be accidentally degenerate and are assigned to the broad band of uncertain contour at 227 cm-l. This assignment is consistent with the near degeneracy of the CF, deformation vibrations and is reasonable since the molecule is roughly a symmetric top. [S] B. P. Susz and J. J. WUHRMANN, He&v. Chim. Acta 40, 722 (1957). [7] J. MASON and J. DUNDERDALE, J. Chem. f&c. 759 (1956).
124
KARL R. Loos and R. C. LORD
The broad featureless band at 50 cm-l is assigned to the torsional vibration because this vibration is expected to have the lowest frequency and to occur in the range 50-150cm-l. THE
BARRIER
TO INTERNAL
ROTATION
The breadth and lack of structure in the band assigned to the torsional oscillation are readily understandable. Since many levels of this mode are significantly populated at room temperature, the transitions 1 c 0,2 c 1,3 c 2,etc. will have comparable intensities. These transitions presumably have slightly different frequencies and thus at the moderate resolution of our spectrometer (~1 cm-l) will not be resolved. The partial overlapping of the various bands may be expected to wash out the type-C rotational structure of the band contour and thus to leave a structureless band of the sort observed. From the observed frequency of the torsional oscillation and assumptions about the shape of the potential-energy curve, one can calculate the height of the potential barrier to rotation. For groups of threefold local symmetry oscillating with respect to an asymmetric framework, the potential energy, V(E), of torsional oscillation is usually written in the form: V(E) = V,(l - cos3a)/2+ V&l - cos6x)/2+ ...
(1)
where cc is the angle of torsion, V, a potential constant which is the barrier height if only the first term in (1)is significant, V, a second potential constant and so on. In some molecules, however, e.g., H&-NO, [S] and H&-BP, [9], the V, term vanishes by symmetry and V, can be evaluated directly. In these two cases it is found to be extremely small (of the order of 5 cm-l or less) compared to V3, which in molecules where it does not vanish by symmetry is often higher than 1000 cm-l. In acetyl fluoride [4],which is isoelectronic with H&?-NO, and H,C-BF,, V3 is about 1040 cm-l, and FATELEY and MILLER [lo] have shown that V6 is less than 3% of 8, for a number of molecules containing methyl groups. While it is certainly possible that VTe/V3 is larger for CF, groups than for CH,, we have no way of evaluating this ratio from our present data and therefore assume that VTsis negligible. If this is so and the value of V3 is large, the torsional vibration will be approximately harmonic. Under such circumstances the potential V(a) simplifies to 9V,cr2/4 for sufficiently small values of CL In CF,CFO, the torsional frequency vt in cm-l is very low and far removed from the frequencies of other vibrations in species A”. It is therefore reasonable to assume that the normal coordinate of this vibration approximates l/ca, inertia of the CF, and CFO groups. Hence
where I, is the reduced moment of
V(a) = 277Vv,2I&?= 9V&/4
(2)
and thus v, = 8?72c%,21,/9
[S]E.TANNENBAUM,R.D.JOHNSON,R.J.MYERS (1954).
~~~W.D.GWINN,
(3) J.Chem.Phya.22,949
[Q] R.E.NAYLOR and E.B. WILSON,JR., J.Chem. Phys.26, 1057 (1957). [IO] W. G.FATELEY and F. A. MILLER,Spectrochim. Acta 19,611 (1963).
Vibrational
spectrum and barrier to internal rotation for CF,CFO
The reduced moment I, is computed Iv
=
I,
from the expression
[
1 -I,
2
125
[ 111
c- 1
(4)
n=1,2,3 I R
where I, is the moment of inertia of CF, about its presumed symmetry axis, the C-C bond, 4, is the angle between this axis and the n’th principal axis of inertia, and I, is the n’th principal moment of CF,CFO. From the parameters assumed earlier, the value of I, is 296 a.m.u.A2. If equation (3) is applicable, the error in V, depends on the precision with which vt is measured and I, is calculated. The error in vt arises from error in measurement of the band maximum of a broad band recorded at a signal-to-noise ratio of about 20: 1. It is estimated as fl cm-r, or 2%. The errors in I, are more difficult to assess, since the geometrical parameters for CF,CFO have been transferred from other molecules. At best it appears that I, is accurate to 5%, and it may be considerably worse. We place the error somewhat arbitrarily at about 15% in IV,, which then leads to an estimate for V3 of about 486 f 75 cm-l or 1390 + 210 cal/ mol. The torsional force constant K, calculated from the relationship k, = 4rr2c2v,*IT is 4.29 x lo-l4 dyn-cm. Converted to more usual units by multiplication with the square of the distance of the F-atoms from the axis of rotation, one finds for k, = k,rCF2 sin2 0, where 0 is the CCF angle, the value O-0275 x lo5 dyn/cm. Barriers have been measured in related molecules as follows: CH,CFO, 1040 cal./mol [4]; CF,CHO, 950 cal./mol [12]. From these data it appears that replacement of CH, by CF, does not raise the barrier greatly, as has been found to be approximately true for molecules in which the barriers are considerably higher, e.g. CH,CF,, 3500 cal./mol vs. CH,CH, 2900 cal./mol. This suggests that the size of the barrier depends more on the nature of the C-C bond rather than on repulsion between non-bonded atoms. AcknowledgmentWe are indebted work (NSF grant G-19637).
to the National
Science Foundation
[ll] D. R. HERSCRBACH, J. Chem. Phys. 31, 91 (1959). [la] R. C. WOODS III, O.S.U. Symposium on Molecular Paper J-9 and private communication.
Structure
for support
and Spectroscopy,
of this
1964.
Vibrational spectrum and barrier to internal rotation for CF,CFO* KARL R. Loos? and R. C. LORD Spectroscopy Laboratory, Massachusetts Institute of Technology, Cambridge, Mass. 02139 (Received 10 April
1964)
Abstract-The
vibrational spectrum of CF,CFO has been investigated over the range 3900-35 cm-l with small grating spectrometers. All fundamental frequencies have been assigned to approximate vibrational modes and the torsional vibration haa been located at 50 cm-l. The principal moments of inertia of the molecule have been estimated from assumed bond angles and distances to be 130, 200, and 248 a.m.u.Aa, and the reduced moment of inertia,for the CF, group to be 29.5 a.m.u.A2. From these values the threefold barrier to internal rotation is calculated to be 1390 f 210 Cal/mole or 486 f 75 cm-l on the basis of a harmonic potential function. The harmonic force constant for torsion has been calculated &80.0275 x IO5dyn/cm. ALTHOUGH much work has been reported
in the literature on barriers to internal rotation in molecules, most of it has been concerned with methyl groups [l]. The present study of the far infrared spectrum of perfluoroacetyl fluoride, CF,CFO, was carried out as part of a program of investigation of barriers to rotation of the txifluoromethyl group. Since no previous vibrational study has been reported for perfluoroacetyl fluoride, its spectrum has also been investigated throughout the mid infrared region. EXPERIMENTAL
Perfluoroacetyl fluoride, a colorless gas (b.p. -60°C) was obtained from the Central Research Department, E. I. du Pont de Nemours and Company, through the courtesy of Dr. S. ANDREADES. The sample was used without further purification. The infrared spectrum of the gas was recorded from 3900 cm-l to 600 cm-l with a Perkin-Elmer Model 421 Dual Grating Spectrometer in the M.I.T. Spectroscopy Laboratory, and from 600 cm-l to 400 cm-l on a Perkin-Elmer Model 421 spectrometer equipped with a cesium bromide interchange at Tufts University through the courtesy of Professor M. K. WILSON. The far infrared spectrum from 400 cm-l to 35 cm-l was obtained with the far infrared spectrometer of LORD and MCCUBBIN [2], modified for vacuum operation as described by C. H. PERRY [3]. Absorption cells of 10 cm and 1 meter path length, equipped with KBr and polyethylene windows respectively, were used, with sample pressures up to 600 mm Hg.
* Based on the S.M. thesis of KARL R. Loos, Department of Chemistry, Massachusetts Institute of Technology, September, 1963. t Corning Glass Works Foundation Fellow, 1962-63. [l] A bibliography for hindered internal rotation covering work reported in the literature up to September, 1962, has been prepared by D. R. HERSCHBACH, University of California Radiation Laboratory report UCRL-10404. [2] R. C. LORD and T. K. MCCUBBIN, JR., J. Opt. Sot. Am. 47, 689 (1957). [3] C. H. PERRY, Quarterly Progress Report No. 70, Research Laboratory of Electronics, M.I.T., July 15 (1963). 119
120
KARL
R. Loos and R. C. LORD
Perfluoroacetyl fluoride is relatively inert and no special sample-handling techniques were required. All fundamentals reported are believed accurate to f 1 cm-l. Recordings of the spectra are shown in Figs. 1 and 2 and numerical results summarized in Tables 1 and 2.
%
Fig. 1. Mid infrared spectrum of CF,CFO as recorded by a Perkin-Elmer 421 Dual Grating Spectrophotometer (spectral slit width -2 cm-l).
Model
ASSIGNMENT OF FUNDAMENTALS Since no microwave or electron-diffraction studies of perfluoroacetyl fluoride have yet been published, the molecular parameters must be estimated. The CF, group has been assumed to orient with one CF bond coplanar with the CFO group and opposed to the CO bond. The assumed bond angles and bond distances in Angstroms for the resultant structure of C, symmetry were: CF distances in CF, group, 1.34; CF distance in CFO group, l-35 ; CC distance, 1.50 ; CO distance, 1.18; FCC angles (CF, group), 107”; CC0 angle, 128’; OCF angle, 122”. These estimates are based on the results obtained by PIERCE and KRISHER [4] from their microwave study of acetyl fluoride, together with data from other studies of molecules containing CF, groups [5]. The resultant moments of inertia in a.m.u.A2 are: I, (axis roughly parallel to CC bond in C, plane), 130; II, (axis roughly perpendicular to CC bond in C, plane), 206; I, (axis perpendicular to C, plane), 248. The vibrations of CF,CFO of the above structure divide into 10A’ and 5A”, A’ being the species of vibrations symmetric to the plane of symmetry and A” that antisymmetric to the plane. Since by symmetry the dipole moment of the A” modes vibrates parallel to the axis of greatest moment of inertia I,, these modes should produce infrared bands with type-C contours. On the other hand, the directions of the vibrating dipole moments in the A’ vibrations are not fixed by symmetry with respect to the principal axes of inertia and therefore the contours of the A’ L. PIERCE and L. C. KRISHER, J. Chem. Phys. 31, 875 (1959). [5] Tables of Interatomic Distances, Chemical Society (London) (1958).
‘41
I
600
I
550
I
500
I
450
I
400
I
350 Wavenumber
I
I
250 300 in cm-’ *
I
200
I
150
I
100
I 50
_
Fig. 2. Mid and far infrared spectrum of CF,CFO observed with the Perkin-Elmer Model 421 Dual Grating Spectrophotometer and a vacuum far infrared spectrometer. Bands below 450 cm-l are computed from single-beam data of the vacuum spectrometer. Spectral slit widths are indicated on the Figure.
0
0
122
R. Loos
KARL Table 1.
and R.
C. LORD
Infrared spectrum of perfluoroacetyl
fluoride
a in 1O-8 cm~/mole
Frequency
RT PI.
a=-log-i-
(cm-‘)
0.09
0.28 0.18 0.42 0.18 0.28 1.1 0.45 1.7 0.35 0.50 108.0 1.2 1.3 0.37 75.0 180.0 192.0 239.0 33.0 67.0 45.0 37.0 37.0 78.0 56.0 48.0 109.0 3.7 12.0 37.0 2.5 4.1 0.25
2980
2645 2575 2530 2480 2410 2280 2180 2120 2040 1895 1792 1780 1560 1339 1267 1206 1096 808 807 804 799 796 760 747 743 690 595 531 519 387 227 50
Species
A'
Aff
Fundamental
Assignment
type
2.8
3760 3220
Table 2.
Band
I,
A
A C? B
A? A?
A?
A
A?
c
2 x 1 = 3790 1 + 2 = 3234 1+ 4 = 2991 1 + 6 = 2655 1 + 7 = 2685 2 + 3 = 2546 1+ 8 = 2490 2 x 3 = 2412 1 + 13 = 2287 2 x 4 = 2192 1+ 14 = 2122 2 + 7 = 2029 1 11 + 12 = 1788 11 + 9 = 1776 6 + 6 = 1567 2 11 3 4 5 + 15 - 15 5 5+2x15-2216 5+3x15-3315 5+4x15-4415 6 7 + 15 - 15 7+2x15-2x15 7 8 12 9 13 10,14 15
vibrations of perfluoroacetyl
fluoride
Vibration number
Approximate description
Frequency (cm-‘)
1 2 3 4 5 6 7 8 9 10
CO stretching CF stretching CF, “degenerate” stretching CF, symmetric stretching CC stretching CF, symmetric deformation FCO deformation FCO rocking CF, “degenerate” deformation CF, wagging
1895 1339 1206 1096 807 760 690 595 519 227
11 12 13 14 15
CF, “degenert~te” stretching CF, “degenerate” deformation FCO wagging
1257 531 387 227 50
CF, wagging CF, torsion
Vibrational
spectrum and barrier to internal rotation for CF,CFO
123
bands will‘vary in type from one band to the next. One can expect band contours of type A, type B and intermediate shapes. Because the masses of the various atoms are comparable, considerable interaction of the atomic displacements is expected in all modes and the descriptions of the modes assigned below are only rough approximations. The carbonyl stretching frequency occurs at 1895 cm-l, a reasonable location in view of the highly electronegative fluorine attached to the carbonyl group. The frequency is found in acetyl fluoride at 1865 cm-l [6]. Of the four CF stretching frequencies, three are associated with the CF, group, an A’ stretching vibration symmetric to the local symmetry of the CF, group, and an A’ stretching and an A” stretching derived from the doubly degenerate stretching vibration of the CF, group. The fourth is the stretching of the CF adjacent to the carbonyl group and is of species A’. The band at 1339 cm-l is assigned to the latter mode as the value for this might be expected to be higher than the other CF The three very intense bands in the region stretchings because of its location. 1300 cm-l-1000 cm-l are associated with the CF, group. The band at 1206 cm-r has a type-B contour and is thus of species A’, while the bands at 1257 cm-l and 1096 cm-l have either type-A or type-C contours. These have been assigned respectively to the A’ “degenerate”, A” “degenerate” and A’ symmetric stretchings by analogy to the spectrum of CF,NO, [7] which is isoelectronic with CF,CFO. The band at 807 cm-l is assigned to the CC stretching mode. This vibration is probably coupled with the surrounding CF stretchings, and subject to considerable interaction with them. There are three CF, deformation vibrations, classified as for the CF stretchings with respect to the local symmetry of the CF, group: a symmetric deformation of species A’, and two modes derived from the CF, degenerate deformation which The band at 760 cm-l is assigned to the belong to species A’ and A” respectively. symmetric A’ deformation and the bands at 519 cm-l and 531 cm-l to the A’ and A” “degenerate” modes. This is in accord with the assignment in CF,NO, where the A, deformation occurs at 751 cm-l and the other two deformations are assumed to occur together at 529 cm-l. The two “degenerate” deformation frequencies in CF,CFO would also be expected to lie close to one another, but the distinction between the A’ and A” modes is not obvious and our assignment might well be interchanged. The FCO group undergoes three bending vibrations: an A’ deformation, an A’ rocking and an A” wagging mode. The FCO deformation is expected to occur at a higher frequency than the others and is assigned to the band at 690 cm-l. The rocking mode is assigned to the band at 595 cm-l, and the wagging mode to the band at 387 cm?. The CF, A’ and A” wagging vibrations are taken to be accidentally degenerate and are assigned to the broad band of uncertain contour at 227 cm-l. This assignment is consistent with the near degeneracy of the CF, deformation vibrations and is reasonable since the molecule is roughly a symmetric top. [S] B. P. Susz and J. J. WUHRMANN, He&v. Chim. Acta 40, 722 (1957). [7] J. MASON and J. DUNDERDALE, J. Chem. f&c. 759 (1956).
124
KARL R. Loos and R. C. LORD
The broad featureless band at 50 cm-l is assigned to the torsional vibration because this vibration is expected to have the lowest frequency and to occur in the range 50-150cm-l. THE
BARRIER
TO INTERNAL
ROTATION
The breadth and lack of structure in the band assigned to the torsional oscillation are readily understandable. Since many levels of this mode are significantly populated at room temperature, the transitions 1 c 0,2 c 1,3 c 2,etc. will have comparable intensities. These transitions presumably have slightly different frequencies and thus at the moderate resolution of our spectrometer (~1 cm-l) will not be resolved. The partial overlapping of the various bands may be expected to wash out the type-C rotational structure of the band contour and thus to leave a structureless band of the sort observed. From the observed frequency of the torsional oscillation and assumptions about the shape of the potential-energy curve, one can calculate the height of the potential barrier to rotation. For groups of threefold local symmetry oscillating with respect to an asymmetric framework, the potential energy, V(E), of torsional oscillation is usually written in the form: V(E) = V,(l - cos3a)/2+ V&l - cos6x)/2+ ...
(1)
where cc is the angle of torsion, V, a potential constant which is the barrier height if only the first term in (1)is significant, V, a second potential constant and so on. In some molecules, however, e.g., H&-NO, [S] and H&-BP, [9], the V, term vanishes by symmetry and V, can be evaluated directly. In these two cases it is found to be extremely small (of the order of 5 cm-l or less) compared to V3, which in molecules where it does not vanish by symmetry is often higher than 1000 cm-l. In acetyl fluoride [4],which is isoelectronic with H&?-NO, and H,C-BF,, V3 is about 1040 cm-l, and FATELEY and MILLER [lo] have shown that V6 is less than 3% of 8, for a number of molecules containing methyl groups. While it is certainly possible that VTe/V3 is larger for CF, groups than for CH,, we have no way of evaluating this ratio from our present data and therefore assume that VTsis negligible. If this is so and the value of V3 is large, the torsional vibration will be approximately harmonic. Under such circumstances the potential V(a) simplifies to 9V,cr2/4 for sufficiently small values of CL In CF,CFO, the torsional frequency vt in cm-l is very low and far removed from the frequencies of other vibrations in species A”. It is therefore reasonable to assume that the normal coordinate of this vibration approximates l/ca, inertia of the CF, and CFO groups. Hence
where I, is the reduced moment of
V(a) = 277Vv,2I&?= 9V&/4
(2)
and thus v, = 8?72c%,21,/9
[S]E.TANNENBAUM,R.D.JOHNSON,R.J.MYERS (1954).
~~~W.D.GWINN,
(3) J.Chem.Phya.22,949
[Q] R.E.NAYLOR and E.B. WILSON,JR., J.Chem. Phys.26, 1057 (1957). [IO] W. G.FATELEY and F. A. MILLER,Spectrochim. Acta 19,611 (1963).
Vibrational
spectrum and barrier to internal rotation for CF,CFO
The reduced moment I, is computed Iv
=
I,
from the expression
[
1 -I,
2
125
[ 111
c- 1
(4)
n=1,2,3 I R
where I, is the moment of inertia of CF, about its presumed symmetry axis, the C-C bond, 4, is the angle between this axis and the n’th principal axis of inertia, and I, is the n’th principal moment of CF,CFO. From the parameters assumed earlier, the value of I, is 296 a.m.u.A2. If equation (3) is applicable, the error in V, depends on the precision with which vt is measured and I, is calculated. The error in vt arises from error in measurement of the band maximum of a broad band recorded at a signal-to-noise ratio of about 20: 1. It is estimated as fl cm-r, or 2%. The errors in I, are more difficult to assess, since the geometrical parameters for CF,CFO have been transferred from other molecules. At best it appears that I, is accurate to 5%, and it may be considerably worse. We place the error somewhat arbitrarily at about 15% in IV,, which then leads to an estimate for V3 of about 486 f 75 cm-l or 1390 + 210 cal/ mol. The torsional force constant K, calculated from the relationship k, = 4rr2c2v,*IT is 4.29 x lo-l4 dyn-cm. Converted to more usual units by multiplication with the square of the distance of the F-atoms from the axis of rotation, one finds for k, = k,rCF2 sin2 0, where 0 is the CCF angle, the value O-0275 x lo5 dyn/cm. Barriers have been measured in related molecules as follows: CH,CFO, 1040 cal./mol [4]; CF,CHO, 950 cal./mol [12]. From these data it appears that replacement of CH, by CF, does not raise the barrier greatly, as has been found to be approximately true for molecules in which the barriers are considerably higher, e.g. CH,CF,, 3500 cal./mol vs. CH,CH, 2900 cal./mol. This suggests that the size of the barrier depends more on the nature of the C-C bond rather than on repulsion between non-bonded atoms. AcknowledgmentWe are indebted work (NSF grant G-19637).
to the National
Science Foundation
[ll] D. R. HERSCRBACH, J. Chem. Phys. 31, 91 (1959). [la] R. C. WOODS III, O.S.U. Symposium on Molecular Paper J-9 and private communication.
Structure
for support
and Spectroscopy,
of this
1964.