An electron diffraction investigation of the structure of hexafluoropropene combined with infrared and microwave spectroscopy

Journal o f Molecular Structure, 53 (1979) 189--196 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

AN ELECTRON DIFFRACTION INVESTIGATION OF THE STRUCTURE OF HEXAFLUOROPROPENE COMBINED WITH INFRARED AND MICROWAVE SPECTROSCOPY

A. H. LOWREY, C. GEORGE, P. D'ANTONIO and J. K A R L E

Naval Research Laboratory, Washington, DC 20375 (U.S.A.) (Received 3 November 1978)

ABSTRACT Electron diffraction studies of gas-phase hexafluoropropene show that it exists at equilibrium in a planar form for all but two fluorine atoms of the CF 3 group. The equilibrium conformation of the CF 3 group has one fluorine atom in the molecular plane in the trans position with respect to the fluorine atom bonded to the central carbon atom. Diffraction experiments at different temperatures indicate that the data can be fit with a three-fold barrier to internal rotation for the CF 3 group with a value for the height of the barrier in the range 1.0--2.0 kcal tool -~ . The comparison theoretical molecular intensity calculations were based on the potential function hindering rotation and an average value for the contributions of the frame vibrations to the amplitudes associated with the rotational motion. The average of the C--F bond distances and the C=C distance is rg= 1.329 + 0.003 A and the C--C single bond distance is rg = 1.513 + 0.003 A. The corresponding vibrational amplitudes at r o o m temperature are < l i ] > '/= = 0.046 -+ 0.004 A and 0.052 + 0.006 A, respectively. The bond angles are LC=C--C = 127.8 -~ 0.7 ~, LF--C=C = 123.9 -+ 1.4 ~ for the CF~ group,/_C=C--F = 120.0 -+ 3 ° for the central fluorine atom and LC--C--F = 110.3 -+ 1.5 ° for the CF 3 group. The structure possesses the symmetry found by microwave and Raman spectroscopy and gives rotational constants that reproduce those obtained from the microwave study. The amplitudes of vibration are in good agreement with those predicted from a diagonal force field and normal coordinate analysis. Predictions from theoretical calculations of NMR coupling constants agree with the trans orientations found for the CF3 equilibrium position. INTRODUCTION An electron diffraction investigation of hexafluoropropene (HFP) affords the opportunity to perform a structure determination using the information obtained from infrared, Raman and microwave spectroscopy. Jacob and Lide [ 1 ] studied HFP with microwave spectroscopy, and concluded that the molecules must have a planar structure with Cs symmetry. Harvey and Nelson [2] investigated the polarization of Raman lines and came to similar conclusions. Ramey and Brey [3] studied HFP with NMR and discussed the effects of temperature change on coupling constants. They concluded that t h r o u g h s p a c e c o u p l i n g f a v o r s a s t r u c t u r e w i t h a trans r e l a t i o n s h i p b e t w e e n

190

fluorine atoms. Hirao et al. [4] predicted a structure for H F P with a fluorine atom in the CF3 group lying in the skeletal plane and trans to the fluorine atom b o n d e d to the central carbon atom. We report a structure that agrees with the rotational constants obtained from the microwave study, has the s y m m e t r y implied by the microwave and Raman study, exhibits the trans relationship predicted by N M R and agrees with the electron diffraction data obtained at several temperatures ranging from 223 K to 373 K. A principal feature of the structure is the inclusion of the large amplitude torsional motion of the CF3 group which exhibits a three-fold rotational barrier in the range 1.0--2.0 kcal m o l - ' . EXPERIMENTAL The sample of HFP was obtained from PCR Inc. Chromatographic analysis indicated that the purity was better than 99%. Continuous gas-stream photographs were made on Kodak electron image plates using the N R L rotating sector apparatus [5]. The sector was fabricated from 5 mil. copper by Buckbee Meets Co. using a photoetching technique. The sector opening was designed to level the scattering from argon. Photographs were taken at a variety of specimen-to-plate distances {10--17 cm) for each of three sample temperatures, 223 K, 293 K and 373 K. The accelerating voltage was continuously monitored with a calibrated resistance bank and was recorded for each photograph. A comparison wavelength calibration was performed using the scattering from CS2 and CC14. The photographs were spun at approximately 200 rpm and analyzed with a recording microdensitometer [6]. The photographic density-to-intensity conversion was made using the procedure described by Foster [7]. Analysis

For each sample temperature, a leveled intensity function was created by combining the intensity data for each scattering range [ 8]. A computer-drawn background line based on the criteria of smoothness and the non-negativity of the resulting radial distribution function obtained from a constant coefficient molecular intensity function was subtracted from the leveled intensity function [9]. The leveled intensity data, the background lines and the resulting molecular intensity functions for the different data sets are given in the supplementary material [10]. The theoretical molecular intensity functions were c o m p u t e d by use of the variable atomic scattering factors given by Cox and Bonham [11] and Bonham and Schaefer [12]. RESULTS The electron diffraction patterns show a distinct change as a function of temperature. The molecular intensity curves for the three different

191

temperatures were put on the same scale before subtraction by making their outer regions equal. This scaling is based on the fact that the vibrational amplitudes of the bonded distances, which predominate in the outer regions of s, are affected relatively little by the range of temperature covered by the experiments. These scaled data sets were subtracted from each other and are shown in Fig. 1. As discussed below, it is possible to account for these differences with a model that incorpora~s the large amplitude torsional motion of the CF3 group. The difference curves between the experimental data and the model molecular intensity function, calculated for the corresponding temperature, are given in Figs. 2 and 3. The experimentally observed differences between the scattering at different temperatures are greater than the differences between the experimental and model intensity functions for the corresponding temperatures. The interpretation of the electron diffraction data is based on the procedure previously described for including constraints based on such information as spectroscopically determined s y m m e t r y [13]. In this case, the procedure was extended to include the molecular rotation constants determined from the microwave structure. To relate the microwave information to the electron diffraction experiment, approximate corrections can be made based on a harmonic force field [14]. A diagonal force field for this molecule gives a reasonable representation of the spectroscopically observed vibrational frequencies. The harmonic corrections for the moments of inertia derived from this force field are small, 0.02%, 0.05% and 0.2% f o r A . B, and C, respectively. Such corrections affect the reported structural model only well within the range of the reported uncertainties. The same rotational constants are characteristic of two different planar moieties for HFP, one with a trans arrangement and one with a cis arrangement for a fluorine atom of the CF3 group with respect to the fluorine atom

Normol residuol

293K minus 373 K

293 K minus 223 K

373 K minus 22.3 K

o --4-

1 21'6 2'o 2'4 2'8 3'2 $

4'o

Fig. 1. Differences between experimental molecular intensity curves obtained at different temperatures.

192

rD(r~

sire(s) 293K

--

295K 293K ~ - - - - ~

373 K 223K

~ ~ 112 115 2'~; 2'4 2'~3312 3'5 4'0 $

@

4

5

Fig. 2. Experimental molecular intensity curve and differences between experimental and theoretical curves for various sample temperatures. Theoretical curves were computed with a potential barrier of 1.5 kcal tool -1 . Fig. 3. Experimental radial distribution curve and transform of molecular intensity differences in Fig. 2 for various sample temperatures. bonded to the central carbon atom. The positions of the peaks at large distances in the radial distribution function affirm that the trans form is the predominant conformation. However, it is n o t possible to describe adequately the electron diffraction data w i t h o u t taking into account the large amplitude torsional m o t i o n of the CF3 group. The equations for including restricted rotation in the electron diffraction analysis are given by Karle [15, 16]. For these calculations, HFP was considered to consist of a single rotor and a planar skeleton. In this approximation, there are 12 interatomic distances affected by the torsional motion. The contribution of these interatomic distances to.. the molecular intensity function can be calculated by numerical integration [ I 7 ] . The contribution of the frame vibrations for each of these distmaces was assumed to be 0.1 A, as indicated by theoretical calculations. Figure 4 illustrates the effect of different barrier heights for a three-fold potential. A value of 1.5 kcal mol-1 gives t h e best difference curve for the radial ¢tistribution function and the m i n i m u m residual for the molecular intensity data. Figures 5 a n d 6 illustrate the transform of the differences between molecuta~ intensity functions, both for the experimental data illustrated in Fig. 1, and for theoretical models calculated with a 1.5-kcal mo1-1 barrier at the experimental temperatures. These figures demonstrate that the magnitude of the torsional effects are much greater than experimental error and t h a t the assumptions used for the theoretical calculations adequately describe the torsional motion. The structural parameters for HFP are given in Table 1 and coordinates for a model in the trans position are given in Table 2 along with the corresponding m o m e n t s of inertia. This model is illustrated in Fig. 7.

193

I000 cal mole -= 1500 col mole -= 2 0 0 0 col mole -I

Fig. 4. Experimental radial distribution curve and differences with theoretical curves computed for various barrier heights.

•a ( • )

rD(r)

Normal residual

293 K minus 373 K

293 K minus 373 K

293 K minus 223 K

293 K minus223 K

373 K minus 22.3 K

373 K minus 2.23K

I '

'

'

'

f

Fig. 5. Fourier transform of differences between experimental molecular intensity curves shown in Fig. 1. Fig. 6. Fourier transform of differences between theoretical molecular intensity curves computed with a potential barrier of 1.5 kcal t o o l - ' . Compare with Fig. 5. VIBRATIONAL ANALYSIS

An analysis o f the infrared and Raman spectra of h e x a f l u o r o p r o p e n e was published in 1952 by Neilson et al. [ 1 8 ] . T h e y r e p o r t e d 17 distinct fundamental frequencies and two overlapped peaks. Many o f the assignments are uncertain and difficult to check because of the low s y m m e t r y o f the molecule. In particular, the r e p o r t e d f u n d a m e n t a l at 609 c m - ' is labeled uncertain and very weak. I f this f unda m ent al is ignored, it is possible to match the frequencies to those c o m p u t e d f r om a diagonal force field with an average deviation o f 11 cm -1 [ 1 9 ] . The force field predicts a torsional f r e q u e n c y of 38 cm -j , a region n o t covered by the 1952 study. Jacob and Lide [1]

194

Fig. 7. Molecular m o d e l and structural parameters (shaded portions are b e l o w the molecular plane). TABLE 1 Structural parameters for hexafluoropropene determined for sample temperature of 293 K a

Parameter rg (CffiC and C--F)

rg (C--C) LC=C--C LF--C=C LCffiC--F LC--C--F < 1~j>1/9 2

1/2

< IU>

1.329 ± 0.003 A 1.513 ± 0.003 A 127.8 ± 0.7 ° 123.9 ± 1.4° (CF~ group) 120.0 *- 5.0 ° (central fluorine) 110.3 ± 1.5 ° (CF3 group) 0.046 ± 0.004 A (CfC and C--F) 0.052 ± 0.006 A (C-C)

aThe uncertainties are limits of error based on satisfying both the electron diffraction data and the rotational constants.

p r e d i c t e d a t o r s i o n a l f r e q u e n c y o f 3 0 ± 15 c m -l. L o o s a n d L o r d [ 2 0 ] r e p o r t e d a t o r s i o n a l f r e q u e n c y o f 50 c m -1 f o r t r i f l u o r o a c e t y l fluoride, a m o l e c u l e w i t h s o m e w h a t similar s t r u c t u r a l a r r a n g e m e n t s in t h e vicinity o f t h e CF3 g r o u p . I t is d i f f i c u l t t o evaluate t h e significance o f t h e f o r c e field because o f t h e small a m o u n t o f e x p e r i m e n t a l d a t a t h a t is available. H o w e v e r , v i b r a t i o n a l

195 TABLE 2

Hexafluoropropene Principal coordinates a c --0.24265 c --1.41429 F --1.54925 F --2.58550 F --0.19827 C 1.13750 F 1.04666 F 1.80739 F 1.80739 Moments ofinertia(amu As ) A 197.58

b

c

0.48181 --0.14347 --1.46286 0.47884 1.81446 --0.13408 --1.45630 0.24837 0.24837

0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 1.07774 --1.07774

B 402.69

C 512.00

a m p l i t u d e s derived f r o m f o r c e field calculations, particularly t h o s e f o r b o n d e d distances, axe o f t e n n o t very sensitive t o small details in t h e calculation. Table 3 s h o w s t h e c a l c u l a t e d a n d observed vibrational a m p l i t u d e s associated with the d i f f e r e n t sample t e m p e r a t u r e s . As e x p e c t e d , t h e y s h o w a general increase with increasing sample t e m p e r a t u r e . The c a l c u l a t e d vibrational a m p l i t u d e s are in g o o d a g r e e m e n t with t h o s e observed in the e x p e r i m e n t s . If the torsional c o n t r i b u t i o n t o the vibrational analysis is restricted b y an artificially large f o r c e c o n s t a n t , an e s t i m a t e o f the f r a m e c o n t r i b u t i o n s t o the vibrational a m p l i t u d e s a f f e c t e d b y t o r s i o n a l m o t i o n m a y be o b t a i n e d . A n average value o f these e s t i m a t e d f r a m e c o n t r i b u t i o n s is in g o o d a g r e e m e n t with the 0.1 A a s s u m e d f o r the torsional c o n t r i b u t i o n s t o the m o l e c u l a r i n t e n s i t y f u n c t i o n . The t o r s i o n a l f r e q u e n c y o f 38 c m -~ p r e d i c t e d b y the f o r c e field is in a p p r o x i m a t e a g r e e m e n t with a three-fold r o t a t i o n a l barrier o f 1.5 kcal tool -1 [ 2 1 ] . TABLE 3 Comparison of calculated a and observed vibrational amplitudes in angstroms Temp. (K)

223 293 373

< I~j>~/~ C--F and C=C

t/2 C--C

Obs.

Calc.

Obs.

Calc.

0.044 0.046 0.047

0.045 0.046 0.047

0.046 0.052 0.055

0.052 0.053 0.055

aFor the combined distance vibrational amplitude, the calculated value is the square root of the mean of the squared amplitudes calculated for the different distances.

196 SUMMARY T h e e l e c t r o n d i f f r a c t i o n investigation o f h e x a f l u o r o p r o p e n e r e c o n f i r m s t h a t it exists in a f o r m t h a t is largely planar with C s s y m m e t r y . A p r e d o m i n a n t f e a t u r e o f the s t r u c t u r e is the torsional m o t i o n o f the CF3 g r o u p a b o u t a t r a n s e q u i l i b r i u m p o s i t i o n for a fluorine a t o m o f this g r o u p with r e s p e c t to the fluorine a t o m o n the central c a r b o n a t o m . T h e d i f f r a c t i o n d a t a are c o n s i s t e n t with a t h r e e - f o l d barrier t o internal r o t a t i o n with a barrier height in the range 1 . 0 - - 2 . 0 kcal m o l -~ . T h e m u l t i p l i c i t y o f t h e p o t e n t i a l f u n c t i o n c a n n o t be u n i q u e l y d e t e r m i n e d b y the d i f f r a c t i o n data, h o w e v e r , n o r m a l c o o r d i n a t e analysis with a diagonal f o r c e field gives calculated vibrational a m p l i t u d e s in g o o d a g r e e m e n t with t h o s e observed in the e l e c t r o n d i f f r a c t i o n e x p e r i m e n t . The final s t r u c t u r e r e p r o d u c e s the r o t a t i o n a l c o n s t a n t s observed f r o m the m i c r o w a v e e x p e r i m e n t [ 1 ] . ACKNOWLEDGEMENTS We a p p r e c i a t e discussions with Drs. Stanley A b r a m o w i t z , K e n n e t h Hedberg, L a w r e n c e H e n c h e r and Dennis K o h l c o n c e r n i n g the n o r m a l c o o r d i n a t e analysis and with Dr. David Lide c o n c e r n i n g his m i c r o w a v e results. REFERENCES 1 2 3 4 5 6 7 8 9

E. J. Jacob and D. R. Lide, Jr., J. Chem. Phys., 59 (1973) 5877. A. B. Harvey and L. Y. Nelson, J. Chem. Phys., 55 {1971) 414S. K. C. R a m e y and W. S. Brey, J. Chem. Phys., 4 ~ (1964) 2349. K. Hirao, H. Nakatsuji, H. Kato and T. Yonezawa, J. A m . Chem. Soc., 94 (1972) 4078; K. Hirao, H. Nakatsuji and H. Kato, J. A m . Chem. Soc., 95 (1973) 31. P. D'Antonio, C. George, A. H. Lowrey and J. Karle, J. Chem. Phys., 55 (1971) 1071. A. H. Lowrey, P. D'Antonio and C. George, J. Chem. Phys., 64 (1976) 2884. H. R. Foster, J. Appl. Phys., 41 (1970) 5355. P. D'Antonio, P. Moore, J. H. Konnert and J. Karle, Trans. A m . Crystallogr. Assoc., 13 (1977) 43. A. H. Lowrey, C. George, P. D'Antonio and J. Karle, J. A m . Chem. Soc., 93

(1971) 6399. 10 The supplementary material has been deposited with the British Library Lending Division as Supplementary Publication Number SUP 26118 (16 pages). 11 H. L. Cox, Jr. and R. A. Bonham, J. Chem. Phys., 64 (1967) 2599. 12 R. A. Bonham and L. Schaefer, International Tables for X-ray Crystallography, Vol. IV, Kynoch Press, Birmingham, England, 1974, p. 176. 13 A. H. Lowrey, P. D'Antonio, C. George and J. Karle, J. Chem. Phys., 58 (1973) 2840. 14 K. Kuchitsu and S. J. Cyvin, in S. J. Cyvin (Ed.), Molecular Structures and Vibrations, Elsevier, Amsterdam, 1972, Ch. 12. 15 J. Karle, J. Chem. Phys., 22 (1954) 1246. 16 J. Karle, J. Chem. Phys., 45 (1966) 4149. 17 The numerical integration was accomplished with a rapid Gaussian Quadrature Routine written by Dr. Daniel Jones. 18 J. R. Nielsen, R. H. Claassen and D. C. Smith, J. Chem. Phys.,.20 (1952) 1916. 19 The authors are indebted to Dr. Dennis Kohl for a copy of his adaptation of the normal coordinate analysis program originally written by Dr. Richard Hilderbrandt. 20 K. R. Loos and R. C.' Lord, Spectrochim. Acta, 21 (1965) 119. 21 J. R. Durig, S. M. Craven and W. C. Harris, in J. R. Durig (Ed.), Vibrational Spectra and Structure, Vol. 1, Marcel Dekker, New York, 1972, Ch. 4.