Rotational isomerism in 1,2-diphenyltetrafluoroethane

Journal of Molecular Structure, 238 (1990) 101-109 Elsevler Science Publishers B V , Amsterdam

101

ROTATIONAL ISOMERISM IN 1,2DIPHENYLTETRAFLUOROETHANE

Y L LAM, Y S CHONG and H H HUANG Chemistry Department, NatLonal UnLuersLtyof Srngapore, Lower Kent RLdgeRoad, Smgapore 0511 (Repubkc of SLngapore) (Received 4 December

1989)

ABSTRACT IR and Raman spectra of 1,2-dlphenyltetrafluoroethane (m the solid and solution states) are reported and assignment of frequencies made Comparison of the Raman and IR spectra m both the sohd and solution states indicates that the compound exists m the non-polar truns conformation m the solid state and as a rotamerlc mixture of the polar gauche and trans conformers m solution Analysis of the dipole moment and Kerr effect data shows that m benzene solution at 25”C, the mixture contains approximately 59% trans and 41% of the gauche rotamer These results are m reasonable agreement with those predicted by an MMP2 calculation (65% trans, 35% gauche)

INTRODUCTION

Dielectric studies showed that m carbon tetrachlorlde solution, 1,2-dlphenyltetrachloroethane consists of approximately 55% of the gauche rotamer and 45% of the tram based on a Lennard-Jones and Pike analysis Replacement of the four Cl atoms by four methyl groups increased the proportion of the gauche rotamer to about 67% [ 1 ] This paper presents the results of a dielectric and spectroscopic study on rotational lsomerlsm m 1,2-dlphenyltetrafluoroethane to investigate the effect of replacing the four Cl atoms m 1,2-dlphenyltetrachloroethane by the smaller and more electronegative fluorine atom We have also undertaken a semlemplrlcal molecular orbital (MO ) and molecular mechanics (MMPB) study of 1,2-dlphenyltetrafluoroethane to determme the relative energies and geometries of the gauche and truns rotamers The calculated results are compared with experimental estimates of the relatlve populations of the gauche and trams rotamers from dipole moment and Kerr effect measurements The dipole moment and Kerr constant of 1,2-dlphenyltetrafluoroethane as well as its IR and Raman spectra have not been reported before

0022-2860/90/$03

50

0 1990 -

Elsevler Science Publishers

BV

102

EXPERIMENTAL

1,2-Diphenyltetrafluoroethane was prepared by fluormation of 1,2-diphenyltetrachloroethane with antimony trifluoride according to the method of Johnson et al [ 21 1,2-Diphenyltetrachloroethane was obtained by free radical dimerization of benzyhdene chloride using di-tert-butyl peroxide as the untiator [3] All solvents were carefully distilled and stored over drying agents The Kerr constant was measured photometrically [ 41 while the dielectric constants were determmed with a heterodyne-beat meter [5] Densities and refractive indices were measured by standard procedures [ 61 Solid-state IR spectra were recorded as NUJO~ mulls and as KBr pressed-disc samples The solution-state spectra were obtained using solvents such as carbon tetrachloride, carbon disulphide, benzene, chloroform and acetomtrile A Perkm-Elmer 1710 IR spectrophotometer was used for all the IR measurements Solution-state Raman measurements were made using the 4880 A line while the sohd state spectrum was obtained with the 5145 a lme of a Spectraphysic argon-ion laser and a JESCO NRlOOO spectrometer Semiempirical molecular orbital calculations were performed usmg the program AMPAC [ 71 AM1 parametrization [ 81 was used and full geometry optimization was performed for each incremental value of the ethane C-C torsion angle. Torsion angles were defined by the atoms C,,-C-C-C,, m the diphenyl-substituted compounds by the convention of Klyne and Prelog [ 91 In the case of 1,2diphenyltetrafluoroethane, the angle which the phenyl rmg plane makes with the plane through the central carbon atoms C2’C,‘C1C2 was fixed at 85”19’ The structure of 1,2-diphenyltetrafluoroethane is given m Fig 1 Interatomic distances and bond angles used were taken from an X-ray diffraction study of this molecule [lo] Molecular mechanics calculations were performed using Alhnger’s MMP2 program [ 11,121 The CSp2-CSp3-C&,3--fluorme torsional parameters required for 1,2diphenyltetrafluoroethane were adopted from the values derived for fluorine substituted /3-carbonyl compounds [ 131 where the C,, is attached to an sp2 carbonyl oxygen instead of a C,z of the phenyl rmg

Fig 1 The structure of 1,2-dlphenyltetrafluoroethane

103 RESULTS AND DISCUSSION

The results of the dlelectrlc measurements are presented m Table 1 with standard notation. The IR and Raman spectra are given m detail elsewhere as Supplementary Data (available from B L L D. as Supplementary Pubhcatlon number SUP 26394 (3 pages) ) Spectroscopy 1,2-Dlphenyltetrafluoroethane can exist as the tram rotamer, the gauche rotamer or as a mixture of both. As the molecule 1s made up of 28 atoms, 78 fundamental modes are expected as given by 3N- 6 where N 1s the number of atoms The trans rotamer belongs to the Cab point group whereas the gauche rotamer belongs to the C, pomt group For restricted rotation of the phenyl rings, the fundamentals correspondmg to the vlbratlonal species are given m Table 2 while the correspondmg values for free rotation of the phenyl rings m the tram conformer are given m parentheses The fundamentals for thegauche TABLE 1 Molar polansatlons, refractions, dlphenyltetrafluoroethane Temp (“C)

Solvent

25 25

C,Hs C,H,,

Cone range

dipole moments

(YE

/3

and Kerr constant

y

6

0051 0015

21195 6220 82 1 226 76 6001

( 105w,) 530- 900 450-1015

3 22 0334 2 48 0094

P2 (cm”)

Rn (cm3)

at mfmlte

dllutlon

of 1,2-

103’pa (Cm)

10z7 (mKz) (m5V-*mol-‘)

893 877

328+ 17

“Calculated on the basis that ,,P= 1 05Ro TABLE 2 Number of fundamentals Vlbratlonal

Tram

species

A,

AU B, B” Gauche

A B

for 1,2-dlphenyltetrafluoroethane Actlvltyb

(restricted

rotation

of phenyl rings)”

No of fundamentals

IR

Raman

ia a la a a a

P :P X

P dp

Skeletal 6 4 3 5

10 8

“Values for free rotation of the phenyl rings m the tram conformer bAbbrevlatlons a, active, la, inactive, p, polarized, dp, depolarized,

Total

Phenyl 16 14 14 16 30 30

(20) (10) (10) (20)

are gwen m parentheses x, forbldden

22 18 17 21 40 38

(26) (14) (13) (25)

104

conformer remam unchanged However, there will be a certain amount of overlappmg and masking of bands The slmphclty of the Raman solid state spectrum is striking when compared with the IR solid spectra. Although there are several bands m the 3100-1000 cm-’ region which appear m both spectra, the rule of mutual exclusion for a centrosymmetrlc molecule holds for the 770-370 cm-’ region These two facts together with the appearance of certain extra bands m both the IR and Raman spectra m solution indicate that this compound exists as the tram rotamer m the solid but as a mixture of tram and gauche rotamers m the solution state Extra absorptlons occur at 1522 cm-l m CH,CN, CS, and CCll, 1279 cm-’ m all the solvents measured, at ca. 1250 cm-’ m some solvents, 1041 cm-’ m CHC&, CS, and CC14,772 cm-’ m CH,CN, 667 cm-l m CH,CN, CS, and CC14, and 305 cm-’ m CH&N, CS, and C&H, These extra bands are attributed to the gauche conformer The assignment of bands for this compound was facilitated by comparison with the spectra of 1,1,2,2_tetrafluoroethane [ 141, 1,2-dlphenyltetrachloroethane [ 15],1,2-dlphenylethane [ 161 and 1,2-dlcyanotetrafluoroethane [ 171 Characteristic phenyl C-H vibrational frequencies may be found m the following approximate frequency ranges 3110-3017 cm-’ for the C-H stretching mode, 1980-1620 cm- ’ for the overtones and combmatlon bands, and probably 1215-1003 cm-’ and 990-698 cm-’ for the respective m-plane and out-ofplane bending vibrations Absorptlons attributable to phenyl C-C stretching modes he m the region 1620-1371 cm-l while the m-plane and out-of-plane ring and C-C-C deformation gives rise to bands in the region 700-430 cm-’ The absorption at 2911 cm-’ is attributed to the central C-C stretching The deformation vibration of this group is however masked by the phenyl C-C stretching at 1451 cm-‘. The C-F stretching mode occurs at 1344 and 1089 cm-’ m the IR spectra and 1360 and 1080 cm-’ m the Raman spectrum of truns-1 f 192,2 3-tetrafluoroethane Gustavsen et al. [ 171 have attributed the IR absorptlons at 1258,1185 and 1166 cm-’ of 1,2-dlcyanotetrafluoroethane to C-F vibrations These stretching and bending modes are expected to be more sensitive to changes m the dihedral angle than the phenyl vibrational frequencies Thus the strong IR bands at 1258 and 1083 cm-’ could be assigned to the C-F stretching of the tram rotamer while the extra absorptlons at 1279 and 1041 cm-’ might be assigned to that of its gauche counterpart The Raman lines at 1280 and 1230 cm-’ might be the remammg two C-F stretch fundamentals for the tram form The strong absorption at 659cm-’ m the solid IR spectra may be attributed to the F-C-F deformation of the tram rotamer while the absorption due to its gauche counterparts is observed at 667 cm-l The bands at 564 and 470 cm-’ of the solid IR spectra are similarly assigned to the C-C-F and F-C-F bending modes of the trans rotamer The mtensltles of the bands due to the gauche conformer relative to the

105

nelghbourmg bands arlsmg from the truns conformer lation of the gauche conformers 1s appreciable

suggest that the popu-

D1pol.emoment and Kerr effect measurements The drpole moment of 1,2-dlphenyltetrafluoroethane was measured m cyclohexane and benzene while the Kerr constant measurement was made only m cyclohexane The molar Kerr constant (mK) of any conformation can be calculated as a function of the dihedral angle using the appropriate bond and group polarlsablhtles (Table 3 ) From dipole moment and Kerr constant measurements, two separate plots of the gauche population X% against dihedral angle can be obtained The point of mtersectlon of these two plots gives the X% and dihedral angle values which satisfy both the observed dipole moment and molar Kerr constant of the equlhbrmm mixture [ 181 Some comment on the polarlsablhtles of the C-F bond 1s necessary Le Fevre et al [ 191 have shown that for gemmal dlchlorldes and trrchlorldes, changes m bond polarlsablhtles occur Thus the bL, bT and bv values for the C-Cl bond of CH&!l are 3 562, 2 449 and 2.449 ( X 10e4’ Cm2 V-l) respectively, m contrast to 4.341,2 003 and 2 003 for those m chloroform. A similar effect may be expected m the case of the C-F bond for gemmally substituted fluorides To make a choice of C-F bond polarlsablhtles for use with 1,2-dlphenyltetrafluoroethane, we may consider the relatlonshlp between longltudmal bond polarlsabihty and bond length [20] The bond lengths for the molecules CHBF, CH2Fz and CF, are 1385,1358 and 1323 A respectively Since the C-F bond length of 1,2-dlphenyltetrafluoroethane 1s 1 374 A [lo], a value closest to that m CH3F, rt would appear more appropriate to use the C-F bond polarlsablhtles derived from CHBF For C-C6H5, two sets of group polarlsablhtles were used one derived from toluene and the other from benzotrlfluorrde to take account of the exaltation m bond polarlsablhty arising from the mesomerlc effects of the CF, group on the C,,-CFB bond The electric moment of benzotrlfluorlde [ 211 mstead of benzyl fluoride was used for the bond moment of the C-F group We have assumed the phenyl rings to be restricted with respect to rotation TABLE 3 AnisotropIc polansabdhes of bonds and groups expressed m units of lo-@ Cm2 V-’ Bond/Group

b,

h

bv

Ref

C-F c-c C-&H6 (from toluene) C-C,HS (from benzotnfluonde)

1391 1091 13 333 12 443

0 456 0 289 11775 12 332

0 456 0 289 7 991 7 880

22 23 24 25

106 TABLE 4 Calculated dipole moments and molar Kerr constants with correspondmggauche ulatlon X% for various conformations of 1,2-dlphenyltetrafluoroethane Parameter

X% (p=8 77X10v3’Cm) Based on toluene X% (mK=328X 10Wz7m5 V-’ mol-‘) Based on benzotrlfluorlde X% (mK=328X 10ez7 m5 V-’ mol-‘)

percentage

pop-

Dihedral angle 60”

70”

80”

90”

38 1

42 6

48 7

57 1

37 9

42 7

49 2

58 1

50 5

50 9

53 0

56 7

about the C&-C axis with the average orlentatron found m the crystal The results of these calculations are presented m Table 4 The relatrve populations of the two rotamers determined from the mtersectlon of the plots of the data m Table 4 were as follows: 0 59 m the truns form and 0 41 m the gauche form (based on the toluene C-C,, bond polarlsablhtres) and 0 44 m the trans form and 0 56 m the gauche form (based on benzotrlfluorrde C-C,, bond polarrsablhtles) The gauche torsion angle was estimated to be approxrmately 67 5 ’ and 90” respectively with correspondmg energy dlfferences (AE=E,-E,) of 2 21 kJ mol-’ and 112 kJ mol-’ The C,Z-CSpS-C,, -F torsional parameters required for MMP2 calculatrons were adopted from the values derived for 3-fluoro-2,2drmethylcyclohexane [ 131 ( V, = - 4 184, V, = 0.000 and V, = 0 837 kJ mol- ’ ) The varlatlon of heat of formation with the dihedral angle obtained from MMP2 calculations 1s given m Fig 2 The energy difference between the tram and gauche forms (LIE= Eg- E,) obtained was 3 3 kJ mol-’ and by use of the Boltzmann dlstrrbutlon, the relative populations of the two rotamers calculated would be 0 65 m the tram form and 0 35 m the gauche form The torsion angle of the gauche rotamer of lowest energy calculated was found to be 68” when the geometry was fully optlmlsed These values are m reasonable overall agreement with the results obtamed from toluene C-C,, bond polarlsablhtles truns form, 0 59, gauche form, 0 41, gauche torsion angle 68”, energy difference between the trans and gauche form, 2 21 kJ mol-’ The results from AM1 calculations are less satisfactory AE = - 10119 kJ mol-‘, glvmg 25% of the truns form and 75% of the gauche form although the dihedral angle remains at 68” Mlyagawa et al [ 261 have pointed out that the relatively hrgh proportion of gauche rotamer for 1,1,2,2-tetrachloroethane may be attributed to an increase m the gemmal Cl-C-Cl angle as a result of mutual repulsions between gemmal chlorine atoms [27,28] This would mean that the proJectron of this angle 1s larger than 120’” and would have the effect of srmultaneously stabihsmg the

107

1,2-DIPHENYLTETRAFLUOROETHANE

I I

0

-1

30

60

00

,

120

TORSIONAL

Fig

2

I

180

160

210

ANGLE

240

270

JO0

330

360

(DEGREES)

The varlatlon of heat of formatlon with the dihedral angle obtained from MMP2 calculations

Fig 3 Newman proJectlon of the trans conformer

of 1,1,2,2-tetrachloroethane

gauche conformer and making the trans form less stable The C-Cl azlmuthal angles will be greater than 60” m the gauche conformer and less than 60” m the tram conformer The effect of the trczns conformer 1s particularly pronounced as a given mcrease m the Cl-C-Cl angle ~111cause a double decrease of each azimuthal angle (Fig 3) The question now arlses as to the extent this effect will be present m 1,2dlphenyltetrafluoroethane

108

The FCF angle m CH,F, and CHF3 from microwave studies [ 27,281 is about 108”) slightly smaller than the tetrahedral angle of 109” 28’ This indicates that repulsions between fluorine atoms are weaker or smaller m magnitude than between chlorme atoms Moreover, the potential barrier of CF3-CF3 [ 291 which 1s 16 39 kJ mol-‘, 1s closer to that of 12.02 kJ mol-’ for CH,-CH, [30] than 45 14 kJ mol-’ for CC&-Ccl, [ 311 This suggests a fluorine atom 1s closer m behavlour to a hydrogen atom than a chlorine atom, m its sterlc mteractlons Thus it would appear that the stablhsmg effect on the gauche rotamer, due to an increase m the gemmal X-C-X angle, 1s less important m tetrafluoroethane and 1,2-dlphenyltetrafluoroethane, than tetrachloroethane Calculations show that if the C-F bond becomes less lsotroplc m going from a lower to a higher substituted halogen0 compound, then the calculated Kerr constants for 1,2-drphenyltetrafluoroethane for the various drhedral angles would increase The solution for the dihedral angle of the gauche form would be numerically smaller Hence, rf the polarlsablhty values for the second set of data with allowance made for the amsotropy of the C,,-CFB bond were used, a more plausible value of the dihedral angle could be obtained

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