Infrared and Raman spectra, conformational stability, and vibrational assignment of divinylfluoroborane

Journal of hfolecular Structure, 112 (1984) 19-30 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

INFRARED AND RAMAN SPECTRA, CONFORhiATIONAL STABILITY, AND VIBRATIONAL ASSIGNMENT OF DMNYLFLUOROBORANE

J. D. ODOM, J. A. SMOOTER Department

of Chemisfry,

SMITH and J. R. DURIG

University

of South

Carolina,

Columbia,

SC 29208

(U.S.A.)

E. J. STAMPF Chemistry

Department,

Lander

College,

Greswood,

SC 29646

(U.S.A.)

(Received 3 May 1983)

ABSTRACT The infrared spectra of gaseous (3500 to 450 cm-‘) and solid (3500 to 50 cm-‘) divinylfluoroborane, (C,H,),BF, and the Raman spectra (3500 to 20 cm-‘) of the liquid and solid have been recorded. All of the normal modes have been assigned, based on band contours, depolarization values and group frequencies- From a comparison of the spectra in the fluid and solid states, it is concluded that the molecule exists in at least two conformations in the fluid phase but mainly as one conformer of C, symmetry in the solid state. The failure to obtain a “good” crystalline sample in the solid state is believed to be due to a very small enthalpy difference between the conformers. The results are compared to the corresponding quantities in several similar molecules. INTRODUCTION

In the past few years, spectroscopic evidence has clearly established that, in tricoordinate organoboron compounds containing an unsaturated organic moiety, there is some interaction between the emptyp orbital on the boron atom and the Ir-system of the organic group [l] _ Spectroscopic techniques which have been used to investigate this phenomenon have included infrared [Z-14], F&man [5, ‘i-141, microwave [7, 8, 12, 151, photoelectron [16] and nuclear magnetic resonance [19-12, 17-241. In addition, these mole cules have attracted significant theoretical interest 125-321. The vinylboranes in particular have proved amenable to theoretical study and a comparison of calculated versus experimental geometries and bonding in this class of molecules has been useful. The divinyl systems appear to have been the least thoroughly studied, especially with regard to the conformation and the barrier to internal rotation about the boron-zarbon bond. As a continuation of our studies of vinylboranes, we have investigated the infrared and Raman spectra of divinylfiuoroborane, (C&H&BF, in order to determine the stable conformation and the barrier to rotation about the boron-vinyl carbon bond. Since limited theoretical studies [26, 291 have been reported for this molecule, we were not only interested in the effect 0022-2860/84/$03.00

o 1984 Elsevier Science Publishers B.V.

of the fluohne atom on the conformation and barrier compared to the corresponding divinylme~hylborane [ll] molecule, but we also wished to compare the experimental results with the theoretical prtictions. The results of this study are reported herein. EXPERIMENTAL

Ali preparative work was carried out in a standard high vacuum system

exnployinggreaselessstopcocks. Divinylfluoroborane was obtained from the

reaction of (C&EI,),Bcl with SbF3 (Alfa) which had previously been “activated” by heating SbFs in the reaction apparatus with a cold flame while pumping. The (&H&BCl was prepared by the reaction of BCl, (Matheson) with (&H,),Sn (Columbia Organic Chemicals, Inc.) and the (C2H&BCl was separated tirn the (C2H3)BC12 by distillation on a variable-temperature vacuum-fractionation column. The purity of the (&H&BF sample was determined by the in&ared [2] and llB NMR spectra 1171. The Raman spectra shown in Fig. 1 were recorded on a Gary model 82

1

Wmenunber

(cm-‘)

Fig. 1. Raman spectra of liquid (-45°C) divin~lfluoroborane.

(upper trace) and giassy (-167%)

(lower trace)

21

spectrophotometer using a Spectra-Physics model 171 argon ion laser operating on the 5145 A line. The monochromator was calibrated against mercury emission Iines so that frequencies of sharp resolvable lines are expected to be accurate to a2 cm-‘. The spectrum of the liquid was recorded from the sampie seaIed in a glass capillary contained in a jacket similar to the one described by Miller and Hamey [33]. The temperature of the sample was monitored with an iron-con&r&an thermocouple referenced to O’C. Depolarization measurements for the liquid were made using the standard Gary accessories. The spectrum of the glass was obtained by condensing the sample onto a blackened copper block maintained at -77 K by boiling liquid nitrogen (Fig. 2). The mid-infrared spectra of divinylfluoroborane shown in Fig. 3 were obtained using a Digilab model FTS14C interferometer equipped with a Ge beamsplitter on a KBr substrate and a Globar source. The sample was conmined in a 12 cm cell equipped with CsI windows. The spectrum of the solid was obtained by condensing the sample onto a CsI substrate cooled by boiling liquid nitrogen. The far-infrared spectrum shown in Pig. 4 was

-97

!

Wovenumber

Fig. 2._Raman wavenumbers.

(cm-‘)

spectra of

divinylfluoroborane

at various temperatures.

A&c&a

is in

500 Wovenumber

(cm-‘)

Fig. 3. Mid-infrared borane.

spectra

Fig. 4. Far-infrared

spectrum

of gaseous

300

Wovenumber

(upper

trace) and solid

(lower

100

(cm-‘)

trace) divinylfluoro-

of solid divinplfluoroborane.

recorded on a Digilab model ITIS-15B intxzferometer equipped with a mercury arc larrp source and a 6.25 p Mylar beamsplitter. The spectrum of the solid was obtained by condensing the sample anti a silicon substrate maintained at -77 K. Both the mid-infrayed and f&r-Srared spectra were obtained at a resolution of 2 cm-‘. RESULTS

A comparison of the Raman spectra of the liquid as the temperature is lowered (see Fig. 2) shows several weak lines disappearing and, by the time the sample become; a glass, these lines are no longer visible in the spectrum. For example, the weak pronounced lines at 681 and 596 cm-’ are essentially absent in the spectrum at -97X, and the broad lines around 460 and 400 cm-‘, which make the identification of the center of the lines at 480 and 375 cm-’ quite difficult in the spectrum recorded at -45%, are greatly

23

reduced in intensity by -97°C so the band centers can now be easily identified. We believe these additional lines are due to the presence of one or more conformers which are less stable than the conformer which remains in the glassy sample. These lines which disappear are in the skeletal stretching and bending region of the spectrum and it is just such modes which should be sensitive to the presence of additional conformers. For example, the 681 cm-’ line could be due to the B-C stretch of a second conformer and certainly the 460 and 400 cm-’ lines are skeletal bending modes. Additional evidence for the presence of more than one conformer in the fluid phases was found in the measured depolarization ratios of several of the Raman lines. For example, the Raman lines at 178, 375, 51@, 988, and 1019 cm-’ have depolarization ratios in the range 0.5 to 0.65 for the measurements taken on the spectrum recorded at -45°C but these same lines have depolarization ratios of 0.75 for the spectrum recorded at -97°C. Therefore it is very probable that the second conformer has only the trivial symmetry C1 which would give rise to all bands being polarized. In fact, if we had not recorded the Raman spectrum of the liquid below -45”C, we would have concluded that the molecule is non-planar based on the depolarization ratios. It is probabie that the second conformer has one or both of the vinyl groups rotated o-ct of the plane which gives rise to a molecule of C, symmetry. Since it was not possible to obtain a Raman spectrum of a good crystalline sample, it is probable that very smalI enthalpy differences esist between the predominant conformer and any additional conformers. Repeated annealing of the sample resulted in no further sharpening of the Raman lines and no evidence for discrete lattice modes could be found in the low-tiequency Raman spectrum. It is clear from the Raman spectrum of the low-temperature liquid that the predominant conformer has at least a plane of symmetry baaed on the number of depolarized Raman lines. Three possible planar conformers are illustrated in Fig. 5. Calculations of the nonbonded distances between the

Fig. 5. Illustrations of some possible planar conformers of divinylfluoroborane. Conformer C has previously been referred to as “seagull” and conformer B as “swastika”.

various hydrogens were carried out and the structure A in Fig. 5 has a distance between the hydrogen atoms which is considerably less than the van der Waals radius for hydrogen, so this structure is very improbable. The other two structures do not contain any unrealistic distances and structure C has Ca,, symmetry whereas structure B has C, symmetry. For the structure vnth CzV symmetry the normal vibrations will span the irreducible representations, llA1 -t- 4& -I- lOBI + 5&, where only the Al modes will give rise to polarized Raman lines. For the structure with C, symmetry the 30 normal modes span the representations, 21A’ + 9A”, where now only nine depolarized Raman lines are expected. Since the low-temperature Raman data appear to be much more consistent with the C, structure, we have interpreted the vibrational data on the basis of structure B, although the presence of a second conformer of C1 symmetry even at low temperatures could have led to more polarized bands than expected for the conformer of GV symmetry. VIBRATIONAL

ASSIGNMENT

Based on reasonable structural parameters transferred from vinyldXluoroborane 183, the P-R separations for the A, 23, and C infrared band contours were calculated to be 14, 11.8 and 19.3 cm-‘, respectiveIy, for the C, conformer. Both the a and b axes lie in the plane of the molecule with the a axis being parallel to the two C=C bonds of the vinyl group whereas the B-F bond lies more along the B axis. Thus, the out-of-plane modes should give rise to C type bands whereas the in-plane modes will give rise to A, B or A/B hybrid bands. Therefore the normal modes were assigned on the basis of Reman depolarization values, infmred band contours and group frequencies.

Ccrbon-h

ydrogen

modes

Eighteen of the normal modes of divinylfluoroborane are carbon-hydrogen modes of the two vinyf groups and these modes, for the most part, have reasonably well characterized group frequencies. For the carbon-hydrogen stretches, the CH2 antisymmetric modes are assigned to the two highest fcequencies (3075, 3069 cm-‘) in the infrared spectrum of the solid whereas the two C-H stretches are assigned to the two lowest frequencies in this region (2990, 2966 cm-l ). The two CH, symmetric stretches are assigned at intermediate frequencies of 3054 and 3008 cm-‘. The two CH2 deformations appear as separate bands in the Raman spectra at 1424 and 1412 cm-’ but as a single band in the infrared speckum of the gas. We have assigned the two C-H in-plane bends at 1306 and 1267 cm-’ and the two CH? wags at 1246 and 1180 cm- ‘. It is possible that the two C-H bends are at 1246 and 1267 cm-l which appear as a single B-type band in the spectrum of the gas but this assignment would require a very large separation between the two CH2 wags. Therefore we prefer the assiggent as given in Table 1.

25 TABLE 1 Observed” infrared and F&man frequencies (cm*) fluoroborane

and vibrational assignment for divinyl-

Anigament

Raman Rd.

Rel.

int.

ReL int. liquid and depol.

(-160°) glass

iat.

3208 3197

w VJ

3208 3197

vw.p w.p

3197

w

3075

s

3074

s.dp

3077

m

3069

s

solid

id.

3086 R 3078Q.A 3072 P 3058 3OlOctr

R”

2986 2978ctr.. 2970 P 1974

1958 1631 R 16268,A 1621 P

s

1262 R 1256 ctz,B 1250 P 1190

vi

APP~

rimate

dexription

v, +v, =3214 2v, =3204 VI. v2

CHI

antisnnm.

symm.stretch

stretches

w.sh3054 3008 m 2990

s m m

u3

3004 2995

sh.w 'c% P

2996

s

V4 v5

CH, symm.stretch CHstretch

s

2966

s

2969

S.P

2968

m

VA

CHstretch

w

w

1974 1959

w v7

vs

1615

vg

1614

vw.p

1620

c=c stretch

1602 1585 1433 1423

s w vs vs

1602 1585 1424

vs,p w.p S.P

1602 1584 1424

C-C stretch “C=C stretch

1415

sh.p

1412 14043

2%. -2s

CH,

=1980 =1954

def.

s

1394 1382 I.350 1320

m m w s

1322

w.p

1322

w

bands ‘OB-F

s s

1306 1301

s 9

1306 1301

m,dp? m.p

1307 1298

m

C-H

m

"B-Fstret&

1267

s

1266

vz.p

1266

W

C-Hbend

5

1246

M

1247

W.P

1243

W

cq

wag

s

1180

s

1183

vw.P

1185

w

CH,

wag

vs

1152

vs

1155

W,P

1157

w

BC2 antisymm.

stretch

1088

VW

conformer

impurity

1018

W

overtones or combination stretch bend

R

1183Q.A 1159 R 1152Q 1096 R 10918 1083 P 1023 R 1018 Q.C 1008Q.C 988 R 982

ReL

CH,

1438 R 14328. A/B1425 P

13398 1314 R 13108 1306ctT 1300 P

(450)

8

975P 9498 755 R 7488 742 P

s

vs m vs

1019 1009 996 990 977

vs s 5 vs y5

759 726

* vs

702

s

1019

w.dp

CH,

II or

twist

CH,twi.st 988

w.dp

986

w

CH,mcks

760 739 724 703

VW

C-Hout-of-planebends

w

s

532 488

VW rw

726 704 681 596 510 -480

w.p m.p vw.p vw.p br.vr.dp? bGW.P

510

VW

w

m

'%C,sY~.sM.ch "BC, sYmm.stcet&

COnfOIIQerII confolmer II FBCbend CCB bend

26 TABLE

1 (continued)

Rel. illt.

Solid Rel. int. 438

w

366 251 187 115

w W w w

(-454

Rel. int.

liquid anddepol.

-460 442 -400 375 265 178 118

W.P

m.p

:;.‘w. m.p

=-ii,dp w. dp

dp

(-1604

Rel.

glass

kit.

441

m

370 266 192 120

br. w vZ3 m V11 W VIP W

vi

conformer

V10

*30

description

Approximate

II

CCB bend conformer II FBC, out-f-plane Be2 SCinOR vmyl

torsion

vinyl

torsion

bend

aAbbreviations used: s, strong; m, medium; w, weak; v, very; br, broad; sb, shoulder; p, polarized; dp, depolarized; P, Q, R, vibrational-rotational branches; A, 33, C, type of band contour.

The remaining six carbon-hydrogen modes are out-of-plane bends and they are usually character&d as the two CH2 twists, two CHI rocks and the two C-H out-of-plane bends although the out-of-plane modes of the three hydrogens on monosubstituted vinyl compounds are stror,gly coupled. The twist and rocks have well-defined group frequencies in the 1020 to 900 cm-’ region and give rise to strong inked bands. The infrared spectrum of divinylfiuoroborane has five strong bands in the infrared spectrum of the solid but only three in the spectrum of the gas. We have assigned the twists and rocks to the 1019, 1009, 990, and 977 cm-’ infrared bands but only two Raman bands are observed in this region where both are definitely depolarized. The C-H out-of-plane bends are usually found in the range 750 to 580 cm+ and we have chosen the two infrared bands at 759 and 726 cm-’ for these two modes. We believe the Raman line at 726 cm-’ is different from the infrared band at the same frequency because it is polar&d and therefore could not be due to the C-H out-of-plane bending mode. Certainly the 726 cm-’ infrared band is too strong for the “BC2 stretch and there are two very weak Raman lines at higher frequencies, 760 and 739 cm-‘, wh;ch probably correspond to the v26 and ~27 fundamentals. Skeletal

modes

There are twelve skeletal modes of which only three are out-of-plane motions and bending modes. The five skeletal stretching modes are, of course, all in-plane motions and can be readily assigned. The two C=C stretches are assigned at 1615 and 1602 cm-’ and the B-F stretch is assigned at 1301 cs-‘. The two boron-carbon stretches can be characterized as an antisymmetric stretch at 1152 cm-’ and a symmetric mode at 702 cm-l. Six of the seven skeletal bending modes can be readily identified in the far-infrared spectrum of the solid at 115, 187, 261, 366,438 and 488 cm-’ and it is believed that the seventh one is at 532 cm” although this frequency seems rather high. -All of these in&xred bands have Raman counterparts

27

with the possible exception of the 488 cm-’ band but it is probable that the 510 cm-’ Raman line, which is broad, is due to both the 488 and 532 cm-’ infrared bands. The crystalhnity of the sample which was used for the farinfrared study appeared to be better than the one which we could obtain for the Raman studies. If the 510 cm-’ Raman line represents a single fundamental then it would be necessary to assign the 120 cm-’ Raman line to both of the vinyl torsions. The exact description for these skeletal bending modes is not possible without a normal coordinate calculation but we have suggested some assignments based on some of our earlier work on vinyl borane molecules. We have assigned the CCB bend at a considerably higher hequency than the corresponding mode in vinyldifluorobomne [S] but it is possible that it should be assigned at a considerably lower frequency. Nevertheless the FBC bend most certainly must be above 400 cm-’ and the FBCp out-of-plane bend must correspond to the depolarized Raman line at 375 cm-‘. Also, the vinyl torsion can be confidently assigned to the Raman line at 120 cm-l but, as pointed out earlier, this may correspond to both vinyl torsions. The Raman line at 266 cm-’ appears to be a reasonable frequency for the BC, scissoring motion but the assignment of the other skeletal bending modes is very speculative. DISCUSSION

Previous conformational studies of vinylboranes have established that vinyldifluoroborane, C2H3BF*, is a planar molecule in both the solid and fluid phases [S] while vinyldimethylborane [lo] and trivinylborane [5] are planar in the solid state but probably exist as more than one conformer in the fluid state. The vibrational spectra of (CzH3)2BF indicate that this molecule also exists as more than one conformer although the major conformer is one of C, symmetry. Assuming a planar heavy atom skeleton, there are three possible conformers of (C*H&BF (Fig. 5). As we have pointed out earlier [ll], conformer A contains non-bonded hydrogen distances considerably less than twice the van der WaaIs radius for hydrogen and is not considered a likely structure. The remaining two structures in Fig. 5 have been termed [26] the ‘%eagull” (C) and “swastika” (B) structures and possess CzV and C, symmetries, respectively. In an LCAO-MO-SCF study of several vinylhaloboranes, Armstrong and Perkins [26] found that a calculation of band energies in the ultra-violet spectra of divinylboranes yielded very similar values for both the “seagull” and ‘cswastika” geometries. However, on the basis of relative intensities of calculated bands, they concluded that the equilibrium configuration of the ground state of the (C2H3)2BCl molecule is chiefly the “swastika” CC,) form. This is entirely consistent with our results for (C2H&BF and suggests that our second conformer is, in all likelihood, the “seagull” (Cz,) conformer. In further support of our experimental work, Fitzpatrick and Matthews

[ 29 J recently published some geometryoptimized ab initio molecular orbital calculations on a series of methyl- and vinylfiuoroboranes (R,BF+,). The conformation of (C2H3)2BF used in these calculations was the “swastika” CC,) conformer and excellent agreement between experimental and calculated values were obtained when a number of regression analyses were performed to study the relationship between tine ab initio orbital population and the published NMR parameters (JIQ+IP~,6 (“F) and 6 (“B)). With the confident assignment of at least one of the vinyl torsions in (&H&BF it is possible to estimate the barrier to internal rotation of the vinyl groups. In our earlier study [B] of vinyldifiuomborane we observed the vinyl torsion at 103 cm-’ in the Raman spectrum of the gas and this mode shifted to 128 cm-l in the Raman spectrum of the liquid. By assuming the same shift factor for the vinyl torsion in divinylfluoroborane, one would predict that the torsional frequency in the gas phase lies at 96 cm-‘. With this frequency and an F number of 1.73 cm-‘, one calculates a twofold barrier to internal rotation of 3.8 kcal mol-’ for the vinyl group. This value is only slightly lower than the 4.17 kcal mol-’ obtained [7] for the vinyl barrier in vinyldifluoroborane and it is consistent with the NMR results 1173 where, for the vinyldifluoroborane molecule, it has been shown that one vinyl group has a greater interaction with the boron orbital than do two vinyl groups. Thus, one would predict the vinyl barrier to be slightly lower in divinylfluoroborane than -& vinyldifluoroborane. The infrared spectrum reported herein agrees remarkably well with that reported [2] earlier. Two bands were previously reported [2] at 1126 and 848 cm-l which were not observed in our spectrum and we suspect that they are due to impurities in the sample studied earlier. The proposed assignment is in excellent agreement with the assignment proposed [S] for the vinyldifluoroborane molecule where c’euterium substitution and normal coordinate calculations were carried out. The two major differences are in the assignments of the CH2 wag and the CCB bending modes. In the current study we have assigned the CH2 wag in the 1250 cm-’ region which cornpares well with the frequency predicted on the basis of group frequencies. However, this mode was assigned at 1027 cm-’ in vinyldifluoroborane [B] although a Raman line was observed at 1266 cm-’ but was not assigned. Also, for the vinyldifluoroborane molecule, the two polarized Raman lines at 427 and 221 cm-’ were shown to consist of approximately 21% and 65% of the CCB bend, respectively. In our current study we have assigned the 442 cm- ’ Raman line as the CCB bend but undoubtedly this motion contributes significantly to the 265 cm-’ Raman line as well. For a more exact description of these low-frequency skeletal modes, a normal coordination calculation is necessary. It is rather surprEing that there appears to be little coupling between the normal modes for tZle two vinyl groups except in the spectrum of the solid. For example, in the ii&ared spectrum of the gas the carbon-hydrogen bending modes all appear to have “good” band contours as if they were due

29

to a “single” vinyl group. Apparently the hydrogen atoms on one vinyl group are sufficiently removed from those on the second vinyl group so that there is little coupling between these modes. Since the out-of-plane modes have well-defined intied band contours, it appears that the planar conformer of C, symmetry is the major constituent in the gas phase. This conclusion is also supported by the relative intensities of the weak bands in the Raman spectrum of the liquid which we have assigned to a second conformer. However, since these lines disappear very slowly as the temperature is lowered, the enthalpy difference between the conformers must be small. Such a small enthalpy difference could be the reason why it was difficult to obtain a “good” crystalline solid. Nevertheless, based on the Raman depolarization data and infrared band contours, it can be concluded that the predominant conformer of divinylfluoroborane is planar with C, symmetry. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-80-13694 and CHE-8215492. E. J. S. acknowledges the help of Mr. J. Carignan and Ms. P. Davidson as well as generous support by the Lander Foundation. The authors also thank Ms. Senja Compton and Mr. Frank Cox who recorded the initial Raman spectrum. REFERENCES 1 J. D. Odom in G. Wilkinson, F. G. A. Stone and E. W. Abel (Eds.), Comprehensive Organometallic Chemistry, Pergamon Press, Oxford, 1982, Chap. 5.1. 2 F. E. Brinckman and F. G. k Stone, J. Am. Chem. Sot., 82 (1960) 6218. 3 D. S. Matteson, 3. Am. Chem. Sot., 82 (1960) 4228. 4 T. D. Coyle, S. L. Stafford and F. G. A. Stone, J. Chem. Sot., (1962) 3103. 5 J. D. Odom, L. W. Hail, S. Riethmiiier anti J. R. Durig, Inorg. Chem., 13 (1974) 170. 6 A. K Ho!iiday, W. Reade, K. R. Seddon and I. A. Steer, J. Organomet. Chem., 67 (1974) 1. 7 J. R. Durig, R. 0. Carter and J. D. Odom, -Inorg. Chem, 13 (1974) 701. 8 J. R Durig, L. W. HaI& R. 0. Carter, C. J. Wurrey, V. F. Kaiasinsky and J. D. Odom, J. Phys. Chem., 80 (1976) 1188. 9 J. R. Durig, E. J. Stampf, J. D. Odom and V. F. Kalasinsky, Inorg. Chem., 16 (1977) 2895. 10 J. D. Odom, T. F. Moore, S. A. Johnston and J. R. Durig, J. Mol. Struct., 54 (1979) 49. 11 J. R. Durig, S. A_ Johnston, T. F. Moore and J. D. Odom, J. Mol. Struct., 72 (1981) 85. 12 J. D. Odom, Z. Szafran. S. A. Johnston, Y. S. Li and J. R. Durig, J. Am. Chem. Sot., 102 (1980) 7173. 13 J. R. Durig, P. L. TrowelI, Z. Szafran, S. A. Johnston and J. D. Odom, J. Mol. Struct., 74 (1981) 85. 14 J. D. Odom, S. V. Saari, A. B. Nease, Z. Szafran and 3. R. Durig, J. Raman Spectrosc, 12 (1982) 111. 15 D. Christen, D. G. Lister and J. Sheridan, J. Chem. Sot., Faraday Trans. 2,70 (1974) 1953.

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