Polarized infrared spectra of an oriented and very thin tolane
crystal have been measured in the 3500--50 cm-1 region. The symmetry
of the infra red active internal vibrations is proposed on the
basis of the oriented gas model. The application of the vibrational
spectroscopy in studying the conformation of the free tolane molecule
is discussed

Ann E. Stanley

Goran Baranović

James R. Durig

Lidija Colombo

English

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CROATICA CHEMICA ACTA CCA-1476 CCACAA 57 (4) 629-636 (1984) YU ISSN 0011-1643 UDC 547.636 Original Scientific Paper Polarized Infrared Spectra of Tolane Single Crystal* Lidija Colombo and Goran Baranovic Rudjer Boskovic Institute, P. P. 1016, 41001 Zagreb, Croatia, Yugoslavia and James R. Durig and Ann E. Stanley Department of Chemistry, University of South Carolina, Columbia, SC 29208, USA Received November 21, 1983 Polarized infrared spectra of an oriented and very thin tolane crystal have been measured in the 3500--50 cm-1 region. The sym-metry of the infra red active internal vibrations is proposed on the basis of the oriented gas model. The application of the vibrational spectroscopy in studying the conformation of the free tolane mole-cule is discussed. INTRODUCTION The conformation of non-rigid molecules containing phenyl rings has attracted the attention of spectroscopists in recent years. The problem of the conformation of the tolane molecule has been raised from the first crystal structure analysis performed by Robertson and Woodward1• Later structural investigations2,3 have shown that the unit cell consists of two crystallogra:phically non-equivalent sets of molecules with similar bond distances and angles, but differing in orientation. The origin of such a rela-tionship in the tolane crystal is still unclear. While the molecules in the crystal are plane within experimental errors, the conformation of the free molecule has not been established yet. In order to investigate this problem, quantum-mechanical calculations were carried out4, and the energy barrier thus obtained (2.7 kJ/mol) corresponds to a quasi free rotation of the phenyl rings around the C-C single bonds. Our aim is to study this conformational problem by vibrational spectroscopy techniques and using normal coordinate analysis. The large number of vibrational frequencies of this molecule requires an approach by different methods. Above all it is advisable to proceed very gradually. In the present paper we report results of the vibrational analysis perfor-med on the tolane single crystal, which enabled us to determine the symmetry of the internal vibrational modes. Molecular and Crystal Structure Tolane crystallizes in a monoclinic system, space group P2i/a with Z = 4 molecules in the unit cell3. The molecules belonging to each set are packed * Dedicated to Professor D. Grdenic on occasion of his 65th birthday. 630 L. COLOMBO ET AL. in layer along the crystallograpihc c-axis (Figure 1). The site symmetry of both non-equivalent tolane molecules is Ci. Their structures are the same with approximate D2h symmetry. Figure 1. Crystal structure of tolane The correlation between the molecular groups, site groups and factor groups (Table I) indicates that the total number of internal vibrations in the TABLE I Selection Rules and Symmetry of Molecular and Crystal Vibrations f't:>lecular group Site group Factor group D2h c2h 12va ag ~Ag 3v, 11. blg~ 33v, R. Ag --------7v, ~ b2g :::::====-- 1 11v, !\, B b3g g 2 x 33v, 6 Ri 2 x 33v, 6 Ri 4v a ~Au llv, TL b:u~ llv, TM b2u --33v, T. Au----1 7v, TN b ---B 3u u 2 x 33v, 5 Ti 2 x 33v, 4 Ti a 1' stands for the vibrational frequencies TO LANE 631 crystal unit cell is 264. In fact, each internal vibration should in principle be doubled in each factor group on account of the two sets of molecules. At room temperature this doubling is not observed. Thus the total number of observed bands is practically reduced by half. One of the possible explanations for this feature will be offered later on in this paper. With regard to the lattice vibrations, we obviously have to consider all four molecules and consequently there are 21 lattice vibrations. EXPERIMENTAL Single crystals of tolane were grown from melt by slow colling. Large and clear single crystals were optically oriented by observing interference patterns in a polarizing microscope. Observation of X-ray diffraction in an oriented sample established that the axis of the lowest refrnctivity is parallel to the crystal binary axis b. The sample for IR absorption measurements must be oriented against the refractivity axes in order to conserve the linear polarization of the incident beam inside the crystal. The preparation of oriented and enough thin samples is the most difficult part of this type of experiments. The necessm·y thinness of the sample can be obtained only if the single crystal is attached to the support - window. Any kind of glue would give its own spectrum and therefore it is better not . to use it. We have solved this problem by attaching the crystal sample to the support by melting a very thin layer of the crystal in front of the window. If the layer of the melt is · very thin in comparison to the thickness of the whole crystal, and if the cooling of the support and the crystal is performed slowly, the molten pairt of the crystal will recrystallize in a perfect single crystal. This can be verified by observing the extinction of polarized light. If the thinning is performed by rubbing the crystal on a wet silk, the deposition of the material around the smnple will fasten the crystal more securely to the support. This feature makes it possible to prepare very thin single crystals. The smne procedure can be used even for FIR measurements. The only limit is set by the melting-point of the polyethylene windows. The spectra measured in the middle infra red region have been recorded with a Perkin Elmer 650 spectrometer (Figure 2.), while the FIR spectra were recorded with a Digilab FTS-15B Fourier transform interferometer. The polarization of the incident beam in the middle infra red region was ensured by a polarizing filter, while the polarization in FIR interferometer was obtained on the Mylar beamsplitter only.s INTERPRETATION OF THE OBSERVED SPECTRA The notion of characteristic frequencies of functional groups like phenyl rings is particularly helpfu,l in the assignment of the vibratiional spectra. The correlation between the point group D6h of benzene and the gr-oup C2v, which is the point group for one half of the tolane molecule, enables us to derive the symmetry of the ring vibrations in tolane. However, the number of frequencies observed in the Raman as well as in the IR spectrum is still very large, and furthermore, these frequencies can be very close. Polarization analysis of scattered light (Raman) or of absorbed light (IR) offers a further experimental possibility of separating transition moments of different symme-try, even in the case of very close frequencies. The number of vibrations and the symmetry species in the crystal spe-ctrum can be derived from the correlation between the molecular group, site group and factor group given in Table I. From this correlation it follows that all molecular vibrations of the »U« species will be active in both the Au and Bu spectrum of the crystal. The molecular modes of the b1u and b2u species describe in-plane vibrations with the transition moment oriented along the longest (L) and middle (M) molecular axes respectively. The transition moment 632 100 90 80 60 1600 :E 11 b Au L. COLOMBO ET AL. 1400 1200 1000 800 600 400 FREQUENCY /cm-1 Figure 2. Polarized IR spectra of a thin tolane single crystal and the definition of -+ -+ the molecular axes. The polarizations used: EI! b (Au), E 11 c* (Bu). of the vibrations of the species b30 is normal (N) to the molecular plane. The definition of the molecular axes is given in Figure 1. The vibrations of the species a0 , which are forbidden in the molecular group Dzh, become active in the crystal spectrum. The corresponding bands can be detected by com-parison of the spectrum of the crystal to the spectrum of solution or vapor, where these bands must be absent if the free molecular group is Dzh· Oriented gas approximation has been often applied in the determination of the symmetry species of the IR active vibrations in the crystal.6 In this approximation the interaction between the molecules in the lattice is neglected and the intensity of the absorbed band is proportional to the square of the direction cosine between transition moment and the principal axes of the optical indicatrix (a*, b, c*). The angles of the molecular axes in the frame of the crystallographic axes are given by X-ray analysis. In the case of mono-clinic crystals, the angles between the crystallographic and the principal axes of the optical indicatrix can be measured on an oriented sample with a polarizing microscope by observing along the binary axis and by measuring the angle between the layers of the cleavage planes and the direction of optical extinction. With respect to the crystal axes as defined in Ref. 3, a* and c* lie in the ac plane with a* 5° from a and c* 18° from c; a*, b and c* are mutually orthogonal. Since the measurement is performed on the crystal sample, the vector of the incident radiation will be interacting wHh the total dipole moment of the crystal which in the present model corresponds to the sum of dipols of all molecules in the crystal. Consequently, if we apply the oriented gas approximat-TO LANE 633 ion to tolane, we have to take into account that there are two sets of mole-cules. From the positions of the two non-equivalent molecules in the frame of the principal axes of the optical indicatrix (see Tables Ila and Ilb) we calculated the intensity matrices for the internal modes. TABLE II The Matrices cI>1 and cI>2 of the Direction Cosines of the Two Non-equivalent Tolane Moleculesa a) cI>1 L M N a* 0.845 -0.417 -0.318 b 0.269 0.852 -0.462 c* 0.462 0.315 0.827 b) cI>z L lVl N a* 0.783 0.338 0.499 b -0.101 0.892 -0.446 c* -0.614 0.299 0.743 • L, M, N are the molecular axes. a*, b, c* are the principal axes of the optical indicatrix. As it was already emphasized (see correlation Table I), a doublet of bands corresponds to each internal mode for a given crystal symmetry class. Since both sets of molecules have a common binary axis b, the calculation performed showed, that if the intensity of one component of that doublet is proportional to the square of the sum of the direction cosines: I1 - I P1 .. + ei'Jl P2 .. 12 ,l] ,l] i =a*, b, c* j = L, M, N or b1u, b2u, b3u then the intensity of one other is proportional to the square of the difference of the same cosines: I 2 - I <P1 .. - ei'Jl p2 ·· 12, ,lJ ,lJ where eiqi is the phase difference of the two nonequivalent molecules which is a priori unknown. To avoid this complication we suppose (which is consistent with the general pattern of the spectra) that the frequency difference between the two components is small so that we actually see the superposiUon is small so that we actually see the superposition of the bands (or their intensities). The dichroic ratio (the relative intensity of the A11 to the B11 component), () (j), is then simply: {! (j) = I P1,2j + e'.'I' P2,2j 1: + I P1.2j - e'.'I' P2,2j 1: ! P1,3j + e•'Jl P2,3j I-+ I Pl,3i - e•'Jl P2,3j I-<P\2i + P 22.2j P\3i + P 22,3i This gives(! (b1u) = 0.14, (! (b2u) = 8.14 and(! (b3u) = 0.34, and means, for example, that intensities of the b1u bands should rise remarkably on changing the polar-ization of the spectrum from Au to Bu. How these predictions are fulfilled can be seen from the Table III. The assignment of internal vibrations given the-634 L. COLOMBO ET AL. TABLE III The Observed and Calculated Dichroic Ratios for Some Vibrational Modes of the Tolane Molecule point group symmetry Wilson notation 8a 19a 9a 18a 12 8b 19b 9b 18b Obc. frequencies in cm-1 dichroic ratio (Au) (B,.) obs. calc. 1599 1594 0.71 1490 doublet 1491 doublet 0.71 1172 1173 0.67 0.14 1020 1021 0.30 992 994 5.01 1568 doublet 1568 3.33 1438 1439 1.00 8.14 1154 1150 0.11 · 1064 doublet 1067 1.12 rein was previously established by investigation of the Raman and in-frared spectra of solutions. Obviously, the agreement is only qualitative for the majority of vibrational modes. For the oriented gas model with its well known deficiencies one could hardly expect more. Applying the same considerations to all observed bands we could determine the symmetry of each molecular vibrational mode active in the IR spectrum as given in Table IV. One of the deficiencies of the oriented gas approximation is that there exists no rule for the determination of the intensity distribution for the au internal modes, but it can be compensated by other types of measurements. In Table IV we listed the frequencies of bands 'observed in the IR spectra of two polarizations for the single crystal samples of different thickness in order to demonstrate the dependence of the relative intensities on the thinness of the sample. Only the spectral region corresponding to the characteristic frequencies of the organic compounds is considered. We also give the spectrum of solution, which can be useful in further determination of the symmetry of some particular modes. CONCLUSION By determination of the symmetry species of each IR active vibrational mode, one of the necessary conditions is created for performing the normal coordinate calculatrons for large molecules like tolane. Other procedures such as comparison to similar molecules (phenylacetylene, biphenyl) or isotopic substitution can be used to improve the preliminary assignment of the observed spectrum and consequently to obtain the most reliable force field for the molecule of tolane. Only then will it be possible to attempt, a solution to the problem of the conformation of these molecules in solution. We are aware of the fact that the present results, although undoubtedly useful, are not TO LANE 635 TABLE IV Oriented Gas Approximation Applied to the Talone Spectrum CRYSTAL (cm-1) Solution E II b (Au) E II c* (Bu) Symmetry (cm-1) Thick Thin Thick Thin 3090S" 3085vS b2u 3070S 3068S 3068vS biu 3042mS 3048m b2u 3030w 3030w 3036S biu 3032S 3023w b2u 1608vS 1599S 1599mb 1594vS 1601S biu 1572m 1567vS 1569mS 1568m 1570w b2u 1530m 1530wb 1529w 1535w combination 1500vS 1495 1495sh 1495sh 1498S biu 1492sh 1488 vs 1490m 1492vS 1492vS biu 1446vS 1438vSb 1439vS 1439vSf 1443mf b2u 1435sh 1420sh 1425sh b2u 1382m 1382mSb 1384m 1382w b2u 1328w 1326Sf 1325m 1327mS 1330w b 2u 1310S 1305Sf 1306w 1308vSf 1312m biu 1280m 1276vS 1278mS 1275mS 1280w b2u solvent 1328w biu solvent 1220w 1222sh ? 1179w 1172mS 1170vw 1175S 1179w biu 1160w 1153w 1150vS 1158Sm b3u llOOm 1098m 1095m 1098m 1102w b2u 1070vS 1068sh 1067vS 1070S b 3u 1064vS 1065vS 1063sh b2u 1028S 1020m 1020vw 102lvS 1027S biu solvent 992m 992vw 992vw b2u 980S 980m 980m absent in solution, au? 916S 912vS 918vS b3u ? 905S 908m b2u 898w 890mw absent in solution, au? 878w 868m 868w combination, b2u 848m 845m 845w 850mS 850vw combination, biu solvent 815m 815m b3u 760vS 750vSb 750S 752vvs 752vS biu 735sh 745vS 740sh combination, biu 725sh 740sh 730sh combination, ? 699sh 689vS 680vS b3u 690vS 682vS 682vS 685vS b2u 670m 660mw 660m 665S 665mw biu 630w 620w combinffiion, ? 539S 534mw 530m 530vS 535S bau 512S 505vS 505S 502vS 505vS bau 463vS 463S 462m 462w absent in solution, au? 400vS" 405mb 405m" ? 284sh" 288m" 288S" b3u 255m" 253w" 258m" b3u a s, m, w, v, b, f, sh denote strong, medium, weak, very, broad, sharp, shoulder respectively. " FIR measurements. 636 L . COLOMBO ET AL. sufficient in themselves for a appropriate solution in vibrational spectroscopic studies on molecular conformations. Acknowledgement. - The authors gratefully acknowledge the financial support of the National Science Foundation (Project YOR81/029) and of the SIZ for Scientific Research of Croatia. REFERENCES 1. J. M. Robertson, J. Woo d ward, Proc. Roy. Soc. Al64 (1938), 436. 2. V. D. Samarskaya, R. M. Myasnikova, A. I. Kitaigorodskii, KristaLlografia 13 (1968), 616. 3. A. M a v r i d i s, I. Mo u s t a k a 1 i - M a v r i d i s, Acta Cryst. B33 (1977) 3612. 4. J. A. P o p 1 e, M. G o r d o n, J. Amer. Chem. Soc. 89 (1967), 4253. 5. R. C. Mi 1 ward, A Complete System for Interferometric Fourier Spectroscopy in the Far Infrared, in: Far-Infrared Properties of Solids, S. S. Mitra, S. Nu-d e 1 man (Eds.), New York, Plenum Press, 1970., p. 4. 6. G. C. Pim en t e 1, A. L. Mc C 1e11 an, J. Chem. Phys. 20 (1952), 270. SAZETAK Spektri tolana L. Colombo, G. Baranovic, J. R. Durig i A. E. Stanley Polarizirani infracrveni spektri orijentiranog i vrlo tankog kristala tolana mje-reni su u podrucju od 3500 do 50 cm-1• Infracrveno-aktivne interne vibracije ra<zvr-stane su po simetrijskim klasama na temelju modela orijentiranog plina. Rasprostrena je primjena vibracijske spektroskopije pri proucavanju konformacije slobodne mole-kule tolana. 

Polarized Infrared Spectra of Tolane Single Crystal

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