Biswas, D C

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THE RAMAN SPECTRA OF m- AND p-XYLENE IN THE SOLID STATE AT DIFFERENT LOW TEMPERATURES*Bv I). C. BISW ASOPTICS DliPARTMhNT, INDIAN ASSOCIATION FOR THit C'l’ l/ nVAT !()\  OK S 'IKNl'K, CUA'CTTV 32{ R e c e i v e d  fot  p u b l i C i i t lo n , M a y  6 , 19^4)Plntcs X  A --CABSTRACT. 'ITic Raman spectra of im hi ami para xvlcnes in tfie li()inM state and in the solid .state at different l.w lempernturcs were iec)tdrd R )th the substances in the liipiid state vield a few feeble ^xlra Raman line.s not rep )rted in earlier publications 8 me of the prominent Raman bne^ of b itb these coniponmh shift slightly with solidilicotion of the eompoiiiids, but the number of lines nndergoin'2 ‘>nch changes in their fn'quencv-sliifts is much greatfT in the case of meta xcUme than in (he ease of the para compound.ThiecMievv lines appear in the 1 'w-freqnency region when the meta eotnpound is solidific 1 while two such lines in the low fr (iiiency region ajipe ir in ilie case of the para eom|> Hind. With lowering of temperature f the s »lidified mass to - i8  )"C' the low- frequenrv lines of the former .‘■ iibshinec show little change except that the band at Si em * s h ift to  go cm * while in the case T the othei eomponn l the number of -f e<iuencv lines increases from two to live with the lowering temperature to*-iSo®C' Ih^ probable or'gin of the-e low fr(T|uency lines is di‘-eussed The difference in the RanMii spc'ctra ol thc.se two isomers in the solid state is explained by as.suniing that the inlluence of the intcr- molccnlar field on the moleenles of the sub^taiu'c depen Is on the relative p ysitions of the sub>tituent groups in the benzene ring.I N T R O T) V C T r O NIt was pointed out earlier (Biswas, 1054) that the study of the influence of temperature on tlic intensities and positions of the low-frequency Ramanlines which appear when some organic liquids are solidified may lead to theproper understanding of the origin of such liiicS. 'I'lie results obtained by Swamy (1Q51, IQ52 and 19.531 and Deb (1952, i953i i954  ^ their investiga­tions on the ultraviolet absorption spectra of substituted benzenes and iiapblhaleue compounds show that tlie influence* of iiiteimolecular field on the electronic energy level in the solid stale depends on the relative positions of the substituent groups. A comparison of the Raman spectra of ortho and meta compounds in the solid state at different tempciatures may indicate to what extent the intensities and positions of the lines can be correlated with the relative positions of the substituent groups and also with the interinole- cular field indicated by the results of investigation of the ultraviolet absorption spectra under similar conditions. With this object in view the Raman spectra of meta and paia xylene in the solid state at different low temperatures have been investigated and the results have been uiscussed in the present paper.K'ommunicated by Professor vS. C. Siikar.36304 D. C. BiswasK X P K R I M K N T A LThe liquids of the present investigation were secured from Fischer Scientific Co., New York and were of chemically pure quality. They were distilled in vauum, as usual. The Raman spectra of the substances in the liquid state were photographed and compared witli the Raman shifts of these liquids reported by previous workers to test the purity of the samples. The technique for recoiding the Raman lines of the solidified substances is same as that described earlier (Biswas, 1954). In all the cases the spectra were recorded on Ilford Zenith plates using a Fuess glass spectrograph having a dispersion of about II A"" in the 4047 A"* region. Kach spectrogram contains an iron arc spectrum for comparison.R K vS IT L  T vS A N D  D I vS C tJ vS vS T C) N SThe calculated Raman shifts of the substances both in the liquid state and in the solid state at different temperatures arc given in Tables I and II.T aju.k I»;/-Xylene C f , H 4 .A*'  in cm~^Liquid Solid (present author)Magat (1936) 1 Present author At about — 8o®CT | At - i S o ’ C41 (J)  ^ j 41 (r) e5Q (2) e 59 (2) e81 (6) c, k : 90 (2b) e, k200 (4) 20A (3) + e 1224 (6) 231 (5)±e, k 236 (i) e, k 1 236 (0) e, k295 (3I 282 (3)±e, k 282 (0) e, k j 282 (0) e, k318 (lb) e, k i461 (2) ± e , k !j514 <41 517 (4) e, k 513 (i) e» k 1 513 (0) e, k(6) 54^ ( 7)±t?»k 588 (0) e 647 (x) e 679 (0) e536 (2) e. k ! 534 (i) e, k!725 (ro) 727 (10) +e , k, i 721 (5) e, k 721 (3) e, k77^ 766 (i) e, kRaman Spectra of m- and p- Xylene in Solid State 305TAHUi I—could.LiquidMagat {1936) Present authorSolid (present author)At about-80T I A t - j8o''C819 (3*^ )812 (0) e 828 (3) e, k 887 (lb) e' S26 (u) c*. k 820 (0) c, k9-13 (1) e9^ -)8 (i) eI(K)0 (12) 1000 (12) e, k, i i(»uo (7) c, k 1000 (5) f, kIC»32 (l) 1035 (2) e, k 1035 (0) e, k 1035 (u) c, k1094 (i) 1093 (2) e, k 1093 (0) e, k 1093 (u) e, k1167 (1)i2i«) (2) e,k ijio  (f>) e, k 1210 (i) e, k124ft (4) 1250 (4) e, k 12 jS (2) e, k 1248 (1) e, k1263 (0.5) 1265 (2) e, k 11342 (5) 1349 (1) e1375 (5) 3380(5)0, k ' 1374 (-2) t*. k 1374 (I) h1448 (iW ! 1446 (2b) e, k1592 (^ )1! J592 (3) e, k 1592 e 159*^  b’) ^1613 (3) 1616 (5) e, k I 1610 (1) e, k lOio (1) c, k12571 (0) 1 12731 (0-5) ;I12866 (3)12874 (3b) e, k i 2S68 (1) k 2808 (0) k2917 (8h) 2920 (6b) e, k 2911 (4b) e, k 2911 (2b) e, k2962 (1) k 2962 (1) k8014 (3)3045 (7b) 3058 (4) ^ 3015 (2) k 3^15 (*) ^3217 (iW j306 D. C. BiswasT abi.k II/>-Xylone, C6H4 (CH3I.4Ai' in c 111 *LiquidMagat (1936)170 (O c,)Prf^ent author511 (7) j 3 c* (7) ± k389 (0) 389 (j) e460 (8) 1 4(x) (10) + e, 1S‘\o (0) ((■’) eM^3 {>) i c, k(197 (n) 897 (l) (•7:5 (i) e754 ('S ii (.1) 1 8<>o {')) i  e, k807 (1.) 8j() (12) f t‘, knu)o (1) k11 >30 (i.9 j1112'' (2) e, k1182 (2) 11nSc (2) e. kJ212(8) 1210 (lo) e, k ii 3J<- (:■) j ‘319 (1) e13^0 (0) e1382 (6) , 1380 (6) e, k1-1452 (2b) 1 1450 (2b) e, ki46(> (ib) eSolid (present author) At about-3o"C55 («)1)1 (2b) k316 (1) e, k.]0() (1) c, k0.]7 (i I e, kS(.() (o) e S.H) (^ ) e, k1185J2ln (5) e, kI371 (0) e, k 1448 (o) eAt-iSo%'5'^  U)  ^hS (1) c, k 87 (2) e k10(8 (j) L‘ Iv118(1 b) e 310 ( 1 ) f ,  k 40') (1 ) e, k(1)7 ( 1) e , kS<<(.) (o) t8'^ 9 (3> e ^11S5 n^) e I 2JO (5) e, k1375 (»b) e, k 1448 (lb) e, kBIS ^AS PLATE XAc « : Hvo 00 Iino rn &- T u<Oti u t >X X NUaman sppctraFig. 1. Meta xylene (a) Liquid at 30°C ib) Solid at —8()"C (c) Solid at -ISO ’^CFig 2 Fara xylene(a) Liquid at 3(LC(b) Solid at -3(rC(c) Solid at • 180 CBISWAS PLATE XBFig. 3Fig. 4Miciophotometiic records of Raman spectra Fig. 3. Meta xylene at —80°CFig 4. at - 180“C( W i=41 «»*=59cm‘ ,^ « 8 « 9 0 c m * ^ )BBWAS P U T E  X CI  r . ?  a ■?  iSSMicrophotometric records the Raman spectraFig. 5. Para xylene et —30®CFig. 6.................  at - 180”C(<D,=50cm'^, <«»*=68cm'^ W8"87cm‘ ‘ , w *=108cm "', W4=148cm‘ ^ )T a b i.k II—con id.Raman Spectra of m- and p-Xylene in Solid State 307Solid (present author)273 (2)2867 (5) 2920 (oh)3017 (5* ^3050 (7h)3214 (il>).7,10 (2) e, k 28( 5 (ib) e, k 2870 (3I)) c, k 2Q22 {5b) e k3014 (2) «■, k 3026 (2) e, k 3063 (4) c, k2740 (.)) kjS08 (ib) k 29:1 (jb) e 2942 (C) e 3 ‘I4 (2) t- 3026 (2) e 3061 (3) e, kMagat (1936) Pre^^enl nullioi At abuiit -30*0 i A t~ iSo"C1519 (0) e 111575 (J) 1580 (2) e, k ; ('>) 15S0 (n) e, ki6'6 (C) 1622 (S) c, k ' 1620 ( ’) e 1^20 (3) e, k2*740 (o) k2S6S (lb) k ,*()2i (2b) e, k 2942 (2) e k 3014 (>) k 3026 (2) k 3061 (4) e, kTlie tables also include for comparison the Raman shifts of these substances in the liquid state reported by some previous woikers. The spectrograms are repioduced in figures i and 2, Plate XA . Microphotometric records showing the intensities of the low frequency lines at different temperatures relative to some other lines due to intramolecular oscillations are also reproduced in figures 3—6 (Plates X  B~C ).ia) Raman lines of the substances in the liquid state :Tables I and II show that some of the weak lines observed in the present investigation for these two compounds have not been recorded by previous workers. Such extra lines observed in the case of m-xylene are at 31b (ib), 461 (2), 588 fo). 647 (0 , 812 (0), 887 fib), 943 (ib 96S (i) and 1210 (2) enr^ and in the case of the other compound the extra lines are at 593 7^5754 (ib  1000 (i), 1120 (2), 1360 (o), 1466 (ib), 1519 (0) and 2805 (ib) cm b As the extra lines 461 (2), 647 (i), 812 (o) and 1210 (2) cm'^ observed in the case of w-xylene coincide in positions with the strongest lines of the para com­pound, it is suspected that a small percentage of para compound is present as impurity in the experimental substance used as meta-xylene in this investiga­tion. Similarly, the presence of the extra lines 725 (i) and 1000 (1) cm"** inm D. C. BiswasIthe spectrum of the liquid para xylene indicates the contamination of the substance by minute traces of the meta compound. The other extra lines observed in the present investigation seem to be genuine Raman lines of the respective liquids.Previous authors, on the other hand, have reported the existence of four lines not found in our spectrogram in the case of each of the two xylenes. For meta xylene such extra lines are at 1167 (o), 2571 (0), 2731 (0.5) and 3217 (ib) cm“  ^ while for the other compound they are at 170 0^.5), 540 (0)?, 1036 (0) and 3214 (lb) cm"'. The lines assigned by previous workers a^  2731 cm” ' and 3217 cm"' in the case of meta xylene and as 3214 cm” ' in the case of the para compound cannot be attributed to the C—H valence oscilla­tion, because although there are traces of the lines at their positions corres­ponding to 4046 1  excitation, no trace of them is found at the positions coiresponding to the much stronger excitation by 4358 A Hg line. They can be satisfactorily attributed to the C—C valence vibration and the C—H deformation vibration excited by the 4358 A Ilg line. These lines are assigned as the lines 943 cm"' and 1446 cm” ' of the meta compound and the line 1466 cm~' of the para compound as given in Tables I and II respectively.(6) Raman lines of the substances in the solid state :It can be seen from Tables I and II that meta xylene when solidified and cooled to ~8o°C, (m. p. of 7>i-xylene is~52°C) gives rise to three new Raman lines in the low-frequency region while the para compound in the solid state at -30®C (m. p. of f-'-xylene is i3°C) gives rise to two new lines in the low- frequency region. When the solidified mass is cooled down to — i8o°C, the low-frequency lines of the meta compound show little change in their intensities and positions except the shift of the band at 81 cm"' to 90 cm"'. The low-frequency lines of the para compound, on the other hand, show remarkable changes with the lowering of temperature of the polycrystalline mass to ~i8o®C. Two more low frequency lines at 68 cm"'and 148 cm” ' respectively appear and the broad line at 91 cm"' breaks up into two equally intense lines at 87 cm"' and 108 cm”' when the solid mass is cooled to -i8o®C. Thus, with lowering of temperature of the solid para xylene from ~3o'’C to -i8o®C, the number of low-frequency lines increases from two to five. Moicover, it is evident from the microphotometric records reproduced in figures 3—6 that the intensities of some of the low-frequency lines do not diminish appreciably with the lowering of temperature of the crystals of meta and para xylenes to -  i8o®C. On the other hand, the intensities of some of the low-frequency lines increase while those of the other lines practically do not show any change with lowering of temperature.We can now try to understand the above results of our investigation from the points of view of different theories regarding the origin of these low- frequency lines. The ultraviolet absorption spectra of these xylenes werestudied previously by bwaniy (1952). It was observed that the absorption bands of these substances become sharper with the solidification of the samples and with the lowering of the temperature to ~  i8o®C. He concluded from these results that with the lowering of temperature angular oscillation of the molecules tends to cease. We can, therefore, see that if the low-frequency lines originate from the angular oscillations of the molecules fixed in the crystal lattice as postulated by some previous workers their intensities should invariably diminish with the lowering of temperature of the solid mass. The evidence being on the contrary, we conclude tliat at least some of these lines cannot be due to the angular oscillations of the xylene moleculej fixed in the crystal lattice. An alternative explan ition of the origin of these low- frequency lines was given by Sirkar (1936). According to his theory, the low-frequency lines are due to the intermolecular vibrations in groups of molecules formed by virtual bonds amongst the molecules in each group. In this associated group, the molecules may execute both translational and rotational oscillations giving rise to the new Raman lines in the low-frequeiicy region observed in the case of crystals. The enhancement of the frequency shifts of the low-frequency lines with the lowering of temperature can be easily understood to be the result of the increase in the strength of the virtual bonds which are formed between neighbouring molecules in the solid state with the lowering of temperature. Moreover, at lower temperatures the molecules come closer together and association between next neighbours may take place. These new virtual bonds may be responsible for the increase in the number of low-frequency lines at lower temperature.This hypothesis of association of the molecules in the solid state in this particular case seems to be unavoidable in view of the fact that the intermole­cular field in the liquid state is too weak to make the frequency of angular oscillation as high as 10  ^Vsec. This can be seon from the results of investi­gation on the absorption of U. H. radio waves in meta xylene reported by Ghosh (1953). He observed an absorption peak of frequency 8.2 x lo^/sec in the case of m-xylene at -30 ®C  and this peak was attributed to dimers. The peak due to monomer was expected to have a frequency of the order of 10®/sec. The frequency of the angular oscillation of the single molecule in the solid state, therefore, cannot be greater than io ‘7sec in this case, unless there is a thousandfold increase in the intermolecular field. As the frequencies of the new lines are of the order of 10^^/sec the intermolecular field is much stronger in the solid state than that in the liquid, as pointed out recently bySirkar (1951).A  comparison of the number and positions of the low-frequency lines of the two compounds studied in the present investigation show that the relative positions of the substituents in the benzene ring have considerable influence on the low-frequency lines exhibited by these crystals. Also it is clear that the low-frequency lines of the para compound show considerable changes with lowering of temperature to i8o°C, while those of the meta compoundRaman Spectra of m- and p-Xylene in Solid State 30?i/3 D. C. Biswaspractically do not show such changes under similar condition. This difference in the behaviour of these two compounds with lowering of temperature is certainly related with the relative iiositions of the substituent groups in the benzine ring. The molecule of the meta compound, due to iheir unsymmetri- cal structure, possesses permanent electric moment. Hence the polar molecules of the meta compound get associated more easily even at a temperature just below the freezing point than the symmetrical molecules of the para compound, and with further lowering of temperature only the virtual bond may strengthen a little in the former case. The molecules of the para compound, however, come closer and closer together when the temperature is brought down to ^ i8o”C and the molecules probably disturb the symmetry of their neighbours only when they are sufBciently close to each other. The association of molecules in the case of the para compound is, therefore, complete only at very lew temperatures and not at a temperature just below its freezing point.(ci L ’nes of sni^le molecule in the solid state :All the piominent Raman lines of both the meta and the para compound in the liquid state are found in the Raman spectra of these substances in the solid state and the relative intensities of these lines remain unaltered with solidification. The frequency-shifts of the intense lines 517, 5/|o, 727, 1380, 1616 and 2920 cni*^ diminish slightly while that of t!ie line 231 cm”  ^ increases with solidification of the substance. In the case of the para compound also the lines 310 and 13S0 cm~’ shift respectively to 316 and 1375 cin“ * while a new line at 2942 ern''^ appears with solidification. This probably indicates that the influence of the interniolecular field on the modes of vibration of the molecules in the solid state defends on the symmetry of the molecule and that meta xylene n:olccule being more asymmetiic than the para xylene molecule greater number of vibrations are affected by intermolecular field in the former case than in the latter case.The line at 2874 cm*^ of the meta xylene due to C—H valence oscillation shifts to 2868 cm“  ^ with the solidification of the substance. Another line at 2920 cni^  ^ of meta xylene due to this C—H valence oscillation splits up into two lines at 2911 cm“'^  and 2962 cni~’ with the solidification of the substance. Similarly for the para compound the line at 2922 cm~’ bieaks up into two lines at 2921 cm~  ^ and 2942 cm“  ^ with the solidification of the substance. Such changes in the lines due to C—H valence oscillation of the xylene molecules, when the other intramolecular oscillations of xylene molecule are little affected with solidification, may lead one to conclude that the association of the neighbouring molecules in the solid state in case of xylenes probably takes place through the hydrogen atom. It may be pointed out here that even when polymerization takes place at the expense of a regular C—C bond in in methylmethacrylate, the other lines of the methylmethacrylate molecule donot change appreciably (Roy, 1953). Hence in the case of the xylenes also considerable changes in the positions and intensities of other lines are not expected if association takes place through C— H  bonds.A C K N O W I, K D G M E  N TThe author is indebted to Prof. S. C. Sirkar, D Sc., P \N .I. for his kind interest and constant guidance throughout this work.R K r  R R  E N C ^  vSBiswas, D, C., 1954, ./• Phys., 28, 85Deb, A. R .. J 9 5 2 ,  Ind J .  Phys  . 26, 201 „  i 9 « ; 3 ,  Ind ]  Phy ., 27, 1 8 3„  1954, Jnd. J. Phxs  , 28, 21.Ghosh, D. K , 1953, Ind. J  Phys  , 27, 285.Magat, M.. 1936, Annual Table of constants p. 26-^77.Roy, N. K ., 1953, Ind. J .  Phys., 27, 167.Sirkar, S. C , 3936, Jnd. J .  Phys., 10, i8g.M i 95ii /• Chem. Phys.^ 19| 256.Swamy, H N., 1951, Jnd. J .  Phys., 28, 263.M 1952, Jnd. J. Phys.t  26, 233... 1953, ^nd. J .  Phys., 27, 55.Raman Spectra of m- and p-Xylene in Solid State 3114—lilsaP- 7

The Raman Spectra of m- and p-Xylene in the Solid State at Different Low Temperatures

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The Raman Spectra of m- and p-Xylene in the Solid State at Different Low Temperatures

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