Padhye, M R

English

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THE NEAR ULTRA-VIOLET SPECTRA OF TOLUENE PART I. THE ABSORPTION BANDS WITH PARTI­CULAR REFERENCE TO I HE NEW BANDS OBSERVED.* By  M. R. PADH YU(Received for publication^  Nov, 5^Plate X IV'ABSTRACT. The absorption spectrum of toluene bus been studied between the temperatures o*’C and 250®C and different initial vapour iDressures, It was only after the investigations were being undertaken that a paper by Ginsberg, Matseii and Robert­son appealed. These authors have recorded and analysed a large number of bands. In addition to these bands, about twenty new bands on the longer wavelength side are also recorded 011 our plates. These bands Jiave been taf)ulated and analysed. The main results of tfie work are as follows :(]) The bands involve a combination of two or more quanta of vibrations in the ground state.(3) The intensity consideration leads to the classification of the band at 352O3 cm'* as due to the excitation of a single quantum of vibration with frequem y Thus anew vibration frequency is proposed,(3) Some bands are classified as due to the excitation of the iion-totally synmictrical frequencies not included in the ana^ vsis of the previous workers.(.-j) Incidentally a complete analysis of the already known absorption bands is also given and the results are di.scnsscd,1 N 'r R O D U C T I O NTile absorption spectrum of toluene recorded hy vSavard (1929) was first suggestively analysed by Sponer (T942) who gave also a Iheorelicai dis­cussion. The analysis comprises only a few bands. Soiiietinie after an ex­perimental investigation on these bauds was undertaken hy us, a paper dealing with ths same subject was published by Ginsberg, Malscn and RobertvSon (1946). They have recorded a large number of absorption bands of toluene. T'hey also give a detailed analysis of the bands observed by them wJiich confirms the first suggestive analysis by Sponer. The wavelength data re­corded by them go up to 2760^. In our experiments we have obtained llic data extended on the longer wavelength side up to 2891.S at higher tempera­tures thus recording 20 new bands. As they were thought likely to yield more informations regarding the vibrational levels in the ground state of the molecule, they are measured and reported here. The bands observed by Ginsberg and others are also recorded on our plates but they have not been measured.E X P R R T M K N T A hAn all^quartz absorption cel) of 10 cms. length was used. A few drops of the liquid were put in the cell which was then evacuated and sealed off.36332 M. R . PadhyeFor the clevelopineut of the bands on the longer wavelength side, the cell was sealed off when there was a small amount of liquid still left in it at room temperature It was not possible to estimate the exact pressure ofthe vapour in the cell but no bauds on the longer wavelength side of the already known bands were recorded at room temi)erature. The cell was then enclosed in an electric furnace. When the temperature of the celh; was in the range of Qo"C —iso^C the band-system developed new bands to­wards the longer waves- At still higher temperatures, continuous absorp­tion sets ill. Toluene supplied by the Fine Chemicals Section of this University was used without further purification. A  Hilger inedium quartz spectrograph was the recording instrument and Ha.stman 111-0 and Kodak Special Rapid plates were employed. ,The source of continuous radiation was a locally prepared ir type hyidrogen discharge tube run on a transformer delivering about 6,000 volts. \Two suitable plates were measured on a Hilger comparator (least \coiint .0001 cm.), one of them twice, and the data presented in 'rable I are the mean of the three values thus obtained for each band- The inten«ity ’ data are those visually estimated relatively on the basis of the 0,0 band at 37475”' taken as to. From the average difference in wavenunibeis found in the three readings, it is estimated that the values of wavenumbers recorded arc correct to within ± 3.T HE ORY  OF THE S P E C T R TT MiJ»ue to the substitution of CH3 in place of one of the H atoms of benzene, the symmetry Dba is reduced to This group has three elements olsymmetry namely the tw'o-fold axis and a plane of reflection perpendicular to the molecular plane. Thei'c can be only four different kinds of vibrations possible ; one, a,, being totally symmetrical and the others a<, h, and being non-totally symmetrical. All the vibrations are non-degenerate. The transition of benzene in this symmetry becomes Ai — U. and isallowed, as also the transitions involving «, vibrations which have the transi­tion moment lying in the jilane of the ring in a direction perpendicular to the C -C H a  bond. The direct product Table shows that transitions in­volving bj and types of vibrations are also aliowed since the direct product transforms like a translation but the moments in_lhis case lie in the other two perpendicular directions and hence these transitions give rise to weak bands. The symmetry considerations show that the transitions involving single excitation of b- vibration should be forbidden since tho transition integral is not invariant for any of the three moments.Since this point group has no degenerate vibrations, most of the degen­erate vibrations of benzene are split up into one totally symmetrical a, and one non-totally symmetrical vibrations. As in benzene, it can be expected that these large, number of a, vibrations will give rise to intense bands dhe* Positions of hands are given in units of cm'^ in this paper.Near Ultra-Violet Spectra o j Toluene 333to i»o 01 0-1 tiaiisition and also the couibinatious of these vibrations, i heor^tically we can expect to find weaker bands due to non-totally symme­trical vibrations of the type <12 and bi- An interesting feature of this part of the spectrum is that there is comparatively a strong band corresponding to the frequency of 620.''' This vibration has been classified as the h, part of the split 606 Cff vibration of benzene. In most of the mono-substituted benzenes so far studied, this is the only non-totally symmetrical vibration which has been found to be prominently excited. The other totally symme­trical frequencies are superposed on this. This part of the spectrum is quite analogous to that of benzene and the intensity of this transition show.s the extent to which the substitution has brought aliout perturbation in the six-fold symmetry of the ring. This also shows that some of the charac­teristics of the benzene spectrum are still carried in the spectra of its mono­substituents.R E M A R K S  ON THE A N A L Y S I S  O E THE A B S O R P T I O N  BANDS OF T OL UE NEPitzer and Scott (1943) classified the fundamental frequencies of toluene, as obtained from investigations of infra-red and Raman spectra, according to the various symmetry classes appropriate to the structure of the molecule and have compared the frequencies with corresponding ones of benzene. From the analysis of its absorption spectrum, Ginsberg ct al (1946) have proposed values for the frequencies in the excited state of the molecule. They find that of the ground state frequencies, six ai vibrations are given bv the absorption spectrum and in the excited stale five of these are obtained. These five ground state vibrations which can also be recogni.sed in the excited state with altered frequencies are 514, 785, 1003, 1012 and 1212. The 1176 vibration does not seem to have its excited frequency represented in the absorption spectrum.Among the non-totally symmetrical vibrations in the ground state, a frequency of 620, classified as corre.spondiug to cj 606 frequency of benzene, is found to be very prominent. Its value in the excited state as identified by Ginsberg is 528. Superimposed on the 0 —i and i —0 transitions of this vibration appear the already mentioned totally symmetrica] vibrations. The analysis of the absorption .spectrum shows that some more non-totally symmetrical frequencies are also probably excited. Gin.sberg found that the frequencies 845, 988 aa and 1062 bj were detectable in the spectrum. But .since the bands involving these frequencies also afford a possible analysis in terms of totally symmetrical vibrations, the latter classification is preferred by them.Ginsberg (1936) and others have classified some bands as due to inteicom- bination of thei^ frequencies. They also note that the stronger bauds arc, Frcqneucie.s are given iu cm"’ units.334 M. R . Padhyeuccompanictl towards Lhc' longer wavelengths, by fairly intense bands at spacings of 59 ami 178. These are suggested to be due to difference frequencies. The high intensity of the bands involving these difference' frequencies probably indicates that the bands arise in low frequency vibration. Tliey have thus clas.sified a large number of bands measured by them. Still some inten.se l»ands are left over and in an attenqjt to classify them we find that some of them involve as high as four quanta of different Uy vibi'ations. It is also possible to account for some more bands on the assumption that they involve other non-totally symmetric vibrations. Such new assignments for the recorded bands of Ginsberg are included in Table II.The excitation of the totally symmetric ijart of the c*, frequency 514 whose value in the excited state falls to 456 shows some characteristic differences from other substituted benzene sjrectra (Sponer and Wollman, 1941; and Wollman, 1946). In the case of halogen derivatives it is ^und that the 0 — 1 transition is very weak. But in the case of toluene as\also aniline and phenol it is found that o — I and even 0 — 2 transitions are fAirly iutertse. Similarly the halogen derivatives the o — i transition correspon­ding to bi frequency 620 is weaker than 0 - 2  tiansition but in toluene reverse is true. Sponer (1941) has explained these characteristics of the halogen derivatives on theoretical grounds.N K W HANDS AND T H K 1 R C L A S S 1 I C A 't' 1 O N#Twenty new bands have been mea.sured on our spectrograms which were not recorded by Ginsberg et <1/ (1946). The wavenumbers, intensity and the possible assignment is given in Tabic I. A s the bauds can only l>e observed at higher temperatures, it is probable that the higliei ground level frequencies may be involved in these bands. Analysis indeed shows that in some cases the ground level lias to be loaded with four quanta of different vibration frequencies to account for the bauds.In the present experiments the band at 35202 is the strongest in this part of the sped nun. It is possible to classify it as due to the excitation of 1062 b, together with 1212 Ui vibration in the ground state. An alterna­tive classification can be, exciuilion of two quanta of 620 61 with two quanta of 514 «i. Its high intens'iy is, however, incompatible w’ilb any of these classifications since both involve the excitation of llon-totally symmetric frequencies which ought to be weak. In the second alternative two quanta of non-totally symmetrical vibrations are excited and the intensity of such a transitioB is expected to be weak since the non-totally symmetrical vibrations have their minima one above the other. Hence we are inclined to regard the band as due to a fundamental frequency of 2273 in the ground state. The band at 36160 and 36050 can then be classified as due to superposition of 032 and 964 n, frequencies in (he excited stale over thi.s transition.p.vDHYE PLATE XIV-261HA(Cu)-35202Cm2W9'9A, ,  o  o  oU  O in o« 1 r*HO OVD CT)CX P6 a0 £X-M Ku;Fig. 1.Near Ultra-violet Spectra oj Toluene 335Ih e  bauds uext in intensity are 35087 and 35604. Ihc one at 35087 involves an excitation ot 1176 and 1212 in the {ground state. There are also bands corresponding to further loading of 620 bj and 514 di on this tran.sition. The band at 35604 which is 1871 wavenumbers distant from the 0,0 band is also one which doe.s not afford a convincing cla.ssificationl It can be due to excitation of three quanta of 620 61 freiiuency but intensity considerations do not support such a classification. The alternative is the excitation of 1178 ni and 1212 «i in the ground state with 528 in the excited state. The discrepancy between the observed and calculated values 'in  this case is, however, large.The band at 35496 is in a similar category. It can be regarded as due to excitation of two quanta of 998 vibration but itis fairly high intensity suggests the alternative classification of two quanta; of 1212 a, in ground state with 456 in the excited state. Here also the numerical agreement is not satisfactory. , ,In considering the intensity distribution in this part of the spectrum, it must be borne in mind that this part of the spectrum is developed only at higher temperatures. The intensity, therefore, corresponds to this temperature.There are also a number of bands due to combination of 514 ttj with 620 bi frequencies and also with other totally symmetric vibrations. The complete analysis is shown in Table I.T abi,k INew Bands in the Absorption Spectrum of TolueneY^ttc c in ^ Ini. Assignment Alternativeproposed assignments34574 u — ii7 6 “ 1 2 1 3-“5 I434658 1 —1176“  J012-62034764 2 -1176 — 1012 — 51434876 , 1 .3 — 2 X 13124-456—620 -2 X 988-62034983 3 -2 >c 1 3 1 3 4-4 5 «5—5^4 -2X988-51435087 5 —1176 — 1313 : -2X1176-785+75135202 S -3273 1 —1063-12121 -2X620 — 2X51435288 A — 1170—101235409 't — 2 X 1003 — 10134-964 — 2 X 103035496 2.5 — 2X 1313+456 — 2 X y8835604 5 -1176-1312 + 528 — 3 X62035687 0 -1003-78535720 I — 514 — 2 X 62035756 - 1 2 1 3- 51/13.5827 t> — 620-2X51435893 0 -1586 (i>i)36013 i -5 -1176-J3J3 + 9 3 236050 3 — J 176-T212+C6436160 3 -'2273+964 -  i(»62- 1213+964— 2 X620“ 2 X5144-96436568 0 -785-2x59 iT able IIKxleiidt'd analyisis of the Absorption Bands of Toluene  ^ Bands already analysed by Ginsberg3J6 M. R. Padhyei '^vnv cm ^ Assi^ijmeats cm Inl. Aiisigiiments36268+ - f  1212 37329 vw -620+528-5936301’^ fi ‘>-1176, '“ Joo3-*-178 43 vvw +932-106236415* a i -1003-59 56+ w }  . - 2 x 5 9w )65* v v w B D . —10128i+ ) —620+528.74^ n -1003 84 w V 195 ji ■' ' -988 93 VVW . + 9 32 -10 12136517* If -7 8 5 -17 8 37418+ m s } - 5 9  \21 m s  ) \71 -7 8 5 -2 x 5 9  ^ \VV7U -1003+964 \^6631+  ^ vvw -7 8 5 -5 9 \' 66+  ^ } -976* }» - 620—178 7093''' <  ^ w “ 785 37477'^ v v s  } (o-b). ;^ 37480 S i36729+ vvw -12124456 +  538-51495 vw35 II -6 2 0 -2  >cs9 + 528-2X178,37649 II — 1189-101263 II — 2 X 6204*52837706+ VV7P — ^ 20 +  964—2 X 59»4* vw -5 1 4 -1 7 860+ V2Cf + 4 5 6 -J7896* -6 20 -5985^ VVW HD -620 + 93236857+ , w —6207V — 620+96473 vvmB -1062+45'^'28^ W} 4-520—17836903* vvw - 5 1 4 - 5 9 31 7ej )63+ w' -514 50 vvxvhD — 2 X 1003 — 2 X 118937121* ' vvw — 2 X 178 59 vw*51* it — 456-785 72 V7V , , +456-5937191 vvw +932-1212 75^ >1 + 1189-78537206 II -620 +  528-178 37913 >nt?D — 620+2X52845* vw -1 7 8 -5 9 24"- II -514+964372991 * 33'; ] + .IS6 .ms — 178 36+ m )37302 J37304* vw -3 X 5 9 48+ IV + 5*8 - 5 953 m )IS $f -620+45693'^ + 5*8 - 92 5 9* -1212+2X528 98 s iNear Ultra-violet Spectra of TolueneT abjle II {contd.)337*^Tac Int, Assignments T^fio cin*^ Int. Assignments38cx)S*o8 1 0+528 3873^ m'10 VS i 49" wBD +751+528-980; vw BD +932-334 37+ m> +751+52838173'' vw +751-5959 xn 538212 wD + 2X/156—178 94 vw ' +964+528-17828“^31ni 1 m > +75138805 vvzvBD + 4 s6 +932- 5q,+2X751-178,33 '^ +932-17840 m +3X45659^ +964-178 38869'-m' +456+93203 7V } Sr w + 932+528-59S2 VW +528+456-17R 3803«+ w' +932+528-993 ;^ •> + 9 3 2 - 2  *59 41+ 5 +932 +52838317 '' vvxv + 3 ’‘ 456-3X178 60'- xvBD +964+528-021 if +964-2 X 59 72 *■ s' +964+52838342 vxvV — 1003+2 X932 84+ w +751+932-178, 2X75151^54 VI  >5 +032-59 39065+ vvxv +456+1189—50, + 3 X 52882+ m BD +964-5999 vxv + 751+032-59.91'■ vxv +2X45639124+ wBD +456+118938402'^ m BD +932-944+47vw ) V7V } +2x456+751.+456+528+751-5909^ + 9 3 2 62' m +751+932i r vs SvtBD +964-9,39204-^ s + 1189+52839335+ vxv' +456+3X528-178,41+964+456+932+528-594456m )vw BD  j 41+ m > +2X052456+528 43 m >86 i1 + 1189-1,-8, +3X456-2 X 178 6 7 ^ mBD +032+964-93S539' %nnv +2X528 75+ m sV  J +932+96447" vvivBD + 1189—2x5977‘ 91%v ) IV D +456+528+0327738601+11 +528+751--178m '39406+ +2X964w BD \ +1189-59JO w B D i • vS2 vw 964+1189-17866+ +1189 39513+ V7VBD +2X751+528,72 m s  5 +456+3x^28338T able  II icontd,)M, R. Padhye* hit. Assiguitienls v^tiG cm.’ ^. lilt. AssiginTOwts /,^9543 VU'> +932+1189-59 3 9Q0 3 *' +932 + 964+5^ 83g6o2‘^ tna* •1-932+ Ti8g 30+ XV D + 3X964+52S39636”* s' + 964 “h 1189 45" vxvBD +2x751+96 ,^+751+1189+52862 vtu +751+964+528-59 40135^ til* +932 + 1189+52872 \ vwBD1 +3 X7SI--59ee" w' +2x751 + 11893969 >■'■ m' + 751+932+52S .10306*' xuBD +2x932+9643 7^ 2 S^ w -l-3 >«7.SJ.+528+751+964xvD + 2 X 1189+J28-5937* XV BD + 2x964+03^82 i1V7L> + 3X456+9324 0 '*' vvxvBD +751+932 “HiV 893gSon'‘' w' +2X932+45*181^ »» + 2Xl 189 + 528I vwD +456+751 + 1189-594>i5 5 S \w' +751 + 932+964'^ 5' vxv +932+964+11895 3 " 7U/) 4  2 X1189 40.816 vxv + 2 X J 189 + 96472'' m' +456+751 + 1189, + 528 + 2 X932 41030 VXfXV + 3 X 1189a c k n o w l e d g m k n t sIn conclusion, the author wishes to express his sincere thanks to Prof. R. K . Asundi for his valuable guidance and encouragement. Thanks aie also due to Mr. B. Bhaitacharyya for his help in analysing the lecordcd data on the absorption of toluene and to Dr. Majiundar of Allahabad Univer­sity, for permission to use the Comparator.IlKNAiuiS Hindu UniversitvR R r  K R K N C U SOinsberg, Matsen and Robertson, 1946, /. Chcm, Phy., 14, s n  I'itzer and Scott, 1943, J, Am. Chcm. S o c ., 68, S03.Savard, 1929, Ann. dc Chemic,, 11, 287.Spnner, 1942, /. ChMtt. Phy., 10, 672.Spoiler and Teller, 1941, Rrv. Mod. Phy., 1 3 , 119,Sponer and Wollnian, ,1041, y. Chem. Phy., 9 , 816 Wollman, 194^ , J. Chcm. Phy., H, 123.

The Near Ultra-Violet Spectra of Toluene Part I. the Absorption Bands with Particular Reference to the New Bands Observed

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The Near Ultra-Violet Spectra of Toluene Part I. the Absorption Bands with Particular Reference to the New Bands Observed

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