Rai, D K

Shukla, J V

Upadhya, K N

English

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Raman and infrared spectra of 2,5-difluoro-and 2,4-difluoro nitrobenzeneJ. V. Shttkla, K. N. U p a d h y a  and D. K. R a iDepartment of Physics, Spectroscopy Laboratory, Banaras Hindu University, Varanasi-5.{Received 24 October 1972, revised 16 December 1972)Indian J, Phya. 48, 53-67 (1974)The infrared spectra of 2,4- and 2,5-difluoronitrobonzene in the liquid pliaso have been recorded for the first time in the region 250- 4000 cm” b These studies have been supplemented b}^  their Raman spectra excited by the (5328 A So radiation from a He-Ne Laser. Moaauremont of the state of polarization of tJio Raman lines have been made and the observed vibrational frequencies liave been assigned to different modes.1, I n teo d u ctio nIn comparison to the vast amount of experimental work on the vibrational spectra of disubatituted benzenes containing the NOg group (vide all the references), the work on trisubstituted benzenes is rather scanty. It is also apparent from a survey of the literature that multi-homosubstitutod benzenes arc more thoroughly studied than the heterosubstitutod ones. As a part of our program of studying a survey of the literature that multi-homoaubstituted benzenes are more thoroughly studied than the heterosubstitutod ones. As a part of our programme of studymg the vibrational spectra of substituted nitrobenzenos (SJuikla et al 19G8, Shukla & Upadhya 1969) we have reported here the results of our studies on 2,4- and 2,6-difluoronitrobenzones. The available spectroscopic study of these molecules is concerned with the electronic absorption spectrum in hexane solution.2. E x p e r im e n t a l  M eth o dThe samples of pure grade used in these experiments were obtained fromE. Merck (Germany) and were used without further purification. These are liquids at room temperature and the infrared spectra were recorded using a film of the liquid between two CsBr windoks, with a Perkin-Elmer 621 Grating infra- red spectrophotometer in the range 260-4000 cm~ .^ The Raman spectra were obtained by the courtesy of Dr. N. Yellin, and were excited by the 6328 A radia­tion of No from a He-Ne Laser source. Spectra have been taken of the scattered radiation polarized in the direction parallel and perpendicular to the incident5354 J. V . Shukla, K , N. Upadhya and D. K . Raiexciting beam, by using a polarizer. The polarization measurements were made from the two spectra from the same molecules.3. Results and DiscttssionsThe infrared and Raman frequencies of the two difluoronitrobonzenes are shown in figures 1 and 2*. Table 1 lists the absorption bands and the assignmentsTable 1. Fundamental vibrational frequencies of 2,4- and 2,5-difluoronitroben-zone (cm“^ )2,4-difluoronitrobenzano 2,5-difiuoronitrobenz(^ neRaman Infrared Int.+ Raman Infrared Int.^Assignments293 d 293 2.5430 d464 d 464 ].l613 d 613 3618 d 618 8676 2 .6737 d 736 6748 d 760 7843 p 843 6.9861 8827 5.6976 p 976 91091 p 1090 81141 p 11.39 41165 p 1166 81194 2.51237 p 1237 41280 p 1283 101319 41360 p 1350 101466 51494 p 1493 61636 p 1536 9.61680 p 1610 103081 6140 d370 d 407 d 464 d635 p 643 p 671 d 717 d764 p 780 p820 p830 p943 p 1071 p 1137 p1192 p1265 p1300 p 1349 p1497 p 1441 p1603 p27‘1407466480635642672764776819834872942107011371162119212641284130013471436149614401603307631142.54.6 1C--NO2 twisting C-O-C o.p. bonding C-F o.p. bending C-F o.p. bending C-NO2 o.p. bonding1 C-C-C o.p. bending6 C-C-C i.p. bonding2 C-C-C i.p. bending 4 6 C-C-C o.p. bondingO-H o.p. bonding7 C-F i.p. bending9 C-F i.p. bending9 Ring breathing9 C-NOn i.p. bonding7 C-H o.p. bending5.6 C-C-C trigonal bonding3 C-H i.p. bending7 C-II i.p. bonding2.5 C-H i.p, bending9 C-F stretching9.5 C-F stretchingC-NO2 stretching0 C — C stretching10 N = 0  stretching7 C = C stretchingaa"a**tt"a"a''a'a'a'^a"fir'a'a'a'a"a'a'a'a'a'a'a'a'a'a'10106.676AbbreviationsC *= C stretching a'N as O asymmetric stretching a' C C stretchingC S5= C stretching o'C «= C stretching o'C C stretching o'' depolarized line; p w polarizedo.p. = out-of.plane; i.p. = -in-plane; d : line; int. =« intensity.+ Maximum intensity is 10.tlott T**— r  l^ belod pemendiclar to the incident radia-it L ^ i ^ ^  i f t  imply perpendicular to the electric vectorradiation and paraUel toihe oleotrio vector in the incident radiation reepectively.j, Shukla, K. N. Upadhta akd D. K. Rai Indian J. Phys. Vol. 48, No. 1. 1974Fi^. 1 'rhf Infmivd ami Kaiimn S[u*i*tfa of 2,*l*'liflu()i'>nitrol)<‘JiZH»‘.Fig. 2. The Infrared and Raman Spect ra 2,6-difliioromtrobenzeue,Raman and infrared spectra etc. 66mado. The NOg groups are assumed to lie in the plane of the ring, both these molecules would belong to the Cg point group. The thirty benzene—like funda­mental vibrational modes would then bo divided into 21 planar [a ) and 9 non- planar (u") vibrations. In addition there would bo six fundamentals duo to the internal vibrations of the NOg group (4ff/+2u") making a total of thirty six vibra­tions in all. All vibrations are active in infrared and Raman. Tiie a" vibrations would give rise to depolarized Raman linos. These two types of vibrations are discussed below under separate headings.(A) Benzenp Type Vibraiion.s(1) C~// Vibrcaion^ : Three C-H stretchings, tlu'oe C-H in -piano beiidingsand throe C-H out-of-plano bonding modes are expected to appear in the spectra of those tw(^  molecules. The sirotcliing vibrations are expected in the infrared region of 3000-3100 cin^ and for 2,4-difhioronitrobenzone are assigned at 3075 and 3114 (?in *. Only one peak is observed in tlie region at 3081 cm^ f(jr tJie other compound. in-plane bending vibrations ari) expected to liave magni­tudes h’ing in tlie range 1050 to 1150 cm h and are oxp(^cted to bo polarized in the Raman .spoctriun. On tliis basis tlie frequenci(\s at 1091, 1140 and 1155 cm  ^for 2,4-i.somer, and 1071 and 1102 enr^ for the 2,5-isomer, respectively, have boon astiigiud to those mides. Only two out-oi-plane bending vibrations could be located, at 737 and 827 cm » for the 2,4-isomer and at 717 and 782 cin-^ for 2,5-isomor.(2) Skeletal Vibraiion^ a : The various carbon vibrations in multisubstitiited benzenes are cliarac.terizcd by different amounts of substitution effects, and hence tlie assignments are quite complicabd. Tiio doubly degenerate vibrations all split into two comi>ononts and for Cg symmetry give rise to six polariztd and two non-poJariztd Raman linos. Tlie former arise from tlu^  vibrations at froquen(}it\s 1485, 1585 and 000 om-^ S whereas, the latter arise from tiie 404 cm~  ^vibration. In all oases one component lies at a lower frequoiujy. From analogy with assignments for similar molecules from the group of vibrations at 513, 618, 1494, 1536, 1609 cm ^ for 2,4-difluoronitrobenzono, the first two ai*o assigned to the (C-C-<C) vibration and the last throe to the (0 =- C) mode. One com­ponent of the 1485 cm-^ (ej„) vibration of benzene is observed at 1455 cm~  ^ in the infrared spectrum, but not observed in Roman spectrum. The corresponding frequencies in 2,5-difluoronitrobenzene at 635, 643, 1497, 1541 and 1603 cm^  ^appeared in both the Raman and infrared spectra, wliile the band at 1436 appears only in the infrared spectrum. The assignment of the higher component of 404 om'*^  ( l^u) vibration is not clear os the (C-NO2) rooking and (C~F) bending also appear in this region. The lower component is observed at 293 and 1276 cm~i, respectively, in the 2,4- and 2,6-compounds. Four non-degenerate vibrations give rise to three polarized and one unpolarized Raman lines. In the Raman56 J. V . Shukla, K . N . Upadhya and D. K . Raispectrum of 2,5-difluoronitrobouzojio we liave polarized linos at 820, 943, 1300 cm-i and a non-polarizod lino at 671 cm~^ and we have assigned them to the ring breathing, C-C-C in-plauo bonding, C~C stretching and C-C~C out-of-plane bending, respectively, corre>sponding to the frequencies 992, 1010, 1310 and 703 cm~  ^ of benzene. The similar lines ibr 2,4-difluoronitrobenzene are 843, 976, 1319 and 676 cm"  ^ respectively. Of those the last two are observed only in the infrai*ed spectrum.(3) Vibrations of the Substituted Nitro Group : Excluding the vibrations of the NO  ^ group wo expect six jDolarizod and tliroe depolarized Raman lines, corresponding respectively to the C-F and C-NO2 stretching and planar bending modes and the C~F and ONOg out-of-plane bending modes.Both C-F and O-NO., strotclung vibrations arc found in the neighbourhood of 1250 cm~  ^ and unique assignments of the observed frequencies in this region are not possible. We have assigned the C-NOo stretching vibration at 1282 and 1264 cm-  ^ and the tvo G~F stretching at 1197, 1237 and 1192, 1265 cm~  ^in 2,4- and 2,5-difluortmitrol)enzene molecules. The 2,5-isomer frequencies observed at 830, 780 and 754 enr^ arc assigned to the one C~NOo and tlio two C-F in-plane bending modes. Tlu) C-NO2 planar bonding mode is assigned at 861 cm  ^ in 2,4-isomer but- only <»u<‘ V -K in-plane mo(l(^ . could be assigned at 748 cm A depolarized line is observed at 454 cm~^  in botli the molecules and is assigned to the C-NOg out-of-plano rocking vibration. Only one C-F non- planar bending could bo located at 430 cm^i for the 2,4-isomor, whereas bands observed at 370, 407 cm~i have been assigned to the same mode in the other mole­cule.The six internal vibrations of the NOg gi'oup give rise to four polarized and two depolarized Raman lijies. Polarized Ihw^ s observed at 1350 and 1536 cm’  ^for the 2,4. and at 1349 and 1541 cm "* for the other isomer have been assignf>d to the N -= 0  symmetric and asymmetric stretching modes respectively. The higher frequencies appear as broad peaks in tlio spo(.tra duo to superposition of a C =  C sti-otching vibration arising as the second component of the 1585 cm'-  ^vibration of benzene. A highly depolarized Raman line at 146 cm““i in 2,5- difluoronitrobenzone has boon assigned to the C-NO, t-wisting vibration The other mtemal vibrations could not be assigned due to overlapping in the region of their expected appearance.Ackn owledqmentTh. wfch to oipres, their thmks to Dr. N. YelJin (Hebrew CniverMtyJera.*m ), for «H,ordmg U.e „peetr., -e l  to Prof. N. L. Sirwh f »  h ikind interest in the work.Raman and infrared spectra etc. 67RetbbenobsBobovioh Ya. S. & Balyaevskaya M. M. 1965 Optika i  Spektroskopiya, 19, 198.Borek F. 1963 NcUurwisaenachaften 50, 471.Green J. H., Kynaston W. & Lindsay A. S. 1961 Spectrochim. Acta 17# 486.Kineigasa T. & Nakashima K. 1963 N ippon KagakUy Zaaahi 84, 365.Medhi K. C. 1964 Spectrochim, Acta 20, 675.Stephenson O. V., Coburn (Jr.) W. C. & Wilcox W. 8 . 1961 Spectrochim* Acta 17, 933. Shukla J. V., Singh V, B. & Upadhya K. N. 1968 Indian J , Phya, 42, Oil.Shukla J. V. & Upadhya K. N. 1969 Ind, J , Pure & A p p l, Phya, 7, 83#

Raman and Infrared Spectra of 2,5-Difluoro-and 2,4-Difluoro Nitrobenzene

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Raman and Infrared Spectra of 2,5-Difluoro-and 2,4-Difluoro Nitrobenzene

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