Vibrational and Electronic Absorption Spectra of 2,3- and 2,4-Difluoro Benzaldehydes

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Indian J . P h y s .

66

B (2). 221-225 (1992)

Vibrational and electronic absorption spectra of 2,3- and 2,4-difluorobenzaldehydes

N R Katti, M K Aralakkanavar, P |L Jeergal, Rekha Rao and M A Shashidhar Department of Physics, Kamatak Universit]^ Dharwad 580 003, India

Received 13 May 1991, accepted 12 July 1 9 ^

I

Abstract : Raman, infrared and electron^ absorption spectra have been observed for 2,3> and 2,4- difluorobenzaldehydes and vibrational aikalysis has been made.

Keywords : Raman and infrared specinit electronic absorption, vibrational analysis.

PACS Nos : 33.10. Gx, 33.20. Ea, 33.20. Fb

Many investigators have studied the ultraviolet, infrared and Raman spectfa o f bcnzaldchyde and their derivatives (Buraway and Chamberlain 1952, Chandra and Sharma 1963, Garg, 1953-54, Imanishi e t a l 1952, Jaiswal 1969, Jaiswal and Sharma 1969, Morion and Stubs 1941, Padhye e t a l 1961, Patel 1959, Rao and Rao 1960, Robinson 1954, Singh and Juyal 1965). In benzene and substituted benzenes, the electronic system '■^ig (2600 A) has been studied in great detail. In substituted benzenes

(2100

A) transition has not been studied much in vapour phase. Because o f the few studies on the second system, it was felt that a continuation o f the studies o f electronic, infrared and Raman spectra o f som e more trisubstituted benzenes would be worthwhile, as this would give som e insight into the structure o f the m olecules in their ground and excited states. A s a part o f our research programme, the present note reports the vibrational analysis o f the Raman spectra, infrared and electronic absoiption spectra o f 2,3- and 2,4- difluorobenzaldehydes.

The experimental techniques used to record the vapour absorption electronic spectra and infrared and Raman spectra in the pure liquid phase and accuracy in the positions o f the bands reported are the same as given in our earlier work (Shashidhar 1971).

2,3- and 2,4-difIuorobenzaldehydes may be considered as belonging to the point Q ,if we co n sid ^ -CHO group to behave as a single mass point and coplanar with the benzene ring. The total n u m b ^ o f 36 normal m odes o f vibration will have the distribution 25a’ + 1 la ”. The Raman and infrared abswption spectra have been analysed keeping in view o f the symmetry o f the m olecules, the analysed infrared and Raman data o f benzaldehydc and substituted benzaldehydes. It m ust how ever be em phasized that many m odes are consideraUy mixed precluding in fact, the use o f terms such as stretching,or deformation. It

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222 N R Katti et al

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N R K a t t i e t a l

might be noted that a ring stretch also implies a certain amount o f C-H deformation and vice versa. Thus with some assignments, a certain vibration turns out to be predominenily a stretch or a deformation whereas in others this might not be the case. The fundamental vibrational frequencies so chosen are listed in Table 1, along with their assignments.

The electronic absorption spectra o f 2,3- and 2,4- difluorobenzaldehydes show two n

« - n systems, one consisting o f rather well defined bands around 2 900 A (System I) and the other having som e weak bands around 2300 A (System II). Even under the w idely varied experimental conditions with the specuographs used, it was not possible to improve

the

System II. So this system w as a lso recorded on a H itachi U -3 2 0 0 UV/VIS spectrophotometer in vapour phase and in this record, som e bands were observed in

the

region 2300 to 2100

A.

The System I has been identified as corresponding to

the

forbidden electronic transition 'U

2

u<~ '•4ig and the System II to the u-ansition ' Bju

benzene.

‘4ig of

Ultraviolet spectra o f System I o f the molecules lie approximately in the region 2900 A. The effect o f temperature on the band system has facilitated the choice o f the bands at 33750 cm"' and 35445 cm"’ as the 0, .0 band in 2,3- and 2,4-difluorobenzaldehyde respectively. The identification o f the fundamentals chosen in compounds has been done ■ mainly on the basis o f intensities o f the bands and the available literature on the analyses of Qther trisubstituted benzenes. The strong bands on the longer wavelength side with spacing

68

cm"’ and 66 cm"* in 2,3- and 2,4-difluorobenzaldehydes hqve been identified as sequence intervals. The band data are with the authors. The analysis o f the electronic spectra of the m olecules shows a red shift in the order o f 2,3- > 2,4-difluorobenzaldehyde with respect to benzene. Such a trend has also I m i iq x a ied in literature (Padhye and Varadarajan 1963).

In System II, bands observed towards the lo n g ^ wavelength side at 41706 cm"' and 41654 cm"’ in 2,3- and 2,4-difIuorobenzaldehydes have been identified as 0, 0 bands. The identified excited state fundamentals are also listed in Table 1 and they are correlated to the fundamentals o f System I. The existence o f such a System II has been reported in the literature (Amma e t a l 1%9, Goel and Agarwal 1982).

References

Amma R A, Nair K P R and Rai D K 1969 Appl. Spectrosc. 23 616 Burawoy A and Chamberlain J T 1952 7. Chem. Soc. 2310 Chandra K and Sharma D 1963 Indian J. Phys. 37 405 Garg S N 1953-54 J, Sci. Res. 4 42

Goel R K and Agarwal M1 1982 Spectrochim. Acta 38A 583 Imanishi S, Semba K Ito M and Anno J 1952 Butt. Chem. Soc. 25 150 Jaiswal R M P 1969 Indian J. Pure Appl. Phys. 7 47

Jaiswal R M P and Sharma D 1969 Indian J. Pure Appl. Phys. 7 338 Morton R A and Stubs A L 1941 J. Chem foe. 1347

Padhye M R and Varadarajan T S 1963 Indian J. Pure Appl. Phys. X 25 Padhye M R and Viladkar B G 1961 J. Sci. Indus Res. 20B 530

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Paicl

J c 1959

/ . Sci Indus Res.

18B 265

Rao I A 1961 J. Sci. Indus Res. 20B 523 Rao IA and Rao V R 1960 Indian J. Phys. 34 196 Robinson G W 1954 J. Chem. Phys. 22 1384 Shashidhar M A 1971 Specirochim. Acta 21A 2363

Singh N L and Juyal D P 1965 Indian J. Pure Appl. Phys. 3 342

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