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Electronic absorption spectra of 5- and 6- fluoroindoles
Spectrochimica Acre, Vol. 42A, No. 7, pp. 781-783, 1986.
0584 8539/86 $3.00+0.00 Pergamon Journals Ltd.
Printed in Great Britain.
Electronic absorption spectra of 5- and 6-fluoroindoles N. H. AYACHIT, A. M. HURALIKOPPI and M. A. SHASHIDHAR* Department of Physics, Karnatak University, Dharwad 580003, Karnataka, India (Received 4 July 1985; in final form 6 November 1985; accepted 23 January 1986) Abstract--The electronic absorption spectra of 5- and 6-fluoroindoles, corresponding to the 2 2850 A system ofindole, have been recorded in the vapour phase and analysed, assuming C, symmetry for the molecules. The observed band system in both the molecules which lies in the region 23100-2680 A has been identified as a ~*,-- n transition corresponding to a ~A' ,- 1A' transition. The i.r. spectra of these molecules have also been recorded and analysed and the results are used in the analyses of the electronic spectra. These show that there is not much change in the shape and very little change in size of either of these molecules in the excited electronic states.
INTRODUCTION
The interpretation and correlation of spectra of compounds containing one or two benzene rings fused to a five-membered heterocyclic compound is found to be difficult because of their inherently complicated nature and the problem of overlapping bands of different electronic transitions. Solution electronic spectra of such molecules will be very difficult to interpret, while the study of their vapour spectra taken at low vapour pressures is expected to give some information about the sizes and shapes of these molecules in their excited states. Indole is one such compound in which a benzene ring has been attached to a five-membered nitrogen heterocycle. The electronic absorption spectrum of this molecule which lies around 2 2850 A and which shows a well-defined vibrational structure has been studied in detail by HOLLAS [1] and HOLLAS and KHALIL]POUR [2] and has been attributed to a n* *--n transition. The results of investigations on the vapour electronic absorption spectra of some substituted indoles are also available in the literature [3-131, most of which have been reported from our laboratory. These studies were carried out with a view to investigating the effect of different substituents at different positions on the 2 2 8 5 0 A system of indole and knowing about their structures in the ground and excited states. In spite of this, we still find that work on the spectra of such molecules, particularly in the vapour phase, is not extensive enough to enable us to come to a worthwhile conclusion. In view of this, it has been thought worthwhile to carry out studies on the spectra of more such molecules. Therefore, we have recorded and interpreted the vapour phase electronic spectra and i.r. spectra in KBr pellets and Nujol mulls of 5- and 6-ftuoroindoles. The results are reported and discussed in this paper. EXPERIMENTAL
Solid samples of 5- and 6-fluoroindoles supplied by the Aldrich Chemical Co. U.S.A. were used without further purification to record their vapour phase electronic absorption spectra and i.r. absorption spectra. The vapour phase absorption spectra were recorded at various temperatures in
the range 0-150°C, with a large quartz spectrograph on Agfa Gevaert 34B50 photographic plates using different absorption path-lengths ranging from 5 to 100 cm. The accuracy of the measurements of the positions of the bands in the spectra is a little better than + 5 cm- t for sharp and strong bands and about + 10 cm-t for weak lines. The i.r. absorption spectra were recorded using a Perkin-Elmer double beam automatic i.r. spectrophotometer using KBr pellets and Nujol mulls in the range 200-4000 cm-t. The accuracy of the positions of the i.r. bands is better than + 5 cm - t in the range 200-2000 cm -t and is better than + 10cm -t in the range 2000-4000 cm - 1. RESULTS AND DISCUSSION
The i.r. bands of 5- and 6-fluoroindoles are listed in Table 1. The analysis of these spectra was carried out assuming C s symmetry for the molecules. The 42 normal modes of vibration for both the molecules are then classified among a' and a" species as 2 9 a ' + 13a", and both a' and a" species are active in both i.r. and Raman spectra. The proposed vibrational analyses of these spectra are also given in Table 1. The electronic spectra of substituted indoles available in the literature show that some of them permit vibrational analysis since in such cases the spectrum extends over a large region on both sides of the O,O band and contains a large number of very sharp, well-defined bands, while a few show diffuse band systems which will not permit vibrational analysis. The spectra of 5- and 6-fluoroindoles recorded in the present work show a well-defined vibrational structure. The spectrum in each case spreads over the region 2 3100-2630 A. Studying the effect of temperature on the band system has facilitated the choice of the band at 34 342 cm-1 in the spectrum of 5-fluoroindole and the band at 34 984 c m - 1 in the spectrum of 6fluoroindole as the O,O bands. The O,O band is the strongest in the electronic system of each molecule and this, along with the general appearance of the band system, shows that the absorption spectra observed in 5- and 6-fluoroindoles belong to the z~* ~- g transition. This is in agreement with the suggested transitions for the spectra of other benzo derivatives [ 12, 13]. In view of the C s symmetry of the molecules, the bands o f the lt*~- n system arise due to I A ' , - I A ' transitions and 781
SA(A)42:7-~
782
N. H. AYACHIT et al. Table 1. Infrared absorption spectra of 5- and 6-fluoroindoies* Fundamental frequencies (cm- i )t 5-Fluoroindole
Assignment Description Species of mode:[:
6-Fluoroindole
255 (w) 335 (m), 370 (s), 405 (sh), 465 (s), 490 (m) 440 (vs) 530 (sh), 600 (s), 635 (m), 730 (s), 950 (ms) 765 (s), 1345 (s), 1420(s), 1460(vs), 1485 (s), t550(m), 1570 (sh), 1580 (s) 785 (m), 800(m), 860 (s), 890 (ms), 935 (m) 1060 (m) 1090(ms), ll15(s), 1135 (s), 1190 (sh), 1280 (s) 1235 (s) 1325 (s) 1620 (s) 3010 (m), 2050 (mw) 3090(mw), 3120(m), 3140 (mw) 3420 (vs)
260 (w) 330 (m), 360 (s), 400 (m), 475 (s), 510 (s), 435 (ms) 540 (ms), 605 (m), 630 (mw), 740 (ms), 950 (vs) 755(s), 1345 (s), 1410(s), 1450(vs), 1495 (vs), 1515 (s), 1570 (m), 1590 (vs) 765 (sh), 800 (s), 845 (s), 900 (mw), 930 (m) 1055 (m) 1090(ms), l120(s), 1140 (vs), 1200 (ms), 1295 (s) 1235 (s) 1320(s) 1625 (vs) 3010 (w), 3040 (w), 3060(m), 3100(s), 3130 (ms) 3400 (vs)
a~ a"
),(C-F) 7 (C--C)
a' a'
fl(C-F) //(C~C)
a'
v(C--C)
a~
?(C-H)
a"
7 (N-H) //(C-H)
a'
a' a'
v (C-F) v (C~N) //(N-H) v (C-H)
a'
v (N-H)
a'
a'
*The i.r. spectra of both the molecules were taken in both KBr and Nujol mulls. In this table the wavenumbers of the bands below 500 c m - 1 are reported from the spectra of the molecules taken in Nujol while the wavenumbers of the bands above 500 cm-1 are reported from the spectra of molecules taken in KBr. i'Letters in the parantheses after the fundamentals give their visual intensities: w -- weak, m = medium, m w = medium weak, s -- strong, vs -- very strong, ms -- medium strong, sh -- shoulder. :[:v -- stretch,//--- in-plane bend, 3, -- out-of-plane bend. are a n a l o g o u s to the b a n d s o f t h e 22850,/~ system o f indole. T h e f u n d a m e n t a l b a n d s are easily picked o u t f r o m the b a n d g r o u p s in t h e electronic spectra o f these molecules b y l o o k i n g a t t h e i r relative intensities. T h e choice o f these f u n d a m e n t a l s was also m a d e k e e p i n g in view t h e d a t a available for indole [1] a n d o t h e r s u b s t i t u t e d indoles [ 3 - 1 2 ] . T h e f u n d a m e n t a l s so
c h o s e n are listed in T a b l e 2 a l o n g with thei r p r o b a b l e assignments. T h e d a t a available f r o m the literature for indole [1] a n d o t h e r h a l o g e n a t e d indoles [4, 9] are given in this table. T h e l o n g e r w a v e l e n g t h side b a n d s with spacings 39, 66, 133 a n d 231 c m -1 f r o m t h e 0 , 0 b a n d in t h e s p e c t r u m o f 5-fluoroindole a n d 12, 46, 61, 147 a n d 227 c m - 1 f r o m the O , 0 b a n d in the s p e c t r u m o f 6 - f l u o r o i n d o l e h a v e b e e n identified as sequence
Table 2. Correlation of fundamental frequencies of some substituted indoles Indole
[11
Fundamental frequencies (cm- ~) 5-Clfloroindole 5-Bromoindole 5-Fluoroindole* [4] [9] Ground Excited Ground Excited Ground Excited state state state state state state
6-Fluoroindole* Assignmentt Ground state
Excited state
-433 540 732 755 952 1108 ----
350 406 455 670 711 875 957 985 1097 1204
Ground state
Excited state
--612 -760 --
--
354 -538 717 736 907 968 989 1120 --
--
1313
--
1316
--
--
--
1318
--
1312
v(C,~)
--
1459
--
1446
--
--
--
1382
--
1380
v(CffiC)
---
--
* Present
4'16 362 610 . 779 1047 1106 1190 -730
.
367 343 540 . 730 904 951 1038 1157 658
1 325 --
__ 255 --
---992 1201 --
744 ------
.
_ 436 530 726 763 950 1085 1129 1 --
work.
t X = substituent,//= in-plane bend, 3' = out-of-plane bend, v = stretch.
372 406 470 669 702 893 938 969 1117 1221
?(C--C) //(C-X) //(C==C) v(C--C) //(C--C) //(C-H) //(C-H) //(C-H) //(C-H) v(C-X)
Spectra of 5- and 6-fluoroindoles
783
Table 3. Position of O,O band and red-shifts of 5- and 6-fluoroindoles in different solvents Solvent
Dielectric constant (e)
5-Fluoroindole Position of Red-shift O,O band (cm - t)
(cm -1) Vapour Hexane n-Heptane Carbon tetrachloride Chloroform
1 1.89 1.96 2.22 4.30
34 342 34 236 34 178 34061 33 888
intervals (v-v transitions). It is found that most of the bands in the respective spectra could be accounted for in terms of identified fundamentals and sequence intervals as combinations and overtones. Indole is highly electronegative among benzo derivatives and any substituent of the same nature makes the whole ring become more electronegative, which is the case in the present work. This alters the transition energies of the molecules. Hence this will affect the intensity and shift the position of the O,O band. This system in the spectra of 5- and 6-fluoroindoles was also recorded in different solvents. The position of the O,O band of this system of these molecules in different solvents is given in Table 3, from which it may be noted that the O,O band in a solvent shifts towards the red and that this red shift increases with increase in the dielectric constant (e) of the solvent. This is consistent with the assignment of the transition of this system observed for these molecules to n* ~-n [14, 15]. Most of the excited state fundamentals identified in the vapour spectra are traced up to two quanta and they form combinations showing that they correspond to totally symmetric vibrations (a'). The ground state fundamentals in the spectra of these molecules are in agreement with their i.r. data. The low frequency vibrations which give rise to sequences are most likely non-totally symmetric and do not occur in this system even in single quanta. The ratio of the intensity of the (O,O) band to that of next intense band has been measured as approximately 1.9 in both molecules, nearly the same result as in indole in which this ratio is 2 [1]. This result, together with the fact that no progressions of more than two members have been observed, shows that there is not much change in shape and very little change in the size of the molecule in its excited state, for both the molecules.
6-Fluoroindole Position of Red-shift O,O band (cm- t)
(cm -1) -106 164 281 454
34 984 34 832 39 772 34 651 34 354
-152 212 333 630
Acknowledgements--The authors thank Professor K. SURYANARAYANARAO, Department of Physics, Karnatak University, Dharwad, India for his encouragement shown in the work and Mr. JAGADEESHTONANNAVARfor his help in carrying out this work. One of the authors (N.H.A.) is also thankful to the University Grants Commission, New Delhi, India, for financial assistance.
REFERENCES
[1] J. M. HOLLAS,Spectrochim. Acta 19, 753 (1963). [-2] J. M. HOLLASand E. KHALILIPOUR,J. molec. Spectrosc. 66, 452 (1977). [.3] M. A. SHASHIDHARand K. SURYANARAYANARAO, Indian J. Phys. 52B, 162 (1977). [.4] N. H. AYACHIT,P. V. SHANABHAG,K. SURYANARAYANA RAO and M. A. SHASHIDHAR,Indian J. pure appl. Phys. 22, 360 (1984). [.5] M. A. SHASH1DHARand K. SURYANARAYANARAO,Curr. Sci., India 42, 313 (1973). [.6] A. M. HURALIKOPPI, N. H. AYACHIT and M. A. SHASHIDHAR,Indian J. pure appl. Phys. 22, 310 (1984). [.7] A. M. HURALIKOPP1, N. n. AYACHIT and M. A. SHASHIDHAR,Indian J. Phys. 58B, 230 (1984). [.8] A. M. HURALIKOPPI,N. H. AYACHIT,J. TONANNAVAR, K. SURYANARAYANARAO and M. A. SHASHIDHAR,Bull. pure appl. Sci., India 3, 59 (1984). [9] N. K. SANYAL,S. L. SRIVASTAVAand S. R. TRIPATHI, Spectrochim. Acta 38A, 933 (1982). [.10] N. K. SANYALand S. R. TRIPATHI, Spectrochim. Acta 39A, 67 (1983). [.11] N. K. SANYALand S. R. TRIPATHI,Indian J. Phys. 58B, 105 (1984). [12] S. C. WALT,JR. and C. A. PINKHAM,J. molec. Spectrosc. 27, 326 (1968). [13] R. C. HECKMAN,J. molec, Spectrosc. 2, 27 (1958). [14] R. D. GORDONand R. F. YANG,Can. J. Chem. 48, 1722 (1970). [.15] n. n. JAFF'Eand M. ORCHIN, Theory and Application of Ultraviolet Spectroscopy. Wiley, New York (1962).
0584 8539/86 $3.00+0.00 Pergamon Journals Ltd.
Printed in Great Britain.
Electronic absorption spectra of 5- and 6-fluoroindoles N. H. AYACHIT, A. M. HURALIKOPPI and M. A. SHASHIDHAR* Department of Physics, Karnatak University, Dharwad 580003, Karnataka, India (Received 4 July 1985; in final form 6 November 1985; accepted 23 January 1986) Abstract--The electronic absorption spectra of 5- and 6-fluoroindoles, corresponding to the 2 2850 A system ofindole, have been recorded in the vapour phase and analysed, assuming C, symmetry for the molecules. The observed band system in both the molecules which lies in the region 23100-2680 A has been identified as a ~*,-- n transition corresponding to a ~A' ,- 1A' transition. The i.r. spectra of these molecules have also been recorded and analysed and the results are used in the analyses of the electronic spectra. These show that there is not much change in the shape and very little change in size of either of these molecules in the excited electronic states.
INTRODUCTION
The interpretation and correlation of spectra of compounds containing one or two benzene rings fused to a five-membered heterocyclic compound is found to be difficult because of their inherently complicated nature and the problem of overlapping bands of different electronic transitions. Solution electronic spectra of such molecules will be very difficult to interpret, while the study of their vapour spectra taken at low vapour pressures is expected to give some information about the sizes and shapes of these molecules in their excited states. Indole is one such compound in which a benzene ring has been attached to a five-membered nitrogen heterocycle. The electronic absorption spectrum of this molecule which lies around 2 2850 A and which shows a well-defined vibrational structure has been studied in detail by HOLLAS [1] and HOLLAS and KHALIL]POUR [2] and has been attributed to a n* *--n transition. The results of investigations on the vapour electronic absorption spectra of some substituted indoles are also available in the literature [3-131, most of which have been reported from our laboratory. These studies were carried out with a view to investigating the effect of different substituents at different positions on the 2 2 8 5 0 A system of indole and knowing about their structures in the ground and excited states. In spite of this, we still find that work on the spectra of such molecules, particularly in the vapour phase, is not extensive enough to enable us to come to a worthwhile conclusion. In view of this, it has been thought worthwhile to carry out studies on the spectra of more such molecules. Therefore, we have recorded and interpreted the vapour phase electronic spectra and i.r. spectra in KBr pellets and Nujol mulls of 5- and 6-ftuoroindoles. The results are reported and discussed in this paper. EXPERIMENTAL
Solid samples of 5- and 6-fluoroindoles supplied by the Aldrich Chemical Co. U.S.A. were used without further purification to record their vapour phase electronic absorption spectra and i.r. absorption spectra. The vapour phase absorption spectra were recorded at various temperatures in
the range 0-150°C, with a large quartz spectrograph on Agfa Gevaert 34B50 photographic plates using different absorption path-lengths ranging from 5 to 100 cm. The accuracy of the measurements of the positions of the bands in the spectra is a little better than + 5 cm- t for sharp and strong bands and about + 10 cm-t for weak lines. The i.r. absorption spectra were recorded using a Perkin-Elmer double beam automatic i.r. spectrophotometer using KBr pellets and Nujol mulls in the range 200-4000 cm-t. The accuracy of the positions of the i.r. bands is better than + 5 cm - t in the range 200-2000 cm -t and is better than + 10cm -t in the range 2000-4000 cm - 1. RESULTS AND DISCUSSION
The i.r. bands of 5- and 6-fluoroindoles are listed in Table 1. The analysis of these spectra was carried out assuming C s symmetry for the molecules. The 42 normal modes of vibration for both the molecules are then classified among a' and a" species as 2 9 a ' + 13a", and both a' and a" species are active in both i.r. and Raman spectra. The proposed vibrational analyses of these spectra are also given in Table 1. The electronic spectra of substituted indoles available in the literature show that some of them permit vibrational analysis since in such cases the spectrum extends over a large region on both sides of the O,O band and contains a large number of very sharp, well-defined bands, while a few show diffuse band systems which will not permit vibrational analysis. The spectra of 5- and 6-fluoroindoles recorded in the present work show a well-defined vibrational structure. The spectrum in each case spreads over the region 2 3100-2630 A. Studying the effect of temperature on the band system has facilitated the choice of the band at 34 342 cm-1 in the spectrum of 5-fluoroindole and the band at 34 984 c m - 1 in the spectrum of 6fluoroindole as the O,O bands. The O,O band is the strongest in the electronic system of each molecule and this, along with the general appearance of the band system, shows that the absorption spectra observed in 5- and 6-fluoroindoles belong to the z~* ~- g transition. This is in agreement with the suggested transitions for the spectra of other benzo derivatives [ 12, 13]. In view of the C s symmetry of the molecules, the bands o f the lt*~- n system arise due to I A ' , - I A ' transitions and 781
SA(A)42:7-~
782
N. H. AYACHIT et al. Table 1. Infrared absorption spectra of 5- and 6-fluoroindoies* Fundamental frequencies (cm- i )t 5-Fluoroindole
Assignment Description Species of mode:[:
6-Fluoroindole
255 (w) 335 (m), 370 (s), 405 (sh), 465 (s), 490 (m) 440 (vs) 530 (sh), 600 (s), 635 (m), 730 (s), 950 (ms) 765 (s), 1345 (s), 1420(s), 1460(vs), 1485 (s), t550(m), 1570 (sh), 1580 (s) 785 (m), 800(m), 860 (s), 890 (ms), 935 (m) 1060 (m) 1090(ms), ll15(s), 1135 (s), 1190 (sh), 1280 (s) 1235 (s) 1325 (s) 1620 (s) 3010 (m), 2050 (mw) 3090(mw), 3120(m), 3140 (mw) 3420 (vs)
260 (w) 330 (m), 360 (s), 400 (m), 475 (s), 510 (s), 435 (ms) 540 (ms), 605 (m), 630 (mw), 740 (ms), 950 (vs) 755(s), 1345 (s), 1410(s), 1450(vs), 1495 (vs), 1515 (s), 1570 (m), 1590 (vs) 765 (sh), 800 (s), 845 (s), 900 (mw), 930 (m) 1055 (m) 1090(ms), l120(s), 1140 (vs), 1200 (ms), 1295 (s) 1235 (s) 1320(s) 1625 (vs) 3010 (w), 3040 (w), 3060(m), 3100(s), 3130 (ms) 3400 (vs)
a~ a"
),(C-F) 7 (C--C)
a' a'
fl(C-F) //(C~C)
a'
v(C--C)
a~
?(C-H)
a"
7 (N-H) //(C-H)
a'
a' a'
v (C-F) v (C~N) //(N-H) v (C-H)
a'
v (N-H)
a'
a'
*The i.r. spectra of both the molecules were taken in both KBr and Nujol mulls. In this table the wavenumbers of the bands below 500 c m - 1 are reported from the spectra of the molecules taken in Nujol while the wavenumbers of the bands above 500 cm-1 are reported from the spectra of molecules taken in KBr. i'Letters in the parantheses after the fundamentals give their visual intensities: w -- weak, m = medium, m w = medium weak, s -- strong, vs -- very strong, ms -- medium strong, sh -- shoulder. :[:v -- stretch,//--- in-plane bend, 3, -- out-of-plane bend. are a n a l o g o u s to the b a n d s o f t h e 22850,/~ system o f indole. T h e f u n d a m e n t a l b a n d s are easily picked o u t f r o m the b a n d g r o u p s in t h e electronic spectra o f these molecules b y l o o k i n g a t t h e i r relative intensities. T h e choice o f these f u n d a m e n t a l s was also m a d e k e e p i n g in view t h e d a t a available for indole [1] a n d o t h e r s u b s t i t u t e d indoles [ 3 - 1 2 ] . T h e f u n d a m e n t a l s so
c h o s e n are listed in T a b l e 2 a l o n g with thei r p r o b a b l e assignments. T h e d a t a available f r o m the literature for indole [1] a n d o t h e r h a l o g e n a t e d indoles [4, 9] are given in this table. T h e l o n g e r w a v e l e n g t h side b a n d s with spacings 39, 66, 133 a n d 231 c m -1 f r o m t h e 0 , 0 b a n d in t h e s p e c t r u m o f 5-fluoroindole a n d 12, 46, 61, 147 a n d 227 c m - 1 f r o m the O , 0 b a n d in the s p e c t r u m o f 6 - f l u o r o i n d o l e h a v e b e e n identified as sequence
Table 2. Correlation of fundamental frequencies of some substituted indoles Indole
[11
Fundamental frequencies (cm- ~) 5-Clfloroindole 5-Bromoindole 5-Fluoroindole* [4] [9] Ground Excited Ground Excited Ground Excited state state state state state state
6-Fluoroindole* Assignmentt Ground state
Excited state
-433 540 732 755 952 1108 ----
350 406 455 670 711 875 957 985 1097 1204
Ground state
Excited state
--612 -760 --
--
354 -538 717 736 907 968 989 1120 --
--
1313
--
1316
--
--
--
1318
--
1312
v(C,~)
--
1459
--
1446
--
--
--
1382
--
1380
v(CffiC)
---
--
* Present
4'16 362 610 . 779 1047 1106 1190 -730
.
367 343 540 . 730 904 951 1038 1157 658
1 325 --
__ 255 --
---992 1201 --
744 ------
.
_ 436 530 726 763 950 1085 1129 1 --
work.
t X = substituent,//= in-plane bend, 3' = out-of-plane bend, v = stretch.
372 406 470 669 702 893 938 969 1117 1221
?(C--C) //(C-X) //(C==C) v(C--C) //(C--C) //(C-H) //(C-H) //(C-H) //(C-H) v(C-X)
Spectra of 5- and 6-fluoroindoles
783
Table 3. Position of O,O band and red-shifts of 5- and 6-fluoroindoles in different solvents Solvent
Dielectric constant (e)
5-Fluoroindole Position of Red-shift O,O band (cm - t)
(cm -1) Vapour Hexane n-Heptane Carbon tetrachloride Chloroform
1 1.89 1.96 2.22 4.30
34 342 34 236 34 178 34061 33 888
intervals (v-v transitions). It is found that most of the bands in the respective spectra could be accounted for in terms of identified fundamentals and sequence intervals as combinations and overtones. Indole is highly electronegative among benzo derivatives and any substituent of the same nature makes the whole ring become more electronegative, which is the case in the present work. This alters the transition energies of the molecules. Hence this will affect the intensity and shift the position of the O,O band. This system in the spectra of 5- and 6-fluoroindoles was also recorded in different solvents. The position of the O,O band of this system of these molecules in different solvents is given in Table 3, from which it may be noted that the O,O band in a solvent shifts towards the red and that this red shift increases with increase in the dielectric constant (e) of the solvent. This is consistent with the assignment of the transition of this system observed for these molecules to n* ~-n [14, 15]. Most of the excited state fundamentals identified in the vapour spectra are traced up to two quanta and they form combinations showing that they correspond to totally symmetric vibrations (a'). The ground state fundamentals in the spectra of these molecules are in agreement with their i.r. data. The low frequency vibrations which give rise to sequences are most likely non-totally symmetric and do not occur in this system even in single quanta. The ratio of the intensity of the (O,O) band to that of next intense band has been measured as approximately 1.9 in both molecules, nearly the same result as in indole in which this ratio is 2 [1]. This result, together with the fact that no progressions of more than two members have been observed, shows that there is not much change in shape and very little change in the size of the molecule in its excited state, for both the molecules.
6-Fluoroindole Position of Red-shift O,O band (cm- t)
(cm -1) -106 164 281 454
34 984 34 832 39 772 34 651 34 354
-152 212 333 630
Acknowledgements--The authors thank Professor K. SURYANARAYANARAO, Department of Physics, Karnatak University, Dharwad, India for his encouragement shown in the work and Mr. JAGADEESHTONANNAVARfor his help in carrying out this work. One of the authors (N.H.A.) is also thankful to the University Grants Commission, New Delhi, India, for financial assistance.
REFERENCES
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