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Vibrational spectra and assignments for 2,3-dibromothiophene and 2,3,5-tribromothiophene
Speclrochimico Acto, Vol. Printed in Great Bntain
39A.
No.
Vibrational
II. pp. 1013-1017.
0584-8539183 $3.00 + 0.00 Pergamon Press Ltd.
1983
spectra and assignments for 2,3-dibromothiophene 2,3,5_tribromothiophene
and
GIULIO PALIANI, ROSARIO CATALIOTTI and MARIAROSARIA PELLEIY Dipartimento di Chimica, Laboratorio di Chimica Fisica dell’Universit8, Via Elce di Sotto, 8-06100 Perugia, Italy (Received 29 April 1983) Abstract-The infrared spectra of 2,3dibromoand 2,3,5tribromothiophene have been studied from 4000 to 200 cm- ‘. The Laser Raman spectra have also been recorded and depolarization values have been measured. An assignment of the 21 fundamental vibrations is proposed based on group frequency correlations, Raman polarization data and comparison with the spectra of parent and some halogeno-substituted molecules.
INTRODUCTION
EXPERIMENTAL
Our laboratory has been carrying out studies of vibrational nature on halogen derivatives of thiophene for some years [l, 21. We had, in the past, been occupied with the vibrational spectrum of this molecule[3] in order to bring out the spectroscopic modifications induced by its low temperature phase transitions. There are two basic reasons for investigating this class of compound. First, we wish to expand the enquiry to as large a series as possible of this type of molecule, both as regards the type of halogen and as regards its position or positions with respect to the hetero-atom. By so doing, we hope to be able to clarify the relationships that exist between similar modes in different but structurally correlated molecules, and in particular the correlations between “group frequency” vibrations. Given the sufficient number of papers already published in the literature in this respect, our intention is to give a systematic description of the vibrational modes, relative to the single case of the halogens, such as the ones that already exist for benzene and its derivatives [4]. Second, analyses of this type are of interest in as far as, in our opinion, the most accurate vibrational assignment is indispensable for obtaining transferable force fields that not only contain a given number of adjustable parameters and lead toa satisfactory reproduction of the observed frequencies, but are also physically reasonable. The series of halogen derivatives of thiophene lends itself well to studies of this type in that many molecules have already been obtained in other laboratories, and, with relatively simple systems, it is possible to obtain almost all the desired thiophene substitutes. This note, then, reports the assignment for 2,3dibromothiophene and 2,3,5_tribromothiophene, for which, to the best of our knowledge, there are no data in the literature. sA,*j
39:11
-
F
1013
The molecules studied were commercial products. 2,3dibromothiophene was purified by double distillation under vacuum, and 2,3&tribromothiophene using repeated crystallization from methyl alcohol. Their purity, tested using G. L. chromatography, was better than 99 %. The i.r. spectra were recorded with a Perkin-Elmer Mod. 983 grating spectrophotometer using conventional cells. Laser Raman snectra were obtained by a JobinYvon HG 2S double monochromator using the 5i45A radiation of a Spectra Physics Model 16406 argon laser as the exciting line. Both i.r. and Raman spectra were estimated accurate to *lcm. RESULTS AND DISCUSSION
The molecules were supposed planar, in analogy with the parent molecule and its mono- and 2,5disubstituted derivatives [5-81. The spectra were thus interpreted in terms of a C, symmetry, the 21 normal vibrational modes being distributed between 15 vibrations of the A’ species and 6 of the A” species, all active both in the Raman and in the i.r. The assignment was made by basing ourselves on the following: (i) the analogy with previous assignments on bromine derivatives of thiophene, bearing in mind that our aim was that of obtaining one single unified assignment for a series of closely correlated molecules rather than a collection of independent ones for each single molecule; (ii) the polarizability and ratios of polarization between Raman lines; (iii) an examination of the relative absorption intensities in the i.r. and Raman spectra. It is then possible to construct “sequences” in which the frequencies and/or intensities of certain absorptions remain stable or vary regularly with the increasing degree of substitution in the thiophene ring. Figures 14 show the spectra obtained and Tables 1 and 2 the proposed assignments. In the following sections comments will be made only on certain general features.
1014
GIULIO PALIANIet al.
f 3000
i~00
I 1200
1600
I 800
I 400
Fig. 1. Infrared spectrum of 2,3-dibromothiophene.
I 500
I 1000
I 1500
3000
Fig. 2. Raman spectrum of 2,3-dibromothiophene.
3200
1600
12OO
800
400
Fig. 3. Infrared spectrum of 2,3,5-tribromothiophene.
3200
Vibrational spectra of thiophene derivatives
1015
r
Fig. 4. Raman spectrum of 2,3,Stribromothiophene.
2,3-Dibromothiophene
The six normal modes involving the hydrogen atoms (2 CH stretchings, 2 CH bend, 2 CH 0.0.~. deformations) correspond in heterocyclic compounds with pentatomic ring, to localized group frequencies, only marginally coupled with the rest of the molecule. They are, in fact, found at more or less the same frequency as
the corresponding modes in the parent molecule. It must, however, be noted that the 7,-n mode, corresponding to vibration 9 of the thiophene, which in thiophene and in the 2-halogen-thiophene gives rise to a very weak absorption, is absent in the molecule under consideration, analogous to what is found for 3halogen-derivatives.
Table 1. Vibrational spectral data and assignments for 2,3-dibromothiophene i.r. (cm-‘)
Raman (cm-l)
P
Assignment*
3108 sh 3089 m 1497 vs 1448 VW 1399 s 1338 vs 1281 VW 1164 m 1148 s 1083 mw 1040 VW 1040 VW 992 vs 920 VW 860 vvs 798 m 701 vvs 651 m 631 VW 581 s 460 mw
3107 ms 3087 sh 1496 w
P P P
“CHA’ ‘CHA’ “Ri”# 1338f110; 798+651
1400 VW 1339 w
0.2 0.5
1163 w 1148 w 1082 m
0.5 0.6 0.3
390 w 326 w 241 w 222 mw
990 m 924 vvw 858 mw 796 ms 700 vvw 649 s 630 w 578 VW 458 w 440 sh 389 vs 325 vvs 240 m 222 mw 148 s 110 vs
0.3
“Ring
A’ A’
7;;Y?58 1 701+460 ‘CH A’ ‘CH A’ ‘CH A’ 651+390; 581 f460 X-Sells (VCRr + Vaing)A’ 2 x 460
0.: 0.1 dp 0.2
*RingA’ ?CHA”
0.;5 0.7
388 + 240 A” ‘iRII-I~ Y&n A” 2x522
0.7 0.2 0.4 0.75 0.75 0.6
*Y: stretching; 6: i.p. deformation; y: 0.0.~. deformation.
“Ring A’
‘Ring
A’
VCd’
&A’ ‘CBr A’ i’CBrA” -?CBrA” ‘CBr A’
Gm~ro PALIANI et al.
1016
Table 2. Vibrational i.r. (cm-‘) 3099 1504 1411 1298 1287 1149 1132 1041 1008 993 968 948 811 729 671 573 476 470 337 275 258 229
spectral
Raman (cm-‘) m KS ms s mw mw ms VW VW vs m VW vs w mw m m s w w VW vvw
3101 1504 1412 1199
mw VW vs mw
1149 VW 1132 m
data and assignments
P P
for 2,3,Stribromothiophene
Assignment 'CHA' ‘Ring”
0; 0.2
“Ring *’ “Ring*’
1132+159; 811+476 993+159:811+337 0.2
992 mw 969 VW
P P
810 vvw
dp
670 vs
0.1
572 VW 477 m
dp 0.2
337 277 256 229 159 123 108
0.3 0.2 0.4 0.1 0.74 0.7 0.6
6,; A'
811+229;573+470 671 f337 X-sens (vCar+ vRine)A’ X-sens (vCBr + vRing)A’ 671+ 275 YCHA” 573+159;470+258 *Ring *’ YRmR
YRing
s w VW vvs ms s s
Three modes that may be associated with skeletal stretchings are found around 1500, 1400 and 1350 cm- ’ (corresponding to the 14,4 and 5 modes of thiophene); they are not influenced by the position or by the nature of the substituent. The two remaining ring stretchings (3 and 17 in thiophene) are, on the other hand, strongly coupled with the C-Br vibrations, especially when a halogen is present in the CIposition with respect to the heteroatom (2-X thiophenes and 2,5-X thiophenes). In our opinion, the two intense i.r. bands at 990 and 860cm-i, which find their correspondence in medium intensity polarized Raman lines, must be correlated to these modes. The four ring deformations (2 in-plane and 2 out-ofplane) are found at frequencies that are more or less unaltered with respect to 3-bromothiophene. The remaining in-plane modes correspond to very intense polarized Raman lines and are associated with the stretchings and deformations of the C-Br bonds. The two lowest-lying depolarized Raman bands are undoubtedly connected with the 0.0.~. deformations of the C-Br bonds.
2,3,5-Tribromothiophene
The three vibrations involving movements of the hydrogen atom are easily localized at 3102, 1132 and 811 cm-’ and, again analogous to 2,3-dibromothiophene, we have the assignment of the three skeletal modes corresponding to the 14, 4 and 5 vibrations of thiophene and of the remaining out-of-plane yR and ycBRvibrations.
A”
BRing *’
A”
VCB*A’
"CBrA’
&B, A’ ‘CBr
A’
YCBr A” YCB~*” ‘k~r
A’
The attribution of the remaining 10 in-plane mcdes (2 ring stretchings, 2 ring deformations, 3 C-Br stretchings and 3 C-Br deformations) requires a more detailed discussion. Their assignments are based on more speculative arguments in so far as these vibrations do not correspond to “good group frequencies”, since they are mixed in various ways among themselves dependent on the position and/or the number of the substituents present in the ring. In the region below 400 cm- ‘, there are five polarized Raman peaks in the spectra; these may be tentatively correlated with the six modes involving movement of the bromine atoms, essentially because far-i.r. absorptions for similar molecules investigated up to now seem to have this as their origin. At this point, the four skeletal modes, predicted to absorb in the 45(rlOOOcm-’ region [8810] are still to be assigned. In fact, four i.r. absorptions of noticeable intensity, corresponding to polarized Raman lines are present in the spectra of 2,3,5-tribromothiophene at 477, 670, 969 and 992cm-‘. A comparison with the spectra of 2,5dibromothiophene in this region is of particular interest; the vibrational behaviour of this molecule was studied by GREEN[9] and its assignment seems to us to be more or less definitive. In the 2,5_dibromothiophene molecule, the modes in question are correlated with the absorptions at 418,646,948 and 982, which are only a little less in frequency than the absorptions we observed in 2,3,5_tribromothiophene. Therefore the assignment we are proposing finds support in this close analogy in spectral behaviour.
Vibrational spectra of thiophene derivatives CONCLUSIONS At this point, it would undoubtedly be of interest to compare the vibrational assignments of the various bromothiophenes investigated (2-bromo-, 3-bromo-, 2,3-dibromo-, 2,5-dibromo-, 2,3,5_tribromothiophene) and it is already possible to draw some conclusions that are both sufficiently reliable and of certain interest from a chemical point of view. We note: (i) the existence of a certain number of “stable” group frequencies; (ii) the tendency of a halogen in the a position to increase the degree of mixing between the ring modes and the substituents’ own modes, while a substituent in the fi position tends to lessen this mixing; (iii) the small effect produced by the rise in mass consequent on a higher degree of substitution. However, we think necessary to obtain further data examining other bi-, tri- and tetra- substituted halogen thiophenes (work in progress) before attempting a rationalization of the effects observed up to now. Acknowledgements-We
thank Prof. F. FRINGUELLIand Dr.
1017
F. PIZZO for their help in the purification of the molecules investigated. The financial assistance of the Consiglio Nazionale delle Ricerche (CNR, Roma) is also greatly acknowledged. REFERENCES [l] G. PALIANIand R. CATALIO~I,Spectrochim. Acta 37A, 707 (1981). [2] G. PALIANIand R. CATALIOT~I, Spectrochim. Acta 38A, 751 (1982). [3] G. PALIANI,A. PoLErnand R. CATALIOTTI, Chem. phys. Lett. 18, 525 (1973). [4] J. R. SCHERERand J. C. EVANS,Spectrochim. Acta 19, 1739 (1963). [5] M. RICO, J. M. ORZAand J. MORCILLO,Spectrochim. Acta 21A, 689 (1965) and references cited therein. [6] G. PALIANI, R. CATALIOITI, A. POLETTI, F. FRINGUELLI, A. TATICCHI and M. G. GIORGINI, Specwochim. Acta 32A, 1089 (1976). [7] M. ORAK, I. J. HYAMS and E. R. LIPPINCOTT, Spectrochim. Acta 22, 1355 (1966). [S] I. J. PERON, P. SAUMAGNE and J. M. LEBAS, Spectrochim. Acta 26A, 1651 (1970) and references cited therein. [9] J. H. S. GREEN, Spectrochim. Acta 27A, 2015 (1971). [lo] A. ROGSTAD,Spectrochim. Acta 31A, 1749 (1975).
39A.
No.
Vibrational
II. pp. 1013-1017.
0584-8539183 $3.00 + 0.00 Pergamon Press Ltd.
1983
spectra and assignments for 2,3-dibromothiophene 2,3,5_tribromothiophene
and
GIULIO PALIANI, ROSARIO CATALIOTTI and MARIAROSARIA PELLEIY Dipartimento di Chimica, Laboratorio di Chimica Fisica dell’Universit8, Via Elce di Sotto, 8-06100 Perugia, Italy (Received 29 April 1983) Abstract-The infrared spectra of 2,3dibromoand 2,3,5tribromothiophene have been studied from 4000 to 200 cm- ‘. The Laser Raman spectra have also been recorded and depolarization values have been measured. An assignment of the 21 fundamental vibrations is proposed based on group frequency correlations, Raman polarization data and comparison with the spectra of parent and some halogeno-substituted molecules.
INTRODUCTION
EXPERIMENTAL
Our laboratory has been carrying out studies of vibrational nature on halogen derivatives of thiophene for some years [l, 21. We had, in the past, been occupied with the vibrational spectrum of this molecule[3] in order to bring out the spectroscopic modifications induced by its low temperature phase transitions. There are two basic reasons for investigating this class of compound. First, we wish to expand the enquiry to as large a series as possible of this type of molecule, both as regards the type of halogen and as regards its position or positions with respect to the hetero-atom. By so doing, we hope to be able to clarify the relationships that exist between similar modes in different but structurally correlated molecules, and in particular the correlations between “group frequency” vibrations. Given the sufficient number of papers already published in the literature in this respect, our intention is to give a systematic description of the vibrational modes, relative to the single case of the halogens, such as the ones that already exist for benzene and its derivatives [4]. Second, analyses of this type are of interest in as far as, in our opinion, the most accurate vibrational assignment is indispensable for obtaining transferable force fields that not only contain a given number of adjustable parameters and lead toa satisfactory reproduction of the observed frequencies, but are also physically reasonable. The series of halogen derivatives of thiophene lends itself well to studies of this type in that many molecules have already been obtained in other laboratories, and, with relatively simple systems, it is possible to obtain almost all the desired thiophene substitutes. This note, then, reports the assignment for 2,3dibromothiophene and 2,3,5_tribromothiophene, for which, to the best of our knowledge, there are no data in the literature. sA,*j
39:11
-
F
1013
The molecules studied were commercial products. 2,3dibromothiophene was purified by double distillation under vacuum, and 2,3&tribromothiophene using repeated crystallization from methyl alcohol. Their purity, tested using G. L. chromatography, was better than 99 %. The i.r. spectra were recorded with a Perkin-Elmer Mod. 983 grating spectrophotometer using conventional cells. Laser Raman snectra were obtained by a JobinYvon HG 2S double monochromator using the 5i45A radiation of a Spectra Physics Model 16406 argon laser as the exciting line. Both i.r. and Raman spectra were estimated accurate to *lcm. RESULTS AND DISCUSSION
The molecules were supposed planar, in analogy with the parent molecule and its mono- and 2,5disubstituted derivatives [5-81. The spectra were thus interpreted in terms of a C, symmetry, the 21 normal vibrational modes being distributed between 15 vibrations of the A’ species and 6 of the A” species, all active both in the Raman and in the i.r. The assignment was made by basing ourselves on the following: (i) the analogy with previous assignments on bromine derivatives of thiophene, bearing in mind that our aim was that of obtaining one single unified assignment for a series of closely correlated molecules rather than a collection of independent ones for each single molecule; (ii) the polarizability and ratios of polarization between Raman lines; (iii) an examination of the relative absorption intensities in the i.r. and Raman spectra. It is then possible to construct “sequences” in which the frequencies and/or intensities of certain absorptions remain stable or vary regularly with the increasing degree of substitution in the thiophene ring. Figures 14 show the spectra obtained and Tables 1 and 2 the proposed assignments. In the following sections comments will be made only on certain general features.
1014
GIULIO PALIANIet al.
f 3000
i~00
I 1200
1600
I 800
I 400
Fig. 1. Infrared spectrum of 2,3-dibromothiophene.
I 500
I 1000
I 1500
3000
Fig. 2. Raman spectrum of 2,3-dibromothiophene.
3200
1600
12OO
800
400
Fig. 3. Infrared spectrum of 2,3,5-tribromothiophene.
3200
Vibrational spectra of thiophene derivatives
1015
r
Fig. 4. Raman spectrum of 2,3,Stribromothiophene.
2,3-Dibromothiophene
The six normal modes involving the hydrogen atoms (2 CH stretchings, 2 CH bend, 2 CH 0.0.~. deformations) correspond in heterocyclic compounds with pentatomic ring, to localized group frequencies, only marginally coupled with the rest of the molecule. They are, in fact, found at more or less the same frequency as
the corresponding modes in the parent molecule. It must, however, be noted that the 7,-n mode, corresponding to vibration 9 of the thiophene, which in thiophene and in the 2-halogen-thiophene gives rise to a very weak absorption, is absent in the molecule under consideration, analogous to what is found for 3halogen-derivatives.
Table 1. Vibrational spectral data and assignments for 2,3-dibromothiophene i.r. (cm-‘)
Raman (cm-l)
P
Assignment*
3108 sh 3089 m 1497 vs 1448 VW 1399 s 1338 vs 1281 VW 1164 m 1148 s 1083 mw 1040 VW 1040 VW 992 vs 920 VW 860 vvs 798 m 701 vvs 651 m 631 VW 581 s 460 mw
3107 ms 3087 sh 1496 w
P P P
“CHA’ ‘CHA’ “Ri”# 1338f110; 798+651
1400 VW 1339 w
0.2 0.5
1163 w 1148 w 1082 m
0.5 0.6 0.3
390 w 326 w 241 w 222 mw
990 m 924 vvw 858 mw 796 ms 700 vvw 649 s 630 w 578 VW 458 w 440 sh 389 vs 325 vvs 240 m 222 mw 148 s 110 vs
0.3
“Ring
A’ A’
7;;Y?58 1 701+460 ‘CH A’ ‘CH A’ ‘CH A’ 651+390; 581 f460 X-Sells (VCRr + Vaing)A’ 2 x 460
0.: 0.1 dp 0.2
*RingA’ ?CHA”
0.;5 0.7
388 + 240 A” ‘iRII-I~ Y&n A” 2x522
0.7 0.2 0.4 0.75 0.75 0.6
*Y: stretching; 6: i.p. deformation; y: 0.0.~. deformation.
“Ring A’
‘Ring
A’
VCd’
&A’ ‘CBr A’ i’CBrA” -?CBrA” ‘CBr A’
Gm~ro PALIANI et al.
1016
Table 2. Vibrational i.r. (cm-‘) 3099 1504 1411 1298 1287 1149 1132 1041 1008 993 968 948 811 729 671 573 476 470 337 275 258 229
spectral
Raman (cm-‘) m KS ms s mw mw ms VW VW vs m VW vs w mw m m s w w VW vvw
3101 1504 1412 1199
mw VW vs mw
1149 VW 1132 m
data and assignments
P P
for 2,3,Stribromothiophene
Assignment 'CHA' ‘Ring”
0; 0.2
“Ring *’ “Ring*’
1132+159; 811+476 993+159:811+337 0.2
992 mw 969 VW
P P
810 vvw
dp
670 vs
0.1
572 VW 477 m
dp 0.2
337 277 256 229 159 123 108
0.3 0.2 0.4 0.1 0.74 0.7 0.6
6,; A'
811+229;573+470 671 f337 X-sens (vCar+ vRine)A’ X-sens (vCBr + vRing)A’ 671+ 275 YCHA” 573+159;470+258 *Ring *’ YRmR
YRing
s w VW vvs ms s s
Three modes that may be associated with skeletal stretchings are found around 1500, 1400 and 1350 cm- ’ (corresponding to the 14,4 and 5 modes of thiophene); they are not influenced by the position or by the nature of the substituent. The two remaining ring stretchings (3 and 17 in thiophene) are, on the other hand, strongly coupled with the C-Br vibrations, especially when a halogen is present in the CIposition with respect to the heteroatom (2-X thiophenes and 2,5-X thiophenes). In our opinion, the two intense i.r. bands at 990 and 860cm-i, which find their correspondence in medium intensity polarized Raman lines, must be correlated to these modes. The four ring deformations (2 in-plane and 2 out-ofplane) are found at frequencies that are more or less unaltered with respect to 3-bromothiophene. The remaining in-plane modes correspond to very intense polarized Raman lines and are associated with the stretchings and deformations of the C-Br bonds. The two lowest-lying depolarized Raman bands are undoubtedly connected with the 0.0.~. deformations of the C-Br bonds.
2,3,5-Tribromothiophene
The three vibrations involving movements of the hydrogen atom are easily localized at 3102, 1132 and 811 cm-’ and, again analogous to 2,3-dibromothiophene, we have the assignment of the three skeletal modes corresponding to the 14, 4 and 5 vibrations of thiophene and of the remaining out-of-plane yR and ycBRvibrations.
A”
BRing *’
A”
VCB*A’
"CBrA’
&B, A’ ‘CBr
A’
YCBr A” YCB~*” ‘k~r
A’
The attribution of the remaining 10 in-plane mcdes (2 ring stretchings, 2 ring deformations, 3 C-Br stretchings and 3 C-Br deformations) requires a more detailed discussion. Their assignments are based on more speculative arguments in so far as these vibrations do not correspond to “good group frequencies”, since they are mixed in various ways among themselves dependent on the position and/or the number of the substituents present in the ring. In the region below 400 cm- ‘, there are five polarized Raman peaks in the spectra; these may be tentatively correlated with the six modes involving movement of the bromine atoms, essentially because far-i.r. absorptions for similar molecules investigated up to now seem to have this as their origin. At this point, the four skeletal modes, predicted to absorb in the 45(rlOOOcm-’ region [8810] are still to be assigned. In fact, four i.r. absorptions of noticeable intensity, corresponding to polarized Raman lines are present in the spectra of 2,3,5-tribromothiophene at 477, 670, 969 and 992cm-‘. A comparison with the spectra of 2,5dibromothiophene in this region is of particular interest; the vibrational behaviour of this molecule was studied by GREEN[9] and its assignment seems to us to be more or less definitive. In the 2,5_dibromothiophene molecule, the modes in question are correlated with the absorptions at 418,646,948 and 982, which are only a little less in frequency than the absorptions we observed in 2,3,5_tribromothiophene. Therefore the assignment we are proposing finds support in this close analogy in spectral behaviour.
Vibrational spectra of thiophene derivatives CONCLUSIONS At this point, it would undoubtedly be of interest to compare the vibrational assignments of the various bromothiophenes investigated (2-bromo-, 3-bromo-, 2,3-dibromo-, 2,5-dibromo-, 2,3,5_tribromothiophene) and it is already possible to draw some conclusions that are both sufficiently reliable and of certain interest from a chemical point of view. We note: (i) the existence of a certain number of “stable” group frequencies; (ii) the tendency of a halogen in the a position to increase the degree of mixing between the ring modes and the substituents’ own modes, while a substituent in the fi position tends to lessen this mixing; (iii) the small effect produced by the rise in mass consequent on a higher degree of substitution. However, we think necessary to obtain further data examining other bi-, tri- and tetra- substituted halogen thiophenes (work in progress) before attempting a rationalization of the effects observed up to now. Acknowledgements-We
thank Prof. F. FRINGUELLIand Dr.
1017
F. PIZZO for their help in the purification of the molecules investigated. The financial assistance of the Consiglio Nazionale delle Ricerche (CNR, Roma) is also greatly acknowledged. REFERENCES [l] G. PALIANIand R. CATALIO~I,Spectrochim. Acta 37A, 707 (1981). [2] G. PALIANIand R. CATALIOT~I, Spectrochim. Acta 38A, 751 (1982). [3] G. PALIANI,A. PoLErnand R. CATALIOTTI, Chem. phys. Lett. 18, 525 (1973). [4] J. R. SCHERERand J. C. EVANS,Spectrochim. Acta 19, 1739 (1963). [5] M. RICO, J. M. ORZAand J. MORCILLO,Spectrochim. Acta 21A, 689 (1965) and references cited therein. [6] G. PALIANI, R. CATALIOITI, A. POLETTI, F. FRINGUELLI, A. TATICCHI and M. G. GIORGINI, Specwochim. Acta 32A, 1089 (1976). [7] M. ORAK, I. J. HYAMS and E. R. LIPPINCOTT, Spectrochim. Acta 22, 1355 (1966). [S] I. J. PERON, P. SAUMAGNE and J. M. LEBAS, Spectrochim. Acta 26A, 1651 (1970) and references cited therein. [9] J. H. S. GREEN, Spectrochim. Acta 27A, 2015 (1971). [lo] A. ROGSTAD,Spectrochim. Acta 31A, 1749 (1975).