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The C=S stretching frequency and the “−N−C=S bands” in the infrared
The C=S stretching frequency and the "-N-C=S bands" in the infrared * C. N. R. RAO and R. VENKATARAGHAVAN Department of Inorganic and Physical Chemistry, Department of Organic Chemistry, Indian Institute of Science, Bangalore-12, India
(Received 24 October 1961) Abstract-A correlation of the infrared spectra ofthiocarbonyl derivatives based on the literature data has been carried out. Assignments haye also been made in some new systems. Since simple alkyl thioketones are unstable, we have prepared thiofenchone in order to obtain a reference C=S stretching frequency. The C=S stretching frequency in thiofenchone has been found around 1180 cm-I which is in fair agreement with the value calculated for thioformaldehyde. In the case of the thiocarbonyl derivatives where the C=S group is linked to elements other than nitrogen, the stretching frequency is generally found in the region 1025-1225 em-I. Strong vibrational coupling is operative in the case of the nitrogen containing thiocarbonyl derivatives and three bands seem to consistently appear in the regions 1395-1570 em-I, 1260-1420 em-I, 940-1140 cm- I due to the mixed vibrations. These bands, which may be tentatively designated as the "-N-C=S I, II and III bands", could be useful in qualitative analysis.
INTRODUCTION Frequencies ranging from 850 to 1550 cm-I have been attributed to the C=S stretching frequency in the literature and there seems to be no adequate correlation of the literature data [1]. A careful examination of the data reveals that the assignments of very high or low frequencies are always in the nitrogen-containing thiocarbonyl compounds. It therefore becomes necessary to distinguish two groups of thiocarbonyl derivatives: the first group, where the thiocarbonyl group is linked to atoms such as carbon, sulphur, oxygen and chlorine and the second group, where the thiocarbonyl group is linked to one or two nitrogen atoms. An unambiguous assignment of the C=S stretching frequency seems to be possible only when the C=S group is attached to atoms other than nitrogen. In this communication we have carried out a correlation of the infrared spectra of thiocarbonyl derivatives based on the literature data and have also reported assignments in some new systems. There was, however, need for a reference C=S stretching frequency in a simple saturated thioketone. Since simple alkyl thioketones are unstable, we have prepared thiofenchone for this purpose. EXPERDlE :"'l'AL
Thiofenehone was prepared by reftuxing fenchone with phosphorous pentasulphide in heptane. The other compounds investigated are dithio-oxamide,
* Material taken from the Ph.D. thesis of H. VENKATAllAGHAVAN to be submitted to the Indian Institute of Science under the guidance of C. N. R HAO. [1] L. J. BELLAMY, The Infrared Spectra of Complex Jfolecules, l\fethuen, London (1958).
Reprinted from Spectrochimica Acta. Vol. 18. pp. 541 - 547, 1962. Pergamon Press Ltd.
299
300
C. N. R. RAo and R. VENKATARAGHAVAN
2-mercapto 3,4-dimethylthiazole, thiozolidine-2-thione, 2-mercaptobenzthiazole, thiosemicarbazides, tetrazolinethiones and guanylthioureas, of which the last two series of compounds were prepared in connection '\\ith other studies [2, 3]. The others were commercially available. Infrared spectra were recorded employing Perkin-Elmer spectrometers, models 21 and 137B. The samples were in the form of solutions in CCI, or CHCla or solids dispersed in Nujol mulls or KBr pellets. RESULTS AND DISCUSSION
Thiocarbonyl derivatives where the C=S group is linked to elements other than· nitrogen The C=S stretching frequency in thiofenchone is found around 1180 cm-l . This is in fair agreement with the calculated frequency in thioformaldehyde [4]. The various thiocarbonyl derivatives for which the infrared data are available are ethylenetrithiocarbonate [5, 6], alkyl and perfluoroalkyl trithiocarbonates [7], dithioesters [8], 2,4-dihydroxy dithiobenzoic acid [9], thiobenzophenones [10], pyr-4-thione, 2,6-dimethyl pyr-4-thione and thiopyr-4-thione [11], pyrid-4-thione [12], y-mercaptoaza compounds [13], alkyl and perfluoroalkyl chloroformates [7], thiophosgene [9] and dianthogens and xanthates [14, 15]. The assignments in these derivatives have been shown in the form of a correlation diagram in Fig. l. In the case of the xanthate compounds, the assignment in the range 1020-1070 cm-l [14] has been preferred. The assignment in the region 1140-1265 cm-l [15] probably corresponds to the C-O stretching vibration. The correlation clearly shows that the C=S stretching frequency in these derivatives falls within the range 1125 ± 100 cm-l • No simple correlation of the C=S stretching frequency with electronegativities of the atoms directly linked to the group is apparent. The linear relationship found by DAASCH [16] is not considered to be generally valid. In substituted phenyl thiocarbonyl derivatives {8, 10] the C=S stretching frequency does not show any simple relationship with the Hammett (1 or (1+ constants of the substituents [17]. The thiocarbonyl group is not at all similar to the carbonyl group [2] E. LIEBER, C. N. R. RAo, C. N. PILLAI, J. RAMACHANDRAN and R. D. HITES, Can. J. Chem. 36,801 (1958). [3] K. S. SURESH, J. RAMACHANDRAN and C. N. R. RAO,J. Sci. Ind. Res. (India) 20B, 203 (1961). [4] E. SPINNER, Spectrochim. Acta 15,95 (1959). [5] R. MECKE, R. MECKE and A. LUTTRINGHAUS, Chem. Ber. 90, 975 (1957). [6] L. J. BELLAMY and P. E. ROGASCH, J. Chem. Soc. 2218 (1960). [7] R. N. llAzELIDINE and J. M. KIDD, J. Ohem. Soc. 3871 (1955). [8] B. BAK, L. HASEN·NYGAARD and C. PEDERSEN, Acta. Ohem. Scand. 12, 1451 (1958). [9] J. I. JONES, W. KYNASTON and L. J. HALES, J. Chem. Soc. 614 (1957). [10] N. LOZACH and G. GUILLOUZO, Bull. BOC. chim. France 1221 (1957). [11] A. R. KATRITZKY and R. A. JONES, Spectrochim. Acta 17, 64 (1961). [12] A. R. KATRITZKY and R. A. JONES, J. Chem. Soc. 2947 (1960). [13] E. SPINNER, J. Chem. Soc. 1237 (1960). [14] L. H. LITTLE, G. W. POLING and J. LEJA, Can. J. Chem. 39, 745 (1961). [15] M. L. SHANKARANARAYANA and C. C. PATEL, Can. J. Chem. 39, 1633 (1961). [16] L. W. DAASCH, Spectrochim. Acta 13, 257 (1958). [17] C. N. R. RAO and R. VENKATARAGHAVAN, Can. J. Ohem. 39,1757 (1961).
301
The C=S stretching frequency and the "-N-C=S bands" in the infrared
with regard to bond polarity and electrical effects of substituents [6]. The insensitivity of the thiocarbonyl stretching frequency to electrical effects is further confirmed by the fact that the calculated frequency of 1120 ± 40 cm-l for thioformaldehyde is not very different from the observed frequency of 1140 cm-l for thiophosgene [4] and 1180 cm-l for thiofenchone. The only cases where the C=S frequency falls outside the limits of this correlation are xanthione, thioxanthione and N-methyl thioacridone, all of which are x
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Fig. 1. The C=S stretching frequencies in thiocarbonyl derivatives where the group is linked to elements other than nitrogen. X and Y refer to the elements directly linked to the thiocarbonyl group.
heteroaromatic ketones. The at 1330 ± 30 cm-l fI8].
C~=S
stretching frequency in these systems is found
Thiocarbonyl derivatives where the C=S gr01lp is linked to one or two nitrogen atoms There has been great indefiniteness with regard to the assignment of the C=S stretching frequency in nitrogen containing compounds. The assignment in these compounds varies in the wide range of R50 to 1570 em-I. RANDALL et al. [19] first observed that a strong band is present in the region 1471-1613 cm-l in compounds where the N-C=S unit is present. Several authors [1, 2, 20, 21] have made a [18] T. BERGMANN, J. Am. Chem. Soc. 77, 1549 (1955). [19] H. M. RANDALL, R. G. FO\VLER, X. FUSON, J. R. DA~GL, Infrared Determination of Organic Structures. D. Van Nostrand, New York (1949). [20] J. MANN, Trans Inst. Rubber Ind. 27,232 (1951). [21] J. CHATT, L. A. DUNCANSON and L. M. YENANZI, Suomen J(emistilehti 29B, 75 (1956).
302
C. N. R.
RAO
and R.
VENKATARAGHAVAN
similar assignment in a number of such systems. Subsequently it has been shown by ELMORE [22] that this thioureide band results from the coupling of the C-N stretching vibration and the NH deformation vibration. The extreme variations in the assignment of the C=S stretching frequency in nitrogen-containing thiocarbonyl derivatives is undoubtedly due to vibrational coupling effects. In most of these systems the C=S stretching vibration is not localized. The vibrations that interact will naturally vary with the system. Thus in thiourea the vibrations due to C-N stretching, C=S stretching and NH2 rocking can interact to give the observed infrared frequencies [4, 23]. In the thioamide system, coupling can take place among C-N stretching, C=S stretching and NH deformation vibrations. The observed infrared 'bands in nitrogen-containing thiocarbonyl compounds are therefore due to mixed vibrations and an estimate of the mixing effects will only be possible by normal co-ordinate analysis similar to that reported for N -methyl acetamide by MIYAZAWA et al. [24]. Many of the earlier assignments of the C=S frequencies in compounds such as thioamides [6], dithio-oxamides [25], thiadiazoles [26], tetramethylthiurammonosulphide [27], thiosemicarbazides and tetrozolinethiones [2], thiobenzanilide [28] and thiohydantoins [22] appear to be partial in the sense that these might have had some contribution from the C=S stretching. The assignments of the C=S stretching frequencies in thioacetamide and tetramethylthiurammonosulphide in the region 960-980 cm-1 by BELLAMY and ROGASH [6J, based on solvent effects, are also considered to be partial assignments. The observed solvent effects are small and such effects may not be conclusive in establishing assignments. It is however possible that the 960-980 cm-1 band in these compounds has appreciable contribution from C=S stretching and therefore show solvent shifts similar to other simpler thiocarbonyl derivatives. Although it appears impossible at first sight to obtain any kind of correlation of the infrared spectra of nitrogen-containing thiocarbonyl derivatives, a critical survey of the existing data indicates that some generalizations are possible. In such a correlation one should only consider molecules with similar structural units. Thus, a strict comparison of the spectra of thiourea and thioacetamide cannot be made since the interacting vibrations would be different. In the case of thiourea, the bands at 1472, 1415 and 1086 cm-1 may be considered as composite bands of NH2 bending, C-N stretching and C=S stretching. The band at 730 cm-1 may have some contribution from the NCS bending vibration. In thioacetamide [4,6, 29J, the frequencies at 1393,1303 and 974 cm-1 are probably due to the mixing of NH bending, C-N stretching and C=S stretching vibrations, the band at 715 cm-1 having some contribution from NCS bending. A similar [22] D. T. ELMORE, J. Chem. Soc. 3489 (1958). [23] A. YAMAGUCHI, R. B. PENLAND, :So MIZUSHIMA, T. J. LANE, C. CURRAN and J. V. QUAGLIANO J. Am. Chem. Soc. SO, 527 (1958). [24] T. MIYAZAWA, T. SHIMANOlTCHI and S. MIZUSHIMA, J. Chem. Phys. 29, 611 (1958). [25] T. A. SCOTT JR. and E. L. WAGNER, J. Chem. Phys. 30, 465 (1959). [26] G. D. THORN, Can. J. Chem. 38, 1439 (1960). [27] G. D. THORN, Can. J. Chem. 38, 2349 (1960). [28] D. HADZI, J. Chem. Soc. 847 (1957). [29] W. KUTZELNIOG and R MECKE, Spectrochim. Acta 17, 530 (1961).
The C=S stretching frequency and the "-N-C=S bands" in the infrared
303
assignment has been made in the case ofthioformamide by DAVIES and JONES [30] for the bands at 1432 and 1287 cm-I , who have rightly called them the asymmetric and symmetric NCS stretching vibration frequencies. The absorption bands reported by MARVEL et al. [31] for N-substituted thioamides may similarly be reassigned to the various mixed vibrations. In N -n butyl thioacetamide the bands at 1336, 1305, 1181, 1110 and 930 cm-I are likely to be due to composite vibrations of NH bending, C-N stretching, C=S stretching, C-C stretching and CH 3-C stretching. In N-thioacetylpiperidine where there is no contribution from NH bending the spectrum seems to be slightly different [31]. In this molecule the highest composite vibration frequency occurs at 1284 cm-l which probably has the greatest contribution from C-N stretching. The infrared bands of dithiooxamide [9, 25] may also be reinterpreted in terms of coupled vibrations [32]. The NH in-plane bending in the range 1503-1534 cm-l assigned by MILLIGAN et al. [32] in substituted dithiooxamides is likely to have considerable contribution from C-N stretching. The recent assignments for the absorption bands of 2,5-dimercapto-l, 3,4-thiadiazole derivatives by THORN [26] should also be interpreted in terms of coupled vibrations. Similar assignments are possible for the recent infrared data on thiosemicarbazones [33]. The frequencies commonly found in a number of thioamides and similar derivatives have been presented in the form of a correlation diagram in Fig. 2. The systems included in the correlation are thioformamide [30], thioacetamide [4,29], dithiooxamides [~, 25, 32], 2,5-dimercapto-l,3,4-thiadiazoles [26], 2-mercapto-3,4-dimethylthiazole, thiozolidine-2-thione, tetramethylthiurammonosulphide [27], thiourea [4], thiosemicarbazides [2J, thiohydantoins [22J, tetrazolinethiones [2J, guanylthioureas [:n 2-mercaptobenzthiazole [3J and thiosemicarbazones [33]. The correlation clearly shows that the three bands in the regions 940--1140 cm-l , 1260-1420 cm-1 and 1:l!li)-1i)70 cm- 1 are common to most of the systems. MECKE et al. [5J assigned the ('=8 stretching fl'equency to the region 10;')0-1200 cm-l in some cyclic thioamides. An examination of the charts reveals that there are sev.eral other strong bands in the 1:100-1400 cm- 1 region which may also be assigned to some coupled vibrations. SPIx:~n;R (l:J, :14.] seems to feel that there is no vibrational mixing in the case of pyridthiones and related compounds and assigns the C=8 stretching frequency in the vicinity of 1140 cm- 1 , on the basis that it is t.he only strong band in the spectrum. An examination of hiR data· seems to indicate fairly strong bands in the three regions obtained in the correlation in Fig. 2 and it is felt that an exclusive assignnlt'nt of the fl'eqllency for the localized ('=8 vibrations in these compounds is not rlear cnt. ( 'OXCLUSIOXS
As a result of these correlations it is possible to assign the range 102ti-l:!:!!) cm-1 • Ref. 13 Appendix. Personal ('ommunication. M. DAVIES and W. J ..JONEs,.f. Chem. Soc. 955 (1958).
[30] [31] [32] r33] [34]
C. S. MARVEL, P. DE HADZITSKY and .f. ,J. BRADER,.f. Am. Ohem. ,""0/'.77, ;)997 (1955). B. MILLIGAN, E. f\PINNEIt and .f.l\1. HWAX,.f. Chrm. Sf)/'. 1919 (1901). P. W. SADLER,'!. Chern. Soc. 9:)7 (1961). E. SPINNER,.!. Org. Chem. 23. 20:J7 (19Mj).
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• Data in the region 940-1140 cm-1 are not available for thiosemicarbazones, and it is difficult to make any specific assignment in the case of 2-mercaptobenzthiazole.
Fig. 2. The C=S stretching frequency in thiocarbonyl derivatives where the group is linked to one or two nitrogen atoms. The dotted lines represent the three distinct regions of the "-N-C=S absorptions".
THIOSEIWIICAAB"ZONES •
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The C=S stretching frequency and the "-N-C=S bands" in the infrared
305
to the C=S stretching vibration in simple derivatives where there are no appreciable coupling effects (Le. the thiocarbonyl group is linked to elements other than nitrogen). In compounds where the thiocarbonyl group is linked to one or two nitrogen atoms strong vibrational coupling effects are possible and the C=S vibration is not localized. However three bands seem to consistently appear in the regions 1395-1570 cm-I , 1260-1420 cm-1 and 940-1140 cm-I in these nitrogen'containing derivatives due to the mixed vibrations. These bands which ma.y be tentatively designated as the "-N-C=S I, II and III bands" could be useful in qualitative analysis. Aoknowledgement-The authors are most grateful to Professor D. K. BANERJEE and Professor M. R. A. RAO for their keen interest, in the work and to Dr. J. RAMACHANDRAN for his assistance in the early stages of the work.
SA(Al8) SUPP-U
(Received 24 October 1961) Abstract-A correlation of the infrared spectra ofthiocarbonyl derivatives based on the literature data has been carried out. Assignments haye also been made in some new systems. Since simple alkyl thioketones are unstable, we have prepared thiofenchone in order to obtain a reference C=S stretching frequency. The C=S stretching frequency in thiofenchone has been found around 1180 cm-I which is in fair agreement with the value calculated for thioformaldehyde. In the case of the thiocarbonyl derivatives where the C=S group is linked to elements other than nitrogen, the stretching frequency is generally found in the region 1025-1225 em-I. Strong vibrational coupling is operative in the case of the nitrogen containing thiocarbonyl derivatives and three bands seem to consistently appear in the regions 1395-1570 em-I, 1260-1420 em-I, 940-1140 cm- I due to the mixed vibrations. These bands, which may be tentatively designated as the "-N-C=S I, II and III bands", could be useful in qualitative analysis.
INTRODUCTION Frequencies ranging from 850 to 1550 cm-I have been attributed to the C=S stretching frequency in the literature and there seems to be no adequate correlation of the literature data [1]. A careful examination of the data reveals that the assignments of very high or low frequencies are always in the nitrogen-containing thiocarbonyl compounds. It therefore becomes necessary to distinguish two groups of thiocarbonyl derivatives: the first group, where the thiocarbonyl group is linked to atoms such as carbon, sulphur, oxygen and chlorine and the second group, where the thiocarbonyl group is linked to one or two nitrogen atoms. An unambiguous assignment of the C=S stretching frequency seems to be possible only when the C=S group is attached to atoms other than nitrogen. In this communication we have carried out a correlation of the infrared spectra of thiocarbonyl derivatives based on the literature data and have also reported assignments in some new systems. There was, however, need for a reference C=S stretching frequency in a simple saturated thioketone. Since simple alkyl thioketones are unstable, we have prepared thiofenchone for this purpose. EXPERDlE :"'l'AL
Thiofenehone was prepared by reftuxing fenchone with phosphorous pentasulphide in heptane. The other compounds investigated are dithio-oxamide,
* Material taken from the Ph.D. thesis of H. VENKATAllAGHAVAN to be submitted to the Indian Institute of Science under the guidance of C. N. R HAO. [1] L. J. BELLAMY, The Infrared Spectra of Complex Jfolecules, l\fethuen, London (1958).
Reprinted from Spectrochimica Acta. Vol. 18. pp. 541 - 547, 1962. Pergamon Press Ltd.
299
300
C. N. R. RAo and R. VENKATARAGHAVAN
2-mercapto 3,4-dimethylthiazole, thiozolidine-2-thione, 2-mercaptobenzthiazole, thiosemicarbazides, tetrazolinethiones and guanylthioureas, of which the last two series of compounds were prepared in connection '\\ith other studies [2, 3]. The others were commercially available. Infrared spectra were recorded employing Perkin-Elmer spectrometers, models 21 and 137B. The samples were in the form of solutions in CCI, or CHCla or solids dispersed in Nujol mulls or KBr pellets. RESULTS AND DISCUSSION
Thiocarbonyl derivatives where the C=S group is linked to elements other than· nitrogen The C=S stretching frequency in thiofenchone is found around 1180 cm-l . This is in fair agreement with the calculated frequency in thioformaldehyde [4]. The various thiocarbonyl derivatives for which the infrared data are available are ethylenetrithiocarbonate [5, 6], alkyl and perfluoroalkyl trithiocarbonates [7], dithioesters [8], 2,4-dihydroxy dithiobenzoic acid [9], thiobenzophenones [10], pyr-4-thione, 2,6-dimethyl pyr-4-thione and thiopyr-4-thione [11], pyrid-4-thione [12], y-mercaptoaza compounds [13], alkyl and perfluoroalkyl chloroformates [7], thiophosgene [9] and dianthogens and xanthates [14, 15]. The assignments in these derivatives have been shown in the form of a correlation diagram in Fig. l. In the case of the xanthate compounds, the assignment in the range 1020-1070 cm-l [14] has been preferred. The assignment in the region 1140-1265 cm-l [15] probably corresponds to the C-O stretching vibration. The correlation clearly shows that the C=S stretching frequency in these derivatives falls within the range 1125 ± 100 cm-l • No simple correlation of the C=S stretching frequency with electronegativities of the atoms directly linked to the group is apparent. The linear relationship found by DAASCH [16] is not considered to be generally valid. In substituted phenyl thiocarbonyl derivatives {8, 10] the C=S stretching frequency does not show any simple relationship with the Hammett (1 or (1+ constants of the substituents [17]. The thiocarbonyl group is not at all similar to the carbonyl group [2] E. LIEBER, C. N. R. RAo, C. N. PILLAI, J. RAMACHANDRAN and R. D. HITES, Can. J. Chem. 36,801 (1958). [3] K. S. SURESH, J. RAMACHANDRAN and C. N. R. RAO,J. Sci. Ind. Res. (India) 20B, 203 (1961). [4] E. SPINNER, Spectrochim. Acta 15,95 (1959). [5] R. MECKE, R. MECKE and A. LUTTRINGHAUS, Chem. Ber. 90, 975 (1957). [6] L. J. BELLAMY and P. E. ROGASCH, J. Chem. Soc. 2218 (1960). [7] R. N. llAzELIDINE and J. M. KIDD, J. Ohem. Soc. 3871 (1955). [8] B. BAK, L. HASEN·NYGAARD and C. PEDERSEN, Acta. Ohem. Scand. 12, 1451 (1958). [9] J. I. JONES, W. KYNASTON and L. J. HALES, J. Chem. Soc. 614 (1957). [10] N. LOZACH and G. GUILLOUZO, Bull. BOC. chim. France 1221 (1957). [11] A. R. KATRITZKY and R. A. JONES, Spectrochim. Acta 17, 64 (1961). [12] A. R. KATRITZKY and R. A. JONES, J. Chem. Soc. 2947 (1960). [13] E. SPINNER, J. Chem. Soc. 1237 (1960). [14] L. H. LITTLE, G. W. POLING and J. LEJA, Can. J. Chem. 39, 745 (1961). [15] M. L. SHANKARANARAYANA and C. C. PATEL, Can. J. Chem. 39, 1633 (1961). [16] L. W. DAASCH, Spectrochim. Acta 13, 257 (1958). [17] C. N. R. RAO and R. VENKATARAGHAVAN, Can. J. Ohem. 39,1757 (1961).
301
The C=S stretching frequency and the "-N-C=S bands" in the infrared
with regard to bond polarity and electrical effects of substituents [6]. The insensitivity of the thiocarbonyl stretching frequency to electrical effects is further confirmed by the fact that the calculated frequency of 1120 ± 40 cm-l for thioformaldehyde is not very different from the observed frequency of 1140 cm-l for thiophosgene [4] and 1180 cm-l for thiofenchone. The only cases where the C=S frequency falls outside the limits of this correlation are xanthione, thioxanthione and N-methyl thioacridone, all of which are x
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Fig. 1. The C=S stretching frequencies in thiocarbonyl derivatives where the group is linked to elements other than nitrogen. X and Y refer to the elements directly linked to the thiocarbonyl group.
heteroaromatic ketones. The at 1330 ± 30 cm-l fI8].
C~=S
stretching frequency in these systems is found
Thiocarbonyl derivatives where the C=S gr01lp is linked to one or two nitrogen atoms There has been great indefiniteness with regard to the assignment of the C=S stretching frequency in nitrogen containing compounds. The assignment in these compounds varies in the wide range of R50 to 1570 em-I. RANDALL et al. [19] first observed that a strong band is present in the region 1471-1613 cm-l in compounds where the N-C=S unit is present. Several authors [1, 2, 20, 21] have made a [18] T. BERGMANN, J. Am. Chem. Soc. 77, 1549 (1955). [19] H. M. RANDALL, R. G. FO\VLER, X. FUSON, J. R. DA~GL, Infrared Determination of Organic Structures. D. Van Nostrand, New York (1949). [20] J. MANN, Trans Inst. Rubber Ind. 27,232 (1951). [21] J. CHATT, L. A. DUNCANSON and L. M. YENANZI, Suomen J(emistilehti 29B, 75 (1956).
302
C. N. R.
RAO
and R.
VENKATARAGHAVAN
similar assignment in a number of such systems. Subsequently it has been shown by ELMORE [22] that this thioureide band results from the coupling of the C-N stretching vibration and the NH deformation vibration. The extreme variations in the assignment of the C=S stretching frequency in nitrogen-containing thiocarbonyl derivatives is undoubtedly due to vibrational coupling effects. In most of these systems the C=S stretching vibration is not localized. The vibrations that interact will naturally vary with the system. Thus in thiourea the vibrations due to C-N stretching, C=S stretching and NH2 rocking can interact to give the observed infrared frequencies [4, 23]. In the thioamide system, coupling can take place among C-N stretching, C=S stretching and NH deformation vibrations. The observed infrared 'bands in nitrogen-containing thiocarbonyl compounds are therefore due to mixed vibrations and an estimate of the mixing effects will only be possible by normal co-ordinate analysis similar to that reported for N -methyl acetamide by MIYAZAWA et al. [24]. Many of the earlier assignments of the C=S frequencies in compounds such as thioamides [6], dithio-oxamides [25], thiadiazoles [26], tetramethylthiurammonosulphide [27], thiosemicarbazides and tetrozolinethiones [2], thiobenzanilide [28] and thiohydantoins [22] appear to be partial in the sense that these might have had some contribution from the C=S stretching. The assignments of the C=S stretching frequencies in thioacetamide and tetramethylthiurammonosulphide in the region 960-980 cm-1 by BELLAMY and ROGASH [6J, based on solvent effects, are also considered to be partial assignments. The observed solvent effects are small and such effects may not be conclusive in establishing assignments. It is however possible that the 960-980 cm-1 band in these compounds has appreciable contribution from C=S stretching and therefore show solvent shifts similar to other simpler thiocarbonyl derivatives. Although it appears impossible at first sight to obtain any kind of correlation of the infrared spectra of nitrogen-containing thiocarbonyl derivatives, a critical survey of the existing data indicates that some generalizations are possible. In such a correlation one should only consider molecules with similar structural units. Thus, a strict comparison of the spectra of thiourea and thioacetamide cannot be made since the interacting vibrations would be different. In the case of thiourea, the bands at 1472, 1415 and 1086 cm-1 may be considered as composite bands of NH2 bending, C-N stretching and C=S stretching. The band at 730 cm-1 may have some contribution from the NCS bending vibration. In thioacetamide [4,6, 29J, the frequencies at 1393,1303 and 974 cm-1 are probably due to the mixing of NH bending, C-N stretching and C=S stretching vibrations, the band at 715 cm-1 having some contribution from NCS bending. A similar [22] D. T. ELMORE, J. Chem. Soc. 3489 (1958). [23] A. YAMAGUCHI, R. B. PENLAND, :So MIZUSHIMA, T. J. LANE, C. CURRAN and J. V. QUAGLIANO J. Am. Chem. Soc. SO, 527 (1958). [24] T. MIYAZAWA, T. SHIMANOlTCHI and S. MIZUSHIMA, J. Chem. Phys. 29, 611 (1958). [25] T. A. SCOTT JR. and E. L. WAGNER, J. Chem. Phys. 30, 465 (1959). [26] G. D. THORN, Can. J. Chem. 38, 1439 (1960). [27] G. D. THORN, Can. J. Chem. 38, 2349 (1960). [28] D. HADZI, J. Chem. Soc. 847 (1957). [29] W. KUTZELNIOG and R MECKE, Spectrochim. Acta 17, 530 (1961).
The C=S stretching frequency and the "-N-C=S bands" in the infrared
303
assignment has been made in the case ofthioformamide by DAVIES and JONES [30] for the bands at 1432 and 1287 cm-I , who have rightly called them the asymmetric and symmetric NCS stretching vibration frequencies. The absorption bands reported by MARVEL et al. [31] for N-substituted thioamides may similarly be reassigned to the various mixed vibrations. In N -n butyl thioacetamide the bands at 1336, 1305, 1181, 1110 and 930 cm-I are likely to be due to composite vibrations of NH bending, C-N stretching, C=S stretching, C-C stretching and CH 3-C stretching. In N-thioacetylpiperidine where there is no contribution from NH bending the spectrum seems to be slightly different [31]. In this molecule the highest composite vibration frequency occurs at 1284 cm-l which probably has the greatest contribution from C-N stretching. The infrared bands of dithiooxamide [9, 25] may also be reinterpreted in terms of coupled vibrations [32]. The NH in-plane bending in the range 1503-1534 cm-l assigned by MILLIGAN et al. [32] in substituted dithiooxamides is likely to have considerable contribution from C-N stretching. The recent assignments for the absorption bands of 2,5-dimercapto-l, 3,4-thiadiazole derivatives by THORN [26] should also be interpreted in terms of coupled vibrations. Similar assignments are possible for the recent infrared data on thiosemicarbazones [33]. The frequencies commonly found in a number of thioamides and similar derivatives have been presented in the form of a correlation diagram in Fig. 2. The systems included in the correlation are thioformamide [30], thioacetamide [4,29], dithiooxamides [~, 25, 32], 2,5-dimercapto-l,3,4-thiadiazoles [26], 2-mercapto-3,4-dimethylthiazole, thiozolidine-2-thione, tetramethylthiurammonosulphide [27], thiourea [4], thiosemicarbazides [2J, thiohydantoins [22J, tetrazolinethiones [2J, guanylthioureas [:n 2-mercaptobenzthiazole [3J and thiosemicarbazones [33]. The correlation clearly shows that the three bands in the regions 940--1140 cm-l , 1260-1420 cm-1 and 1:l!li)-1i)70 cm- 1 are common to most of the systems. MECKE et al. [5J assigned the ('=8 stretching fl'equency to the region 10;')0-1200 cm-l in some cyclic thioamides. An examination of the charts reveals that there are sev.eral other strong bands in the 1:100-1400 cm- 1 region which may also be assigned to some coupled vibrations. SPIx:~n;R (l:J, :14.] seems to feel that there is no vibrational mixing in the case of pyridthiones and related compounds and assigns the C=8 stretching frequency in the vicinity of 1140 cm- 1 , on the basis that it is t.he only strong band in the spectrum. An examination of hiR data· seems to indicate fairly strong bands in the three regions obtained in the correlation in Fig. 2 and it is felt that an exclusive assignnlt'nt of the fl'eqllency for the localized ('=8 vibrations in these compounds is not rlear cnt. ( 'OXCLUSIOXS
As a result of these correlations it is possible to assign the range 102ti-l:!:!!) cm-1 • Ref. 13 Appendix. Personal ('ommunication. M. DAVIES and W. J ..JONEs,.f. Chem. Soc. 955 (1958).
[30] [31] [32] r33] [34]
C. S. MARVEL, P. DE HADZITSKY and .f. ,J. BRADER,.f. Am. Ohem. ,""0/'.77, ;)997 (1955). B. MILLIGAN, E. f\PINNEIt and .f.l\1. HWAX,.f. Chrm. Sf)/'. 1919 (1901). P. W. SADLER,'!. Chern. Soc. 9:)7 (1961). E. SPINNER,.!. Org. Chem. 23. 20:J7 (19Mj).
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Fig. 2. The C=S stretching frequency in thiocarbonyl derivatives where the group is linked to one or two nitrogen atoms. The dotted lines represent the three distinct regions of the "-N-C=S absorptions".
THIOSEIWIICAAB"ZONES •
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The C=S stretching frequency and the "-N-C=S bands" in the infrared
305
to the C=S stretching vibration in simple derivatives where there are no appreciable coupling effects (Le. the thiocarbonyl group is linked to elements other than nitrogen). In compounds where the thiocarbonyl group is linked to one or two nitrogen atoms strong vibrational coupling effects are possible and the C=S vibration is not localized. However three bands seem to consistently appear in the regions 1395-1570 cm-I , 1260-1420 cm-1 and 940-1140 cm-I in these nitrogen'containing derivatives due to the mixed vibrations. These bands which ma.y be tentatively designated as the "-N-C=S I, II and III bands" could be useful in qualitative analysis. Aoknowledgement-The authors are most grateful to Professor D. K. BANERJEE and Professor M. R. A. RAO for their keen interest, in the work and to Dr. J. RAMACHANDRAN for his assistance in the early stages of the work.
SA(Al8) SUPP-U