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The infrared spectra of organic thiocyanates and isothiocyanates
Spectrochimica Aeta, 1959,Vol. 13, pp.296 to 299. PergamonPressLtd. Printed in NorthernIreland
The infrared spectra of organic thiocyanates
and isothiocyanates
E. LIEBER,* C. N. R. RAO and J. RAMACHANDRAN Department of Chemistry, De Paul University, Chicago 14, Tl!inois, U.S.A. (Rec&ed
21 July
1968)
Abstract-The infrared spectra of several organic thiocyanates and isothiocyanates have been studied. The thiocyanates and the isothiocyanates can be distinguished by their characteristic vibration frequencies around 2140 cm-l and between 2060-2105 cm-l, respectively. These respective frequencies serve as an analytical tool for distinguishing these isomers both singly and in mixtures.
Introduction have been several isolated reports of the infrared spectra of organic thioWe have now conducted a systematic investigation cyanates and isothiocyanates. of the infrared spectra of these compounds. Since thiocyanates and isothiocyanates are isomeric it was considered that the infrared spectra may yield a simple analytical tool for their identification.
THERE
Experimental The compounds used in this study were obtained commercially and when necessary purified Spectrometer, to the constants reported for them. The spectra were recorded on a Perkin-Elmer Model 21, with sodium chloride prism. - The spectra of liquids, using liquid cells, and of solids in Nujol mulls, were taken. In a few cases the spectra were also taken in carbon tetrachloride The positions of the absorption maxima are listed in cm-l in Tables 1 and 2, with solutions. the intensities being indicated by the symbols: (vs), very strong; (a), strong; (m), medium; The infrared data on some isothiocyanates from the (mw), medium weak; (w), weak. literature [5, 10, 111 have also been included in Table 2 for purposes of comparison.
Discussion Organic thiocyanates. The important absorption frequencies of several organic thiocyanates have been summarized in Table 1. The well known vibration frequencies (e.g. C-H, etc.) have not been tabulated. All the organic thiocyanates show a sharp strong peak due to the nitrile stretching vibration [l] around 2140 Table R
CH, C,H, n-C,H9 n-‘&H, F,
C,H,CH,
1. Infrared
spectra of organic thiocyanates
S-CrN
2141 2141 2137 2137 2141
(R-S-C=X)
Other bands
s s s s s
1316 m 1269 m 1272 mw 1275 m
1225 m 1238mw 1233 m
1062 1095 1101 1072
s w w s
1053 mw 1058 w
9% 968 916 914
VW s s w
* To whom all correspondence and requests for reprints should be addressed. 296
696 772 742 722
w s ml\ m
072 m 683 w 700 w
The infrared spectra of organic thiocyanates and isothiocyanates
cm-l. The nitrile band is observed around this region in diazoguanidine cyanide [2] and cyanogen halides [3]. Methyl thiocyanate shows a medium intensity band at 1316 cm-i which can be ascribed to the CH, deformation cl]. The other alkyl thiocyanates also exhibit a medium intensity band around 1270 cm-l which may also be due to the CH, Table 2. Infrared spectra of organic isothiocpanates R
C’%
w%
n-C,H, n-c,%J
CEI,=CHCH, CsH, 4-NH,C,H, t-C,H, [51 ally1 carbinyl [IO] trans-crotyl [lo] cis-crotyl [ 101
methosybenzyl
[I l]
Other bands
pI;=C=S
N=C=-R
1172 s
1089 1062 1081 1038 1081 986 1067 1070
s s s
1120 1115
992 1078 1075
2079 s
1153
1050
2188 s 2174 s 2155 s
2092 x-s 2092 vs 2058 vs
2155 2151 2160 2160
2088 2079 2060 2105
x-s vs vs \‘s
2090 2105 2062 2105
s
s s s s
(R--N=C=S)
1120 mu 1116 w 1121 w
w m w w n’ m w u-
938 m 977 mw 942 w 921 926 946 913 887 955 889
m nw w mw
795 mw 745 s 725 707 750 751
w m s mw
772 746 739
deformation frequency in these compounds. Also, one of the bands exhibited in the region of 683-722 cm-l may be due to the -C-Svibration. Organic isothiocyanates. The infrared spectra of organic isothiocyanates are listed in Table 2. The well known vibration frequencies like the CH vibrations, etc., have not been tabulated. All the organic isotliiocyanates exhibit a broad and very strong band centred around 2100 cm-l due to the isothiocyanate group. At lower concentrations (smaller layer thickness) this band shows finer resolution and gives a strong peak or shoulder between 2080-2105 cm-l (Fig. 1). This is in agreement with the observation of WILLIAMS [4] on phenyl isothiocyanate. However, no weak band was apparent around 1950-2000 cm-i [4, 51. The infrared spectra of organic isocyanates (R-N=C=O) has been extensively studied and the characteristic isocyanate band is found between 2269-2275 cm-l [6, 7, 81. It is interesting to note that the isothiocyanate group shows its characteristic band at a lower frequency (2060-2190 cm-l) than the isocyanate groups. While going from the normal thiocyanate to the isothiocyanate there is a decrease in the characteristic frequency just as in the example of nitrile to isonitrile [l, 91. This suggests t,hat the contribution of the resonance form: R-N=C-S @
E
is greater than that of the others [4]. All the isothiocyanates also show other bands of which the weak or medium intensity band between 1050-1089 cm-l occurs 297
E. LIEBER, C. N. R. RAO and J. RAMACEANDBAN
constantly in all of them. stretching vibration.
This may be due to the isothiocyanate
(0)
2500
symmetric
b)
2500
cm-’
cm“
Fig. 1. Infrared spectra of methyl thiocyanate and methyl tiothiocyanate. methyl &othiocyanate, - - - - - methyl thiocyanate. (b) A mixture (a) of methyl thiocyanate and methyl tiothiocyanate in carbon tetrachloride solution.
From the results discussed above it may be noted that the organic thiocyanates and isothiocyanatea can be distinguished by their characteristic vibration frequencies around 2140 cm-l and between 2060 and 2105 cm-l respectively. Fig. 1 also includes the infrared spectrum of a mixture of methyl thiocyanate and methyl isothiocyanate in carbon tetrachloride solution. The peaks due to the thiocyanate and the isothiocyanate groups are clearly distinct. Acknowledgement-The authors gratefully acknowledge grants-in-aid from the Frederick Gardner Cottrell Program of the Research Corporation, New York, U.S.A. and the Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana, U.S.A., which made this study possible.
298
The infrared spectra of organic thiocyanates and tiothiocyanates
References [I] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll]
BELLAMY L. J., The Infrared Spectra of Complex Moleculea Methuen, London 1954. LIEBER E., LEVERINQ D. R. tend PATTERSON L., Adyt. Chem. 1951 23 1594. WEST W. and FARNSWORTH M., J. Chem. Phys. 1933 1 402. WILLIAMS D., J. Chem. Phys. 1940 8 513. LUSKIN L. S., GAMTERT G. E. and CRAIG W. E., J. Anzer. Chem. Sot. 1956 78 4965. HOYER H. Chem. Ber. 1956 89 2677. BORTNICK N., LUSKIN L. S. and HUROWITZ M. D., J. .4mer. Ch,em. Sot. 1956 78 4358. DAVISON W. H. T., J. Chem. Sot. 1953 3712. GORDY W. and WILLIAMS D., J. Chem. Phys. 1936 4 85. ETTLINGER M. G. and HODGKINS J. E., J. Amer. Chem. Sot. 1955 77 1831. ETTLINGER M. G. and LUNDEEN A. J., J. Amer. Chem. Sot. 1956 78 1952.
299
The infrared spectra of organic thiocyanates
and isothiocyanates
E. LIEBER,* C. N. R. RAO and J. RAMACHANDRAN Department of Chemistry, De Paul University, Chicago 14, Tl!inois, U.S.A. (Rec&ed
21 July
1968)
Abstract-The infrared spectra of several organic thiocyanates and isothiocyanates have been studied. The thiocyanates and the isothiocyanates can be distinguished by their characteristic vibration frequencies around 2140 cm-l and between 2060-2105 cm-l, respectively. These respective frequencies serve as an analytical tool for distinguishing these isomers both singly and in mixtures.
Introduction have been several isolated reports of the infrared spectra of organic thioWe have now conducted a systematic investigation cyanates and isothiocyanates. of the infrared spectra of these compounds. Since thiocyanates and isothiocyanates are isomeric it was considered that the infrared spectra may yield a simple analytical tool for their identification.
THERE
Experimental The compounds used in this study were obtained commercially and when necessary purified Spectrometer, to the constants reported for them. The spectra were recorded on a Perkin-Elmer Model 21, with sodium chloride prism. - The spectra of liquids, using liquid cells, and of solids in Nujol mulls, were taken. In a few cases the spectra were also taken in carbon tetrachloride The positions of the absorption maxima are listed in cm-l in Tables 1 and 2, with solutions. the intensities being indicated by the symbols: (vs), very strong; (a), strong; (m), medium; The infrared data on some isothiocyanates from the (mw), medium weak; (w), weak. literature [5, 10, 111 have also been included in Table 2 for purposes of comparison.
Discussion Organic thiocyanates. The important absorption frequencies of several organic thiocyanates have been summarized in Table 1. The well known vibration frequencies (e.g. C-H, etc.) have not been tabulated. All the organic thiocyanates show a sharp strong peak due to the nitrile stretching vibration [l] around 2140 Table R
CH, C,H, n-C,H9 n-‘&H, F,
C,H,CH,
1. Infrared
spectra of organic thiocyanates
S-CrN
2141 2141 2137 2137 2141
(R-S-C=X)
Other bands
s s s s s
1316 m 1269 m 1272 mw 1275 m
1225 m 1238mw 1233 m
1062 1095 1101 1072
s w w s
1053 mw 1058 w
9% 968 916 914
VW s s w
* To whom all correspondence and requests for reprints should be addressed. 296
696 772 742 722
w s ml\ m
072 m 683 w 700 w
The infrared spectra of organic thiocyanates and isothiocyanates
cm-l. The nitrile band is observed around this region in diazoguanidine cyanide [2] and cyanogen halides [3]. Methyl thiocyanate shows a medium intensity band at 1316 cm-i which can be ascribed to the CH, deformation cl]. The other alkyl thiocyanates also exhibit a medium intensity band around 1270 cm-l which may also be due to the CH, Table 2. Infrared spectra of organic isothiocpanates R
C’%
w%
n-C,H, n-c,%J
CEI,=CHCH, CsH, 4-NH,C,H, t-C,H, [51 ally1 carbinyl [IO] trans-crotyl [lo] cis-crotyl [ 101
methosybenzyl
[I l]
Other bands
pI;=C=S
N=C=-R
1172 s
1089 1062 1081 1038 1081 986 1067 1070
s s s
1120 1115
992 1078 1075
2079 s
1153
1050
2188 s 2174 s 2155 s
2092 x-s 2092 vs 2058 vs
2155 2151 2160 2160
2088 2079 2060 2105
x-s vs vs \‘s
2090 2105 2062 2105
s
s s s s
(R--N=C=S)
1120 mu 1116 w 1121 w
w m w w n’ m w u-
938 m 977 mw 942 w 921 926 946 913 887 955 889
m nw w mw
795 mw 745 s 725 707 750 751
w m s mw
772 746 739
deformation frequency in these compounds. Also, one of the bands exhibited in the region of 683-722 cm-l may be due to the -C-Svibration. Organic isothiocyanates. The infrared spectra of organic isothiocyanates are listed in Table 2. The well known vibration frequencies like the CH vibrations, etc., have not been tabulated. All the organic isotliiocyanates exhibit a broad and very strong band centred around 2100 cm-l due to the isothiocyanate group. At lower concentrations (smaller layer thickness) this band shows finer resolution and gives a strong peak or shoulder between 2080-2105 cm-l (Fig. 1). This is in agreement with the observation of WILLIAMS [4] on phenyl isothiocyanate. However, no weak band was apparent around 1950-2000 cm-i [4, 51. The infrared spectra of organic isocyanates (R-N=C=O) has been extensively studied and the characteristic isocyanate band is found between 2269-2275 cm-l [6, 7, 81. It is interesting to note that the isothiocyanate group shows its characteristic band at a lower frequency (2060-2190 cm-l) than the isocyanate groups. While going from the normal thiocyanate to the isothiocyanate there is a decrease in the characteristic frequency just as in the example of nitrile to isonitrile [l, 91. This suggests t,hat the contribution of the resonance form: R-N=C-S @
E
is greater than that of the others [4]. All the isothiocyanates also show other bands of which the weak or medium intensity band between 1050-1089 cm-l occurs 297
E. LIEBER, C. N. R. RAO and J. RAMACEANDBAN
constantly in all of them. stretching vibration.
This may be due to the isothiocyanate
(0)
2500
symmetric
b)
2500
cm-’
cm“
Fig. 1. Infrared spectra of methyl thiocyanate and methyl tiothiocyanate. methyl &othiocyanate, - - - - - methyl thiocyanate. (b) A mixture (a) of methyl thiocyanate and methyl tiothiocyanate in carbon tetrachloride solution.
From the results discussed above it may be noted that the organic thiocyanates and isothiocyanatea can be distinguished by their characteristic vibration frequencies around 2140 cm-l and between 2060 and 2105 cm-l respectively. Fig. 1 also includes the infrared spectrum of a mixture of methyl thiocyanate and methyl isothiocyanate in carbon tetrachloride solution. The peaks due to the thiocyanate and the isothiocyanate groups are clearly distinct. Acknowledgement-The authors gratefully acknowledge grants-in-aid from the Frederick Gardner Cottrell Program of the Research Corporation, New York, U.S.A. and the Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana, U.S.A., which made this study possible.
298
The infrared spectra of organic thiocyanates and tiothiocyanates
References [I] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll]
BELLAMY L. J., The Infrared Spectra of Complex Moleculea Methuen, London 1954. LIEBER E., LEVERINQ D. R. tend PATTERSON L., Adyt. Chem. 1951 23 1594. WEST W. and FARNSWORTH M., J. Chem. Phys. 1933 1 402. WILLIAMS D., J. Chem. Phys. 1940 8 513. LUSKIN L. S., GAMTERT G. E. and CRAIG W. E., J. Anzer. Chem. Sot. 1956 78 4965. HOYER H. Chem. Ber. 1956 89 2677. BORTNICK N., LUSKIN L. S. and HUROWITZ M. D., J. .4mer. Ch,em. Sot. 1956 78 4358. DAVISON W. H. T., J. Chem. Sot. 1953 3712. GORDY W. and WILLIAMS D., J. Chem. Phys. 1936 4 85. ETTLINGER M. G. and HODGKINS J. E., J. Amer. Chem. Sot. 1955 77 1831. ETTLINGER M. G. and LUNDEEN A. J., J. Amer. Chem. Sot. 1956 78 1952.
299