The vibrational spectra of phenyl thiocyanate and benzoyl isothiocyanate

Research notes

The vibrational spectra of phenyl thiocyanate and bemoyl isothiocyanate (Received 21 February 1960)

IN A previous study of the vibrational spectra of some isothiocyanates [l] we concluded that NCS groups in different types of environment show the following characteristic bands: (1) Aliphatic (-CH,NCS) and aromatic (ArNCS) isothiocyanates-2100 cm-l, a broad and strong infra-red band with integrated intensity B about 15 x lo4 mole-l 1. cmm2,halfwidth about 100 cm-l, and a moderately strong, broad and double Raman band. (2) Aliphatic isothiocyanates-1090 cm-l, a strong band in both the Reman and i&a-red spectra. (3) Aromatic isothiocyanates-1250 cm-l, a very strong Raman and very weak infra-red band and 930 cm-l, a strong infra-red and moderately weak Raman band. The infra-red spectrum of a benzoyl isothiocyanate solution in the 2000 cm-’ region has been obtained by other workers [2], and we now report fuller i&a-red and Raman spectra of this compound, together with the infra-red and Raman spectra of phenyl thiocyanate. These spectra confirm the above identification of the characteristic isothiocyanate bands. Benzoyl isothiocyanate was prepared by the method of ELMOREand OGLE[3] and was vacuum-sublimed before use. The sample of phenyl thiocyanate was kindly provided by Dr. D. WILLIS. The spectroscopic techniques used are described in our previous paper [l]. The infra-red spectra of these compounds, together with that of phenyl isothiocyanate. are shown in Fig. 1 and their Raman frequencies are listed in Table 1. Table 1. Reman frequencies (cm-‘) Benzoyl isothiocyanatd

,

168 VW, 195 VW, 490 w, 612 m, 704 m br, 846 VW br, 990 vs sharp, 1020 8, 1062 m, 1082 s, 1157 w, 1177 w, 1238 m br, 1294 m, 1425 w diff, 1444 w, 1591 vvs, 1690 vs br asymm.

I

Phenyl thiocyanate

-

205vw,

276w,

393 w, 689 w, 9968,

1017 m, 1086m,

158Ow,

2150m

Ph,enyZ thiocyanute, C,H,SCN

The infra-red spectrum of this compound shows a sharp, moderately intense band at 2175 cm-l, which is characteristic of organic thiocyanates [4]. The integrated intensity B in carbon tetrachloride solution is 0.21 x 104 mole-l 1. cm-2, apparent half-width 11 cm-l and apparent emax 47. Furthermore there is, as expected for a thiocyanate, no strong Raman line at 1250 cm-l and no strong infra-red band at 930 cm-l. The moderately strong Raman and infra-red band at 1086 cm-l, which is not observed in methyl thiocyanate [I], is assigned to WHIFFEN’Sq-vibration, which is a combination of an in-plane C-H deformation and the G-S stretching vibration [5]. [I] N. S. HAM and J. B. WILLIS, Spectrochim. Acta 16, 279 (1960). [2] ;;MT;)REELMoRE and J. R. OGLE, Tetrak&on 3, 310 (1958); and personal communication from D. T. [3] D. T. ELMOREand J. R. OGLE, .7. Chew. Sot. 1141 (1958). [4] E. LIEBER, C. N. R. RAO and J. RAMACHANDRAN, S~ectrochim. Acta 13, 296 (1959). [5] D. H. WHIFFEN, J. Chem. Sot. 1350 (1956).

FREQULNCY

IN

WAVLNUMILRS

Fig. 1. Tnfra-red spectra of benzoyl isothiocyanate (capillary film), phenyl isothiocyanate (0.063 mm layer) and phenyl thiocyanate (0*063 mm layer and capillary film). In the infra-red spectrum of phenyl thiocyanate, the broad peak at 2075 cm-r and shoulder at 2125 cm-r suggest the presence of about 3 per cent of an isothiocyanate. The intensity values quoted above have been corrected to allow for this.

This acyl isothiocyanate has its broad strong NCS infra-red band at 1995 cm-r, with a half-width of about 100 cm-l and a subsidiary peak at 1950 cm-r, which is probably due to a combination band of the out-of-plane C-H deformations. The integrated intensity B in carbon tetrac~oride solution is 21.0 x 104 mole-r 1. cm-2, with an Earn of 830, and these figures are higher than those reported for any other isothiocyanate [l, 6,7]. The lowering of frequency and increase of intensity, compared with other isothiocyanates, suggest that there is inter&ction between the NCS and CO groups [2] and this inference is supported by study of the carbonyl band,* whose frequency (1703 cm-r) and intensity (B = 3.7 x lo4 mole-l 1. om-2) resemble not so much the values for acetophenone (1692 cm-l, 2-l x 10” mole-l 1. cmm2)as those for acetanilide (1705 cm-l, 3.1 x lo* mole 1. cme2) [8]. The compound differs from aliphatic and aromatic isothiocyanates in showing no sign of the 1995 cm-r band in the Raman spectrum, but resembles methyl, ethyl and phenyl * This band has a slight asymmetry on the high-frequency side (near 1715 cm-l) which is perhaps the first overtone of the strong band occurring at 850 cm-1 in the liquid, intensified by Fermi resonance.

394

Research notes

isocyanates, where the Raman counterpart of the strong 2260 cm-l infra-red band has not been observed. [9l The NCS group here is not connected directly to the aromatic ring, so there is no strong Raman line at 1250 cm-l and no strong infra-red band at 930 cm-l. The strong infra-red band at 1245 cm-l is due to the CO group and is related to the strong infra-red band at 1205 cm-l in benzoyl chloride, while the weak infra-red band at 930 cm-l is due to an out-of-plane C-H deformation vibration which also gives a band at 930 cm-l in benzoyl isothiocyanates chloride. Since the group C-NCS is similar to that in aliphatic (-CH,NCS) a strong Raman band is found, as expected, at 1082 cm-l. [6] [7] [S] [9]

G. L. CALDOWand H. W. THOMPSON,Spectrochim. Acta 13, 212 (1958). E. SVATEK, R. ZAHRADNIXend A. KJAER, Acta Chesn.Scantl. 13, 442 (1959). G. M. BARROW,J. Chews. Phys. 21, 2008 (1953). fCi4Zj F. KOHLRAUSCH,Ramanqxktren pp. 298 and 381. Akademische Verlagsgesellschaft, Leipzig

Division of Chemical Physics C.S.I.R.O., Chemical Research Laboratories Melbourne, Australia

NORMAN S. HAM J. B. WILLIS