The infrared spectra of alkyl isothiocyanates

Abstract

A comparison of solid and liquid state spectra of several alkyl isothiocyanates has resolved some of the interpretative difficulties in assigning ν s (NCS) and ν(C αN). The C αN stretching vibration is assigned to absorption bands at about 640 cm −1 on the basis of a “splitting” observed in this region in n-propyl isothiocyanate as contrasted to ethyl isothiocyanate. The ν s (NCS) vibration is then assigned to the bands in the 1100 to 980 cm −1 region. Complete vibrational assignments are presented for methyl, ethyl, isopropyl and tert-butyl isothiocyanate. Restricted rotation about the C αN bond and the associated rotational isomerism has been observed for the alkyl isothiocyanates. At room temperature, the methyl group rotates freely with respect to the isothiocyanate reference frame since the barrier in the methyl compound is only 0.275 kcal/mole. This free rotation is reflected in the fine structure observed for ν as (CH 3) and δ as (CH 3) in the vapor-state spectrum.