Hindered rotation in thioureas: Steric effects and conformations

Abstract

Abstract The temperature dependent spectra of several mono‐, di‐ and trialkylthioureas have been recorded. Free energy barriers to internal rotation about the CN bonds have been calculated. In thioureas that were unsymmetrically substituted, free energy barriers were found to be different for each CN bond with the more substituted amino group exhibiting the higher barrier. The monosubstituted thioureas showed different rotational barriers for the NH 2 groups of the cis and trans isomers, respectively. The free energy barriers for the trans isomers were found to be substituent dependent and substantiate the reassignment of the high and low field substituent signals to the trans and cis isomers, respectively. The spectrum of 1‐methyl‐3‐ t ‐butylthiourea indicates restricted rotation of the t ‐butyl group at temperatures below 200 K.