Steric Effects of Alkyl Substituents on the N–H Stretching Absorptions and Rotational Isomerism of N,N′-Dialkylthioureas

Abstract

Abstract The fundamental N–H stretching absorptions of twenty nine N,N′-dialkylthioureas were observed in dilute solutions. The observed frequencies were classified into five groups assignable to the out, three different types of the trans, and the cis forms. The steric hindrance between the bulky t-butyl group and the thiocarbonyl sulfur, which is larger than for N,N′-dialkylureas, was suggested to cause the out form. Dialkylthioureas of RTUtB type seem to exist to a great extent in the trans-out conformation in solutions. It is suggested that it is not the N–H group with the bulky t-butyl group but the N–H group in the opposite part which is out of the skeletal plane. The energy difference between the out and the trans forms was found to be ΔH=430±100 cal mol−1.