Restricted carbon–nitrogen bond rotation in some ureas, thioureas, and thiuronium salts

Abstract
Some NN-diaryl-NN′-dimethylureas, their thio-analogues, and their S-methylthiuronium salts exhibit a kinetic phenomenon, observable by n.m.r. spectroscopy, which has a free energy of activation, ΔG Tc, of between 40 and 70 kJ mol–1. The origin of this process, probably slow rotation about the C–NMe2 bond, is discussed in terms of the variation of ΔG Tc with structure.

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