Bandshape analysis with internal resolution standard. Barriers to internal rotation in 3,5‐dimethyl‐4‐(N,N‐dimethylthio‐carbamoyl)pyrazoles

Abstract

The barriers to rotation of the dimethylamino group in a series of 4-N,N-dimethylthiocarbamoylpyrazoles have been determined by the 'I3n.m.r. technique. For one of the compounds the rate constants have been determined over a temperature interval of 113 degrees by monitoring the exchange rate in the fast and slow exchange regions by the intensity of the resonances of the exchanging protons relative to an internal reference signal, and in the intermediate region by the conventional complete bandshape technique. The entropy and enthalpy of activation have been calculated, and the values obtained by the intensity method are in reasonable agreement with those obtained by the complete bandshape analysis. For the other compounds the usual coalescence approximation has been used. The thiocarbamoylpyrazoles were prepared by reaction between N,N-dimethyl-a,a-diacetylthioacetamide and the appropriate hydrazines.