A LOW-TEMPERATURE MATRIX-ISOLATION INFRARED STUDY OF ACETAMIDES .2. THIOACETAMIDE AND SOME DEUTERATED DERIVATIVES

The IR spectra of thioacetamide and some deuterated derivatives isolat ed in an argon matrix (14 K) are reported. In comparison with acetamid e the CN torsional barrier and the frequencies of the NH2, CH3 and CN stretchings are explained by the influence of the chalcogen atom on th e relative weights of resonance structures in the valence bond scheme. The calculated value of the CN barrier of 128 kJ mol-1 is found to be larger than that obtained for acetamide, 81 kJ mol-1. The results are in good agreement with those reported from NMR measurements for solut ions and the trend is contrary to that obtained from solid state IR fr equencies.