The rates of the thermal isomerizations around N equals N (Process A) and C equals N (Process B) of substituted triphenylformazans were followed spectrophotometrically at various temperatures and under high pressures, and the activation parameters were determined. The values of the activation energy, entropy, and volume were found to be ca. 70 kJ mol** minus **1, ca. minus 20 J K** minus **1 mol** minus **1, and ca. minus 5. 0 cm**3 mol** minus **1 respectively for Process A, and ca. 40 kJ mol** minus **1, ca. minus 200 J K** minus **1 mol** minus **1, and ca. minus 10 cm**3 mol** minus **1 respectively for Process B. It is suggested that the thermal isomerizations both for Processes A and B proceed by the inversion mechanism.
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