Kinetic Pressure and Substituent Effects on the Thermal Isomerization of Triphenylformazans

The rates of the thermal isomerizations around N=N (Process A) and C=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−1, ca. −20 J K−1 mol−1, and ca. −5.0 cm3 mol−1 respectively for Process A, and ca. 40 kJ mol−1, ca. −200 J K−1 mol−1, and ca. −10 cm3 mol−1 respectively for Process B. It was suggested that the thermal isomerizations both for Processes A and B proceed by the inversion mechanism. Furthermore, in the transition state of Process B, a quasi six-membered ring was assumed to form. The very large negative entropy of Process B was discussed in terms of the restricted internal rotations, and others.