Substituent and solvent effects on the thermal isomerisation of 7H-pyrazolo[5,1-c]-1,2,4-triazole azomethine dyes

Activation parameters for the thermal syn–anti isomerisation of 25 pyrazolotriazole azomethine dyes in ethanol have been obtained, and the effect of solvent on the rate of isomerisation examined for two of these compounds. Substituent and solvent variations have only a minor influence upon the rate constants, and steric crowding about the CN bond is the major factor in determining the isomerisation rate. For substituent variations in the 3-position there is a reasonable Hammett correlation between rate constants and σp substituent constants and an isokinetic relationship, with an ill-defined isokinetic temperature, at ca. 520–1200 K. These data are discussed in terms of the inversion, rotation and biradical mechanisms for isomerisation.