Kinetic Studies of Solvent and Pressure Effects on Thermochromic Behavior of 6-Nitrospiropyran

Abstract

Abstract The thermal conversion of 6-nitrospiropyran from a merocyanine form to a spiro form was studied in various solvents and under high pressures. The rates of the thermal conversions were remarkably retarded in polar solvents, and a Kirkwood plot of the rate constants showed a large downward deviation as the solvent polarity increased. The rate constants (on a logarithmic scale) were linearly related to Kosower’s Z-values. The values of the activation volumes were positive and divided into two groups: ΔV≠≈10 cm3 mol−1 (in nonpolar solvents) and ΔV≠≈20 cm3 mol−1 (in polar solvents). These findings suggest that for the reaction in polar solvents, specific interactions are operating and that the thermal conversion proceeds via a transition state which is much less polar than the initial state.