Thermal Isomerization of 1′,3′,3′-Trimethyl-8-nitrospiro[2H-1-benzopyran-2,2′-indoline]-6-carboxylic Acid

The rates of thermal isomerization of 1′,3′,3′-trimethyl-8-nitrospiro[2H-1-benzopyran-2,2′-indoline]-6-carboxylic acid (6-COOH-8-NO2-SP) from its spiro form (SP) to its merocyanine form (MC) have been measured in some solvents and at high pressures. The equilibrium constants between these isomers at atmospheric pressure and at 30 °C are in the range of 0.25 to 2.52 depending on solvents. The activation volumes are 0.2–3.4 cm3 mol−1, while the reaction volumes are −7.9–−22.3 cm3 mol−1. Thus the volume profile was found to have a maximum, as predicted previously.