UNIVERSALITY IN ISOMERIZATION-REACTIONS IN POLAR-SOLVENTS

Recently it has been shown by Singh and Robinson (SR) that universalit y and scaling occur in the van der Zwan-Hynes (ZH) model of dipole iso merization reactions near a critical point. This advance in the unders tanding of chemical reactions was accomplished through the discovery o f an analogy of this critical point with the one in the van der Waals equation of state for an imperfect gas. In the present paper, the expe rimental data of Onganer et al. on the cis-trans photoisomerization of merocyanine 540 in n-alkyl alcohol and n-alkanenitrile solvents, as a function of the solvent shear viscosity and temperature, is analyzed in terms of the ZH model. This model, based on the Grote-Hynes theory, uses a non-Markovian solvent friction characterized by a reactant-sol vent coupling strength parameter beta, the solvent inertial response t ime r, and the solvent frictional or viscous response time x. It is fo und that the photoisomerization data with alcohol solvents fit the ZH model reasonably well, and the fitted parameters beta, r, and x are fo und to lie in the regime near the singularity or the critical point of the ZH model. Utilizing the results of the SR study mentioned above, the experimental isomerization rate is shown to obey a universal scali ng relation involving the two scaled variables, gamma = (1 - beta)r an d z = (1 - beta)x(2/3), of the ZH theory in the critical regime. A fra ctional power law dependence of rate on the inverse of viscosity arise s naturally from the theory. The two-variable experimental scaling fun ction is shown to agree with the theoretical scaling function of the Z H model, thus demonstrating the existence of scaling and universality in these experimental data.