Universality in isomerization reactions in polar solvents

Mary Vedamuthu, Surjit Singh, Yavuz Onganer, David R. Bessire, Mary Yin, Edward L. Quitevis, G. Wilse Robinson

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Recently it has been shown by Singh and Robinson (SR) that universality and scaling occur in the van der Zwan-Hynes (ZH) model of dipole isomerization reactions near a critical point. This advance in the understanding of chemical reactions was accomplished through the discovery of 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 experimental 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-solvent coupling strength parameter β, the solvent inertial response time r, and the solvent frictional or viscous response time x. It is found that the photoisomerization data with alcohol solvents fit the ZH model reasonably well, and the fitted parameters β, r, and x are found 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 scaling relation involving the two scaled variables, y = (1 - β)r and z = (1 - β)x2/3, of the ZH theory in the critical regime. A fractional power law dependence of rate on the inverse of viscosity arises naturally from the theory. The two-variable experimental scaling function is shown to agree with the theoretical scaling function of the ZH model, thus demonstrating the existence of scaling and universality in these experimental data.

Original languageEnglish
Pages (from-to)11907-11913
Number of pages7
JournalJournal of physical chemistry
Issue number29
StatePublished - Jul 18 1996


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