Understanding solvent effects on structure and reactivity of organic intermediates: a Raman study

Abstract

The effect of solvent on chemical reactivity has generally been explained on the basis of the dielectric constant and viscosity. However a number of spectroscopic studies, including UV-VIS, IR and Raman, has led to numerous empirical parameters to define solvent effect based on either solvating ability or polarity scale. These parameters include solvent polarizability, dipolarity, Lewis acidity and Lewis basicity, E_T(30), π*, α, β etc. However, from a structural point of view, we can separate solvation as static and dynamic processes. The static solvation basically relates to stabilization of the molecular structure by the solvent to attain the equilibrium structure, both in the intermediate and ground state. Dynamic solvation relates to solvent reorganization-induced dynamics prior to the structural reorganization to reach the equilibrium state. In this paper, we present (a) structural distortions induced by the solvent due to preferential solvation of the triplet excited state, and (b) the importance of dynamic solvation induced by vibronic coupling (pseudo-Jahn-Teller coupling). The examples include the effect of solvent on structure and reactivity of excited states of 2,2,2-trifluoroacetophenone (TFA). Based on the comparison of time resolved resonance Raman (TR3) data of TFA and other substituted acetophenone systems, it was found that change in solvent polarity indeed results in electronic state switching and structural changes in the excited state, which explains the trend in reactivity. Further, a TR3 study of fluoranil (FA) in the triplet excited state in solvents of varying polarities indicates that the structure of FA in the triplet excited state is determined by vibronic coupling effects and thus distorted structure. These experimental results have been well supported by density functional theoretical computational studies.