Solvent effects on the absorption spectrum and first hyperpolarizability of keto-enol tautomeric forms of anil derivatives: A Monte Carlo/quantum mechanics study

Abstract

Theoretical results for the absorption spectrum and electric properties of the enol and keto tautomeric forms of anil derivatives in the gas-phase and in solution are presented. The electronic properties in chloroform, acetonitrile, methanol, and water were determined by carrying out sequential Monte Carlo simulations and quantum mechanics calculations based on the time dependent density functional theory and on the second-order Møller-Plesset perturbation theory method. The results illustrate the role played by electrostatic interactions in the electronic properties of anil derivatives in a liquid environment. There is a significant increase of the dipole moment in solution (20%-100%) relative to the gas-phase value. Solvent effects are mild for the absorption spectrum and linear polarizability but they can be particularly important for first hyperpolarizability. A large first hyperpolarizability contrast between the enol and keto forms is observed when absorption spectra present intense lowest-energy absorption bands. Dynamic results for the first hyperpolarizability are in qualitative agreement with the available experimental results.

Effect of Solvent Polarizability on the Keto/Enol Equilibrium of Selected Bioactive Molecules from the 1,3,4-Thiadiazole Group with a 2,4-Hydroxyphenyl Function

“…Numerous reports refer to the solvent polarity as the main factor influencing the keto/enol equilibrium . The strong solvation effect is considered as a main cause of the dominance of the keto form in polar solvents, while in nonpolar solvents the tautomeric equilibrium is usually shifted toward formation of enol forms, most likely due to the formation of internal hydrogen bonds. ,, On the other hand, studies on some specific Schiff bases revealed an opposite solvent-dependent effect taking place in a range of polar and nonpolar solvents . The present work aimed to determine the influence of polar and nonpolar solvents on the enol/keto equilibrium, using solvents with different electric polarizabilities.…”