Theoretical Calculation of a Solution Phase Torsional Free Energy Profile. π-Ethylimidazole in Water

Abstract

Abstract Ab initio molecular orbital calculations, in conjunction with Monte Carlo statistical mechanics simulations, have been used to calculate the torsional free energy profile for π-ethylimidazole in aqueous solution. Geometry optimisations were carried out with 3–21G and 6–31G* basis sets on various conformational forms of π-ethylimidazole. Electron correlation energies were calculated from the 6–31G* optimised geometries using Moller-Plesset second order perturbation theory and single point calculations with the STO-3G basis set were used to obtain an estimate of atomic polarisation effects. Gas phase torsional energy barriers were found to be small; ∼7–8kJmol−1 for the principal barrier and ∼4.5–5.5 kJmol−1 for the subsidiary barrier. Free energy perturbation theory was then applied in Monte Carlo simulations to examine the effect of solvation on the calculated gas phase torsional energy profile. The variable torsion angle of a single π-ethylimidazole molecule in a periodic box of 505 TIP4P water m...