Gas Phase Free Energies of Formation and Free Energies of Solution ofαC-Centered Free Radicals from Alcohols: A Quantum Mechanical−Monte Carlo Study

Abstract

The gas phase free energies of formation of the αC-centered radicals of methanol (0.1 kJ mol-1), ethanol (−11.2 kJ mol-1), 1-propanol (−1.8 kJ mol-1), and 2-propanol (−23.2 kJ mol-1) were derived from a combination of experimental data and theoretical procedures. Enthalpies of formation were taken from experiment or derived from ΔfH°(g) of the parent alcohols and theoretical BDEs (radicals of 1- and 2-propanol). Entropies were obtained from B3LYP/6-31G(D) geometries and vibrational frequencies, and the rigid rotator harmonic oscillator approximation, taking account of the conformational mix of the free radicals. These results were combined with experimental free energies of formation in water to yield free energies of solution. The BOSS Monte Carlo discrete solution simulation package, combined with quantum mechanical calculations (QM+BOSS), was used to derive free energies of solution of the αC-centered free radicals of methanol, ethanol, 1-propanol, and 2-propanol in water. The absolute free energies of solution are quantitatively described by QM+BOSS with TIP4P water (in kJ mol-1): methanol radical, expt −17.3, calc −16.2; ethanol radical, expt −11.8, calc −12.3; 2-propanol radical, expt −12.3, calc −13.3. A value is predicted for the 1-propanol radical, −15.4 kJ mol-1. The results are not sensitive to the choice of Lennard-Jones parameters for the radical center. The recommended procedure involves geometry optimization and frequencies at the B3LYP/6-31G(D) level in the gas phase, followed by a single point SCRF−SCIPCM calculation to obtain CHELPG charges. Omission of the SCRF step yields free energies of solution that are too low compared to experiment. The radicals are less solvated than the parent alcohols. Examination of the CHELPG charges suggests that the reason lies in the lower polarity of the C−O bond and lower H-bond acceptor ability of the oxygen atom.