The puzzling structure and bonding of the methanol radical cation.

Electron spin resonance indicates that the unpaired electron in the methanol radical cation is delocalized, however, the molecular geometry has not been experimentally resolved. In this work, high level, state-of-the-art computations at the finite temperature density functional theory and highly correlated CCSD(T) levels indicate that a syn-periplanar conformation of the H-C-O-H bonds, in which the C-H and O-H bonds eclipse each other, is a three-fold global minimum in the potential energy surface for internal rotation of the O-H bond. We show that vicinal hyperconjugation between the orbitals in the C-H bonds and in the oxygen atom is responsible for this puzzling conformational preference. The transition state for the rotation yields an ≈0.6 kcal/mol rotational barrier, which matches the thermal energy at room conditions and, therefore, renders the O-H bond a free rotor. The molecular wave function has a moderate multireference character with the oxygen atom acting as the preferred spot for static correlation.