Bond-function analysis of rotational barriers: Methanol

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Abstract
Our previous bond-function analysis of internal rotation in ethane is extended to methanol with the essential result that the dominant contribution to the barrier in these two molecules has the same origin, namely the overlap (exclusion-principle) interaction between closed-shell, localized bonds. The role of the CH3OH lone-pair orbitals is investigated. It is concluded that their main effect on the barrier is in partially shielding the oxygen nuclear charge from the bonding electrons. The amount of screening they provide is nearly independent of conformation. The potential energy functions of the staggered and eclipsed forms of methanol are studied with a double-zeta Slater basis. The calculated angles of methyl tilt are 3.4° and − 1.8°, respectively; the inclusion of tilt raises the barrier height by about 25%. The dipole moment is determined as a function of the rotational angle.