Role of Lone-Pairs in Internal Rotation Barriers

Abstract

An in-depth understanding of internal rotation barrier formation and energetics for dimethyl ether, protonated dimethyl ether, methanol, and their sulfur analogs is provided by dissecting the barrier into Pauli exchange steric repulsion, σ-lone-pair reorganization, and π hyperconjugation. The combined natural bond orbital, symmetry, and relaxation analysis demonstrates that oxygen σ lone-pairs play an important (sometimes dominant) role in controlling barrier heights. In dimethyl ether the increased steric contact brought about by simultaneous rotation of the methyl groups causes the COC angle to widen, in turn increasing the σ lone-pair p character, which leads to large lone-pair reorganization energy. Steric repulsion contributes to the dimethyl ether >4 kcal/mol barrier energy in only a minor way even though the steric contact can be looked at as the origin of the lone-pair increased p character. Absence of a σ lone-pair in acid media predicts a drastically lowered barrier (i.e. ∼1 kcal/mol). In methanol the increase in O atom lone-pair p character and associated lone-pair reorganization energy is strongly reduced, leading to an also greatly lowered barrier. Lone-pair σ reorganization effects are smaller in the sulfur-containing compounds, and C-S (σ) bond weakening is predicted to become the dominant barrier energy controlling term.

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