Where does the dimethyl ether internal rotation barrier come from?

The energetic consequences of skeletal relaxation and natural bond orbital and symmetry decomposition of the barrier energy for dimethyl ether internal rotation is analysed. The largest contribution to the barrier energy is found to involve increased p character in the oxygen σ lone-pair on going to the barrier top. Opening of the COC angle, occurring because of increased Pauli exchange repulsion between in-plane C-H methyl orbitals is responsible for the p character increase. However, the Pauli repulsion does not contribute importantly to the barrier energy. π-interaction effects are found to give important, but not dominant barrier energy contributions.