Knowledge Bank

The barrier to methyle internal rotation in dimethyle ether is analysed through decomposition of the barrier energy by symmety partitioning and natural bond orbital analysis. The largest single contribution to the barrier is the energy change of the $\sigma$ lone pair orbital, largely due to the rehybridization at-the-top of the barrier (staggered) conformation. The main structural changes, leading to the decrease in s and increase in p character of this orbital, is the openinng of the COC angle, occuring because of the increased Pauli exchange repulsion between C-H(in plane) orbital. This interaction is among the major barrier forming terms, but is far less important than the changes in the energy of the lone pair $\sigma$ orbital on oxygen. Change transfer energy contribution are also important, but through the contribution of many small barrier forming terms. This work was supported by National Science Foundation, Petroleum Research Fund and through a grant of C-90 computer time from Pittsburgh Supercomputer Center.