Abstract
AbstractA linear‐combination‐of‐bond‐orbitals (LCBO) formulation of INDO–SCF–MO theory is employed to investigate the origin of rotation barrier coupling in propane and related molecules. The dominant contributions to rotor coupling in propane are identified to be (i) a direct steric (bond–bond) interaction, involving the close approach of two methyl CH bonds in the eclipsed–eclipsed geometry, and (ii) a nonvicinal bond–antibond effect, which arises when the side of a σCH bond orbital in an eclipsed methyl group comes into favorable overlap with the “backside” lobe of the coplanar σ antibond in the staggered–eclipsed geometry. A smaller nonadditive interaction of each methyl rotor with the central group is noted to be of importance when the central atom contains lone pairs. Aspects of this qualitative picture, and its dependence on geometry changes, are tested in applications to (CH3)2CHF, (CH3)2NH, (CH3)2O, (CH3)2CCH2, (CH3)2CO, and C(CH3)4.
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