EFFECT OF CARBON-ATOM PYRAMIDALIZATION ON THE BONDING IN ETHYLENE

The changes in the bonding that occur on syn and anti carbon atom pyramidalization in ethylene are analyzed from the perspective of the second-order Jahn-Teller effect. Situations in which a pyramidalized geometry might actually be favored over a planar one are discussed, and a preference for anti over syn pyramidalization is predicted. The results of ab initio SCF calculations on planar and pyramidalized geometries are reported. It is found that the energy difference between the lowest singlet and triplet state decreases monotonically with increasing pyramidalization. The C-C bond lengthens and the HCH angle opens on pyramidalization; and, as expected, the anti mode is found to be less energetically costly than the syn. The changes in the bonding that occur on pyramidalization are elucidated by population analyses, which show that orbital following of the motions of the hydrogens is far from complete. © 1979, American Chemical Society. All rights reserved.