SUDDEN POLARIZATION - PYRAMIDALIZATION OF TWISTED ETHYLENE

Nonempirical molecular electronic structure theory has been used to investigate the concept of “sudden polarization”. In particular, the departure of twisted ethylene from ideal D2d geometries has been studied. Initially, configuration interaction (CI) studies of all four electronic states (of twisted C2H4) arising from the e2 orbital occupancy were completed. The predicted C-C bond distances and energies relative to the planar ground state follow:1B1 (N state) 1.49 Å, 2.6 eV;3A2 (T state) 1.49 A, 2.7 eV; B2 (V state) 1.40 A, 5.9 eV; A, (Z state) 1.40 A, 5.8 eV. A major theoretical problem for the zwitterionic Z and V states is to formulate a type of wave function which passes smoothly and correctly from D2d to non-D2d geometries. This problem has been solved and the ensuing calculations predict rather large dipole moments as the Z and V states become pyramidalized. For example, with one of the CH2 groups bent out of its D2d plane by only 5°, the Z and V state dipole moments µ(along the C-C axis) are +1.25 and -1.18 D, respectively. The differing dipole moment signs are meant to imply that for the Z state, the pyramidalized methylene carries the negative charge. The Z state, the lower of the two zwitterionic states, has a pyramidalization angle 8 of 31 0 at its equilibrium geometry. © 1979, American Chemical Society. All rights reserved.