Electron momentum distributions in the π orbitals of small hydrocarbons from the (e,2e) experiment

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Abstract
Momentum distributions have been obtained by the (e,2e) technique for the π electrons of acetylene, ethylene, and butadiene, and for the pseudo‐π electrons of cyclopropane. The first and second ionization potentials of butadiene are unambiguously assigned to π orbitals from the shape of the associated momentum distributions. Experimental momentum distributions are compared with those calculated from a number of approximate wave functions. Hückel, INDO approximate SCF, and minimum basis set ab initio SCF MO wave functions all predict maxima in momentum distributions at values about 0.2 a0−1 higher than observed experimentally for the compounds studied, although differences between compounds are described correctly. Double zeta basis ab initio SCF calculations give considerably more accurate momentum distributions, although some discrepancies between calculation and experiment remain for the more diffuse of the π orbitals. The expected linear relation between the positions of the momentum distribution maxima and orbital energies is observed.

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