SCF LCAO Study of Linear and Bent Acetylene

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Abstract
The self‐consistent field molecular orbital method has been applied to linear acetylene, and to the cis‐ and trans‐bent molecule, for various values of the bending angle. All 14 electrons have been taken into account, and free inner‐shell outer‐shell mixing has been allowed. As for the multicenter integrals, they have been evaluated by the Mulliken approximation. Reasonable values are obtained for the first ionization potential and for the excitation energy corresponding to the first π→π* transition. The variation of the orbital energies induced by the distortion brings a confirmation of the qualitative predictions made by Walsh on the basis of the simple molecular orbital theory, but the treatment fails to predict the correct shape of the molecule in its first excited state, very likely because of the approximation of the three‐ and four‐center integrals. The difference in energy between the cis‐ and the trans‐configurations is predicted correctly, and there is a rough agreement between the calculated force constants and the experimental values. The results of an electron population analysis agree with experimental evidence and with the current views on hybridization, and they are confirmed by an examination of the equivalent orbital representation.