Theoretical Study of the Rotational Barriers of Allene, Ethylene, and Related Systems

Abstract

The allene and ethylene rotational potential surfaces are investigated by means of ab initio SCF MO calculations employing a basis set of Gaussian group orbitals. It is pointed out that the MO's of both systems can be divided into two categories, σ or π , according to the behavior of their component carbon AO's upon rotation about the C–C bonds. A general qualitative theory capable of dealing with the barriers of allene and ethylene and related systems in the same terms is thus developed on the basis of two observations: First, that the π MO's in both allene and ethylene are essentially unchanged by hydrogen rotation and secondly, that there is a one‐to‐one correspondence between each of the four most stable π MO'x of the two molecules. Subsequently it is shown that the barrier description of both systems is significantly altered upon the introduction of CI techniques to the quantitative calculations and consequently the qualitative theory, based strictly upon consideration of the closed shell SCF wavefunctions, can be improved in a simple manner to take account of the modifications of the rotational potential surfaces dictated by configuration interaction. Finally, the resulting integrated theory is applied to the discussion of other geometrical phenomena in AnH4 systems and also to the study of related trends in molecules containing a different number of hydrogen atoms, such as acetylene, hydrogen peroxide, and ethane.