Ab initio energy calculations for the intramolecular rotational potential of polyacetylene

Using ab initio methods, calculations have been performed on the rotational states of polyacetylene. Using a 4--31G basis set a gauche state is found involving a 400 rotation from the cis state. It is 0.6 kcal/mol lower in energy than the cis state. The trans state is the lowest energy state with an energy 2.60 kcal/mol lower than the gauche. Trans--cisoid to trans isomerization is proposed to occur by a rotation about the single C--C bonds in the backbone with a rotational barrier of 2.61 kcal/mol. The lack of sensitivity which arises when using a STO-3G poor basis set is also demonstrated