Effect of heteroatoms in determining the rotational barrier around carbon-carbon double bond in substituted ethylenes. An MO ab initio theoretical study

Internal rotational barriers around the exocyclic partial C=C double bond w ere calculated for a wide group of heterocyclic derivatives as a function o f the different structure of the heterocyclic ring and of the nature of het eroatoms, by means of MO abinitio theory. The rotational barriers refer to the two-fold potential energy, V-2, obtained from single-determinant Hartre e-Fock (HF) wave functions and at MP2/6-31G*//HF/6-31G* level of theory. Th e V-2 values seem to represent homogeneously the rotational barrier in subs tituted ethylenes with varying polar character of the C=C bond, ranging fro m unpolarized ethylene to molecules having a marked push-pull character, an d thus allow the effect of substituents on the height of the barrier to be compared. Sets of parameters quantifying the heteroatom effect of constantl y modulating the barrier in the different heterocyclic rings were extracted , and two heteroatoms were found to have an additive effect when acting in the same ring. The difference between the V-2 values and the rotational bar riers calculated from the HF energy of the perpendicular conformation is di scussed in the light of a qualitative approach based on calculated contribu tions of singlet excited states to the electronic configuration of the perp endicular conformation. (C) 2001 Elsevier Science BN. All rights reserved.