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 were calculated for a wide group of heterocyclic derivatives as a function of the different structure of the heterocyclic ring and of the nature of heteroatoms, by means of MO abinitio theory. The rotational barriers refer to the two-fold potential energy, V-2, obtained from single-determinant Hartree-Fock (HF) wave functions and at MP2/6-31G*//HF/6-31G* level of theory. The V-2 values seem to represent homogeneously the rotational barrier in substituted ethylenes with varying polar character of the C=C bond, ranging from unpolarized ethylene to molecules having a marked push-pull character, and thus allow the effect of substituents on the height of the barrier to be compared. Sets of parameters quantifying the heteroatom effect of constantly 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 barriers calculated from the HF energy of the perpendicular conformation is discussed in the light of a qualitative approach based on calculated contributions of singlet excited states to the electronic configuration of the perpendicular conformation. (C) 2001 Elsevier Science BN. All rights reserved.