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

摘要

Internal rotational barriers around the exocyclic partial C=C double bond were calculated for a wide group of heterocycic derivatives as a function of the different structure of the heterocyclic ring and of nature of heteroatoms, by 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 insubsituted barrier in substituted ethylenes with varing polar character of the C=C bond,ranging from unpolarized ethylene to molecules having a parameters 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 differentheterocyclic 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.