Ground-state molecular stabilization of substituted ethylenes. A theoretical mo ab-initio thermochemical study

The origin of the rotational barrier around the partial C=C double bond in substituted ethylenes is discussed with reference to the stabilization of the conformational minimum (GS) and of the rotational transition state (TS). Molecules with different polar character of the double bond were chosen, ranging from ethylene to olefins with strong push-pull character. The enthalpies of hydrogenation, DeltaH(hydr.), and of formation, DeltaH(f)(0), of these molecules were employed to obtain the stability of GS; these thermochemical properties were calculated with MO ab-initio theory at HF/6-311G**//HF/6-311G** and MP2/6-311G**//HF/6-311G** levels and with CBS-4M model chemistry. The stabilization of TS was derived from the torsional potential for rotation around the C=C bond. The lowering of the energy content of GS of substituted ethylenes, referring to ethylene, is accompanied by an even greater stabilization of TS, thus a lowering of the rotational barrier with respect to ethylene is generally found in these molecules. (C) 2001 Elsevier Science B.V. All rights reserved.