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 t he conformational minimum (GS) and of the rotational transition state (TS). Molecules with different polar character of the double bond were chosen, r anging from ethylene to olefins with strong push-pull character. The enthal pies of hydrogenation, DeltaH(hydr.), and of formation, DeltaH(f)(0), of th ese molecules were employed to obtain the stability of GS; these thermochem ical 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 chemis try. The stabilization of TS was derived from the torsional potential for r otation around the C=C bond. The lowering of the energy content of GS of su bstituted ethylenes, referring to ethylene, is accompanied by an even great er stabilization of TS, thus a lowering of the rotational barrier with resp ect to ethylene is generally found in these molecules. (C) 2001 Elsevier Sc ience B.V. All rights reserved.