Abstract
Ab initio molecular orbital calculations using MP2 and DFT/B3LYP methods at the 6-311++G(d,p) and aug-cc-pvdz basis sets were applied to characterise the kinetics of the thermal dissociation of HC≡CCH
2
NHX [where X = OH(I), F(II) and Cl(III)] to produce Z- and E-prop-2-ynylideneamines (HC≡CCH=NH) (IV and V, respectively), which tautomerise to vinyl cyanide (CH
2
=CHC≡N) (VI). The optimised geometries and electronic energies of reactants, transition states and products were estimated and discussed. A concerted proton migration and HX abstraction mechanism was proposed for the imine formation. The reliance of these properties on the elected levels of theory was discussed. The activation energies and barrier heights for the Z- and E-forms and their vinyl cyanide tautomers were estimated and analysed. The Z-form was computed to be more stable than the E-form. Using natural bond orbital calculations, the origin of the preference of the Z-form was attributed mainly to the N lone pair delocalisations. Vinyl cyanide was located to have a lower energy (33-35 kcal/mol) than prop-2-ynylideneamine. The provenance of the preference of the former and its tautomerisation mechanism will be addressed in a separate publication.