The origins of the isomerization barriers for the isomerization of vinylidene (H2C=C), fluorovinylidene (HFC=C), and difluorovinylidene (F2C=C) to their respective acetylenes are explored in this paper. The bonding interactions present in the transition states of each isomerization pathway are analyzed within the framework of electron density deformations and the atoms-in-molecules method using densities obtained from quadratic configuration interaction calculations. The high isomerization barrier for F2C=C is a consequence of the large energetic penalty associated with the C-F bond cleavage to give a covalently unbound fluorine in the transition state. In the case of H2C=C isomerization, analysis by the atoms-in-molecules method reveals that a strong covalent bond exists between the migrating hydrogen and the C=C bond critical point. Concerted C-H bond cleavage and formation in the hydrogen migration process is expected to yield a low energetic requirement for H2C=C isomerization. The observed difference in the bonding interactions present in the transition states for fluorine and hydrogen atom migration can be rationalized in terms of the difference in directionality of the hybrid orbital on the migrating atom. Calculations carried out for both fluorine and hydrogen migration in HFC=C revealed bonding interactions in the transition states that are reminiscent of those observed in the isomerization of F2C=C and H2C=C. An alternative account for the observed violation of Hammond's postulate in the H2C=C isomerization pathway is also provided. Finally, we demonstrate that the conceptual framework defined in this work may be used to explain the kinetic stabilities of other species that can undergo 1,2-atom shift reactions across an unsaturated bond. (C) 2003 American Institute of Physics.