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The NOx(x=1 or 2)-catalyzed isomerization of cis-2-butene was investigated by means of the INDO-UHF theory, with a particular view to the energetic and conformational aspects of the reaction. The NO2-addition process to form the intermediate radical (R1) had no appreciable activation barrier, although the NO2-fission process from the intermediate radical (R2) had the C-NO2 dissociation energy of 44.6 kcal/mol. Some characteristic features of the stretched C-C bond in the intermediate radical (R1) were; (1) rcc=1.45 Å, which was roughly equal to the value (1.481 Å) in cis-2-butene at the transition state of its thermal isomerization; (2) a pseudo π-conjugation with a small π-character (about 13% of ECCR) in comparison with that (about 29% of ECCR) in cis-2-butene, and (3) approximately the same magnitude of the hfsc value (34.40–34.47 G) for the sp2-hydrid carbon as that (experimental value of 39.07 G) for the said atom in the ethyl radical. The calculated internal-rotation barrier of the intermediate radical (R1) around the C–C bond (1.75–3.01 kcal/mol in the two rotating directions) was smaller than that (around 4 kcal/mol) obtained from the thermodynamic estimations. The energetically most stable or unstable rotamer was found in the staggered or the eclipsed configuration respectively; a hydrogen-bonding effect on the stabilization was observed in staggered configuration, although a remarkable effect of the homoconjugation or the repulsive interaction of unpaired and lone-paired electrons can be disregarded in the present internal-rotamer.

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