Model calculations of chemical interactions. Part 3.—Rotational energy profiles in simple molecules: evaluation, additivity and role of bond–bond, bond–lone-pair and lone-pair–lone-pair interactions

Abstract

Rotational isomerism in ethane, methylamine, methanol, fluoromethylamine, hydrazine, hydroxylamine, hydrogen peroxide and difluorine dioxide has been analysed using basis sets of symmetrically orthogonalized hybrid AOs. Total vicinal delocalization, which has either an exclusive or a relevant role in determining the rotational energy profile, shows good additivity of the effects of individual interactions and invariance with respect to alternative hybridizations of lone pairs. Bond–σ-lone-pair interactions which obtain maximum stabilization in their anti-periplanar orientations are significant even in anticlinal and syn-periplanar orientations and contribute, in comparison with bond–bond interactions, a much lower differential energy between eclipsed and staggered conformations. This finding explains interesting features of the rotational energy profiles, like the ‘unusual’ stability of some eclipsed rotamers. Bond–π-lone-pair interactions are noticeably effective in their syn(anti)-periplanar orientations and are responsible for the stability of the skew forms of HOOH and FOOF; the exceptionally large OF–O_{lp, π} interaction in difluorine dioxide is shown to be responsible for the unusually short O—O and long O—F bond lengths, in agreement with a previous explanation where the same structural features had been attributed to the large three-centre four-electron character of the FOO systems.