Bay-type H···H "bonding" in cis-2-butene and related species: QTAIM versus NBO description.

We use comparative natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) methods to analyze the proximal bay-type H···H interactions in cis-2-butene and related species, which lead to controversial interpretation as attractive "HH bonding" in the QTAIM framework. We address the challenging questions concerning well established structural, conformational, and vibrational properties of such species that appear to be sharply at odds with the QTAIM interpretation. In contrast to the purported "HH bonding" of QTAIM theory, NBO-based evaluation of steric (donor-donor) and hyperconjugative (donor-acceptor) interactions unambiguously portrays such H···H contacts as dominated by steric clashes that are only partially softened by weak secondary hyperconjugative interactions, contributing negligibly (bHH  < 0.

On the large σ-hyperconjugation in alkanes and alkenes.

The conventional view that the σCC and σCH bonds in alkanes and unsaturated hydrocarbons are so highly localized that their non-steric interactions are negligible is scrutinized by the block-localized wavefunction (BLW) method. Even molecules considered conventionally to be "strain free" and "unperturbed" have surprisingly large and quite significant total σ-BLW-delocalization energies (DEs) due to their geminal and vicinal hyperconjugative interactions. Thus, the computed BLW-DEs (in kcal mol(-1)) for the antiperiplanar conformations of the n-alkanes (C(N)H(2N+2), N = 1-10) range from 11.