On the large sigma-hyperconjugation in alkanes and alkenes
Wu, Judy I-Chia
McKee, William Chadwick
Schleyer, Paul von Rague
- 化学化工－已发表论文 
The conventional view that the sigma(CC) and sigma(CH) 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 sigma-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 (CNH2N+2, N=1-10) range from 11.6 for ethane to 82.2 for n-decane and are 50.9 for cyclohexane and 91.0 for adamantane. Although sigma-electron delocalization in unsaturated hydrocarbons usually is ignored, the sigma-BLW-DEs (in kcal mol(-1)) are substantial, as exemplified by D-2h ethylene (9.0), triplet D-2d ethylene (16.4), allene (19.3), butadiene (19.0), hexatriene (28.3), benzene (28.1), and cyclobutadiene (21.1). While each individual geminal and vicinal hyperconjugative interaction between hydrocarbon sigma-bonding and sigma-antibonding orbitals tends to be smaller than an individual p conjugative interaction (e. g., 10.2 kcal mol(-1) in anti-1,3-butadiene, the presence of many sigma-hyperconjugative interactions (e. g., a total of 12 in anti-1,3-butadiene, see text), result in substantial total sigma-stabilization energies (e. g., 19.0 kcal mol(-1) for butadiene), which may surpass those from the pi interactions. Although large in magnitude, sigma-electron delocalization energies often are obscured by cancellation when two hydrocarbons are compared. Rather than being strain-free, cyclohexane, adamantane, and diamantane suffer from their increasing number of intramolecular 1,4-C center dot center dot center dot C repulsions resulting in elongated C-C bond lengths and reduced sigma-hyperconjugation, compared to the (skew-free) antiperiplanar n-alkane conformers. Instead of being inconsequential, sigma-bond interactions are important and merit consideration.