Stereoelectronic Effects and General Trends in Hyperconjugative Acceptor Ability of σ Bonds

A systematic study of general trends in σ acceptor properties of C−X bonds where X is a main group element from groups IVa−IIa is presented. The acceptor ability of the C−X σ bonds in monosubstituted ethanes increases when going to the end of a period and down a group. Enhancement of acceptor ability of C−X σ bonds as one moves from left to right in periods parallels the increase in electronegativity of X, whereas augmentation of acceptor ability in groups is opposite to the changes in electronegativity of X and in the C−X bond polarization, following instead the decrease in the energy of σ*C-X orbitals when one moves from the top to the bottom within a group. This simple picture of acceptor ability of σ bonds being controlled by electronegativity in periods and by σ* orbital energy in groups is changed in monosubstituted ethenes where the role of electronegativity of the substituent X becomes more important due to increased overlap between σ orbitals. The combination of several effects of similar magnitude influences acceptor ability of σ bonds in monosubstituted ethenes in a complex way. As a result, the acceptor ability of σ bonds can be significantly modified by substitution and is conformer dependent. Stereoelectronic effects displayed by C−X bonds with X from second and third periods are highly anisotropic. For example, C-chalcogen bonds are excellent σ acceptors at the carbon end but poor σ acceptors at the chalcogen end. This effect can be relied upon in the design of molecular diodes with σ bridges with unidirectional electron conductivity. While the general trends revealed in this work should be useful for the qualitative understanding of stereoelectronic effects, one should bear in mind that the magnitude of hyperconjugative effects is extremely sensitive to small variations in structure and in substitution. This advocates for the increased role of theoretical methods in analysis of stereoelectronic effects.