Stereoelectronic Effects and General Trends in Hyperconjugative Acceptor Ability of σ Bonds Igor V. Alabugin* and Tarek A. Zeidan Contribution from the Department of Chemistry and Biochemistry, Florida State UniVersity, Tallahassee, Florida 32306-4390 Received November 30, 2001 Abstract: 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. Introduction Chemical reactions involve interactions between electronic orbitals accounting for the increasingly important role of the concept of stereoelectronic effects in modern organic chemistry. 1 Stereoelectronic interactions involving π-bonds (conjugation) are generally regarded as being among the most important chemical phenomena. Interactions between σ orbitals (hyper- conjugation) have received less attention, although as early as 1941 Robert Mulliken pointed out its importance and, indeed, hyperconjugative stereoelectronic effects were later found to be ubiquitous in chemistry. Depending on the nature of interacting orbitals, hyperconjugative stereoelectronic interac- tions can provide electron density to electron-deficient centers 2 or withdraw it from electron-rich centers, and may stabilize incipient bonds and radical centers. 3 These effects influence conformational equilibria (anomeric effect, 4,5 conformational behavior of the phosphodiester backbone in nucleic acids, 6 conformational stability of collagens, 7 and torsional barrier in ethane 8 and other molecules 9 ). Hyperconjugation has been shown to modify reactivity, 10 control selectivity, 11 and play an * To whom correspondence should be addressed. E-mail: alabugin@ chem.fsu.edu. (1) Deslongchamps, P. Stereoelectronic Effects in Organic Chemistry; Perga- mon: Oxford, 1983. Deslongchamps, P. Tetrahedron 1975, 31, 2463. 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