title: Negative hyperconjugation and red-, blue- or zero-shift in X-Z∙∙∙Y complexes.
abstract: A generalized explanation is provided for the existence of the red- and blue-shifting nature of X-Z bonds (Z=H, halogens, chalcogens, pnicogens, etc.) in X-Z∙∙∙Y complexes based on computational studies on a selected set of weakly bonded complexes and analysis of existing literature data. The additional electrons and orbitals available on Z in comparison to H make for dramatic differences between the H-bond and the rest of the Z-bonds. The nature of the X-group and its influence on the X-Z bond length in the parent X-Z molecule largely controls the change in the X-Z bond length on X-Z∙∙∙Y bond formation; the Y-group usually influences only the magnitude of the effects controlled by X. The major factors which control the X-Z bond length change are: (a) negative hyperconjugative donation of electron density from X-group to X-Z σ* antibonding molecular orbital (ABMO) in the parent X-Z, (b) induced negative hyperconjugation from the lone pair of electrons on Z to the antibonding orbitals of the X-group, and (c) charge transfer (CT) from the Y-group to the X-Z σ* orbital. The exchange repulsion from the Y-group that shifts partial electron density at the X-Z σ* ABMO back to X leads to blue-shifting and the CT from the Y-group to the σ* ABMO of X-Z leads to red-shifting. The balance between these two opposing forces decides red-, zero- or blue-shifting. A continuum of behaviour of X-Z bond length variation is inevitable in X-Z∙∙∙Y complexes.
authors: School of Chemistry, Indian Institute of Science Education and Research-Thiruvananthapuram, Thiruvananthapuram, Kerala 695016, India.
authors: Jyothish
authors: J
authors: Joy
authors: Eluvathingal D
authors: ED
authors: Jemmis
authors: Kaipanchery
authors: K
authors: Vidya
mesh_terms: D018011:Chalcogens / Q000737:chemistry*; D004583:Electrons; D006219:Halogens / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D008956:Models, Chemical; D011789:Quantum Theory; D055672:Static Electricity; D013816:Thermodynamics
chemical_list: D018011:Chalcogens; D006219:Halogens; D006859:Hydrogen