title: On the nature of blueshifting hydrogen bonds.
abstract: The block-localized wave function (BLW) method can derive the energetic, geometrical, and spectral changes with the deactivation of electron delocalization, and thus provide a unique way to elucidate the origin of improper, blueshifting hydrogen bonds versus proper, redshifting hydrogen bonds. A detailed analysis of the interactions of F(3)CH with NH(3) and OH(2) shows that blueshifting is a long-range phenomenon. Since among the various energy components contributing to hydrogen bonds, only the electrostatic interaction has long-range characteristics, we conclude that the contraction and blueshifting of a hydrogen bond is largely caused by electrostatic interactions. On the other hand, lengthening and redshifting is primarily due to the short-range n(Y)→σ*(X-H) hyperconjugation. The competition between these two opposing factors determines the final frequency change direction, for example, redshifting in F(3)CH⋅⋅⋅NH(3) and blueshifting in F(3)CH⋅⋅⋅OH(2). This mechanism works well in the series F(n)Cl(3)-n CH⋅⋅⋅Y (n=0-3, Y=NH(3), OH(2), SH(2)) and other systems. One exception is the complex of water and benzene. We observe the lengthening and redshifting of the O-H bond of water even with the electron transfer between benzene and water completely quenched. A distance-dependent analysis for this system reveals that the long-range electrostatic interaction is again responsible for the initial lengthening and redshifting.
authors.affiliation: Department of Chemistry, Western Michigan University, Kalamazoo, MI 49008 (USA), Fax: (+1) 269-387-2909. ymo@wmich.edu.
authors.forename: Yirong
authors.initials: Y
authors.lastname: Mo
authors.forename: Changwei
authors.initials: C
authors.lastname: Wang
authors.forename: Liangyu
authors.initials: L
authors.lastname: Guan
authors.forename: Benoît
authors.initials: B
authors.lastname: Braïda
authors.forename: Philippe C
authors.initials: PC
authors.lastname: Hiberty
authors.forename: Wei
authors.initials: W
authors.lastname: Wu
mesh_terms: D000641:Ammonia / Q000737:chemistry; D006845:Hydrocarbons, Fluorinated / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D006878:Hydroxides / Q000737:chemistry; D011789:Quantum Theory; D055672:Static Electricity; D013816:Thermodynamics
chemical_list: D006845:Hydrocarbons, Fluorinated; D006878:Hydroxides; C029999:fluoromethane; D000641:Ammonia; D006859:Hydrogen
keywords: density functional calculations; electron transfer; hydrogen bonds; hyperconjugation; vibrational spectroscopy