On the Physical Origin of Blue-Shifted Hydrogen Bonds

Abstract

For blue-shifted hydrogen-bonded systems, the hydrogen stretching frequency increases rather than decreases on complexation. In computations at various levels of theory, the blue-shift in the archetypical system, F(3)C-H.FH, is reproduced at the Hartree-Fock level, indicating that electron correlation is not the primary cause. Calculations also demonstrate that a blue-shift does not require either a carbon center or the absence of a lone pair on the proton donor, because F(3)Si-H.OH(2), F(2)NH.FH, F(2)PH.NH(3), and F(2)PH.OH(2) have substantial blue-shifts. Orbital interactions are shown to lengthen the X-H bond and lower its vibrational frequency, and thus cannot be the source of the blue-shift. In the F(3)CH.FH system, the charge redistribution in F(3)CH can be reproduced very well by replacing the FH with a simple dipole, which suggests that the interactions are predominantly electrostatic. When modeled with a point charge for the proton acceptor, attractive electrostatic interactions elongate the F(3)C-H, while repulsive interactions shorten it. At the equilibrium geometry of a hydrogen-bonded complex, the electrostatic attraction between the dipole moments of the proton donor and proton acceptor must be balanced by the Pauli repulsion between the two fragments. In the absence of orbital interactions that cause bond elongation, this repulsive interaction leads to compression of the X-H bond and a blue-shift in its vibrational frequency.

Defining the hydrogen bond: An account (IUPAC Technical Report)

“…Also, one may need to look at group electronegativities rather than that of atoms [95]. The importance of group electronegativity can be highlighted by the hydrogenbonded complex F 3 SiHؒؒؒOH 2 , where X is Si atom having a lower electronegativity [96].…”

Interaction of rare-gas-containing molecules with nitrogen: Matrix-isolation and ab initio study of HArF⋯N2, HKrF⋯N2, and HKrCl⋯N2 complexes

“…Recently, Li and co-workers have suggested that the mechanisms behind the blueshifting and redshifting hydrogen bonds are similar. 3 They noticed that electrostatic inter- action with corresponding Pauli repulsion term is the natural source of the blueshift whereas the strong orbital interaction causes the elongation of the H-X bonds and hence a decrease of the vibrational frequency. These effects are competing upon complexation: ''classical'' redshifting hydrogen bonds originate from a large orbital interaction while a weaker orbital interaction ͑or stronger electrostatic interaction and Pauli repulsion͒ in complexes can result in a blueshift of the vibrational frequency.…”

“…2 In a recent theoretical study, it was suggested that the physical origin of a blueshift is based on the same mechanisms as that of a redshift: an interplay between attractive electrostatic forces, Pauli repulsion and orbital interactions. 3 It was concluded that the orbital interactions are stronger in the redshifting case causing lengthening of the bond and hence lowering of the frequency. Recently, we have reported a large experimental blueshift of more than 100 cm Ϫ1 for the H-Kr stretching frequency in HKrCl upon complexation with N 2 .…”

Nonclassical forces: Seemingly insignificant but a powerful tool to control macromolecular structures

“…Many research groups had recently reported several theoretical studies showing the weak X-H/Y hydrogen bond formation giving rise to an unexpected blue shift in the X-H stretching frequency, shortening the X-H bond. [67][68][69][70] This fact was valid only for weak hydrogen bonds, in which rehybridization factor (responsible for shortening of the X-H bond) was more pronounced than hyperconjugation-induced X-H bond lengthening. These studies also gave experimental evidence of blue-shifted hydrogen bonds in matrix-isolation and low temperatures.…”