On the Physical Origin of Blue-Shifted Hydrogen Bonds

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, F₃C−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 F₃Si−H···OH₂, F₂NH···FH, F₂PH···NH₃, and F₂PH···OH₂ 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 F₃CH···FH system, the charge redistribution in F₃CH 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 F₃C−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.