On the nature of blueshifting hydrogen bonds: Ab initio and density functional studies of several fluoroform complexes

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Abstract
Potential energy hypersurfaces (PESs) for four fluoroform complexes (with acetonitrile, ethyleneoxide, formaldehyde, and water) were explored at the HF, MP2, and B 3 LYP /6-311++ G (d,p) levels of theory. Anharmonic C–H stretching vibrational frequency shifts are reported for all minima located on the studied PESs. In all cases, the lowest-energy minimum occurs for a C–H ⋯ O ( N ) hydrogen-bonded arrangement and is characterized by a significant C–H frequency blueshift (upshift), while additional minima [for “reversed” orientations, in which there is no direct C–H ⋯ O ( N ) contact] show only small C–H frequency upshifts. The large blueshifts found for the hydrogen-bonded arrangements are predominantly caused by the electronic exchange interaction, as revealed by Kitaura–Morokuma (KM) analysis, while the purely electrostatic + polarization interaction leads to C–H frequency redshifts, which was proven both by the KM analysis and the charge field perturbational (CFP) approach. The large net blueshifting effect of the exchange contribution is only possible thanks to the smallness of the redshifting electrostatic + polarization contribution, which, in turn, is a consequence of the fact that dμ (0) /dr CH is negative for the fluoroform molecule. In all cases the charge transfer as well as the dispersion energy contributions lead to frequency redshifts. The small C–H blueshifts for the reversed orientations are almost completely governed by the electrostatic interaction, as shown by the KM and CFP analyses. All these characteristics of blueshifting H bonds are in line with a previously outlined general model concerning “standard” and “blueshifting” H-bonding interactions [K. Hermansson, J. Phys. Chem. A 106, 4695 (2002)].