Probing Blue-Shifting Hydrogen Bonds with Adiabatic Energy Decomposition Analysis

The
physical origin of blue-shifting hydrogen bonds remains a subject
of debate, although many plausible explanations have been proposed.
Using a molecular property decomposition analysis based on absolutely
localized molecular orbitals, we investigated several representative
F3CH···Y (Y = H2O, NH3, Cl–) complexes. We reveal that features of a
blue-shifting H-bond already appear on the frozen surface where both
polarization and charge transfer (CT) are “turned off”,
and that the final frequency shift observed depends on the strength
of CT. Further decomposition of forces at the frozen level shows that
Pauli repulsion is the only component that shortens the C–H
bond in the short-range, while both permanent electrostatics and dispersion
lengthen the bond. The effects of these forces from the medium to
long-range are also discussed. Our analysis provides a complete picture
for blue-shifting H-bonds and suggests two necessary conditions for
their features to be observed at equilibrium structures: (i) stronger
Pauli repulsion than the combination of electrostatic and dispersion
forces; (ii) relatively weak CT that is insufficient to compensate
for the blue-shifting effect of the frozen interaction.