Crossover from hydrogen to chemical bonding

Hydrogen bonds (H-bonds) can be interpreted as a classical electrostatic interaction or as a covalent chemical bond if the interaction is strong enough. As a result, short strong H-bonds exist at an intersection between qualitatively different bonding descriptions, with few experimental methods to understand this dichotomy. The [F-H-F](-) ion represents a bare short H-bond, whose distinctive vibrational potential in water is revealed with femtosecond two-dimensional infrared spectroscopy. It shows the superharmonic behavior of the proton motion, which is strongly coupled to the donor-acceptor stretching and disappears on H-bond bending. In combination with high-level quantum-chemical calculations, we demonstrate a distinct crossover in spectroscopic properties from conventional to short strong H-bonds, which identify where hydrogen bonding ends and chemical bonding begins.

Automated summary

The study focuses on the vibrational potential of a bare short H-bond in water, revealing superharmonic behavior of proton motion strongly coupled with donor-acceptor stretching and disappearing on H-bond bending. By combining high-level quantum-chemical calculations, a distinct crossover in spectroscopic properties from conventional to short strong H-bonds is demonstrated, identifying where hydrogen bonding ends and chemical bonding begins.