# # Crossover from hydrogen to chemical bonding

The bonds in aqueous FHF^− are neither simple hydrogen nor covalent bonds

During chemical conversion, covalent bonds often have to convert to weaker H-bonds [1], and proton transport in water relies on the continuous interconversion of covalent and H-bonds.

How to define the strength of a H-bond?

The abstract of ref. [1] says the strength of "classical H-bond" is around 3-5 kcal/mol. What's it stands for?

A proton sandwiched between two water molecules, is an example of a hybrid between covalent and hydrogen bonding but is extremely short-lived in liquid water. 'Short-lived' does not mean 'weak'. The F-H-F$^-$ anion provides a long-lived proxy to study this structure. An extensive conputational study revealed that the bonds are neither simply covalent nor a simple H-bond. The existence of a hybrid covalent-hydrogen bonded state not only challenges our current understanding of what a chemical bond exactly is but also offers the opportunity to better understand chemical reactions, where “intermediate reaction states” are often invoked but rarely studied directly. This findings also have direct implications for proton transfer reactions in proton transfer in water. [2] Simulating an excess proton in liquid bulk water has proved to be immensely challenging.[2]

TIP

The ablility to obtain a electron of F is much stronger than O.

Future work will undoubtedly focus on the dynamics of the interconversion in different systems.Many exciting questions remain open, including what role the environment (for example, the fluctuating solvent) plays in such interconversion, what molecular motions trigger these interconversions, and how these systems can be steered in a desired direction. Forthcoming answers to these questions should be relevant for proton transport in biology and technologies such as fuel-cell membranes.

Questions

What causes the proton transfer in water? To answer this quesiton, I can find lots of proton transfer processes. Then analysis the dynamic properties of this processes. Maybe I can conclude the answer. How these systems can be steered in a desired direction? This question is about how to control those processes, and also need to better understand the properties of this process.

Considering short strong H-bonds (SHBs) [3], they can have interaction strengths similar to those of covalent bonds, protons appearing shared between D and A, and could involve subsitantial electronic redistribution across the bonded atoms. Where do hydrogen bonds end and covalent bonds begin?

Where do hydrogen bonds end and covalent bonds begin?

Maybe this is a question about when, but they use where.

[1] R. Srinivasan, J. S. Feenstra, S. T. Park, S. Xu, A. H. Zewail, J. Am. Chem. Soc. 126, 2266 (2004)
[2] D. Marx, M. E. Tuckerman, J. Hutter, M. Parrinello, Nature 397, 601 (1999)
[3] Dereka et al., Science 371, 160–164 (2021)