The lecture presented at the III Reunión de la Asociación Latinoamericana de Cristalografía y I Encuentro de la AChCr was entitled:
Electron density and the hydrogen bond
Hydrogen bond is considered the strongest intermolecular interaction, with the only exception of ionic forces in molecular salts. The common view is that hydrogen bond is an electrostatic interaction with some degree of covalency. This view has been challenged many times from theoretical models and from experimental evidence, as for example the high frequency of hydrogen bonds between equally charged ions.
The analysis of the electron density in intermolecular regions, either from diffraction experiments in crystals or from quantum chemistry calculations in molecular aggregates, is an important tool in the study of the hydrogen bond. Indeed, the topological analysis1 of the electron density demonstrates that the nature of the hydrogen bond evolves continuously from purely electrostatic to covalent as the bond distance decreases.2 It also shows that there are close correlations between the bond distance, properties derived from the electron density, and the bond energy, which allow the fast estimation of this last quantity from the geometry of the interaction.3
The analyses of the electrostatic potential and the electric field lines, both quantities derived from the electron density, shows that, with very few exceptions, electrostatics is the main contribution to the hydrogen bond. Indeed, it is demonstrated that even in interactions between equally charged ions, where electrostatic repulsion is expected, electrostatic attractive forces appear in the hydrogen bond region.4
In this communication, our trajectory in the analysis of the electron density in the hydrogen bond will be summarized, and ongoing research will be presented.
Acknowledgements: This work was supported by the Ministerio de Economía, Industria y Competitividad (MINECO), Spain (Grants ENE2015-63969 and SEV2015-0496).
1. R. F. W. Bader, Atoms in Molecules: A Quantum Theory (1990)
2. E. Espinosa, I. Alkorta, J. Elguero, E. Molins, J. Chem. Phys. 117, 5529 (2002)
3. I. Mata, I. Alkorta, E. Espinosa, E. Molins, Chem. Phys. Lett. 507, 185 (2011)
4. I. Alkorta, I. Mata, E. Molins, E. Espinosa, Chem. Eur. J. 22, 9226 (2016)
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