Kinetic energy and chemical bonding

R. A. Boto
Institut du Calculet de la Simulation et Laboratoire de Chimie Théorique, Université Pierre et Marie Curie, Paris, France.
Mercredi 11 Mai 2016, 11h00
bibliothèque LCT, tour 12 - 13, 4e étage

There is no doubt that chemical bonding is a fundamental concept in chemistry. Its elusive nature may be traced back to quantum mechanics. Contrary to other observables such as energy or momentum, the chemical bond is not a physical observable, and thus a pure quantum mechanical definition of chemical bond is missed. Other cornerstone concepts in chemistry such as atomic shell, lone pair, (hyper-)conjugation , etc suffer from the same problem. Nevertheless, all of them constitute such a rich set of "fuzzy", yet invaluably set of concepts, that many efforts have been devoted first to understand their underlying mechanistic nature, and then to visualise them. In this regard, kinetic energy densities have played a paramount role. It is known that covalent bonding occurs through a lowering of the kinetic energy [1], therefore, any function able to map these variations should be a promising bonding descriptor, this is the case of the positive defined kinetic energy density τ(r). Experience has shown that τ(r)-based bonding descriptors only shed light on covalent interactions where electron sharing is the driving force, and are not able to properly reveal non-covalent interactions (NCIs). To circumvent this problem different descriptors based on the von Weizsäcker kinetic energy density, τw, instead of τ(r), have been proposed [2]. Moreover, this last bundle of functions enables the possibility of analysing and visualising simultaneously covalent and non-covalent interactions.
In this work we revisit different well known bonding descriptors such as the electron localisation function (ELF), the localised orbital locator (LOL) or the reduced density gradient from a kinematic perspective.

[Picture]

Representation of the reduced density gradient along the internuclear axis for N2.


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[1] Ruedenberg, K. and Schmidt, M. W. J. Comput. Chem. 28, 391-410 (2007)
[2] Boto, A. R., Contreras-García J., Tierny J. and Piquemal J.-P. Mol. Phys. (2015)