Solid State Applications of QTAIM and the Source Function – Molecular Crystals, Surfaces, Host–Guest Systems and Molecular Complexes

Abstract

IntroductionThis chapter deals with the application of QTAIM to the solid state, except for the last section, in which the source function -a recently developed tool for the QTAIM study of the chemical bond from a somewhat original viewpoint -is introduced.The chapter starts with an illustration of TOPOND software, which implements QTAIM for systems periodic in 0 to 3 dimensions, which covers polymers, surfaces, and crystals, besides molecules. The interface of TOPOND to the multipolar package XD is also mentioned, because it enables QTAIM analysis of experimentally derived electron densities. The chapter continues with an example of a didactic application of TOPOND to a study of crystal field effects on bonding and on molecular properties in the urea molecular crystal. Clean and chemisorbed semiconductor surfaces then serve as an example of the wealth of information provided by QTAIM about the effect of surface formation and reconstruction on the bonding and atomic properties of first surface layers. Guest-host systems are discussed as a last example, with emphasis on guest to/from host electron transfer and on the peculiar features of guest-host bonding interactions. The relevance of these key issues to materials science applications is briefly touched upon for thermoelectric materials.In the last section, the source function (SF) is introduced and examples of its preliminary and future potential applications and extensions are presented. This function enables the value of the density at any point within a system to be equated to a sum of atomic contributions, thus enabling the properties of the density to be viewed from a totally new perspective. Although depending on the whole set of interactions within a system, a bond path is topologically associated only with the two atoms it connects. In contrast, the source function details how all the other atoms in a system, in addition to the two linked atoms, contribute to the accumulation of electron density along a bond path and, in particular, to BCP. It thus discloses nonlocal information on bonding and on complex bonding pat- 165The Quantum Theory of Atoms in Molecules. Edited

Challenging chemical concepts through charge density of molecules and crystals

“…Upon crystallization, we found changes for the intramolecular bonds from 0 up to 3% in ρ b , the value of the electron density at the bcps, and as big as up to 40-60% in ∇ 2 ρ b and λ 3b , i.e. in the values of the Laplacian of the density and of the positive density curvature along the BP, also measured at the bcps [93] 1 . Being related to second derivatives of the electron density, changes in the ∇ 2 ρ b and λ 3b values were definitely greater than those in ρ b and prove to be extremely sensitive indicators of crystal field effects.…”

“…However, when applied on a relative basis [5], it provides a set of indices whose quantitative variations along a series of chemically related compounds or, like in the present case, upon a change of phase of a given compound neatly identify the effect these chemical perturbations have on the nature of a bond. More details of this issue may be found in the original paper [67] and in [93], including the fact that, contrary to the intramolecular bonds, the bcp properties of the HBs in the bulk are very closely matched by the simple model of the non-interacting molecular densities, that is, by the electron density given by the sum of the densities of molecules placed at the same position as of the molecules in the bulk, but without allowing the molecules to interact among each other and to distort their densities. This was an important result as it indicated that the most interesting changes induced by packing were to be found within the molecules rather than in the regions between molecules (see below and [76,93]).…”

The boron–boron single bond in diborane(4) as a non-classical electron donor for hydrogen bonding

“…However, several chapters in a recent book discuss NNAs in other situations. [39][40][41] As the editors note in the opening chapter, 39 local maxima in the electron density can occur occasionally at positions other than those of atomic nuclei, especially in metals and semiconductors. The non-nuclear maxima, or non-nuclear attractors (NNAs), are topologically indistinguishable from nuclear maxima.…”

Chemical structure and reactivity by means of quantum chemical topology analysis

“…In addition, other topological analysis of scalar functions have been also developed such as the source function [98], the momentum density [99], the electron pair density [100], the nuclear potential energy field [101], the virial field [102], the Laplacian of charge density [66,[103][104][105], and the electron localizability indicator [106]. Da Silva and…”