NEW INSIGHT IN ELECTRON DENSITY AND ELECTRON SPIN DENSITY THROUGH TOPOLOGICAL DESCRIPTORS BASED ON BADER'S THEORY OF ATOM IN MOLECULES

This Ph.D. thesis is focused on the application of quantum theory of atoms in molecules (QTAIM) based chemical descriptors to challenging chemical test-cases, as well as on the development of novel topological descriptors, like the Source Function for the spin density. The thesis is organized as follows: In chapter 1 the electron density (ED) of a very unusual structural feature in a synthetic beta–sultamic analogue (DTC), has been explored by both low-T single–crystal X–ray diffraction and quantum mechanical simulations to gain insights into the subtle interplay between structure, electron delocalization and crystal field polarization effects. The core chemical moiety in DTC is an uncommon 4–membered thiazete–1,1–dioxide heterocycle, where the formally single N–C bond is, on average, 0.018 Å shorter than the formally double N=C bond. Both local and non–local topological descriptors provided by QTAIM have been employed in the analysis of DTC in comparison with chemically related derivatives and possible implications from the viewpoint of the accurate in silico modelling of crystal structures are discussed. Particular attention is dedicated on such kind of issues in chemical and pharmaceutical industries, because the control of the crystal structure is really problematic in some cases; in fact different polymorphs of the same substance have different intensive physical properties, such as solubility, refraction index and conductivity and problems may arise in industrial processes related to the synthesis of chemicals and drugs on large scale. In chapter 2, we focused on the source function (SF) QTAIM based topological descriptor. The electron density (ED) at any point r within a system may be regarded as consisting of a sum of SF contributions S(r; X) representing a measure of how the various atomic basins (X) or groups of atomic basins defined through QTAIM contribute to determine the ρ(r) at r. Recently it was shown that the SF is able to reveal electron delocalization effects in planar electron conjugated systems, in terms of an increased capability of determining the ED along a given bond by the distant, though through-bonds connected, atomic basins and, at the same time, into a decreased ability to do so by the two atoms directly involved in the bond. Such an adjustment of sources then translates into a pictorial pattern of enhanced and reduced atomic SF contributions from, respectively, distant and nearby atoms, compared to the case of a partially or fully saturated network of bonds. Then we have extended such an analysis to the non planar conjugated systems, where the usual electron separation does no longer apply. Being based on the total ED, the SF analysis may be safely applied also in these less conventional electron delocalized systems. In the present Ph. D. thesis we have extended the SF reconstruction approach also to the electron density spin counterparts in vacuo. Such reconstruction was investigated both on simple (but chemically meaningful) spin-polarized molecular systems and on more complex single-molecule magnets. This investigation has showed that the difference between the two spin counterparts of electron density distribution can be reconstructed with a sufficient accuracy, analogously to the case of the total ED. Moreover, it was found that the SF for the electron spin density brings in precious chemical information, neatly distinguishing the quite different roles played by the unpaired electrons ED and the spin polarized ED due to the remaining electrons. Furthermore, quantitative answers to questions related to the transferability of the spin density in alkyl radicals or to the transmission of spin information in metal(s)-ligand systems were provided. Understanding, from a real space perspective, by which mechanisms spin information transmits, might be of relevance to interpret the fundamental magnetic interactions present in complex materials, such as for example coordination polymers or Heussler and half-Heussler alloys. As these interactions have a key role in spintronics, characterization of the chemical bond and interpretation of the electron spin density distributions in these systems through the SF analysis, could hopefully disclose structure-property relationships extremely useful for the design of materials with particular physical properties.