Electron Pairing and Chemical Bonds. Anatomy of Bond Formation

Several methods of the correlated pair density analysis are introduced. Based on these analyses, the splitting of the chemical bond in H2 has been investigated in detail. It has been found that the splitting does not proceed as a smooth continuous process but rather that it has the character of an abrupt change

Multicentre Bond Indices from the Generalized Population Analysis of Higher Order Densities

Based on the generalized population analysis of appropriate functionals derived from higher order densities, the multicentre bond indices were introduced at both SCF and post-SCF levels. The corresponding indices were applied to the visualization of bonding in several simple molecules both without and with three-centre bonds. The ability of the se indices to detect and to localize the presence of three-centre bonds was tested on the analysis of bonding in several simple boranes B2H6, B4HlO, B5H9\u27 The three centre bonds were always detected in complete agreement with expectations. The approach is quite general and can be applied to the visulization of bonding in other cases of molecules with complex bonding patterns

From the Wave Function to Structural Formula

The recently proposed pairon population analysis based on geminal expansion of pair densities is applied to the investigation of the molecular structure of several simple molecules. It has been shown that the values of pairon populations straightforwardly reproduce the classical structural formula and, thus provide a new and surprisingly simple means of extracting structural information form the wave function

Similarity Approach to Chemical Reactivity. Spin Recoupling in Chemical Reactions

The idea of similarity index was extended to the investigation of the effect of spin recoupling in chemical reactions. The approach was applied at the semiempirical AM1 level to a series of selected pericyclic reactions involving model examples of electrocyclic reactions, cycloadditions and valence isomerizations. Both allowed and forbidden reaction mechanisms were analyzed in each case with the aim to assess the role of spin recoupling in discriminating between these two process types

Position Invariant Index for Assessment of Molecular Similarity

In this study the idea of the topological similarity index has been generalized so that its applicability reaches beyond a simple HMO model to semi- empirical or even ab initio methods. The greatest advantage of this new index is its complete positional invariance, which makes it possible to avoid time consuming optimization with respect to the mutual position of molecules. As a consequence of this invariance, the index can find a number of interesting applications, especially in the design of compounds with desired biological properties. The applicability of the index has been numerically tested on a series of isosteric and isoelectronic molecules