Topographical Analysis of Electron Density and Electrostatic Potential

Lone pairs: An Electrostatic Perspective

Using the topological characteristics of molecular electrostatic potential (MESP), we offered a clear-cut quantitative definition of lone pairs. The largest eigenvalue and corresponding eigenvector of the Hessian at the minima are shown to distinguish lone pair regions from the other types of electron localization (such as π bonds). A comparative study of lone pairs as depicted by various other scalar fields such as the Laplacian of electron density and electron localization function is made. Further, an attempt has been made to generalize the definition of lone pairs to the case of cations. Our article is used as reference to define lone pair by Wikipedia. en.wikipedia.org/wiki/Lone_pair

Electric Field of molecules

The zero-flux surface (ZFS) of the electric field of the molecules introduces the concept of Atoms in Molecules (AIM) based on the field of molecular electrostatic potential (MESP). ZFS manifests the MESP-based atomic basins and consequently brings out the asymmetric electronic distribution of the molecule. For example, an electron-rich atom among the two bonded atoms possesses a completely closed MESP-based atomic basin. Electronic behaviour of molecules such as BF, BH3, AlCl3, B2H6, and Al2Cl6, and a Lewis acid–base pair, viz. NH3BH3, that cannot be explained by classical definition of electronegativity difference can be clearly illustrated through the nature of atomic basins. MESP-based atomic basins can also be used to explain the intricate charge transfer in transition metal complexes, viz. Ni(CO)4, Fe(CO)5, Cr(CO)6, Mn2(CO)10, Co2(CO)8, Fe(η5-C5H5)2, Co(η3-C3H5), and Co(η3-C3H5)(CO)3. It can also provide qualitative explanation of the shielding or deshielding effects revealed by NMR data as well as susceptibility of an atomic region towards an electrophilic or nucleophilic attack. The topographical features of MESP and MESP-based atomic basins can be used as an auxiliary tool for the portrayal of asymmetry in molecular charge distribution.