Electrostatics for Probing Lone Pairs and Their Interactions

Abstract

The value of the molecular electrostatic potential minimum (Vmin) and its topographical features (position, as well as the eigenvalues and eigenvectors of the corresponding Hessian matrix) are recently proposed as the criteria for characterizing a lone pair (Kumar A. et al., J. Phys. Chem. 2014, A118, 526). This electrostatic characterization of lone pairs is examined for a large number of small molecules employing MP4/6- 31111G(d,p)//MP2/6-31111G(d,p) theory. The eigenvector of the Hessian matrix corresponding to its largest eigenvalue (kmax), is found to be directed toward the lone pair-bearingatom, with kmax showing a strong linear correlation with Vmin. Large magnitudes of Vmin and kmax indicate a charge-dense lone pair. The topographical features of Vmin are seen to provide insights into the interactive behavior of the molecules with model electrophiles, viz. HF, CO2, and Li1. In all the complexes of HF and majority of the other complexes, the interaction energy (Eint) correlates well with the respective Vmin, value, but for some deviations occurring due to other competing secondary interactions. The electrostatic interactions are found to be highly directional in nature as the orientation of interacting atom correlates strongly to the position of lone pair. In summary, the present study on a large number of test molecules shows that electrostatics is able to probe lone pairs in molecules and offers a simple interpretation of chemical reactivity.