A Modified Townes-Dailey Model for Interpretation and Visualization of Nuclear Quadrupole Coupling Tensors in Molecules

Abstract

We propose a modified Townes-Dailey (TD) model to help interpret and visualize experimentally measureable nuclear quadrupole coupling tensors (thus the electric field gradient tensors) in molecules. We show that, within the framework of the TD model, each principal component of the nuclear quadrupole coupling tensor is directly related to a new quantity termed as the valence p-orbital population anisotropy (VPPA or ΔP) in the same direction. While the proposed model is a simple reformulation of the original TD model thus does not introduce new physics, the concept of VPPA makes it possible to directly interpret as well as visualize, in a much straightforward way, the experimentally determined nuclear quadrupole coupling tensors in molecules. We illustrate the utilization of VPPA using nuclear quadrupole coupling tensors for 11B, 14N, 17O, 35Cl, 87Br, and 127I nuclei in a variety of molecules. We propose to use VPPA or ΔP ellipsoid representation as a means of visualizing/displaying nuclear quadrupole coupling tensors in the molecular frame. We show the usefulness of the VPPA concept in providing a unifying explanation for seemingly different types of molecular interactions such as hydrogen bonding, halogen bonding, and frustrated Lewis pairs. We further suggest that VPPA can be used as a universal measure of the ability of any element in the entire p-block of the periodic table (groups 13-16) to interact with nucleophiles (e.g., formation of chalcogen, pnictogen, tetrel, and triel bonds).