New findings from old data: A semi-experimental value for the eQq of the nitrogen atom

Nuclear quadrupole coupling arises from the interaction of the nuclear quadrupole moment with the electric field gradient. Thus, it is associated with electron occupancy and the electronic structure of molecules. We demonstrate a simple method for planar molecules based on a direct correlation between the out-of-plane quadrupole coupling constant and the electron occupancy in the p orbital perpendicular to the molecular plane. This method is applied to 98 molecular systems containing a ¹⁴N quadrupolar nucleus using data from more than 40 years of rotational spectroscopy and comparing the performance of three levels of theory from quantum-chemical computations. From this extensive dataset, we have analyzed chemical properties of molecules, such as the hybridization of the atom, and we could quantify the extent of polarization and resonance processes as well as physical characteristics of the quadrupolar nucleus, such as eQq. This is a constant, which represents the interaction in the hypothetical case of having a single electron in an electronic orbital at the isolated nucleus, and its value has been under debate for a long time. Here, the eQq value has been determined for the ¹⁴N nucleus, and the methodology to calculate it for other nuclei is provided.