The nuclear quadrupole coupling of the halogen pseudohalides: A comparison of ab initio and DFT studies with spectroscopic data

Electric field gradients (in both principal (PA) and inertial (IA) axes) have been calculated at the equilibrium structures of the halogen azides, isocyanates and isothiocyanates; these have been converted into the corresponding nuclear quadrupole coupling constants (NQCC). Möller-Plesset (MP2) and DFT (B3LYP) methodologies, together with cc-pVTZ and TZVP basis sets, enable comparison of the effects of methodology and basis set, and experiment. The experimental and theoretical 35Cl, 79Br, 127I NQCC are closely related in linear relationships for both methodologies. The NQCC results also correlate with Pauling electronegativity of the halogen (also including H). The availability of experimental data for the present compounds at the 14N centres is low, and some seems suspect. The study was extended to include a variety of related model compounds, where the small coupling constants have been evaluated by FT-MW spectroscopy. Both methodologies give good correlations with these model compound experimental results, but the B3LYP 14N results are significantly more successful with the central N atom of the azides. These relationships confirm our previous studies, which show that very good agreement between theory and experiment is obtained for all these nuclei. The largest magnitude 14N Nα NQCC-PA tensor elements are the tangential terms χT, whereas the smallest elements lie close to the direction of the lone pair of electrons (χLPN), in contrast to many amines; the out-of-plane (χπ), and in-plane (χT) elements vary smoothly as the halogen size increases. The NQCC-PA magnitude varies significantly with halogen change at the Nγ atoms in the azides, but at Nβ there are almost unchanging values for each of the tensor elements, under these conditions.