The Effect of Homogeneous and Inhomogeneous Electric Fields on NMR Properties of Imidazole and Pyrrole Under.

In this study, we examine the influence of homogeneous and highly inhomogeneous electric fields (HIEFs) on NMR parameters such as isotropic spin-spin couplings and nuclear magnetic shieldings. The model compounds imidazole and pyrrole were chosen because of their distinct symmetries with respect to the applied fields. Calculations were performed at the CAM-B3LYP and second-order polarization propagator approximation (SOPPA) levels using the aug-cc-pVTZ-J basis set. The results show that the influence of the external HIEF is comparable in magnitude with that produced by solvation in water. The coupling constants exhibit angular dependence that partially resembles Karplus-type relationships, with a slight asymmetry, with nitrogen nuclei showing the most pronounced responses. One-bond couplings are mainly governed by their Fermi contact contributions, so that the isotropic coupling is almost indistinguishable from the Fermi contact term. In contrast, the two-bond couplings display a clear separation between the Fermi contact and the isotropic values, influenced by paramagnetic spin-orbit or spin-dipolar contributions. The nuclear shieldings, in turn, exhibit a more harmonic dependence, following the variations of the applied field.