B3LYP density functional calculations of 1H and 13C nuclear shielding constants of some unsymmetric N, N′-dipyridyl ureas and a further insight into their molecular conformations

Abstract

Isotropic 1H and 13C nuclear magnetic shielding constants of N′-(2-pyridyl), N-(3-pyridyl) ureas and N′-(2-pyridyl), N-(4-pyridyl) ureas have been calculated employing the direct (recomputation of two-integrals) implementation of the gauge-including-atomic-orbitals (GIAO) method at the B3LYP density functional level of theory.The geometry of each conformer has been optimized employing a 6-31G∗ basis set. A D95∗∗ basis set has been chosen to perform the nuclear shielding calculations. The theoretical trends are compared with experimental data taken from the bibliography. The optimization of the geometries confirm that the molecular conformation of these compounds has the 2-pyridyl-ring coplanar with the urea plane, with the N–H bonded to the nitrogen of the 2-pyridyl-ring on the other urea nitrogen. The rapid rotation of the 3-/4-pyridyl group about the N–C3/N–C4 bond proposed in the experimental study of other authors is investigated by the NMR shielding calculations and by the SCF energy barriers calculated between the conformers present for each compound.