Rationalization of the anomalous 1H NMR chemical shifts in 1,3-diheterocyclohexanes

Abstract

Ab initio calculations, within the frame of density functional theory, were carried out on cyclohexane, 1,3-dioxane, 1,3-dithiane and 1,3-oxathiane. The 1H and 13C NMR shifts were estimated according to the recently developed sum-over-states density functional perturbation theory (SOS-DFTPT) as implemented in a modified deMon-KS program. The calculations reproduced the observed NMR chemical shifts, quantitatively in the case of 1H nuclei, and qualitatively in the case of 13C nuclei. Analysis of the relevant orbital contributions to the NMR shielding factors does not provide support of the stereoelectronic interactions which have been generally ascribed to the so-called ‘Perlin effects’ and ‘reverse Perlin’ effects.