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

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 H-1 and C-13 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 H-1 nuclei, and qualitatively in the case of C-13 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. (C) 1997 Elsevier Science B.V.