1H NMR chemical shifts of cyclopropane and cyclobutane: a theoretical study.

This study was undertaken in order to rationalize the peculiar (1)H NMR chemical shifts of cyclopropane (δ 0.22) and cyclobutane (δ 1.98) which are shifted upfield and downfield with respect to larger cycloalkanes (δ 1.44-1.54). This is conventionally accounted for by shielding contributions arising from an aromatic-like ring current in cyclopropane, involving six electrons in the three C-C bonds, and deshielding coming from the σ antiaromatic CC framework of cyclobutane. The shielding pattern arising from the cyclopropane and cyclobutane CC framework response to a perpendicular magnetic field was visualized as two-dimensional grid distribution of NICS values. Further insight into the origin of chemical shift values was obtained by the NCS-NBO analysis of proton shielding tensor. In the case of cyclopropane, the CC framework shielding pattern implies the existence of both delocalized and localized currents that have a dominant shielding effect on protons. The magnitude of C-H bonds shielding effect is significant, too. Unlike the conventional interpretation, the CC framework shields cyclobutane hydrogens, and its response to a perpendicular magnetic field is quite similar to responses of other planar σ CC frameworks.