Analysis of the origin of through-space proton NMR deshielding by selectedorganic functional groups

[GRAPHICS] GIAO-HF and IGLO-DFT computations of isotropic magnetic shieldings were use d to map the NMR shielding environments of small molecules exemplifying sel ected organic functional groups. Two different probes were employed: a meth ane molecule and NICS (nucleus-independent chemical shifts) based on comput ed absolute isotropic shieldings. The reason for the different results obta ined using these two probes is perturbation of the wave function by the pro ximity of methane to the pi bond, as analyzed by the localized orbital cont ributions to the shieldings.