Modeling through-space magnetic shielding over ethynyl, cyano, and nitro groups

In a strong magnetic field, covalently bonded hydrogen nuclei located over a pi bonded functional group experience magnetic shielding (or deshielding) that results from the combined effect of the magnetic anisotropy of the pi bond and various other intramolecular shielding effects. Gauge including atomic orbital (GIAO)-HF in Gaussian 98 was employed to calculate isotropic shielding values and to predict the net through-space proton NMR shielding increment for a simple model system: the proximate proton of methane held in various positions over simple molecules that contain a carbon-carbon triple bond, a carbon-nitrogen triple bond, or a nitro group. These net shielding increments of the proximate proton of methane, plotted against their Cartesian coordinates, led to the development of a single empirical equation for predicting the NMR shielding experienced by a covalently bonded proton over each group. The predictive capability of each equation has been validated by calculating shielding increments of protons over the functional group in known structures. These shielding increments are then used to adjust predicted chemical shifts for through-space shielding effects, and the adjusted values are compared to experimentally observed chemical shifts. The algorithms for predicting the shielding increment for a proton over these functional groups can be used in a spreadsheet or incorporated into software that estimates chemical shifts using additive substituent constants or a database of structures. Their use can substantially improve the accuracy of the estimated chemical shift of a proton in the vicinity of these functional groups, and thus assist in spectral assignments and in correct structure determination.