Resumen de Understanding Chemical Shifts in pi-Systems:: 13C, 15N, 17O NMR
In the NMR spectra of nuclei other than 1H, pi-systems show strong shielding or deshielding effects that are not explained by the simple consideration of electron densities. This is mainly because, in the presence of a magnetic field, the low-lying unoccupied pi* orbital allows combinations with energy-rich occupied orbitals of appropriate spatial orientation. The underlying principles can be understood using simple qualitative MO discussions, which follow, in principle, the same reasoning as the very successful ab initio calculations. The typical argument is developed for three examples taken from the literature. First, in phenyllithium the ipso carbon atom is strongly deshielded in a direction in the ring plane perpendicular to the C-Li bond; this is due to the n-pi* orbital combination. Second, in carbonyl compounds R-CO-X the influence of X is systematically felt on the chemical shift of 17O, but not on the chemical shift of 13C; this is attributed to the ny-pi* orbital combination, magnetically acting at O in the direction of the C-O bond, but lacking at C. Third, in linear pi-systems (acetylene, CO2, isocyanate ion, etc.) all atoms are strongly shielded with respect to a nonlinear comparison; this is because, for reasons of orbital symmetry, a deshielding contribution in the direction of the molecular axis has vanished.
