Quantification of σ-Acceptor and π-Donor Stabilization in O, S and Hal Analogues of N-Heterocyclic Carbenes (NHCs) on the Magnetic Criterion.

The spatial magnetic properties, through-space NMR shieldings (TSNMRSs), of stable O, S and Hal analogues of N-heterocyclic carbenes (NHCs) have been calculated using the GIAO perturbation method employing the nucleus-independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSSs) of various sizes and directions. The TSNMRS values (actually the anisotropy effects measurable in 1H NMR spectroscopy) are employed to qualify and quantify the position of the present mesomeric equilibria (carbenes ↔ ylides). The results are confirmed by geometry (bond angles and bond lengths), IR spectra, UV spectra, and 13C chemical shifts of the electron-deficient carbon centers.