SENSITIVITY OF THE H/D ISOTOPE SHIFTS OF THE 13-C NUCLEAR SHIELDING TO THE NITROGEN LONE PAIR DELOCALIZATION IN ANILINES AND ACETANILIDES

Two- and three-bond deuterium isotope shifts (2 DELTA and 3 DELTA) of the C-13 nuclear shielding were measured for a series of para-substituted anilines and acetanilides, in which the amide NH or the amine NH2 hydrogens were partially exchanged with deuterium. The results show that when strong electron-donating groups are present at the para position in acetanilides, the 2-DELTA(C-1) isotope shifts are smaller than the 3-DELTA(C-2,6) isotope shifts. Empirical correlations that relate 2-DELTA isotope shifts with 1H(NH), 13C(C-ipso), 15N(NH2 or NH), 17O(C = O) NMR chemical shifts and with 3-DELTA isotope shifts are presented. The results are interpreted in each series as depending on the characteristic nitrogen lone pair delocalization.