Two-dimensional NMR studies on the additivity rules for carbon-13 chemical shifts in tetra-n-alkylammonium halides

Abstract

The 13C chemical shifts of symmetric tetra-n-alkylammonium cations with n = 1–18 were identified unambiguously using two-dimensional homonuclear (1H) chemical shift correlation (COSY) and heteronuclear (1H, 13C) chemical shift correlation spectroscopy (XHCORR). The 13C chemical shifts in small tetra-n-alkylammonium cations with less than five carbons in a chain are found to decrease from C-1 (bonded to the nitrogen) (52-60 ppm) to the methyl chain end (7-14 ppm). For the cations with longer chains the values of chemical shifts are not in sequence with the carbon positions in an alkyl chain. The chemical shifts of C-2 (22.5 ppm) are close to that of the methylene carbon next to the methyl group. The C-3 carbon resonates at 26.4 ppm and is more shielded than the third carbon from the chain end (31.4 ppm). The chemical shifts of the remaining carbons are very similar to those of carbons in a corresponding linear alkane. The differences of the chemical shifts in alkylammonium salts from the corresponding n-alkanes are largest for C-1, C-2 and C-3, and this can be ascribed to the effects of the positive charge on C-1, and the restricted segmental motion for the innermost region of the cations. Empirical additivity rules for the chemical shifts of C-1, C-2, and C-3 are derived from the experimental results on 26 symmetric tetra-n-alkylammonium salts, and are tested for some asymmetrical systems with unequal chain lengths in the cation.