A solid-state NMR approach for distinguishing between RNH₂ and RNH₃⁺ sites

A necessary step in characterizing solid-phase organic materials is the accurate assignment of the ionization state at acidic and basic sites. Solution phase pK_a's are not always reliable reference points because local environments can significantly change pK_a values in solids. Herein, an approach for distinguishing R-NH₂ and R-NH₃⁺ is described based on experimental ¹⁵N chemical shift tensors principal values for a given site (i.e. δ₁₁, δ₂₂ and δ₃₃) from 18 model compounds. Those ¹⁵N sites that are present as R-NH₃⁺ have anisotropies between 5 and 15 ppm. In contrast, all R-NH₂ sites have anisotropies between 14 and 115 ppm. These R-NH₂ moieties can be further categorized into three subgroups. The differences observed are postulated to arise from differences in the symmetry of the intermolecular hydrogen bonding environment, or the direct attachment of the NH₂ to an aromatic ring.