Pattern recognition of geminal proton nonequivalence and second‐order coupling in 13C single‐frequency off‐resonance decoupled spectra. Assignment criteria and structure elucidation

Abstract 13C n.m.r. single‐frequency off‐resonance decoupled (sford) CH multiplet resonance which deviate from normally assumed, isolated AnX spin descriptions have been characterized. Methylene carbons whose appended protons are nonequivalent generate doublets of doublets in sford spectra provided the proton chemical shift difference exceeds c. 3 × (Jgem). The isolated moiety–CHH′–closely approximates an AMX spin system. The strong chemical shift nonequivalence of geminal aminomethylene protons induced by the nitrogen lone pair in some N‐substituted piperidine derivatives is accompanied by preferential modification of the one‐bond CH coupling constants of the α‐amino protons. In N‐methyl‐4‐t‐butylpiperidine the natural coupling constants were shown to differ by 6–12 Hz. The 13C sford resonances of protonated carbons display varying degrees of second‐order coupling depending on the coupling strength of a carbon's appended protons with a strongly coupled spin set, the number of nuclei in the spin set, and the magnitude of the carbon's residual one‐bond splitting. Qualitative criteria that allow the classification of degrees of second‐order coupling have been established and utilized in signal assignment and structure analysis.