ON THE USE OF NUCLEAR MAGNETIC RESONANCE FOR THE DETECTION, ESTIMATION, AND CHARACTERIZATION OF N-METHYL GROUPS

Abstract

A useful n.m.r. technique for the detection, estimation, and characterization of N-methyl groups is described. The method depends on the decreased shielding of N_basic-methyl protons when the solvent is changed from deuterochloroform to perdeuteroacetic (PDA) (or acetic) acid to trifluoroacetic acid (TFA). The decreased shielding results in a downfield shift of the N-methyl resonance of ca. 0.3–1.3 p.p.m. depending on the type of amine. Most N-methyl groups on neutral nitrogen atoms show little movement. For the majority of aliphatic amines in trifluoroacetic acid the NH exchange rate is slow enough at 38° to permit observation of coupling of H(N) with the N-methyl protons, and thus determination of whether the amine is secondary or tertiary. In some cases information can be obtained about methylene and methine groups attached to basic nitrogen. In acid solution compounds with basic nitrogen attached to, or part of, an aromatic ring show considerable shift of the aromatic proton resonance signals to lower magnetic field. Combination of the n.m.r. technique with the Herzig-Meyer analysis can also reveal the number of methyl groups attached to neutral nitrogen.