Natural abundance nitrogen‐15 n.m.r. spectroscopy. Spin–lattice relaxation in organic compounds

Abstract

AbstractThe small negative magnetogyric ratio (γ) of the 15N nucleus decreases the efficiency of 15N1H dipole‐dipole relaxation to about 25% of that for an analogous 13C nucleus. This may lead to greater competition from other relaxation mechanisms in 15N n.m.r. and consequent partial or total quenching of the negative nuclear Overhauser effect (NOE). In unfavorable circumstances nulling of the 15N resonance can occur. Previous 15N relaxation studies have examined isotopically enriched, low molecular weight compounds. The present study examines several small to intermediate size organic compounds containing nitrogen at natural isotopic abundance. In contrast to some of the earlier studies, 15N1H dipolar relaxation was found to be dominant for protonated nitrogen atoms, even for two tertiary nitrogens (the tertiary amine nitrogen in 1,2,3,4,6,7,12,12b‐octahydroindolo[2,3‐a] quinolizine and the oxime nitrogen in 3‐methyl‐2‐pentanone ketoxime). The magnitude of the NOE and the moderate value of T1 indicate effective dipolar relaxation from neighboring but not directly bonded protons in these cases. Nitro groups were found, as expected, to have predominant contributions from non‐dipolar mechanisms, and in one case (2‐methyl‐2‐nitro‐1, 3‐propanediol) signal nulling (NOE of η = −1) was observed. The effect of paramagnetic impurities was demonstrated for ethanolamine, which contains a basic nitrogen. In this case T1DD(15N1H) = 4·3 s; added Ni(acac)2 at 1 × 10−4 Molar reduced the 15N T1 to 0·065 s and consequently the NOE to η = 0.