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Natural abundance nitrogen‐15 n.m.r. spectroscopy. Spin–lattice relaxation in organic compounds
Author(s) -
Levy George C.,
Holloway Clive E.,
Rosanske Richard C.,
Hewitt James M.,
Bradley Craig H.
Publication year - 1976
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270081211
Subject(s) - chemistry , nitrogen , relaxation (psychology) , protonation , spin–lattice relaxation , crystallography , analytical chemistry (journal) , nuclear magnetic resonance , organic chemistry , nuclear quadrupole resonance , psychology , social psychology , ion , physics
The small negative magnetogyric ratio (γ) of the 15 N nucleus decreases the efficiency of 15 N 1 H dipole‐dipole relaxation to about 25% of that for an analogous 13 C nucleus. This may lead to greater competition from other relaxation mechanisms in 15 N n.m.r. and consequent partial or total quenching of the negative nuclear Overhauser effect (NOE). In unfavorable circumstances nulling of the 15 N resonance can occur. Previous 15 N 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, 15 N 1 H 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 T 1 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 T 1 DD ( 15 N 1 H) = 4·3 s; added Ni(acac) 2 at 1 × 10 −4 Molar reduced the 15 N T 1 to 0·065 s and consequently the NOE to η = 0.