The effect of paramagnetic species on the nuclear magnetic resonance spectra of nitrogen-15

Abstract

In addition to appreciable gains in sensitivity resulting from reduction of long spin-lattice relaxation times, the addition of small amounts of paramagnetic material eliminates the possibility of low or zero intensities arising from partial quenching of the negative nuclear Overhauser effect when relaxation mechanisms other than dipole-dipole interactions are present.VARIOUS workers have recently demonstrated the possibility of observing n.m.r. spectra of the nitrogen-15 isotope in its natural abundance.l Apart from the obvious limitations imposed by the low sensitivity of this nucleus, the results have shown that there are two major problems inhibiting its routine detection. One arises as a result of the long nitrogen-15 spin-lattice relaxation times, particularly notable in cases where there are no protons directly bonded to the 15N nucleus.2 The problem is equally severe when the pulsed-mode is employed, since the small flip angles and/or long delays between successive pulses needed to obtain the free induction decay reduce the TABLE 1. Variation in l5N signal intensities as a function of added paramagnetic for benzonitrile (1) and 4-bromobenzonitrile (2)

The effect of paramagnetic relaxation reagents on nitrogen-15 spin relaxation and the use of Gd(dpm)3 as a nitrogen-15 nuclear magnetic resonance spin label

“…On the other hand, as is shown in the equation 1 + (i>s2Te2 J (7) the contribution of this mechanism to T2 is several orders of magnitude more efficient. Previous workers15 have noted that great care should be taken in using NMR line width information to evaluate spin-labeling.…”

Nitrogen-15 nuclear magnetic resonance spectroscopy. Nitrogen-15 chemical shifts determined from natural-abundance spectra

“…Now that Fourier-transform techniques are available, investigation of natural-abundance 15N chemical shifts of tertiary amines should be more practical, as the greater sensitivity of these techniques will offset the sacrifice of some or all of the nuclear Overhauser enhancement that results from use of paramagnetic material to shorten spin-lattice relaxation times. 10 The stability of our Fourier transform system with internal deuterium lock, relative to our former continuous-wave system employing an external water lock, allowed for much better resolution and precise determination of chemicalshift effects. For example, Fourier-transform spectra of di-2-butylamine and di-2-hexylamine each showed two peaks, separated by 0,56 and 0.3 ppm, respectively, as corresponds to the meso and D,L forms, although at this time, we do not know which peaks should be assigned to each.…”

Water-soluble paramagnetic relaxation reagents for carbon-13 nuclear magnetic resonance spectrometry

“…If the NOE is completely suppressed, quantitative integration of peaks in the 13C NMR spectrum can be performed. This application has been studied rather extensively with the organic soluble relaxation reagents (33)(34)(35)(36)(37). Even if complete suppression of the NOE is not obtained, the faster relaxation times allow for spectra to be run much faster with the gated decoupling technique.…”