^(15)N Nuclear Magnetic Resonance Spectroscopy. XII. Nuclear Overhauser Enhancement of ^(15)N Resonances

Abstract

The enhancement of the ^(15)N signal intensity of ammonium chloride by means of the nuclear Overhauser effect (NOE) has been measured as a function of pH. At low pH under conditions of complete proton decoupling, the ratio of the coupled to decoupled signal intensities is about 56 of the theoretical maximum and the signal is inverted. As the pH is raised, the decoupled signal progressively diminishes in intensity, disappears, and reappears upright. The observations are explained in terms of contributions to relaxation from spin-rotation interactions and from chemical modulation of the N-H spin-spin coupling constant. Implications for natural abundance ^(15)N spectroscopy are discussed.

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