Natural Abundance Nitrogen-15 Nuclear Magnetic Resonance-Liquid Nitrogen

Abstract

Following the estimation by Ramsey of the absolute value for the paramagnetic contribution (σ_p) to the absolute shielding of ^(15)N_2 relative to the bare nucleus ^(15)N, considerable interest has been expressed3 in establishing an absolute shielding scale for ^(14)N (and ^(15)N) nuclear magnetic resonance. This requires ^(15)N chemical shifts referenced to the ^(15)N_2 resonance via some secondary standard, usually ^(15)NH_4^+ or ^(15)NO_3^- in aqueous solution. To date, two measurement of the resonance position of ^(14)N_2 have been published as 14 and 70 ppm upfield from ^(14)NO_3^-, but doubts have been expressed about the accuracy of these values. One source of uncertainty in the reported chemical shifts arises from the line widths of the ^(14)N resonances. From the relaxation time data for liquid ^(14)N_2 at 77ºK of Armstrong and Speight, we estimate the width at half height of the ^(14)N resonance to be ca. 30 Hz (9.5 and 7 ppm at 10 and 13 kG, respectively.

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