Nitrogen-15 nuclear magnetic resonance spectroscopy (nitrogen-15 NMR spectroscopy, or just simply <sup>15</sup>N NMR) is a version of nuclear magnetic resonance spectroscopy that examines samples containing the <sup>15</sup>N nucleus. <sup>15</sup>N NMR differs in several ways from the more common <sup>13</sup>C and <sup>1</sup>H NMR. To circumvent the difficulties associated with measurement of the quadrupolar, spin-1 <sup>14</sup>N nuclide, <sup>15</sup>N NMR is employed in samples for detection since it has a ground-state spin of ý. Since<sup>14</sup>N is 99.64% abundant, incorporation of <sup>15</sup>N into samples often requires novel synthetic techniques.
Nitrogen-15 is frequently used in nuclear magnetic resonance spectroscopy (NMR), because unlike the more abundant nitrogen-14, that has an integer nuclear spin and thus a quadrupole moment, <sup>15</sup>N has a fractional nuclear spin of one-half, which offers advantages for NMR like narrower line width. Proteins can be isotopically labeled by cultivating them in a medium containing nitrogen-15 as the only source of nitrogen. In addition, nitrogen-15 is used to label proteins in quantitative proteomics (e.g. SILAC).
<sup>15</sup>N NMR has complications not encountered in <sup>1</sup>H and <sup>13</sup>C NMR spectroscopy. The 0.36% natural abundance of <sup>15</sup>N results in a major sensitivity penalty. Sensitivity is made worse by its low gyromagnetic ratio (ó = âÂÂ27.126 à10<sup>6</sup> T<sup>âÂÂ1</sup>s<sup>âÂÂ1</sup>), which is 10.14% that of <sup>1</sup>H. The signal-to-noise ratio for <sup>1</sup>H is about 300-fold greater than <sup>15</sup>N at the same magnetic field strength.
The physical properties of <sup>15</sup>N are quite different from other nuclei. Its properties along with several common nuclei are summarized in the below table.
From these data, one can see that at full enrichment, <sup>15</sup>N is about one tenth (-27.126/267.522) as sensitive as <sup>1</sup>H.
The International Union of Pure and Applied Chemistry (IUPAC) recommends using CH<sub>3</sub>NO<sub>2</sub> as the experimental standard; however in practice many spectroscopists utilize pressurized NH<sub>3</sub>(l) instead. For <sup>15</sup>N, chemical shifts referenced with NH<sub>3</sub>(l) are 380.5 ppm upfield from CH<sub>3</sub>NO<sub>2</sub> (ô<sub>NH<sub>3</sub></sub> = ô<sub>CH<sub>3</sub>NO<sub>2</sub></sub> - 380.5 ppm). Chemical shifts for <sup>15</sup>N are somewhat erratic but typically they span a range of -400 ppm to 1100 ppm with respect to CH<sub>3</sub>NO<sub>2</sub>. Below is a summary of <sup>15</sup>N chemical shifts for common organic groups referenced with respect to NH<sub>3</sub>, whose chemical shift is assigned 0 ppm.
Unlike most nuclei, the gyromagnetic ratio for <sup>15</sup>N is negative. With the spin precession phenomenon, the sign of ó determines the sense (clockwise vs counterclockwise) of precession. Most common nuclei have positive gyromagnetic ratios such as <sup>1</sup>H and <sup>13</sup>C.
<sup>15</sup>N NMR is used in a wide array of areas from biological to inorganic techniques. A famous application in organic synthesis is to utilize <sup>15</sup>N to monitor tautomerization equilibria in heteroaromatics because of the dramatic change in <sup>15</sup>N shifts between tautomers.
<sup>15</sup>N NMR is also extremely valuable in protein NMR investigations. Most notably, the introduction of three-dimensional experiments with <sup>15</sup>N lifts the ambiguity in <sup>13</sup>CâÂÂ<sup>13</sup>C two-dimensional experiments. In solid-state nuclear magnetic resonance (ssNMR), for example, <sup>15</sup>N is most commonly utilized in NCACX, NCOCX, and CANcoCX pulse sequences.
<sup>15</sup>N NMR is the most effective method for investigation of structure of heterocycles with a high content of nitrogen atoms (tetrazoles, triazines and their annelated analogs). <sup>15</sup>N labeling followed by analysis of <sup>13</sup>CâÂÂ<sup>15</sup>N and <sup>1</sup>HâÂÂ<sup>15</sup>N couplings may be used for establishing structures and chemical transformations of nitrogen heterocycles.
Insensitive nuclei enhanced by polarization transfer (INEPT) is a signal resolution enhancement method. Because <sup>15</sup>N has a gyromagnetic ratio that is small in magnitude, the resolution is quite poor. A common pulse sequence which dramatically improves the resolution for <sup>15</sup>N is INEPT. The INEPT is an elegant solution in most cases because it increases the Boltzmann polarization and lowers T<sub>1</sub> values (thus scans are shorter). Additionally, INEPT can accommodate negative gyromagnetic ratios, whereas the common nuclear Overhauser effect (NOE) cannot.