:Nitrogen-15 nuclear magnetic resonance spectroscopy

¹⁵N NMR has complications not encountered in ¹H and ¹³C NMR spectroscopy. The 0.36% natural abundance of ¹⁵N results in a major sensitivity penalty. Sensitivity is made worse by its low gyromagnetic ratio (γ = −27.126 × 10⁶ T⁻¹s⁻¹), which is 10.14% that of ¹H. The signal-to-noise ratio for ¹H is about 300-fold greater than ¹⁵N at the same magnetic field strength.{{cite book| author=Arthur G Palmer| title=Protein NMR Spectroscopy| publisher=Elsevier Academic Press| date=2007 | isbn = 978-0121644918}}

The physical properties of ¹⁵N 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, ¹⁵N is about one tenth (-27.126/267.522) as sensitive as ¹H.

File:N15 Chemical Shifts Chart.png

The International Union of Pure and Applied Chemistry (IUPAC) recommends using CH₃NO₂ as the experimental standard; however in practice many spectroscopists utilize pressurized NH₃(l) instead. For ¹⁵N, chemical shifts referenced with NH₃(l) are 380.5 ppm upfield from CH₃NO₂ (δ_NH3 = δ_CH3NO2 + 380.5 ppm). Chemical shifts for ¹⁵N are somewhat erratic but typically they span a range of -400 ppm to 1100 ppm with respect to CH₃NO₂. Below is a summary of ¹⁵N chemical shifts for common organic groups referenced with respect to NH₃, whose chemical shift is assigned 0 ppm.

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File:Gyromagnetic Nuclei Precession.png

Unlike most nuclei, the gyromagnetic ratio for ¹⁵N is negative. With the spin precession phenomenon, the sign of γ determines the sense (clockwise vs counterclockwise) of precession. Most common nuclei have positive gyromagnetic ratios such as ¹H and ¹³C.{{clear}}

File:Tautomerization and 15N-NMR.png

¹⁵N NMR is used in a wide array of areas from biological to inorganic techniques. A famous application in organic synthesis is to utilize ¹⁵N to monitor tautomerization equilibria in heteroaromatics because of the dramatic change in ¹⁵N shifts between tautomers.

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File:Backbone Walk for NCACX, NCOCX, and CANcoCX for SS-NMR.png

¹⁵N NMR is also extremely valuable in protein NMR investigations. Most notably, the introduction of three-dimensional experiments with ¹⁵N lifts the ambiguity in ¹³C–¹³C two-dimensional experiments. In solid-state nuclear magnetic resonance (ssNMR), for example, ¹⁵N is most commonly utilized in NCACX, NCOCX, and CANcoCX pulse sequences.

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¹⁵N 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).{{Cite journal|last1=Shestakova|first1=Tatyana S.|last2=Shenkarev|first2=Zakhar O.|last3=Deev|first3=Sergey L.|last4=Chupakhin|first4=Oleg N.|last5=Khalymbadzha|first5=Igor A.|last6=Rusinov|first6=Vladimir L.|last7=Arseniev|first7=Alexander S.|date=2013-06-27|title=Long-Range 1H–15N J Couplings Providing a Method for Direct Studies of the Structure and Azide–Tetrazole Equilibrium in a Series of Azido-1,2,4-triazines and Azidopyrimidines|journal=The Journal of Organic Chemistry|language=EN|volume=78|issue=14|pages=6975–6982|doi=10.1021/jo4008207|pmid=23751069|issn=0022-3263|hdl=10995/27205|url=http://elar.urfu.ru/bitstream/10995/27205/1/scopus-2013-0375.pdf|hdl-access=free}}{{Cite journal|last1=Deev|first1=Sergey L|last2=Paramonov|first2=Alexander S|last3=Shestakova|first3=Tatyana S|last4=Khalymbadzha|first4=Igor A|last5=Chupakhin|first5=Oleg N|last6=Subbotina|first6=Julia O|last7=Eltsov|first7=Oleg S|last8=Slepukhin|first8=Pavel A|last9=Rusinov|first9=Vladimir L|date=2017-11-29|title=15N-Labelling and structure determination of adamantylated azolo-azines in solution|journal=Beilstein Journal of Organic Chemistry|language=en|volume=13|issue=1|pages=2535–2548|doi=10.3762/bjoc.13.250|pmid=29259663|pmc=5727827|issn=1860-5397}} ¹⁵N labeling followed by analysis of ¹³C–¹⁵N and ¹H–¹⁵N couplings may be used for establishing structures and chemical transformations of nitrogen heterocycles.{{Cite journal|last1=Deev|first1=Sergey L.|last2=Khalymbadzha|first2=Igor A.|last3=Shestakova|first3=Tatyana S.|last4=Charushin|first4=Valery N.|last5=Chupakhin|first5=Oleg N.|date=2019-08-23|title=15 N labeling and analysis of 13C–15N and 1H–15N couplings in studies of the structures and chemical transformations of nitrogen heterocycles|journal=RSC Advances|language=en|volume=9|issue=46|pages=26856–26879|doi=10.1039/C9RA04825A|pmid=35528595 |pmc=9070671 |bibcode=2019RSCAd...926856D |issn=2046-2069|doi-access=free}}

File:INEPT Pulse Sequence.png

Insensitive nuclei enhanced by polarization transfer (INEPT) is a signal resolution enhancement method. Because ¹⁵N has a gyromagnetic ratio that is small in magnitude, the resolution is quite poor. A common pulse sequence which dramatically improves the resolution for ¹⁵N is INEPT. The INEPT is an elegant solution in most cases because it increases the Boltzmann polarization and lowers T₁ values (thus scans are shorter). Additionally, INEPT can accommodate negative gyromagnetic ratios, whereas the common nuclear Overhauser effect (NOE) cannot.

• Heteronuclear single quantum coherence spectroscopy (HSQC)
• Two-dimensional nuclear magnetic resonance spectroscopy
• Triple-resonance nuclear magnetic resonance spectroscopy

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{{NMR by isotope}}

Category:Nuclear magnetic resonance