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Nitrogen-15 nuclear magnetic resonance spectroscopy

Nitrogen-15 nuclear magnetic resonance spectroscopy is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Nitrogen-15 nuclear magnetic resonance spectroscopy rather than just read about it. In short: Nitrogen-15 nuclear magnetic resonance spectroscopy (nitrogen-15 NMR spectroscopy, or just simply 15N NMR) is a version of nuclear magnetic resonance spectroscopy that examines samples containing the 15N nucleus. 15N NMR differs in several ways from the more common 13C and 1H NMR. To circumvent the difficulties associated with measurement of the quadrupolar, spin-1 14N nuclide, 15N NMR is employed in samples for det…

Nitrogen-15 nuclear magnetic resonance spectroscopy — main illustration
Nitrogen-15 nuclear magnetic resonance spectroscopy — illustration

Key takeaways

  • Nitrogen-15 nuclear magnetic resonance spectroscopy belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Nitrogen-15 nuclear magnetic resonance spectroscopy to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Nitrogen-15 nuclear magnetic resonance spectroscopy from memory before moving on to harder problems.

Reference excerpt

Nitrogen-15 nuclear magnetic resonance spectroscopy (nitrogen-15 NMR spectroscopy, or just simply 15N NMR) is a version of nuclear magnetic resonance spectroscopy that examines samples containing the 15N nucleus. 15N NMR differs in several ways from the more common 13C and 1H NMR. To circumvent the difficulties associated with measurement of the quadrupolar, spin-1 14N nuclide, 15N NMR is employed in samples for detection since it has a ground-state spin of ½. Since14N is 99.64% abundant, incorporation of 15N 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, 15N 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).

Implementation 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.

Physical properties 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.

Chemical shift trends

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 (δNH3 = δCH3NO2 - 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.

Gyromagnetic ratio

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.

Applications

Tautomerization

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.

Protein NMR

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.

Investigation of nitrogen-containing heterocycles 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). 15N labeling followed by analysis of 13C–15N and 1H–15N couplings may be used for establishing structures and chemical transformations of nitrogen heterocycles.

INEPT

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.

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

References

Illustrations

Nitrogen-15 nuclear magnetic resonance spectroscopy: The sign of the gyromagnetic ratio, γ, determines the sense of precession. Nuclei such as 1H and 13C are said to have clockwise precession whereas 15N has counterclockwise precession.[3][4]
The sign of the gyromagnetic ratio, γ, determines the sense of precession. Nuclei such as 1H and 13C are said to have clockwise precession whereas 15N has counterclockwise precession.[3][4]
Nitrogen-15 nuclear magnetic resonance spectroscopy: Example 15N chemical shifts for tautomers undergoing tautomerization.[6]
Example 15N chemical shifts for tautomers undergoing tautomerization.[6]
Nitrogen-15 nuclear magnetic resonance spectroscopy: The ssNMR polarization pathways for the NCACX, NCOCX, and CANcoCX experiments respectively. In each case, all carbon and nitrogen atoms are either uniformly or partially isotopically labeled with 13C and 15N.
The ssNMR polarization pathways for the NCACX, NCOCX, and CANcoCX experiments respectively. In each case, all carbon and nitrogen atoms are either uniformly or partially isotopically labeled with 13C and 15N.
Nitrogen-15 nuclear magnetic resonance spectroscopy: Graphical representation of the INEPT NMR pulse sequence. INEPT is utilized often to improve 15N resolution because it can accommodate negative gyromagnetic ratios, increases Boltzmann polarization, and decreases T1 relaxation.[3]
Graphical representation of the INEPT NMR pulse sequence. INEPT is utilized often to improve 15N resolution because it can accommodate negative gyromagnetic ratios, increases Boltzmann polarization, and decreases T1 relaxation.[3]

Worked examples

Example 1 — a first encounter with Nitrogen-15 nuclear magnetic resonance spectroscopy

Start with the simplest possible case. Write down what Nitrogen-15 nuclear magnetic resonance spectroscopy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nitrogen-15 nuclear magnetic resonance spectroscopy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nitrogen-15 nuclear magnetic resonance spectroscopy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nitrogen-15 nuclear magnetic resonance spectroscopy

In research
Nitrogen-15 nuclear magnetic resonance spectroscopy appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Nitrogen-15 nuclear magnetic resonance spectroscopy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Nitrogen-15 nuclear magnetic resonance spectroscopy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Nuclear magnetic resonance, so understanding it makes those chapters shorter.
In everyday life
Look for Nitrogen-15 nuclear magnetic resonance spectroscopy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Nitrogen-15 nuclear magnetic resonance spectroscopy in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
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  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
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Frequently asked questions

What is Nitrogen-15 nuclear magnetic resonance spectroscopy in simple terms?

Nitrogen-15 nuclear magnetic resonance spectroscopy (nitrogen-15 NMR spectroscopy, or just simply 15N NMR) is a version of nuclear magnetic resonance spectroscopy that examines samples containing the 15N nucleus. 15N NMR differs in several ways from the more common 13C and 1H NMR. To circumvent the…

Why does Nitrogen-15 nuclear magnetic resonance spectroscopy matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nitrogen-15 nuclear magnetic resonance spectroscopy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nitrogen-15 nuclear magnetic resonance spectroscopy.

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  • Nuclear magnetic resonance

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