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	Multinuclear NMR

Multinuclear NMR Spectroscopy

General Information

Presumably, all of us are familiar with NMR (Nuclear Magnetic Resonance) spectroscopy from our organic and analytical laboratory courses/experience. Such experience is generally limited to ¹H and maybe ¹³C NMR. There are actually dozens of other nuclei that are also amenable to study using NMR. Part of the joy of Inorganic Chemistry is that we need not limit ourselves to C and H in either syntheses or spectroscopy!

NMR spectra of nuclei with a spin quantum number, I, of 1/2 are easiest for us to understand because they obey the same kind of coupling behavior we see for the most common I = 1/2 nucleus, ¹H. Spins with I = 1 to 9/2 are also amenable to study and their interpretation is not all that much more difficult.

Just because a nucleus has a non-zero spin does not automatically mean that we can obtain an NMR spectrum of that nucleus. There are at least four other factors we must consider:

1. Isotopic Abundance. You can't see it if it isn't there. Some spin active nuclei such as ¹⁹F are 100% abundant (¹H is 99.985%), but others such as ¹⁷O have such a low abundance (0.037%) that we can't expect to get much of a signal unless we isotopically enrich the sample. Consider: ¹³C is only 1.1% abundant -- that's one of the reasons we need to use a lot more sample and take more scans to obtain a ¹³C spectrum versus a ¹H spectrum.

2. Sensitivity. This is typically scaled to ¹H = 1.0. There is only one (rather uncommon) isotope with a sensitivity >1.0; all other isotopes have sensitivities <1.0. The lower the sensititivity the greater the amount of time and sample it will take to get a signal. Some sensitivities are so low, such as ¹⁰³Rh (100% abundant but only 0.000031 sensitivity), that obtaining a spectrum for the nucleus is generally impractical. However, the nucleus can still couple to other spin-active nuclei and provide useful information provided it has good abundance. In the case of rhodium, ¹⁰³Rh coupling is easily observed in the ¹H and ¹³C spectra and the J_RhX can often be used to assign structures.

3. Nuclear quadrapole. For spins greater than 1/2, the nuclear quadropole moment is usually larger and the line widths may become excessively large. This can sometimes be overcome by running the sample at low temperature.

4. Relaxation time. Two factors govern the rate at which the excited nucleus relaxes its spin. Spin-lattice relaxation, T₁, is the time it takes for a nucleus to relax. If the spin population has not re-equilibrated before we pulse again, then we will not have measured the full signal. Spin-spin relaxation, T₂, refers to how long it takes for a set of aligned nuclei to lose their phase coherence. T₁ is usually much greater than T₂, so we normally only concern ourselves with T₁.

The combination of these four factors governs whether a given nucleus will give a useful NMR spectrum. Some of the more common I = 1/2 nuclei used by chemists are:

Nucleus	Natural Abundance	Relative Sensitivity
1H	99.985	1.0
13C	1.108	0.016
19F	100	0.83
31P	100	0.07

There are several very important nuclei that we will discuss in detail later in this document. I'll post some links to comprehensive NMR data tables in the near future.

Spin-spin Coupling

The most important aspect of multinuclear NMR is that all spin active nuclei can couple to each other and that the multiplicity of the coupling is given by 2nI + 1 where n = the number of equivalent nuclei that are being coupled to. For example, if a proton is adjacent to two equivalent protons, the resonance will appear as a triplet because 2nI + 1 = 2(2)(1/2) + 1 = 3. If the two adjacent nuclei were fluorines instead of hydrogens the resonance would still be triplet because I = 1/2 for fluorine and ¹⁹F is 100% abundant. The only difference in these two cases would be the magnitude of the coupling constants, J_HH versus J_HF.

Decoupling

When we obtain an NMR spectrum all the spin active nuclei in the system will couple to each other. For example, a methyl group appears as a quartet in a ¹³C spectrum because both ¹³C and ¹H have I = 1/2 and the coupling follows the 2nI + 1 rule described previously. For a standard ¹³C spectrum we call this condition proton-coupled although some use the perverse term, "undecoupled".

We can suppress this coupling by using an electronic decoupler to selectively remove the coupling from a specific kind of nucleus during our data acquisition. Thus, if we proton decouple and take a ¹³C spectrum of our methyl group it would appear as a singlet because we've suppressed the J_CH. The advantage of this technique is that we can simplify crowded spectra and reduce the amount of time required to obtain a good signal. The drawback is that we lose information about how many protons are attached to our carbon atom.

We are not limited to the heteronuclear decoupling experiments. We can also perform selective homonuclear decoupling. For example, suppose our ¹H NMR spectrum has a multiplet that we suspect is coupled to a doublet. We can irradiate the multiplet with our homonuclear decoupler while collecting our NMR spectrum. If the doublet collapses to a singlet upon decoupling, then the multiplet had to have been coupled to the doublet. If not, then we know the doublet is not coupled to the multiplet.

Homonuclear coupling is extremely easy to do and requires no more time than taking a regular NMR spectrum. For this reason it is a technique that should be in the repertoire of any synthetic chemist.

Finally, recognize that a variety of two (and even three) dimensional NMR techniques such as HETCOR and COSY can be utilized by the Inorganic chemist as easily as the Organic chemist. Discussion of these is beyond the scope of the current chapter.

Notable Nuclei

19F		Extremely facile technique - almost as easy as 1H NMR and can often be done without switching or tuning probes. Large distinctive couplings and a very broad and useful chemical shift range make subtle structural distinctions possible, expecially in with fluorophosphines (1,2JF-P is very diagnostic). Very useful in determining whether a fluorine-containing counterion is unassociated or coordinated.
Spin	1/2
Natural Abundance	100 %
Relative Sensitivity (H = 1.0)	0.83
Typical J values (absolute value)	2JH-F (gem) ~ 45 Hz 3JH-F (trans) ~ 17 Hz 3JH-F (cis) ~ 6 Hz 2JF-F (gem) ~ 300 Hz 3JF-F ~ 1 - 27 Hz

29Si		Although most chemists will not find a general need for this technique, as a solution technique it is only slightly more difficult than a 13C experiment in most cases (solid state Si NMR is a valuable tool to zeolite and solid state chemists). If you are having trouble getting a good signal to noise ratio, addition of a relaxation reagent (such as 1-2 crystals of Cr(acac)3) may help. The inductive effective of Si typically moves 1H NMR aliphatic resonances upfield to approximately 0 to 0.5 ppm, making assignment of Si-containing groups rather easy. In addition, both carbon and proton spectra display Si satellites comprising 4.7% of the signal intensity, although these can sometimes be a bit hard to distinguish from noise, spinning sidebands and impurities. To rule out spinning sidebands, measure the frequency difference between the central peak and the suspected satellite. If this is not equal to your spin rate and both satellites are equidistant from the central peak, then they probably are satellites. Try changing your spin rate significantly and then rescan - if these are spinning sidebands the distance should change. Note: the difference between the two satellite bands is the J value, not the distance between the central peak and satellite!
Spin	1/2
Natural Abundance	4.70 %
Relative Sensitivity (H = 1.0)	0.00784
Typical J values (absolute value)	Haven't had time to look these up. If anyone has some good references, let me know.

31P		Another very useful technique that is easier than 13C. Although the chemical shift range is not as diagnostic as with other nuclei, the magnitude of the X-P coupling constants is terrific for the assignment of structures. The Karplus angle relationship works quite well for organometallic phosphine complexes. For example, in the complex below, the 2JH-P is 153.5 Hz for the phosphine trans to the hydride, but only 19.8 Hz to the (chemically equivalent) cis phosphines. See Selnau, H. E.; Merola, J. S. Organometallics, 1993, 5, 1583-1591.
Spin	1/2
Natural Abundance	100 %
Relative Sensitivity (H = 1.0)	0.07
Typical J values (absolute value)	1JH-P ~ 200 Hz 2JH-P 2 - 20 Hz 1JP-P ~ 110 Hz 1JP-F 1200 - 1400 Hz 3JP-P ~ 1 - 27 Hz

103Rh		An utterly useless NMR technique because of the terrible sensitivity, but coupling to the I = 1/2 Rh nucleus is a very powerful tool, particularly in Rh-phosphine and carbonyl complexes. For example, in the 13C NMR spectrum of this linked Cp, tricarbonyl Rh dimer at 240K (the dimer undergoes fluxional bridge-terminal exchange at higher temperatures), the bridging carbonyl is observed at d232.53 and is a triplet with 1JRh-C = 46 Hz. The equivalent terminal carbonyls occur as a doublet at d190.18 with 1JRh-C = 84 Hz: See Bitterwolf, T. E., Gambaro, A., Gottardi, F., Valle G Organometallics, 1991, 6, 1416-1420.
Spin	1/2
Natural Abundance	100 %
Relative Sensitivity (H = 1.0)	0.000031
Typical J values (absolute value)	1JRh-C ~ 40 - 100 Hz 1JRh-C (Cp ring) ~ 4 Hz

I'll add to this table as time permits. Sn is on my To Do list. Suggestions and submissions are welcome (include references, please).

Suggested Reading

Introduction to NMR Spectroscopy
R. J. Abraham; J. Fischer; P. L. Loftus / Paperback / Published 1992 / 286 pages
A good introduction to the topic, including 2-D spectra.
Approximate Price: $59.95

Problems and Solutions in Organometallic Chemistry
Susan E. Kegley and Allan R. Pinhas / Paperback / Published 1986 / 322 pages
Study manual to accompany Collman. Excellent questions/answers at a hard-to-beat price! NMR section is pages 4 to 19.
Approximate Price: $26.50

NMR, NQR, EPR and Mössbauer Spectroscopy in Inorganic Chemistry
R.V. Parish / Hardcover / Published 1990
Great coverage, a great reference for the Inorganic chemist. Chapter 2 will be quite useful.
Approximate Price: (out of print, but you can try)

Advanced Applications of Nmr to Organometallic Chemistry (Physical Organometallic Chemistry, Vol 1)
M. Gielen; Rudolph Willem ; Bernd Wrackmeyer (Editors) / Paperback / Published 1996 / 396 pages
Twelve contemporary reviews. Includes 1D and 2D techniques as well as organotin, organolead, and tantalum spectra.
Approximate Price: $285.00 (ouch!)

Phosphorus-31 NMR Spectral Properties in Compound Characterization and Structural Analysis
Louis D. Quin; John G. Verkade (Editors) / Hardcover / Published 1994 / 472 pages.
Phosphorus lovers unite!
Approximate Price: $210.00

Handy NMR Links

I'll be adding these in the near future, honest.

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This page was last updated Tuesday, March 31, 2015
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