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:
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.
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, JHH versus JHF.
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 JCH. 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.
| 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. |
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|---|---|---|---|---|---|---|---|
| Spin | 1/2 | ||||||
| Natural Abundance | 100 % | ||||||
| Relative Sensitivity (H = 1.0) | 0.83 | ||||||
| Typical J values (absolute value) |
| ||||||
| 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. |
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|---|---|---|---|---|---|---|---|
| Spin | 1/2 | ||||||
| Natural Abundance | 100 % | ||||||
| Relative Sensitivity (H = 1.0) | 0.07 | ||||||
| Typical J values (absolute value) |
| ||||||
| 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) |
|
I'll try to add to this table as time permits. Sn is on my To Do list. Suggestions and submissions are welcome (include references, please).
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.
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. Out of print, though.
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. Out of print, though.
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.
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!
I will add a list here someday.
