NMR Spectrum

The Nuclear Magnetic Resonance spectrum proves to be of great utility in structure elucidation because the properties it displays can be related to the chemist's perception of molecular structure. The chemical shift of a particular nucleus can be correlated with its chemical environment, the scalar coupling (or J-coupling) indicates an indirect interaction between individual nuclei, mediated by electrons in a chemical bond, and, under suitable conditions, the area of a resonance is related to the number of nuclei giving rise to it. Thus, when we speak of "assigning a spectrum" we are really trying to identify a chemical structure that is consistent with the spectrum; we may have a fairly good idea of what this structure is likely to be, or we may have very little. There are a number of techniques in common use that help with this aim.

The 1H nucleus is the most commonly observed nucleus in NMR spectroscopy. Hydrogen is found throughout most organic molecules and, fortunately for chemists, the proton has high intrinsic sensitivity as well as being almost 100% abundant in nature, all of which make it a favourable nucleus to observe. The proton spectrum contains a wealth of chemical shift and coupling information and is the starting point for most structure determinations.

In addition to these data, the area of a resonance, usually presented as its relative integral, relates to the number of protons giving rise to the signal, giving further information as to a possible structural fragment. However, for most routine acquisitions, the accuracy of such integrals is not high and may commonly show errors of 10% or more. This level of accuracy is usually sufficient if you wish to decide whether it is one or two protons that are giving rise to a particular resonance, but are unsuitable if you wish to get an accurate determination of, say, the relative proportion of two isomers in a mixture.


Homonuclear decoupling in 1H NMR
The appearance of multiplet fine structure on NMR resonances is due to the presence of scalar coupling with another nucleus. In proton spectroscopy, such coupling will usually occur over two or three bonds (geminal or vicinal coupling respectively) and its presence provides direct evidence of a bond within a structure; it indicates connectivity. If the connectivity between all atoms in a structure is known, the gross structure is, therefore, defined. Coupling partners can often be identified by direct analysis of multiplet fine structure; if proton A shows a coupling to proton B of 7 Hz, then the coupling from B to A is also 7 Hz. However, it is often not
possible to identify coupling partners in this way; the multiplet may be too complex to determine all coupling constants; one, or both, protons may be hidden under another resonance or many multiplets may display a coupling similar in magnitude to the one of interest. Homonuclear decoupling (which belongs to a class of
experiments known as double-resonance experiments) offers a simple and effective means to identify coupled protons. The idea is to selectively saturate one multiplet in the spectrum with a radio-frequency during acquisition. This causes a loss of all couplings with the saturated proton and, hence, the multiplet structure of its partners will change. The example below illustrates this point. The spectrum of compound [1] shows a double-doublet at 5.44 ppm due to H5 and its coupling to the C6 protons, which resonate in the 2-3 ppm region. The multiplet structures of the resonances in this area are quite complex and do not lend themselves to direct
analysis. A simple homonuclear decoupling experiment with saturation of the C5 proton unambiguously identifies the C6 protons by removal of the vicinal 5-6 couplings.

http://www.chem.ox.ac.uk/spectroscopy/nmr/nmrfacility.htm


NMR offers the perfect accompaniment to the other major structural determination technique, x-ray crystallography. As well as 3D structure determination NMR can be used to investigate solution dynamics and interaction interfaces. Although still limited to some extent by protein size it is now more and more plausible to look at large proteins using higher field magnets and selective labelling techniques. NMR can be used to investigate proteins, nucleic acids, carbohydrates, lipids and metabolites.

950Mhz Magnet

We currently house a 22.3 T (950 MHz 1H Larmor frequency) superconducting magnet. Delivery and installation of the magnet from Oxford Instruments took place in October 2005 and final field was reached in December. We have built and continue to refine an inverse 1H/15N/13C triple resonance probehead with actively shielded triple axis gradients; and a multichannel console capable of executing essentially all the available pulse sequences.

http://www2.bioch.ox.ac.uk/~rrnmr/NMR-life_2007.pdf