While it is probably true that the majority of experiments are solution-state, proton (1H) NMR, there are many other types of NMR experiments available. First, almost every chemical element has at least one NMR-active isotope. Isotopes are categorized by their ground state nuclear spin quantum numbers, I, which are always n/2 where n is an integer. I is equal to zero for isotopes that have even atomic numbers and even mass numbers (e.g. 12C) and these nuclei do not have NMR spectra; isotopes with odd atomic number but even mass numbers have n even (e.g. 14N), while an isotope with odd atomic number and odd mass number has n odd (e.g. 9Be, 35Cl). The most commonly studied group is the I = 1/2 nuclei, particularly 1H and 13C.
Other NMR-useful isotopes include 19F, 31P, 57Fe, 119Sn, 199Hg (all I = 1/2); 7Li, 33S, 79Br, 131Xe, and 177Hf (I = 3/2); 17O, 27Al, 127I (I = 5/2); 10B (I = 3); 43Ca, 123Sb (I = 7/2); 73Ge, 93Nb (I = 9/2). Nuclei with I > 1/2 are called quadrupolar.
The chemical shifts of the resonances (peaks) in an NMR spectrum are normally reported relative to a reference compound, in units of ppm. The chemical shift depends on the shielding constant which, in turn, depends on the electron density (i.e. the other atoms and their arrangement) around the nucleus being studied. Each chemically distinct nucleus in the sample will produce a unique resonance, and the pattern of these gives detailed structural information. Most NMRs today are Fourier transform instruments, in which a radiofrequency pulse of a few microseconds is applied to the sample followed by data collection as the nuclei relax to their ground states. This is repeated many times to average out the noise. The voltages induced in the receiver coil by the relaxing nuclei produce the free induction decay spectrum and Fourier transformation of this FID gives the more readable frequency domain spectrum.
If two nuclei are near each other (i.e. usually three chemical bonds apart or less), they may couple, which causes the resonances of each nucleus to be split. The spectrum of a spin 1/2 nucleus will be split into a (q + 1) multiplet by q equivalent spin 1/2 nuclei. The relative intensities of the multiplet for equivalent spin 1/2 nuclei follow the pattern of Pascal's triangle. In a system AtBw, where A and B are spin 1/2 nuclei and t and w are the numbers of each nuclei, the spectrum of A will be split into a (w + 1) multiplet and the spectrum of B will be split into a (t + 1) multiplet. Therefore, observation of both spectra enables one to count how many of each nuclei are present in the system.
Coupling to quadrupolar nuclei may also occur, in which case a spin of n/2 causes splitting into n +1 lines of equal intensity.
In addition to the normal, single resonance, experiments, many other NMR techniques have been developed. By applying a second radiofrequency at right angles to the magnetic field, the observed spectrum may be perturbed, from which more information about the system may be gathered indirectly. Internuclear double resonance (INDOR) is especially useful when a multinuclear NMR instrument is unavailable, because it gives details about one type of nucleus while irradiating only a different type. Other techniques include spin-tickling, spin decoupling (reduces the complexity of the spectrum by removing the effects of the irradiated nuclei), triple resonance, and multi pulse methods. Two dimensional NMR spectra are also obtainable. Correlated spectroscopy (COSY) gives details about couplings between nuclei of a single isotope. Heteronuclear correlation (HETCOR or HCOR) spectroscopy represents the couplings of two nuclei, with each axis corresponding to chemical shifts of one of the nuclei. The NOESY (Nuclear Overhauser Effect spectroscopy) and HOESY (heteronuclear NOESY) techniques are two others.
Besides spectra of liquid compounds and solutions, NMR can be used to gain information about the structures of gaseous molecules, liquid crystals and solids. Solid samples are associated with additional difficulties, however, due to the immobility of the nuclei. First, long range couplings produce very broad resonances. Since chemical shifts depend on the orientation of the molecule to the magnetic field, anisotropy in solids also produces broadened lines. Third, long nuclear relaxation times decrease the signal-to-noise ratio. In a liquid, the random orientation and movement of molecules with respect to each other cancels out most long range couplings and chemical shift anisotropy; relaxation times are generally shorter, too.
One way to reduce the linewidths of a solid sample is by using magic angle sample spinning (MASS or MAS). The effects of chemical shift anisotropy are averaged out by rotating the sample about an axis that is tilted at an angle of 54.7 ° to the magnetic field. (The equation for line broadening due to chemical shift anisotropy includes a term (3cos2θ - 1) which vanishes when θ = 54.7 °.) In some samples, especially those with heavy elements, the mechanical strength of the sample container does not allow the sample to be rotated fast enough to completely eliminate line broadening caused by chemical shift anisotropy. In this case, each resonance in the spectrum will be replaced by a central line and several spinning sidebands.
MAS can also remove long range couplings by averaging them to zero. The rotation rate must be greater than the linewidth or the couplings will not be completely eliminated.
The problem of long relaxation times can only be overcome for some nuclei using the technique of cross-polarization (CP). Other, more complicated techniques are necessary in most cases.
References:
Ebsworth, E. A.; Rankin, D. W. H.; Cradock, S. Structural Methods in Inorganic Chemistry, 2nd ed.; CRC Press: Boca Raton, FL, USA; 1991; Chapter 2.
Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis 4th ed.; Saunders: Fort Worth, TX, USA; 1992; Chapter 14.
Solomons, T. W. G. Organic Chemistry, 5th ed.; John Wiley and Sons: New York, NY, USA; 1992; Chapter 14.