4 Nuclear Magnetic Resonance Spectroscopy
Electrons, protons, and neutrons all have spin quantum numbers of ½ and can therefore interact with magnetic fields externally imposed or arising from nearby particles. Nuclear magnetic resonance (NMR) occurs when NMR‐active nuclei in a magnetic field absorb and re‐emit electromagnetic radiation at a specific resonance frequency. This frequency depends on the strength of the magnetic field and the magnetic properties of the nuclei of interest. The sample (~1–500 mg) is perturbed while in the magnetic field by a short radiofrequency (RF) pulse (usually 90° to the applied magnetic field), which releases the energy of the resonance transition measured as a free induction decay (FID) curve in the time domain. This curve is then Fourier transformed to the frequency domain to obtain the NMR spectrum. Because the FID is very weak, usually multiple curves are collected and time‐averaged. Also of importance is the time taken for the excited spin states to return to their equilibrium distributions; the spin lattice (T1) or longitudinal magnetic relaxation, as well as the time for the FID to reach 1/e of its initial amplitude, the spin–spin (T2) or transverse relaxation. The line width of an NMR signal is determined by T2 (a long T2 means sharper lines) and the maximum repetition rate during acquisition of an NMR signal is governed by T1 (short T1 means a spectrum can be acquired in less time). Additional information about the perturbed system can be obtained when one uses multiple and often complex RF pulse sequences and investigates the time and frequency dependence of the relaxation times. The reader should see [10] and references therein for a much more detailed description of solid‐state NMR.
What makes this technique important is that the measured spin energies are affected by interactions with other electrons and nuclei in the sample and, consequently, by the local chemical environment around the element of interest. Furthermore, one can also probe the dynamics of these interactions on timescales of seconds to nanoseconds that makes its application to high‐temperature studies particularly useful.
Although NMR spectroscopy is used to investigate site‐specific information on elements with nonzero spin quantum numbers, it is most commonly carried out for elements whose numbers are ½ or multiples of ½ such as 3/2, 5/2, 7/2, and 9/2. Little information can usually be drawn from the very broad NMR spectra of solids that result from the strong anisotropy of the NMR frequencies caused by variations in electron distributions. To reduce this broadening, the sample may be spun at an angle with the applied magnetic field resulting in what is termed magic angle spinning (MAS) NMR. Although MAS does not remove all the broadening effects, one can further reduce them by using multiple quantum (MQ) MAS NMR or (MQMAS) NMR for quadrupolar nuclei (those with spin quantum numbers > ½). Triple quantum (3Q) is most common but 5 and 7Q can also be done. In the two‐dimensional spectra recorded, one dimension is the equivalent of the MAS NMR spectrum and the other is the high‐resolution isotropic dimension, which is thus free of anisotropic broadening but has peak positions shifted from the isotropic chemical shift as a result of the quadrupolar broadening (cf. Figure 6). A common use of MAS NMR spectra in glass science is to discriminate the coordination environment of a number of nuclides such as four‐, five‐, and sixfold Al, as illustrated in Figure 6a where the existence of distinct sites are revealed in Al‐bearing sodium silicate glasses quenched from high pressure (6 GPa) [10].
Figure 6 Structure determinations of sodium silicate glasses from 27Al and 17O 3Q MAS NMR spectra. (a) Aluminum coordination in samples quenches from 6 GPa. (b) Bridging oxygen linkages in the network.
Source: Reproduced with permission from [10].
Other factors can also broaden the signal such as unpaired electrons, which is why NMR is of limited usefulness in the presence of transition metals or magnetic rare earth elements in glasses although progress is being made to improve the situation. On the other hand, NMR techniques have diversified so much in recent years to tackle such a wide variety of problems that it is impossible to summarize here these advances, or even to mention the technical terminology used by NMR experts. Rather, the reader is referred to the detailed discussion of NMR methods and applications given by Stebbins and Xue [10].
Some of the common elements studied in glasses are 31P, 29Si, 27Al, 23Na, 19F, 11B, 17O, 1H, and 2H. The abundance of the NMR‐active nuclide is important for determining the applicability of the technique. In some cases it is necessary to enrich the sample (usually powdered, although glass chips can also be used) with the appropriate nuclide if its natural abundance is low (e.g. 17O). With appropriate nuclides, however, the sensitivity of NMR can be as low as 1% in crystalline materials and informative spectra can be collected for glasses and crystals with concentrations of the NMR nuclide well below 1%.
Typically, NMR spectroscopy provides element‐specific information on short‐ and intermediate‐range structure. The types of short‐range information include coordination, Q speciation (Si tetrahedra with 1–4 bridging oxygens [BO] attached), and information on bond angles such as Si─O─Si. Intermediate‐range information primarily deals with the type and nature of second neighbors, involving the connectivity of next‐nearest neighbors and in some cases the connectivity of species out to fourth or higher nearest neighbors. In addition, NMR can provide dynamical information on timescales of ~0.1 to 1 ns and thus can investigate to some extent chemical kinetics and crystallization processes. As found with other spectroscopic techniques, NMR spectra are inherently broader for glasses than for crystals because of their intrinsically disordered nature. Double‐resonance NMR methods can provide unique insights into distances between and interactions among multiple NMR‐active nuclides, quite unique for NMR relative to other spectroscopies.
Spectra themselves are usual plotted as intensity versus a relative frequency scale plotted as parts per million (ppm). The frequency is reported relative to a standard and normalized by the excitation frequency:
(4)
The range of frequencies observed depends on the element being studied and sample composition. It is often referred to as the “chemical shift” although technically use of this term should be restricted to frequency shifts only affected by the chemical shift interaction. For 29Si in glasses and minerals, it is usually in the range of −60 to −200 ppm. The chemical shift is sensitive to the coordination about the cation, higher coordinations giving rise to lower chemical shifts. Because the coordination is also strongly correlated with the cation–oxygen bond distance, the chemical shift generally decreases with increasing bond length. At constant coordination the changes in chemical shift are much more subtle and may be partially overlapping. The chemical shifts of Q n species (n = number of bridging oxygens) used to characterize SiO4 tetrahedra, for instance, change by ~ +10 ppm with every decrease in n, and by ~+5 ppm for every silicon nearest neighbor that is replaced by Al. In addition, the chemical shift depends on Si─O─Si and Si─O─Al angles, becoming more negative as the angles increase.
One of the most exciting nuclei has recently been 17O NMR although the wide range of possible bonding environments for oxygen complicates interpretation of the spectra. However, 17O NMR can be used to identify the cation neighbors to the non‐bridging oxygens (NBOs), proportion of NBO vs BO, the coordination of the oxygens, and the nature