NMR sensitive nuclei

The most common nuclei examined by NMR are and 13C, as these are the NMR sensitive nuclei of the most abundant elements in organic materials. H represents over 99% of all hydrogen atoms, while 13C is only just over 1% of all carbon atoms further, H is much more sensitive than 1 C on an equal nuclei basis. Until fairly recently, instruments did not have sufficient sensitivity for routine 13C NMR, and H was the only practical technique. Most of the time it is solutions that are characterized by NMR, although 13C NMR is possible for some solids, but at substantially lower resolution than for solutions. 

Although limited by sensitivity, chemical reaction monitoring via less sensitive nuclei (such as 13C) has also been reported. In 1987 Albert et al. monitored the electrochemical reaction of 2,4,6-tri-t-butylphenol by continuous flow 13C NMR [4]. More recently, Hunger and Horvath studied the conversion of vapor propan-2-ol (13C labeled) on zeolites using 1H and 13C in situ magic angle spinning (MAS) NMR spectroscopy under continuous-flow conditions [15]. 

Because the sensitivity of NMR is the highest for protons compared to other nuclei, all examples of quantitation work described in this chapter are based on proton NMR data. The signals from other NMR active nuclei such as 19F or 13C may also be used for quantitation. The quantification of TFA using 19F NMR is a good example. However, except for 19F, the sensitivities and detection limits are usually compromised in these measurements because nuclei other than H and 19F typically have a lower natural abundance and a lower magnetogyric ratio. 

When using NMR as a detector for online separations, additional consideration must be given to how the sensitivity is affected by the movement of nuclei past the detector cell. Aside from the physical hardware setup, the chromatographic and spectroscopic parameters also play a role in the quality of the resulting data. Flow rate, solvent composition, and residence and acquisition times can be optimized to provide optimal results. NMR sensitivity and chromatographic resolution tend to have an inverse relationship with respect to online LC-NMR experiments. By slowing the flow rate, more scans can be acquired for a particular analyte in the flow cell, but... 

Turning now to lithium, we have two nuclides available for NMR measurements Li and Li. Both are quadrupolar nuclei with spin quantum number / of 1 and 3/2, respectively. The natural abundance of Li (92.6%) provides enough NMR sensitivity for direct measurements, but also Li (7.4%) can easily be observed without enrichment. However, isotopic enrichment poses no practical problem and is advantageous if sensitivity is important, as for measurements of spin-spin coupling constants in solution and of quadrupole coupling constants in the sohd state. 

This pulsed wave process provides simultaneous information on all the frequencies present. The generalisation of this process, which can be computerised and allows the study of less sensitive nuclei such as 13C, has led to major developments in NMR. 

Better chromatographic peak performance is obtained with an NMR detection volume of 60 pi, although NMR sensitivity values suffer from the low amount of nuclei in the detection cell. Thus, despite its degraded chromatographic performance the 120 pi flow cell seems to be a good compromise... 

Magnetic resonance imaging (MRI) is sensitive to any NMR-active nuclei, such as protons. This allows one to distinguish different chemical environments in which these nuclei find themselves, including oil versus water. Figure 2.15 shows a NMR reconstructed image slice taken through the centre of a complex, oil-continuous emulsion sample [97]. Additional examples can be found in [98],... 

An important objective in materials science is the establishment of relationships between the microscopic structure or molecular dynamics and the resulting macroscopic properties. Once established, this knowledge then allows the design of improved materials. Thus, the availability of powerful analytical tools such as nuclear magnetic resonance (NMR) spectroscopy [1-6] is one of the key issues in polymer science. Its unique chemical selectivity and high flexibility allows one to study structure, chain conformation and molecular dynamics in much detail and depth. NMR in its different variants provides information from the molecular to the macroscopic length scale and on molecular motions from the 1 Hz to 1010 Hz. It can be applied to crystalline as well as to amorphous samples which is of particular importance for the study of polymers. Moreover, NMR can be conveniently applied to polymers since they contain predominantly nuclei that are NMR sensitive such as H and 13C. 

The most common probe head is a switchable probe head, which can be used to observe H and all NMR-active nuclei from the low-frequency limit up to the frequency of 31P. The proton coil can be tuned for the observation of 19F. The switch-able probe head is designed for either direct or inverse observation. The direct observation probe head is most sensitive for 1-D experiments on 13C and 31P. The inverse probe head in turn is most sensitive for the direct observation of H and indirect detection, for example of 31P, in 2-D experiments, taking advantage of polarization-transfer phenomena. 

Solid-state NMR is one of the most powerful spectroscopic techniques for the characterisation of molecular structures and dynamics.1 This is because NMR parameters are highly sensitive to local chemical environments and molecular properties. One advantage of solid-state NMR is that it enables dealing with quadrupolar nuclei, which most of the NMR-accessible nuclei are in the periodic table. Moreover, it provides an opportunity to obtain information regarding the orientation dependence of the fundamental NMR parameters. In principle, such NMR parameters are expressed by second-rank tensors and it is the anisotropy that is capable of yielding more detailed information concerning the molecular properties. 

The ability to manipulate spins in two-dimensional experiments and to transfer magnetization between spins has made it possible to use a sensitive nucleus (primarily H) to measure the spectral features of less sensitive nuclei, such as 13C and 15N. Several methods are commonly used, but each begins with a H pulse sequence, often resembling the one in INEPT (Section 9.7). As in INEPT, a combination of H and X pulses transfers polarization to the X spin system. In some instances further transfers are made to another spin system (Y), then back through X to H, where the signal is detected. Thus, the large polarization of the proton is used as the basis for the experiment, and the high sensitivity of H NMR is exploited for detection. Such indirect detection methods use two-, three-, and sometimes four-dimensional NMR. 

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