Active nuclei

Of the NMR-active nuclei around tluee-quarters have / > 1 so that the quadnipole interaction can affect their spectra. The quadnipole inter action can be significant relative to the Zeeman splitting. The splitting of the energy levels by the quadnipole interaction alone gives rise to pure nuclear quadnipole resonance (NQR) spectroscopy. This chapter will only deal with the case when the quadnipole interaction can be regarded as simply a perturbation of the Zeeman levels. 

The first step for any structure elucidation is the assignment of the frequencies (chemical shifts) of the protons and other NMR-active nuclei ( C, N). Although the frequencies of the nuclei in the magnetic field depend on the local electronic environment produced by the three-dimensional structure, a direct correlation to structure is very complicated. The application of chemical shift in structure calculation has been limited to final structure refinements, using empirical relations [14,15] for proton and chemical shifts and ab initio calculation for chemical shifts of certain residues [16]. 

Selective labeling of the initiator with 13C allow s substantial enhancement of the signals of the initiator residues relative to signals due to the backbone in l3C NMR spectra. Initiators labeled with or containing NMR active nuclei such as 19F or J P can also be applied. These methods are described in Section 3.5.4.2,... 

Magnetogyric Ratios of Some Important NMR-Active Nuclei... 

In principle it is possible with many modern spectrometers to carry out correlation experiments using any two NMR-active nuclei, and we shall demonstrate this below by discussing P,C and P,P correlations. 

NMR-active nuclei with spin > Vi (these include, as we mentioned previously, deuterium) have an electric quadrupole moment and are thus referred to as quadrupolar nuclei. 

These nuclei (and they form by far the majority of the NMR-active nuclei ) are subject to relaxation mechanisms which involve interactions with the quadrupole moment. The relaxation times Tj and T2 (T2 is a second relaxation variable called the spin-spin relaxation time) of such nuclei are very short, so that very broad NMR lines are normally observed. The relaxation times, and the linewidths, depend on the symmetry of the electronic environment. If the charge distribution is spherically symmetrical the lines are sharp, but if it is ellipsoidal they are broad. 

In principle it is possible (with a suitably configured spectrometer) to carry out correlation experiments between any pair of NMR-active nuclei. How-... 

Before we start, let us remind ourselves of the basic difference between the NMR-active nuclei. First there are the good nuclei, those with a spin of Vi. These lead to narrow lines with a linewidth of the order of 1 Hz (often considerably less, not often much more). Only two of these, by the way, are singleisotope elements phosphorus-31 and fluorine-19. As we shall see, the spin-Vi nuclei are those which are of more use in structure determination. 

Hydrogen has two NMR-active nuclei 1H, always known as the proton (thus proton NMR ), making up 99.98%, and 2H, normally referred to as D for deuterium. 

The measurement procedure is known as the pulse sequence, and always starts with a delay prior to switching on the irradiation pulse. The irradiation pulse only lasts a few microseconds, and its length determines its power. The NMR-active nuclei (here protons) absorb energy from the pulse, generating a signal. 

When taken together with measurements on the other NMR active nuclei, the spectra yield a complete detailed description of these systems. 

Nitrogen-14, with its natural abundance of 99.6%, is one of the most ubiquitous and, until recently, least studied NMR-active nuclei. Due to the integer spin number (/ = 1), its single-quantum transitions are affected by first-order quadrupolar broadening, which in most materials is on the order of a few megahertz. A new class of 2D HETCOR protocols has been recently developed, which makes it possible to indirectly observe well-resolved 14N sites via their spin-1/2 neighbors and obtain the related parameters of the quadrupolar tensors. 

As an example of the measurement of cross-correlated relaxation between CSA and dipolar couplings, we choose the J-resolved constant time experiment [30] (Fig. 7.26 a) that measures the cross-correlated relaxation of 1H,13C-dipolar coupling and 31P-chemical shift anisotropy to determine the phosphodiester backbone angles a and in RNA. Since 31P is not bound to NMR-active nuclei, NOE information for the backbone of RNA is sparse, and vicinal scalar coupling constants cannot be exploited. The cross-correlated relaxation rates can be obtained from the relative scaling (shown schematically in Fig. 7.19d) of the two submultiplet intensities derived from an H-coupled constant time spectrum of 13C,31P double- and zero-quantum coherence [DQC (double-quantum coherence) and ZQC (zero-quantum coherence), respectively]. These traces are shown in Fig. 7.26c. The desired cross-correlated relaxation rate can be extracted from the intensities of the cross peaks according to ... 

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. 

---