Abundant-spin species

The "decrease of the spin temperature means an increase of population difference between the upper and lower energy spin states and consequently an increased sensitivity of the NMR experiment. From Equation (25), the temperature of dilute spins has been lowered by a factor 7x/y1 h, that is, V4 when X = 13C. This means an increased sensitivity of the FID resonance experiment equal to about 4 for the 13C nuclei. Because the X signal is created from the magnetization of dilute nuclei, the repetition time of NMR experiment depends on the spin-lattice relaxation time of the abundant spin species, protons, which is usually much shorter than the spin-lattice relaxation times of the dilute nuclei. This, a further advantage of cross polarization, delay between two scans can be very short, even in the order of few tens of milliseconds. 

Cross polarization is an rf-mediated transfer of magnetization from an abundant spin species /, most often protons, to a rare spin species S, for example 13C, 31P,... 

Although dipolar decoupling removes dipolar interactions, it does not remove CSA, nor does it permit observation of abundant spin species such as protons, since observation and decoupling cannot be done at the same time. Different, more selective means are needed to remove these interactions. 

Figure 3.5. (A) JH NMR spectrum of the Brooksville fulvic acid (BFA) dissolved in d6-DMSO and (B) HR-MAS NMR spectrum of the BFA-clay complex swollen in g 6-DMSO. Inset shows that lower abundance aromatic species are present in the spectrum in part B. Reprinted from Simpson, A. I, Simpson, M. I, Kingery, W. L., Lefebvre, B. A., Moser, A., Williams, A. I, Kvasha, M., and Kelleher, B. R (2006). The application of 1H high-resolution magic-angle spinning NMR for the study of clay-organic associations in natural and synthetic complexes. Langmuir 22,4498 1503, with permission from the American Chemical Society.

The benefits are primarily an intensity enhancement of the dilute spin signal by a factor of 7 abundant/7 dilute and a reduction of the recycle time between experiments since the ratedetermining relaxation time is now that of the abundant species, rather than that of the rare spins. Usually, the relaxation of the abundant spins are much faster than the dilute spin relaxation (13). The cross polarization experiment may thus be repeated with much shorter intervals, leading to a further increase of the signal-to-noise ratio of the rare spin NMR spectmm within a given period of time. The effectiveness of the cross polarization experiment depends on the strength of the dipolar interaction between the abundant and rare spin systems, i.e. on the distance between the actual nuclei (proportional to r, where r is the distance between the abundant and the dilute nuclei) (11). The efficiency of magnetization transfer decreases extremely fast as the distance between the abundant and rare spins increase. One should emphasis, that under normal conditions, the CP experiment does not provide quantitative results. Finally, the cross polarization sequence does not influence the line width. 

Owing to the chemical composition of silica, two nuclei are usually analyzed. They are H, as abundant spin and Si as a rare one. Numerous experiments have been done by cross polarization (CP) between proton and silicon nuclei and magic angle spinning (MAS). As it is a classical method analyzed elsewhere [2], it is not described here in details. It is only necessary to notice that the silicon signal is enhanced if the distance to proton is short enough to provide an efficient dipolar interaction. So this means that the method allows to distinguish the different types of hydroxylated silicon species, but also... 

Carbon-13 is a dilute-spin species in the sense that it is unlikely that more than one nucleus will be found in any given small molecule (provided the sample has not been enriched with that isotope the natural abundance of C is only 1.1 per cent). Even in large molecules, although more than one C nucleus may be present, it is unlikely that they will be close enough to give an observable splitting. Hence, it is not normally necessary to take into account spin-... 

For a typical orgaalc molecule the ratio (Mo( H)/Mo( C)) may be of the order of 2000. In addition the spin-lattice relaxation times of the protons will generally be shorter than the corresponding values. Both of these factors make it sensible to generate the dilute-spin ( C) signal from the abundant spins ( H). In order to achieve this cross polarisation process, it is necessary simultaneously to irradiate both spin-species with resonant or near-resonant radiation, the amplitudes of these two rf fields satisfying a particular relationship which is dealt with below. 

The spin properties of the magnetic nuclides of Group IV elements are given in Table 1. All the nuclides (with the possible exception of Pb) may be classified as dilute spin species by virtue of their low natural abundance. Thus, when spectra are obtained under conditions of proton noise decoupling, each chemically distinct environment to the nucleus in question will yield a single line (in the absence of P, etc.),... 

All have zero nuclear spin except (33.8% abundance) which has a nuclear spin quantum number this isotope finds much use in nmr spectroscopy both via direct observation of the Pt resonance and even more by the observation of Pt satellites . Thus, a given nucleus coupled to Pt will be split into a doublet symmetrically placed about the central unsplit resonance arising from those species containing any of the other 5 isotopes of Pt. The relative intensity of the three resonances will be (i X 33.8) 66.2 ( x 33.8), i.e. 1 4 1. 

It should be realized that unlike the study of equilibrium thermodynamics for which a model is often mapped onto Ising system, elementary mechanism of atomic motion plays a deterministic role in the kinetic study. In an actual alloy system, diffusion of an atomic species is mainly driven by vacancy mechanism. The incorporation of the vacancy mechanism into PPM formalism, however, is not readily achieved, since the abundant freedom of microscopic path of atomic movement demands intractable number of variational parameters. The present study is, therefore, limited to a simple spin kinetics, known as Glauber dynamics [14] for which flipping events at fixed lattice points drive the phase transition. Hence, the present study for a spin system is regarded as a precursor to an alloy kinetics. The limitation of the model is critically examined and pointed out in the subsequent sections. 

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