Dilute-spin species

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-... 

With the advent of high resolution techniques in solids it became of interest to measure site-specific relaxation times. For the typical double-resonance type of experiments, there are no real problems in measuring T s under MAR conditions, except possibly the often very large values. When it comes to Tj p, however, for the dilute spin species, then there can be difficulties in interpretation. Suppose we have created some spin-locked magnetisation say for in a solid, probably via cross polarisation. To measure T p we would switch off the... 

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.),... 

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. 

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. 

For chemically or isotopically dilute spins, e.g. C, the signal from the dilute species can be enhanced by transferring nuclear polarization from a species of high abundance, e.g. protons, to the dilute spedes . This requires that the two species are coupled to each other. The technique used for this polarization transfer is based on spin locking of the abundant species and on the spin temperature concept. 

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 relation between the spin density distribution in the products of electron transfer to organic molecules and the behavior of these products in further reactions is considered here. This relation is important, but has not been investigated much so far. It would be especially useful to know the reason for the particular sequence of chemical changes in the groups present simultaneously iu the molecule. Let us compare the results of the oue-electrou aud multielectron reductions of 4,4 -dinitro dibenzoyl. The one-electron reduction leads to a paramagnetic species whose ESR spectra depend on temperature and concentration. As shown (Maruyama and Otsuki 1968), such electrou trausfer products can exist in the monomeric or dimeric form. The monomer is present at a high dilution. The dimer forms at an increased concentration or at a low temperature (see Scheme 3.50). 

In very dilute metal solutions where dissociation is considered complete, the alkali metal ion is considered to be a normal solvated ion, as in electrolytic solutions, and it is generally conceded that the large volume change is to be ascribed chiefly to the solvated electron. As the concentration is increased it is quite obvious from conductance and magnetic data that metal ions interact with electrons to form some sort of an ion pair and also that electrons couple to form spin-paired species. The manner in which these species form is not entirely clear, nor is their... 

The main source of Lorentzian broadening is dipole-dipole interaction between paramagnetic species, often termed spin-spin broadening . This will arise if solutions are too concentrated, and can in general be avoided by suitable dilution. Hence most observed lines are nearly Gaussian in shape. 

It might appear that our concern about a fast relaxing high-spin Fem species was unwarranted, as dilute high-spin Fem complexes usually relax slowly at 4.2 K. However, it is well known that [Feni(H20)6]3 + can aggregate upon (slow) freezing... 

Autoreduction of Fe(III) Porphyrins. Oxidation of Piperidine. The autoreduction of TPPFeCl with neat piperidine is rapid, however the reaction rate can be decreased by dilution with CDC13 or DMSO. The NMR spectrum of TPPFeCl in CDC13, on addition of piperidine, shows resonances consistent with the presence of high spin Fe(III) and low spin TPPFen(Pip)2. A low spin TPPFem(Pip)2 species was not observed. It is likely that the latter complex rapidly reduces at room temperature. 

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