Reducing Spin-Lattice Relaxation Times

The drawbacks already described must be taken into account in the acquisition process of SFC-NMR spectra. One is the increase in the spin-lattice relaxation times T due to the reduced viscosity of the supercritical mobile phase. The other is the fact that with increasing pressure the susceptibility of the supercritical fluid changes, thus causing a high-field shift of the NMR signals. For the latter problem, up until now only the number of transients can be reduced to gain acceptable spectra, while for the problem of the long spin-lattice relax- 

Transmitter and receiver coil with 120 gl NMR-detection cell 

6-Tetramethylpiperidin-1-oxyl radical (TEMPO) immobilized on aminopropyl-modified silica gel 

This is shown in Table 7.2.1, which compares the relative integrals of the compounds with and without the influence of the immobilized free radicals (IFRs). The ratio between the aliphatic and aromatic parts of the molecule corresponds much more with the theoretical values when the spectrum is recorded under the influence of the IFRs. One drawback here is the behaviour of the immobilized free radicals, which act like a separation phase, and so the selectivity, achieved by a conventional separation column, is destroyed by this 

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Experimental data on nitrogen obtained from spin-lattice relaxation time (Ti) in [71] also show that tj is monotonically reduced with condensation. Furthermore, when a gas turns into a liquid or when a liquid changes to the solid state, no breaks occur (Fig. 1.17). The change in density within the temperature interval under analysis is also shown in Fig. 1.17 for comparison. It cannot be ruled out that condensation of the medium results in increase in rotational relaxation rate primarily due to decrease in free volume. In the rigid sphere model used in [72] for nitrogen, this phenomenon is taken into account by introducing the factor g(ri) into the angular momentum relaxation rate... 

Recently, Lipton et al. [25] have used zinc-67 NMR to investigate [Zn(HB(3,5-(CH3)2pz)3)2] complexes which have been doped with traces of paramagnetic [Fe(HB(3,4,5-(CH3)3pz)3)2]. The low-temperature Boltzmann enhanced cross polarization between XH and 67Zn has shown that the paramagnetic iron(II) dopant reduces the proton spin-lattice relaxation time, Tj, of the zinc complexes without changing the proton spin-lattice relaxation time in the Tip rotating time frame. This approach and the resulting structural information has proven very useful in the study of various four-coordinate and six-coordinate zinc(II) poly(pyrazolyl)borate complexes that are useful as enzymatic models. 

This technique involves transfer of polarization from one NMR active nucleus to another [166-168]. Traditionally cross polarization (CP) was employed to transfer polarization from a more abundant nucleus (1) to a less abundant nucleus (S) for two reasons to enhance the signal intensity and to reduce the time needed to acquire spectrum of the less abundant nuclei [168]. Thus CP relies on the magnetization of I nuclei which is large compared to S nuclei. The short spin-lattice relaxation time of the most abundant nuclei (usually proton) compared to the long spin-lattice relaxation time of the less abundant nuclei, allows faster signal averaging (e.g., Si or C). CP is not quantitative as the intensity of S nuclei closer to 1 nuclei are selectively enhanced. Nowadays CP has been extended to other pairs of... 

In a net effect, the pulse repetition times in flowing systems can be reduced to the decrease in the apparent spin-lattice relaxation times Tifiow, whereas at a given detection volume an increase in flow rate leads to an increase in the signal half-width W due to the decrease of r2fi0w... 

One of the mayor drawbacks is that only volatile and temperature-resistant compounds can be investigated. Gases are magnetized faster than liquids, because they have shorter spin-lattice relaxation times (T ), due to an effective spin rotation mechanism. Therefore, pulse repetition times in flow experiments can be in the range of 1 s and some dozen transients can be accumulated per separated peak. Nevertheless, the sample amounts used nowadays in capillary GC are far from the detection limit of NMR spectroscopy, and therefore the sensitivity is low or insufficient, due to the small number of gas molecules per volume at atmospheric pressure in the NMR flow cell. In addition, high-boiling components (> 100 °C) are not easy to handle in NMR flow probes and can condense on colder parts of the apparatus, thus reducing their sensitivity in NMR spectroscopy. 

Figure 53. Mean 2H spin-lattice relaxation times for type A and type B glass formers as a function of reduced reciprocal temperature in a double logarithmic plot. (Compiled from Refs. 177, 178, 323, 422, and 423.)...

Finally, we present the mean spin-lattice relaxation times (T ) of several glass formers as a function of the reduced reciprocal temperature TgjT in Fig. 13.100,101 Since the relaxation becomes non-exponential near Ts, the time constant (T ) as given by the integral over the normalized spin-lattice relaxation function is plotted. Well above Tg, a minimum associated with the a-process is observed for both type A and B glass formers. When approaching Tg, the relaxation of type A glass... 

Fig. 13. Mean2H spin-lattice relaxation time (ft) for several glass formers as a function of the reduced reciprocal temperature Tg/T (compiled from Refs. 68, 69, 100, 146). It is evident that types A and B glass formers can be distinguished based on their relaxation behavior however, see comment in Section 3.2.4.

There is considerable evidence from a variety of techniques for a dendrimer model composed of a relatively soft or spongy interior surrounded by a considerably harder outer molecular surface, the so-called dendritic box. C-NMR measurements of spin lattice relaxation times of specifically tagged PAMAM dendrimers have shown considerably reduced mobility in the outer surface groups relative to the interior segments. Meltzer et performed C-... 

The advantage of the MW-4 sequence in intensity is preserved at times T2e that allows to reduce considerably the detection time of the substances with the spin-lattice relaxation time Ti exceeding several seconds (TNT, PETN, HMX, black powder, etc.). 

Titanium h.f.s. are resolved (500) at 77°K in Ti + doped Al(acac)8. The symmetry here is and the large trigonal distortion (S = 2000-4000 cm ) increases the spin-lattice relaxation time so that resonance is observed at 77°K. The electron is in the a- d 2) orbital in contrast with most other cl ions. Titanium h.f.s. are observed on the F-center line in H2-reduced BaTiOs 664). 

In 0, symmetry, the ground state is T g. This is split by distortions and since the three orbital states will be close in energy and connected by spin-orbit coupling, ESR signals will only be obtained at low temperatures when the spin-lattice relaxation time will be longer. When the symmetry is much reduced (e.g., G ), speetra are readily observed and g values are nearer 2 because of the quenching of the orbital angular momentum. Theory predicts that gf, < 2 and > 2 for this system (see Section IV,C,5). 

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