Spin-rotation mechanism

Live oil with dissolved methane does not follow the above correlations as methane relaxes by a spin-rotation mechanism, even when dissolved in liquid hydrocarbons [13]. The Ti relaxation time as a function of rj/T is illustrated in Figure 3.6.2 for different gas/oil ratios expressed in units of m3 m-3 as a parameter. The solid line is the fit for zero gas/oil ratio and is given by Eq. (1). 

The relaxation of gaseous methane, ethane and propane is by the spin-rotation mechanism and each pure component can be correlated with density and temperature [15]. However, the relaxation rate is also a function of the collision cross section of each component and this must be taken into account for mixtures [16]. This is in contrast to the liquid hydrocarbons and their mixtures that relax by dipole-dipole interactions and thus correlate with the viscosity/temperature ratio. 

The first comparison is based on the T values of gaseous, liquid and adsorbed molecules. Unfortunately, no measurements are available for butenes in the gas or liquid phase. Nevertheless a reasonable parallel can be drawn with propylene where the three different phases were investigated (35) at 295 K for the gas (1 atm) Tj of C2 is 0.095 s in the liquid state (2.6. M in CDCI3) 59.9, 58.7 and 65.2 s for Cj, C2 and C3 respectively adsorbed on NaY zeolite 0.81, 1.6 and 0.81 s. The shortest relaxation times characterize the gas phase where the spin-rotation mechanism (NOE factor n = 0) is very effective (30,35). In the liquid, dipole-dipole and spin-rotation mechanisms both play a role and the total relaxation rate is about three orders of magnitude lower than in the gas phase. The adsorbed molecules show therefore an intermediate behaviour between gas and liquid, as it was also suggested by chemical shift data. 

A general model for electronic relaxation of the Gd3+ S = 7/2 ion in various complexes in solution was presented by Rast el al. [86]. Contrary to the usual assumption, the electronic relaxation in their model is not only due to the effects of the transient zero field splitting, but is also strongly influenced by the static crystal field effect which is modulated by the random Brownian rotation of the complex. Experimental peak-to-peak widths of three gadolinium complexes could be well interpreted as a function of temperature and frequency using three static and one transient crystal field parameters. Moreover, their interpretation of experimental data did not require the addition of any field independent contribution to the line width like the spin-rotation mechanism. 

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. 

Finally, we address the apparent trend of the kj-values for the DCA-PSZ pair to increase slightly at the high held end. Actually, this trend can be reproduced by the contribution k ta of the g tensor anisotropy mechanism, however, using an orienta-tional correlation time Tc of 8 x 10 s that is only about half the value extracted from a ht of the spin-rotational mechanism, as described above. It is possible that distortional motions of the nonplanar azine ring—which has been invoked to account for the deviation of the isotropic hyperhne constants of the azine radicals... 

Since nuclear spin-lattice relaxation times, Ti, are such a critical parameter in determining the recycle time of FT NMR experiments several studies have examined the magnitude and mechanism of Se relaxation For small molecules the spin rotation mechanism dominates and for larger molecules where this mechanism is not so effective, the chemical shift anisotropy mechanism becomes more effective. Interestingly, the dipole-dipole mechanism has not been found to be an efficient relaxation... 

It can be seen that full NOE is present upwards from cyclohexane, whereas the smaller cycloalkanes, especially cyclopropane, relax via dipolar and spin-rotation mechanisms. For the higher cycloalkanes the values become constant. 

For the spin-rotation mechanism, where the magnetic fields which... 

In the absence of unpaired electrons, the most efficient pathways for relaxation are the dipolar and the spin-rotation mechanisms. Because the interaction with protons is the most dominant one in this respect, the discussion will be restricted to this aspect. The dipolar term... 

Very few reports have been published on fluorine relaxation times in most cases the results reported for several different molecules indicate that the spin rotation mechanism is dominant at higher temperatures whereas the intermolecular dipole-dipole mechanism is not negligible at lower ones. The chemical shift anisotropy contribution can also be an important factor, whereas the intramolecular dipole-dipole mechanism and scalar coupling contribution seem to be negligible in F relaxation. Some fluorine relaxation times (Tj, T2 and Tj ) have been used for establishing dynamics in a copolymer of tetrafluoroethene and hexafluoropropene. 

While it is possible for the spin-rotation mechanism to be present in the solid state, (1) can be reduced to R1=R1CSA at high magnetic fields and at moderate temperatures. Under extreme narrowing arguments, and assuming axial symmetry of the chemical shift tensor (CST), the CSA relaxation process is described by [18]... 

Methods-. Another method of establishing an absolute shielding scale is by using the relationship [equation (8)] between a and the spin-rotation constant measured in a molecular beam magnetic or electric resonance experiment or by high-resolution microwave spectroscopy. Combined with an accurate calculation of in the molecule, the absolute shielding of a suitable primary reference molecule can be obtained. For C and "O the primary reference molecule is CO for P it is PH3. The spin-rotation constant may also be obtained from the ratio of the relaxation times of two nuclei in the same molecule in the gas phase when both are wholly relaxed by the spin-rotation mechanism. In SeF, for example... 

Shielding anisotropies can be obtained from measured relaxation times when the chemical shift anisotropy relaxation mechanism or the spin-rotation mechanism is dominant. The correlation time has to be estimated or otherwise derived. There are large uncertainties associated with this method. 

Relaxation by the spin-rotation mechanism can lead to 5/Ti correlations, as observed in Sn resonance for methyltin halides, since is directly related to the spin-rotation tensor, and the spin-rotation relaxation rate to its square. With the use of higher magnetic fields, shielding anisotropy relaxation is more often observed AajTi, T2 relationships are reported for Pt(IV) complexes with Pt anisotropies of 10 ppm. ... 

In the gas phase, above the minimum, the F spin-lattice relaxation time is linear with density p of the gas, and the slope (TJp) is found to be consistent with Ti/p)ccT in cases where temperature-dependent measurements of F relaxation have been carried out. This implies that the relaxation is dominated by the spin-rotation mechanism in the gas phase. In the liquid phase, the results for several molecules indicate that spin-rotation mechanism is dominant, especially at higher temperatures and intermolecular dipole-dipole mechanism contributes to the relaxation at lower temperatures, e.g., below 200 K in FCIO3. Intramolecular dipole-dipole mechanism, chemical shift anisotropy and scalar coupling contributions are usually found to be negligible in F relaxation. Where intermolecular dipole-dipole and spin-rotation mechanisms have been used to interpret results using Arrhenius temperature dependence for both, the activation energies are usually not equal. Some typical values of F spin-lattice relaxation times are shown in Table 4. 

Xe, and Xe. The spin-spin relaxation times T2) have also been measured for liquid and solid Xe. The relaxation of the quadrupolar nuclides are nearly entirely by means of the quadrupolar mechanism (although there is some dipolar contribution to Ne near the melting point), the interaction of the nuclear quadrupole moment with the electric field gradients generated by deformations of the spherical atoms during collisions. The Xe relaxation, on the other hand, is thought to be dominated by the spin-rotation mechanism in the transient diatomic molecule formed in collisions, and He relaxation is by dipolar interactions. ... 

Quite efficient relaxation is possible by the spin-rotation mechanism for small, freely rotating molecules. For example, little else is likely to contribute to the relaxation of Os in liquid OSO4 (1 < Ti < 26 s, 0.7 < 72 < 12 s). This mechanism decreases, however, with decrease in temperature, increase in viscosity, or increase in molecular size, so that relaxation times for Fe, Rh, and Os are liable to be long. Thus for ° Rh, values of Ti greater than 60s have been measured. Note that similar values of Tj imply very narrow lines (<0.01 Hz) in the absence of other line broadening effects. 

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