Nitrogen spin-lattice relaxation time

A recent advance in the quantification of DNP came with the inclusion of nitrogen nuclear relaxation into the saturation factor. Inspired by the measurement of nitrogen spin-lattice relaxation times over a wide range of correlation times by Robinson et al.,52 Armstrong and Han developed a... 

The effect of pressure on the anisotropic reorientation of acetonitrile-c/3 in the liquid state can serve as in illustrative example of high pressure NMR studies of molecular reorientation in liquids. The deuteron and nitrogen spin-lattice relaxation times of acetonitrile-( 3 have been measured as a function of pressure up to 200 MPa at 23°C, using the NMR Tj relaxation technique. Since the quadrupole coupling constants for nitrogen and deuterons in acetonitrile-( 3 are known, the experimental Ti data are interpretable in terms of the rotational diffusion constants for motion perpendicular and parallel D to the symmetry axis. 

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... 

Zi(Air, x) and 7)(N2, x) are spin-lattice relaxation times of nitroxides in samples equilibrated with atmospheric air and nitrogen, respectively. Note that W(x) is normalized to the sample equilibrated with the atmospheric air. W(x) is proportional to the product of the local translational diffusion coefficient D(x) and the local concentration C(x) of oxygen at a depth x in the membrane, which is in equilibrium with the atmospheric air ... 

Spin-lattice relaxation times for the carbon atoms of L-ascorbic acid treated with Chelex-100 (Bio-Rad Laboratories) were determined on 0.25 M solutions in deuterium oxide purged with nitrogen. The inversion-recovery method (33-35) with alternating phases and 8-10 t values was used for the protonated carbons, and the homo-spoil technique with 6-8 t values was used for the non-protonated atoms. The spin-lattice relaxation times (Tj) were calculated from the data using the appropriate Nicolet (25) digital computer programs. The Ti-values for the nonprotonated carbon atoms were repeated using the inversion recovery method and were virtually identical to those reported in Table 1. 

The dynamic nature of the gramicidin channel has been the subject of much interest. The N spin-lattice relaxation time (Tj) of the nitrogen atom at the Leu-4 position has been used to investigate the local djmamics about the Ala-3/Leu-4 linkage. Evidence was obtained that suggests a correlation between the local dynamics and ion transport through the channel. The... 

Fig. 2 shows an ESR spectrum of a sample exhibiting grey colour. The magnified parts (at low and high fields) disclose the presence of a species in extra-framework position in small quantities. As ions have very short spin-lattice relaxation time even at liquid nitrogen tempera-... 

The NMR spectra of these complexes under ambient conditions exhibit sharp, paramagnetically shifted features that can span up to 400 ppm (90, 134). Electron exchange is fast on the NMR time scale, so there is an effective twofold symmetry, which approximately halves the number of distinct features observed. The sharpness of the resonances is due to the short electron spin-lattice relaxation time of the Fe(II) center, which allows even the CH2 protons adjacent to the coordinated nitrogen atoms to be observed. 

The ESR spectrum is easily microwave-power saturated and is obscured somewhat by resonances attributed to an F center (see below). The spectrum is clarified if observations are made at 120°K, since this temperature serves both to thermally bleach the F-center resonance and to make the N° resonance less susceptible to microwave power saturation, due to a shorter spin-lattice relaxation time. The nitrogen atom has an electronic spin of 3/2 due to its three un-... 

Although most films of a-Si H are known to contain substantial amounts of impurities such as oxygen, nitrogen, and carbon, there is remarkably little direct evidence from ESR measurements for the presence of impurity species. Miller and Haneman (1978) have investigated the effect on a-Si of the diffusion of O2 gas through the film. They find that paramagnetic O2 readily diffuses into the films and dramatically reduces the spin-lattice relaxation times for the ESR at g = 2.0055. These results indicate that the spins are located primarily on internal surfaces that are accessible to oxygen diffusion. 

Some appreciation for the nitrogen nuclear characteristics may be obtained from Table 1. Because the relative sensitivities are comparable, the approximately 300-fold higher natural abundance of would seem to make it the nucleus of choice. It is even more sensitive than at natural abundance. However, like all nuclei with spin quantum number I > 1/2, possesses an electric quadrupole moment that arises from a nonspherical electric charge distribution in the nucleus itself. When placed in an electric field gradient, such as that characteristic of most molecular electron distributions, a quadrupolar nucleus experiences random fluctuating electric fields. The characteristic frequencies of these motions have components at the resonance frequency and hence afford an efficient relaxation mechanism. As a result, spin-lattice relaxation times (Tj ) are very short, 0.1-10 ms. Because Tj = To for in most molecules Lie in solution, linewidths are corres-... 

In solution NMR, the spin lattice relaxation Tin of small molecules can vary from seconds to hours.(66) Thus this contributes to even more serious sensitivity problems when one considers the need to wait 4-5 Tin between pulses. In a recent study of the relaxation behavior of nitrogen containing organic material in NaOH-extracts of plant composts with an incubation time up to 541 days, it was found that the extracted compounds behave in a similar manner to biomacro-molecules.(6i,66,75) At a resonance frequency of 30.4 MHz, the Tin s of humic substances derived from degrading plant material range from 1 to 3 s.(6i)... 

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