Nuclear quadrupole relaxation

Nuclear quadrupole relaxation may also contribute significantly to spin-relaxation and thus lead to line broadening. These effects depend upon the electric-field gradient at the nucleus which is reduced if the radical is symmetrical. 

Lindman, B., and Lindqvis, I. (1969). Nuclear quadrupole relaxation of 8SRB in an aqueous solution of a humic acid. Acta Chem. Scand. 23, 2215-2216. 

The spin-lattice relaxation of nuclei with /> 1 /2 is mainly due to nuclear quadrupole relaxation and the relaxation time at a temperature of 77 °K is still fairly short (of the order of milliseconds for many nuclei of interest to chemists). Therefore the experimental methods usually applied do not involve relaxation problems. The full RF fields available — for instance, with a superregenerative NQR spectrometer — may be applied successfully, resulting in an improved signal-to-noise ratio for the resonance at 77 °K as compared with room temperature. There are exceptions to this rule (e.g. 14N NQR spectroscopy where saturation problems come into play at 77 °K). 

Lindman, B. Applications of nuclear quadrupole relaxation to the study of ion binding in solution. Thesis, Lund 1971... 

In addition to the two major relaxation processes, T and T2, two other processes important to the study of nuclear properties are nuclear quadrupole relaxation and the Overhauser effect. 

Nuclear quadrupole relaxation occurs only during nuclear spins with I... 

Nuclear quadrupole relaxation—the dipole-dipole interaction between spin > /2... 

N-protonation the absolute magnitude of the Ad values is larger than for Af-methylation <770MR(9)53>. Nuclear relaxation rates of and have been measured as a function of temperature for neat liquid pyridazine, and nuclear Overhauser enhancement has been used to separate the dipolar and spin rotational contributions to relaxation. Dipolar relaxation rates have been combined with quadrupole relaxation rates to determine rotational correlation times for motion about each principal molecular axis (78MI21200). NMR analysis has been used to determine the structure of phenyllithium-pyridazine adducts and of the corresponding dihydropyridazines obtained by hydrolysis of the adducts <78RTC116>. 

Chemists pay much less attention to the NMR relaxation rates than to the coupling constants and chemical shifts. From the point of view of the NMR spectroscopist, however, the relaxation characteristics are far more basic, and may mean the difference between the observation or not of a signal. For the quadrupolar nucleides such as 14N the relaxation characteristics are dominated by the quadrupole relaxation. This is shown by the absence of any nuclear Overhauser effect for the 14N ammonium ion despite its high symmetry, which ensures that the quadrupole relaxation is minimized. Relaxation properties are governed by motional characteristics normally represented by a correlation time, or several translational, overall rotational and internal rotational, and thus are very different for solids, liquids and solutions. 

The low zero field C value which they observed is probably due to an exceptionally long nuclear quadrupole to lattice relaxation time [144], in... 

T. J. Bastow, Temperature dependence of the nuclear quadrupole coupling and relaxation time of Mg in Mg metal. /. Pkifs. Condens. Matter, 1991, 3,753-756. 

Another important nuclear characteristic is the nuclear quadrupole moment which, possessed by nuclei for which 7 1, has given rise to the important field of nuclear quadrupole resonance spectroscopy. A major importance of the quadrupole moment with respect to NMR absorption resides in the effects of quadrupole coupling constants on nuclear relaxation times and, therefore, on the line widths and saturation characteristics of NMR absorption (9). In addition, in favorable situations, quadrupole coupling constants can be derived from the characteristics of nuclear resonance of quadrupolar nuclei 127). Some examples of these effects will be described in Sections III, IV and VI of this chapter. 

Other effects frequently encountered in inorganic systems that can severely affect line shape involve relaxation processes arising from interactions of nuclear quadrupole moments with electric field gradients. For quad-rupolar nuclei (I 1), the quadrupolar contribution to the spin-lattice relaxation time Ti is given approximately by... 

All nuclear multiplet structures due to coupling of nonequivalent nuclei are, as noted earlier, subject to effects on line shapes by chemical or positional exchange. For those multiplet structures arising from coupling of nuclei, one of which has a nonzero nuclear quadrupole moment, effects of quadrupole relaxation must be considered. For example, if a proton or fluorine atom is bonded to a nitrogen nucleus (I = 1), a triplet resonance will be expected in the proton or fluorine spectrum. For observation of this fine structure it is necessary that the lifetimes of the nuclear spin states of nitrogen (m = 1, 0, —1) be greater than the inverse frequency separation between multiplet components, i.e., t > l/ANx (106). The lifetimes of N14 spin states can become comparable to or less than 1 /A as a result of quadrupole relaxation. When the N14 spin-state lifetimes are comparable... 

S has a moderate quadrupolar moment, eQ, arising from its non-spherical charge distribution. The interaction between the nuclear quadrupole moment and the electric field gradient, which is generated at the nucleus itself by the surrounding electronic distribution, is modulated by molecular reorientational motion and provides the only effective relaxation mechanism in liquid. The resulting relaxation times are on the order of milliseconds. 

While S relaxation behaviour can provide interesting information about interactions in solution, nuclear quadrupole coupling constants are a very sensitive probe in the study of the electronic distribution around the sulphur nucleus. 

S nuclear quadrupole coupling constants have been determined from line width values in some 3- and 4-substituted sodium benzenesulphonates33 63 and in 2-substituted sodium ethanesulphonates.35 Reasonably, in sulphonates R — SO3, (i) t] is near zero due to the tetrahedral symmetry of the electronic distribution at the 33S nucleus, and (ii) qzz is the component of the electric field gradient along the C-S axis. In the benzenesulphonate anion, the correlation time has been obtained from 13C spin-lattice relaxation time and NOE measurements. In substituted benzenesulphonates, it has been obtained by the Debye-Stokes-Einstein relationship, corrected by an empirically determined microviscosity factor. In 2-substituted ethanesulphonates, the molecular correlation time of the sphere having a volume equal to the molecular volume has been considered. 

It is known from nuclear structure theory that nuclei with I > 1 also have an electric quadrupole moment Q, which is a measure of the asphericity of the nucleus. Nuclear quadrupole moments affect the rate of magnetic dipole relaxation, and nuclei with Q 0 are candidates for NQR measurements (see Table 3.3). 

We saw in Chapter 7 that the resonance frequency of a quadrupolar nucleus is dependent on the orientation of the molecule in which it resides. Molecular tumbling now causes fluctuating electric fields, which induce transitions among the nuclear quadrupole energy levels. The resulting nuclear relaxation is observed in the NMR just as though the relaxation had occurred by a magnetic mechanism. 

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