Scalar relaxation of the second kind

In the case of Sn—X bonds and quadmpolar nuclei X, the Sn NMR signal may be broadened without resolved splitting, as a result of scalar relaxation of the second kind. In this case, the coupling constants can be calculated, if the relaxation time of the X nucleus is known.in favorable cases, the pattern of the solid-state n Sn NMR spectra can be analyzed with respect to the isotropic indirect nuclear spin-spin coupling (e.g. in PhsSnCl,, 280 and -1-275... 

Scalar Relaxation of the Second Kind. The linewidths of i95pt resonances from complexes with ligands are determined... 

Scalar relaxation of the second kind differs from the first kind only in that the modulation of the field at spin I arises from the relaxation effects of S. A common example is a coupling to a quadrupolar nucleus undergoing rapid relax-... 

Scalar relaxation due to chemical exchange has the usual influence (broadening) on the appearance of Si NMR spectra. A consequence of such exchange processes, depending on the rate of exchange, is that PT techniques may not work very well or sometimes not at all. Scalar relaxation of the second kind affects the line widths by j. sc(29si) jf silicon atom is bonded to one or more quadrupolar nuclei as in... 

Fig. 5. 62.8MHz ovpbjiH NMR spectra of a cyclic bis(amino)plumbylene (=15% in [Dg]toluene), showing the competition between scalar relaxation of the second kind [short relaxation times caused by partially relaxed scalar coupling C N ... 

A. GryfF-KeUer, S. Molchanov, A. Wodynski, Scalar relaxation of the second kind—a potential source of information on the dynamics of molecular movements. 2. Magnetic dipole moments and magnetic shielding of bromine nuclei, J. Phys. Chem. A 118 (2014) 128-133. 

If chemical exchange or internal rotation causes the spin-spin coupling interaction between two nuclei to become time dependent, then scalar relaxation of the first kind can occur. Scalar relaxation of the second kind relates to the case where the relaxation rate of a coupled nucleus is fast compared with 2nJ. Coupling to a quadrupolar nucleus can give rise to this relaxation mechanism. For scalar coupling relaxation to be operative, it is generally important that the resonance frequencies of the coupled nuclei be similar. This is, perhaps, the least common of the nuclear spin relaxation processes considered. 

This classification was introduced by Abragam (2), and the corresponding equations for the relaxation times are derived in his book on p. 307. A short discussion of this mechanism is also given in references 6, 24, 25, and 43. The recent paper of Briguet, Duplan, and Delmau gives an excellent discussion of scalar relaxation of the second kind in SiCl and SiHCl3 including cross-relaxation terms (31). 

Relaxation data of the quadrupolar halogens are directly obtained from pulsed NMR studies, although most data on T2 are from measurements of linewidth or from peak-to-peak distances of the first derivative of absorption curves. Using pulse methods the lower limit of Tj and T2 depends on the dead time of the spectrometer. Presently the practical limits appear to be ca. 10 ys. A number of the available relaxation data on Cl, Br, and I compounds have been determined through the effects of the halogen relaxation on the relaxation of directly bonded spin 1/2 nuclei through modulation of scalar interaction this mechanism is commonly termed "scalar relaxation of the second kind. Very short... 

Gryff-Keller and co-workers pursued their interest in the scalar relaxation of the second kind (SC2). The spin-lattice relaxation rate of a spin-1/2 nucleus (7 spin) scalar-coupled to a quadrupolar nucleus (S-spin) is given by ... 

Scalar or J coupling of the observed nucleus to a neighboring spin S which is fluctuating rapidly with >7 washes out the splitting and can induce relaxation. This scalar relaxation may arise through chemical exchange of the spin S, with x f>J. Scalar relaxation of the second kind is observed if the S spin is relaxing rapidly because quadrupolar, and has a Larmor frequency close to that of the I = ) spin, as in C- Br. 

Scalar relaxation of the second kind occurs when the S nucleus relaxes rapidly, typically because of its quadrupole. This will have a marked effect on the T 2 of nucleus I, by tending to collapse the I - S scalar coupling in a manner analogous to the effects of exchange. More precisely... 

When a dipolar nucleus ( P in the present case) is bonded directly to a quadmpolar nucleus ( 0 in the present case), the P nucleus will also be relaxed by virtue of its spin - spin coupling with O. This was termed scalar relaxation of the second kind by Abragam (1961). Such a scalar relaxation... 

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