CSA —See Chemical shift

As we shall see, all relaxation rates are expressed as linear combinations of spectral densities. We shall retain the two relaxation mechanisms which are involved in the present study the dipolar interaction and the so-called chemical shift anisotropy (csa) which can be important for carbon-13 relaxation. We shall disregard all other mechanisms because it is very likely that they will not affect carbon-13 relaxation. Let us denote by 1 the inverse of Tt. Rt governs the recovery of the longitudinal component of polarization, Iz, and, of course, the usual nuclear magnetization which is simply the nuclear polarization times the gyromagnetic constant A. The relevant evolution equation is one of the famous Bloch equations,1 valid, in principle, for a single spin but which, in many cases, can be used as a first approximation. 

As a consequence of the small quadrupole moment of Li, the quadrupolar interaction in solid state NMR spectra is much smaller for Li than for Li. This has been used to advantage for the determination of the Li chemical shift anisotropy from the Li static solid state powder spectrum of 2,4,6-tris(isopropyl)phenyllithium (see below) . Applying MAS up to 10 kHz, the CSA contributions to the lineshape can be completely ehminated in most Li spectra of organolithium compounds. If the measurement of the quadrupolar... 

By comparison, a saturated methine carbon (C-H) has a CSA of only 25 ppm because the mobility of electrons around the carbon nucleus is much less in an sp3-hybridized carbon and depends much less on the orientation of the C-H bond with respect to B0. In solution-state NMR we only see the isotropic chemical shift, < iso, and the fixed-position chemical shifts and the CSA value are obtained from solid-state NMR measurements. Although CSA does not affect chemical shifts in solution, it does contribute to NMR relaxation and can be exploited to sharpen peaks of large molecules such as proteins in solution. For large molecules, such as proteins, nucleic acids, and polymers, or in viscous solutions, molecular tumbling is slow and CSA broadens NMR lines due to incomplete averaging of the three principle chemical shift values on the NMR timescale. Like isotropic chemical shifts, CSA in parts per million is independent of magnetic field strength B0 but is proportional to B0 when expressed in hertz. Because linewidths are measured in... 

The H , = Hoo contribution to the effective Hamiltonian Hef / contains only-scaled isotropic chemical-shift terms. The first-order correction to the effective Hamiltonian requires the evaluation of commutators between DD elements, CSA elements and cross-terms DDx CSA. We should remind ourselves that the basic justification for using the van Vleck transformation is that the off-diagonal elements of the interactions are small with respect to the differences between the diagonal elements (see Eqs. 48a and 48b). When that is the case... 

In addition to using /-scalar coupling data (see above), several other methods have been proposed to estimating sugar conformations in RNA based on the 13C-4H dipole-dipole cross-correlated relaxation,289 based on the cross-correlated relaxation rates involving 13C CSA and 13C-4H dipolar interactions,290 and based on the 13C chemical shifts of sugar carbons.291... 

All our attempts to observe Pt NMR signals from either PtCli " or cis-Pt( NH3)aCl2 bound to reduced cytochrome c or ribo-nuclease A (RNase) have so far failed. These platinum complexes are known to bind to the sulphur atoms of exposed methionines (residues 65 and 29 of Cyt c and RNase respectively) as shown by our previous H NMR studies on RNase (28) and those of Boswell et al on Cyt c (29) and x-ray crystallography. We assume therefore that the resonances are broadened beyond detection via chemical shift anisotropy relaxation. The restriction of Pt mobility on the protein will lead to a large increase in (see CSA equation above). The anisotropy term would also be expected to increase. Scalar coupling to N will also contribute to the increase in linewidths if nitrogen binding sites are involved. 

Characteristic line shape functions can be observed in special cases as is shown in Fig. 1 by two examples. The so-called Pake doublet [11] is foimd for isolated spin pairs (see Fig. la). The spectrum shown in Fig. lb is observed if an NMR signal is mainly broadened by the chemical shift anisotropy. The analysis of the line shape function, /(v), allows the direct measurement of parameters like the internuclear distance, the chemical shift anisotropy, Aa, or the asymmetry parameter, i]csa> in these cases. 

---