Linewidth correlation with chemical shift

Among several indicators of motional state of organic solids H linewidths has been used from very beginning of solid-state NMR. However, inherently low resolution of H MAS NMR spectra caused by the very strong homonuclear interactions and relatively low MAS speed lead to a very limited application of H MAS NMR for studies of mobility.1 A 2D wideline separation (WISE) experiment correlates carbon chemical shifts recorded under MAS with broad H lines.1,24 As a result a broad ll lines are separated in the components corresponding to the 13C sites in their close proximity. For mobile domains in a sample, the H - ll coupling are relatively weak which leads to relatively narrow 11 static lines. The opposite is observed for the rigid domains. 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

Next, one should consider the line width of the one-dimensional spectra of both nuclei. The detected nucleus should be the one with the smaller linewidth, since a short T2 leads to signal losses during the pulse sequence. Compared with metal nuclei usually has the smaller linewidth. Because of the chemical shift anisotropy and the temperature dependence of the chemical shift of heavy nuclei, the measurement of is therefore favoured in these cases. For detection, in addition, the large NOE enhancement caused by proton irradiation has to be taken into account. Heteronuclei usually have no directly bound proton their NOE factors are therefore lower. To facilitate further discussion, the y values and the natural abundances of the nuclei used for C-X correlation are given in Table 1. 

The Nb chemical shift in (775-C5H5)Nb(CO)4 in tetrahydrofuran had a thermal sensitivity measured as 0.38 ppmK from 210 to 340 K, " and the same compound in CH2CI2 a value of 0.42 ppmK . A linear decrease in shielding of the Nb nucleus, with increased temperature, of 0.25 ppmK for [CpNb(H)(CO)3] and [CpNb(D)(CO)3] has been measured, and also a non-linear decrease in linewidths with increased temperature for both of these compounds, indicative of increased molecular correlation times. [Et4N][Nb (CO)6] in THF showed a linear temperature dependence with a sensitivity of 0.18 ppmK over a range 203 to 323 K, and an increase in linewidth from 9.8 Hz (323 K) to 23 Hz (203 K). -... 

Chemical shifts may correlate with quadrupolar linewidths (if AE terms are similar, or vary consistently), since a is related to the NQCC, and the linewidth to (NQCC). Examples have been reported in chlorine and in manganese resonance. 

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