Cross solid state measurements

Low natural abundances, low gyromagnetic ratios and high quadrupole moments of several NMR active isotopes of the series from yttrium to cadmium can cause difficulties in solution state NMR. How ever, these formidable obstacles in the solution state can be turned to advantages in solid state NMR. Low natural abundance (i.e. dilute spins) can help in solid state measurements because the problem raised by dipolar interactions between like spins is elimi nated. Sensitivity problems found with low gyromag netic ratios in the solution state can be overcome in the solid state by the cross-polarization (CP) tech nique by which the magnetization of the observed nucleus (S) can be increased by a factor, Th/Ts- Fur ther, a nuclear quadrupole moment can result in con siderable sensitivity to local nuclear site symmetry, thus giving useful structural information. 

It is noteworthy that it is the lower cross-over temperature T 2 that is usually measured. The above simple analysis shows that this temperature is determined by the intermolecular vibration frequencies rather than by the properties of the gas-phase reaction complex or by the static barrier. It is not surprising then, that in most solid state reactions the observed value of T 2 is of order of the Debye temperature of the crystal. Although the result (2.77a) has been obtained in the approximation < ojo, the leading exponential term turns out to be exact for arbitrary cu [Benderskii et al. 1990, 1991a]. It is instructive to compare (2.77a) with (2.27) and see that friction slows tunneling down, while the q mode promotes it. 

We used modifications of the standard solid-state CP-MAS (cross-polarisation, magic-angle spinning) experiment to allow the proton relaxation characteristics to be measured for each peak in the C spectrum. It is known that highly mobile, hydrated polymers can not be seen using either usual CP-MAS C spectrum or solution NMR (6). We found, however, that by a combination of a long-contact experiment and a delayed-contact experiment we could reconstruct a C spectrum of the cell-wall components that are normally too mobile to be visible. With these techniques we were able to determine the mobility of pectins and their approximate spatial location in comparison to cellulose. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

Since the discovery of the nuclear Overhauser effect (NOE, see previous section) [4, 5] and scalar coupling constants [36, 37] decades ago, NMR-derived structure calculations of biomolecules largely depended on the measurement of these two parameters [38]. Recently it became possible to use cross-correlated relaxation (CCR) to directly measure angles between bond vectors [39] (see also Chapt 7). In addition, residual dipolar couplings of weakly aligned molecules were discovered to measure the orientation of bond vectors relative to the alignment tensor (see Sect 16.5). Measurement of cross-correlated relaxation was described experimentally earlier for homonuclear cases [40, 41] and is widely used in solid-state NMR [42 14]. 

Aminomethylpyridine (picolylamine) is an important ligand in respect to spin cross-over, [Fe(2-pic)3]Cl2 being the key compound." Fleat capacity measurements on [Fe(2-pic)3]Cl2 EtOH gave values of 6.14kJmol and 50.59 JK moC for the spin eross-over entropy the determined entropy was analyzed into a spin contribution of 13.38, an ethanol orientational effeet of 8.97, and a vibrational contribution of 28.24 JK mol. " This compound exhibits weak cooperativity in the solid state." The heat capacity of [Fe(2-pic)3]Cl2 MeOH is consistent with very weak cooperativity." [Fe(2-pic)3]Br2 EtOH shows a lattice expansion significantly different from that expected in comparison with earlier-established behavior of [Fe(2-pic)3]Cl2 EtOH." ... 

Solid-state C variable-amplitude cross polarization magic-angle spinning (VACP/MAS) nuclear magnetic resonance (NMR) spectra were acquired for the sorbitol samples. Proton decoupling was achieved by a two-pulse phase modulation (TPPM) sequence. Identical C spectra were measured for the y-form sorbitol samples, and a representative spectrum is shown in Figure 9. 

The results of these measurements are shown below. As expected, polymer 8 undergoes only scission to lower molecular weight fragments upon exposure. On the other hand, 2, which has pendant unsaturation, shows a significant cross-linking component both in solution and in the solid-state. In... 

Yannoni, Macho, and Myhre869 obtained magic-angle spinning cross-polarization 13C NMR spectra of the 13C-enriched 2-norbomyl cation in SbF5 solid matrix down to — 196° C. The solid-state chemical shifts and measured barriers for the 6,1,2-hydrogen shift of 6.1 kcal mol-1 correlate well with the discussed solution data. Subsequently, they even obtained 13C NMR spectra in the solid state at —268°C (5K),870 a remarkable achievement indeed. 

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