Rigid-lattice spectrum

The 19 F NMR spectrum of single crystals of silver trifluoroacetate showed a single resonance at room temperature due to rapid rotation of the CF3 moiety. Cooling to 40 K gave the rigid lattice spectrum which consisted of the six lines expected for the two types of dimer present in the unit cell.250... 

The narrow fluorine resonance of "Ci BF " is quite in contrast to the fluorine absorption found for the product of Illinois 6 coal with BF3. At room temperature, we observe a 0.25 mT (=2.5 G) wide, dipolar-broadened, spectrum not indicative of translation freedom. In contrast to the weakly bound complexes of BF3 with aromatic hydrocarbons, we anticipate BF3 to react strongly with oxygen functionality in the coal, through hydration with water, hydrolysis with acids (13), and ether complex formation (14), to give fluorine absorption lines which are in the rigid lattice condition. 

Experiments described in this section are suited to investigate ultraslow motion with correlation times in the millisecond-to-second range. Here, the NMR spectra are given by their rigid-lattice limit and one correlates the probability to find given NMR frequencies at two different times separated by the so-called mixing time tm [11,72]. A two-dimensional (2D) spectrum results, being a function of two NMR frequencies at t = 0 and t = tm, respectively. Since the NMR frequency reflects the orientation of the molecule, 2D spectra provide a visual representation of the reorientational process. Time- and frequency-domain... 

Figure 23.9(A) shows the quadrupole echo spectrum of an oriented block of [2,2- H2]Gly labeled silk fibroin fibers when the fiber axis was set parallel to the magnetic field direction. The quadrupole echo spectrum, Fig. 23.9(B), of unaligned [2,2- H2]Gly labeled silk fibroin fiber is also observed as a powder pattern. Both spectra are split into doublets, which give the value of the quadrupole splitting, Avq as 117.8 kHz for the [2,2- H2]Gly site the values are the same in each case. The full rigid-lattice width of about 126 kHz should be observed when the motion is frozen [72]. Thus, it is concluded that the motion of the methylene groups of the Gly residue is almost frozen at room temperature, which is in agreement with the prediction from the intermolecular hydrogen bonding network in the silk fibroin backbone chain with an antiparallel /3-sheet conformation.

In a rigid lattice, and in the temperature interval where the rapid relaxation rate is fast enough to collapse the dipolar splitting, but not fast enough to result in full narrowing, the broadening of a nitroxide spin label CW spectrum can be... 

---