Spin saturation

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

These experiments while moving beyond simple structure determination, still depend primarily on structure determination as their primary tool. A number of experiments have moved beyond this to exploit other characteristics of NMR. For example, spin saturation transfer can be used to study internal rotation of molecules (59, 60), and line shape analyses can be used to study intramolecular exchange processes (61), conformational interchange (62) and... 

Kinetics of Internal Rotation of A ,A -Dimethylacetamide A Spin-Saturation Transfer Experiment 60... 

Alkylaryl ethers and diaryl ethers undergo protonation on either oxygen or carbon, depending upon the acidity of the medium87 (Scheme 4.3). Both the C-protonated species 28 and the (9-protonated species 29 have been observed. The evidence mainly comes from NMR and UV data. Sommer et al.88 have used para-anisaldehyde as an indicator in acidity measurements in the superacidity range. The barrier of rotation around the Cipso—0 bond upon (9-protonation has been used as a criterion in such studies. The torsional barrier around the phenyl-alkoxy bond in the C-protonated forms of aromatic ethers has also been measured by spin-saturation transfer measurements.89... 

Saturation transfer difference. The origins of the STD experiment1721 can be traced to the spin-saturation transfer experiment or Forsen-Hoffman experiment from the 1960s.1911 In the STD experiment, a subset of the protein II resonances are saturated by means of a train of frequency-selective radiofrequency pulses applied to a narrow spectral region devoid of ligand resonances. The saturation is transferred by spin diffusion ( H- H crossrelaxation pathways) to the rest of the protein, a process that becomes more efficient with... 

Fluxionality studies have been carried out on 343 (Scheme 44).137 No coalescence of the two allene hydrogens (HI and H6) was observed in temperature ranges where the complex is stable (ca. 100°C). However, spin saturation transfer was observed between 60 and 100°C, which yielded activation parameters (AH = 26.8 1.3 kcal/mol A5J = 15.1 1.3 eu) for migration of the metal between the two double bonds. At 80°C, 343... 

Time-of-flight MRA is based on gradient echo sequences with very short repetition times (Laub and Kaiser 1988). The repeated HF excitations cause a relative spin saturation - i. e. signal reduction - in stationary tissue, while inflowing unsaturated blood is depicted with high signal. A saturation... 

Drawbacks of PC-MRA are seen in its susceptibility for patient movements and its impairment by pulsatile or turbulent flow patterns. The diagnostic sensitivity of 3D PC-MRA in intracranial stenosis is markedly lower than with 3D TOF-MRA (Oelerich et al. 1998). Advantages are the good suppression of stationary tissue signal with the lack of interference with short T1 tissues, and the depiction of large volumes without limitations due to spin saturation effects. 

To overcome known drawbacks of TOF-MRA, fast contrast-enhanced 3D MRA was developed. Application of a paramagnetic contrast agent reduces spin saturation phenomena in large examination volumes... 

Fig. 5.29a,b. Giant aneurysm of the posterior communicating artery in an infant. Due to recirculating blood, there is marked spin saturation within the aneurysm on unenhanced TOF-MRA (a). The entire aneurysm is visualized after contrast enhancement (b)... 

In addition to evaluation of line widths and peak separations, analysis of T values and spin saturation transfer can be performed to access a wide range of timescales. Under conditions of slow exchange between forms A and B and upon irradiation at vb, the time constant associated with state A (ta) can be determined from the measured T of nucleus A in the presence of exchange and the T of nucleus A in the absence of exchange ... 

The activation parameters for the exchange reactions of 17 and 18 were determined by a combination of variable-temperature ll NMR lineshape analysis16 and spin saturation transfer experiments.17 Rate data for 17 were measured over a temperature range of 100 "C. Rates for compound 18 were measured over a 65 °C range. The enthalpy of activation was found to be considerably smaller in the case of 17 (12.2(2) kcal/mol) relative to 18 (17.6(3) kcal/mol). Ion pair dissociation is therefore facilitated by the presence of a lone pair of electrons on the boron substituent. The entropy of activation for 17 is -2.3(6) eu, while that of 18 is 8(1) eu. The more positive entropy of activation measured for 18 may be interpreted as the creation of two independent particles from a closely associated ion pair. 

Nevertheless, Eqs. (102) and (103) do not necessarily reduce to the ordinary TDKS equations (40) and (41) in this limit. This is b ause the spin-dependent xc potentials and not identical except for the case of spin-saturated... 

This is possible only if the unperturbed KS ground-state determinant is spin-saturated since, in this case, = j (r) for all j. 

---