Low-temperature solid-state NMR

A. S. Lipton, R. W. Heck, J. A. Sears and P. D. Ellis, Low temperature solid-state NMR experiments of half-integer quadrupolar nuclides caveats and data analysis. /. Magn. Reson., 2004,168, 66-74. 

P. D. Ellis and A. S. Lipton, Low-temperature solid-state NMR spectroscopy a strategy for the direct observation of quadrupolar nuclides of biological interest. Annu. Rep. NMR Spectrosc., 2006, 60,1-38. 

These results further demonstrate the ability of very low temperature solid-state NMR methods to differentiate rapidly equrtibrating classical ion systems from nonclassical bridged systems, even when the equilibration barrier involved is much less than 3 kcal mol... 

A low-tcmpcraturc, dynamically driven structural transition observed in a polypeptide by solid-state NMR spectroscopy has been reported by Bajaj et At low temperatures, proteins and other biomolecules are generally found to exhibit dynamic as well as structural transitions. This includes a so-called protein glass transition that is universally observed in systems cooled between 200 and 230 K, and which is generally attributed to interactions between hydrating solvent molecules and protein side chains. However, there is also experimental and theoretical evidence for a low-temperature transition in the intrinsic dynamics of the protein itself, absent any solvent. In the study by Bajaj et al., low-temperature solid-state NMR was used to examine site-specific fluctuations in atomic structure and dynamics in the absence of solvents. In particular, they employed MAS NMR to examine a structural phase transition associated with dynamic processes in a solvent-free polypeptide lattice at temperatures as low as 90 K. Several quantitative solid-state NMR experiments were employed to provide site-specific measurements of structural and motional features of the observed transition. 

Complexes [ZnX(Tp,Bu,Me)] (X = Br, Cl, and OH) have been investigated by low-temperature solid-state 67Zn NMR spectroscopy. The value of the quadrupole coupling constant Cq for the zinc increased monotonically with the electronegativity of X, e.g. Br < Cl OH.102... 

Hexafluoroarseniate salts show phase transition behavior. [Fe( 7 -C6Fl6)(77 -CsHs)][AsF6], for instance, undergoes phase transition between three different crystal forms.Variable-temperature solid-state NMR measurements have shown that rotation of the entire cation takes place in a cubic phase above 310 K, while in the intermediate / -phase, the rotational motion is restricted to a 90° in-plane reorientation. Below 270 K, the crystal is in a low-symmetry phase, in which whole-body rotation does not take place though the rings execute jumping motion that persists down to 200 K. Transition from a rotational jumping state to a whole-body reorientation has also been detected from the Mossbauer spectra of the [PF6] salt of the same complex. 

The Tg values of polyferrocenylsilanes, which can be as low as ca. -50 °C, are remarkable for a polymer structure with a bulky unit such as ferrocene in the main chain. Even when the influence of the side groups is small, the Tg values are still close to ambient temperature (e.g., for 3.22 (R=R = H), Tg=16°C). By contrast, the Tg of poly(vinylferrocene) is reported to be high (185 or 233 °C Chapter 2, Section 2.2.1.1). The ability of the iron atom in each ferrocene unit to act as a freely rotating molecular ball-bearing [106] probably plays a key role in generating the observed conformational flexibility in polyferrocenylsilanes. This aspect has been explored by means of variable-temperature solid-state NMR studies on PFSs 3.22, which were specifically deuterated either on the Cp rings or in the side groups [107]. 

T is therefore attributed to the melting of the paraffinic phase composed of alkyl side chains. The slight contraction of the intersheet distance observed for this transition is consistent with a small deviation of the C-C torsion angles from the -trans conformation. Therefore, the phase appearing above T is regarded as a structure with the side chains in a conformation not very different from that present at low temperatures. Solid-state nuclear magnetic resonance (NMR)... 

In order to contribute to answering the open questions, the influence of hydrogen bonding to and protonation of the pyridine ring of PLP on its enolimine—ketoamine tautomerism has been studied by X-ray crystallography [96] and NMR [97] of solid model systems, and the influence of a polar environment by low-temperature liquid state NMR [98]. In these studies, correlations... 

TABLE 14. Characteristic solid state NMR parameters for the high- and low-temperature... 

Monolayer coverage of vanadium oxide on tin oxide support was determined by a simple method of low temperature oxygen chemisorption and was supported by solid-state NMR and ESR techniques. These results clearly indicate the completion of a monolayer formation at about 3.2 wt.% V2O5 on tin oxide support (30 m g" surface area). The oxygen uptake capacity of the catalysts directly correlates with their catalytic activity for the partial oxidation of methanol confirming that the sites responsible for oxygen chemisorption and oxidation activity are one and the same. The monolayer catalysts are the best partial oxidation catalysts. 

Beckham and coworkers studied the dynamic mechanical properties of poly(urethane-crown ether rotaxane)s [138]. No difference was observed between the backbone and polyrotaxane, probably because of the low min value (0.02). However, 13C solid-state NMR detected die presence of the crown ether as a mobile structure at room temperature. The same observation was seen in polyrotaxanes with ether sulfone and ether ketone backbones (77-80) [114]. Although no detailed properties were reported, the detection of the liquid-like crown ether provided very important information in terms of mechanical properties, because these properties are the result of molecular response to external forces. For example, mobile crown ethers can play the role of plasticizers and thus improve impact strength. 

---