Variable temperature polarizing

For thermal reactions a variable temperature probe is necessary since optimum polarized spectra are usually obtained in reactions having a half-life for radical formation in the range 1-5 minutes. Reactant concentrations are usually in the range normally used in n.m.r. spectroscopy, although the enhancement of intensity in the polarized spectrum means that CIDNP can be detected at much lower concentrations. Accumulation of spectra from rapid repetitive scans can sometimes be valuable in detecting weak signals. 

There is a striking contrast between the millennial variability of polar interglacial and glacial temperature histories. Whereas climate changes were frequent and large throughout... 

This chapter will only deal with the possible gas transport mechanisms and their relevance for separation of gas mixtures. Beside the transport mechanisms, process parameters also have a marked influence on the separation efficiency. Effects like backdiffusion and concentration polarization are determined by the operating downstream and upstream pressure, the flow regime, etc. This can decrease the separation efficiency considerably. Since these effects are to some extent treated in literature (Hsieh, Bhave and Fleming 1988, Keizer et al. 1988), they will not be considered here, save for one example at the end of Section 6.2.1. It seemed more important to describe the possibilities of inorganic membranes for gas separation than to deal with optimization of the process. Therefore, this chapter will only describe the possibilities of the several transport mechanisms in inorganic membranes for selective gas separation with high permeability at variable temperature and pressure. 

Probing Metalloproteins Electronic absorption spectroscopy of copper proteins, 226, 1 electronic absorption spectroscopy of nonheme iron proteins, 226, 33 cobalt as probe and label of proteins, 226, 52 biochemical and spectroscopic probes of mercury(ii) coordination environments in proteins, 226, 71 low-temperature optical spectroscopy metalloprotein structure and dynamics, 226, 97 nanosecond transient absorption spectroscopy, 226, 119 nanosecond time-resolved absorption and polarization dichroism spectroscopies, 226, 147 real-time spectroscopic techniques for probing conformational dynamics of heme proteins, 226, 177 variable-temperature magnetic circular dichroism, 226, 199 linear dichroism, 226, 232 infrared spectroscopy, 226, 259 Fourier transform infrared spectroscopy, 226, 289 infrared circular dichroism, 226, 306 Raman and resonance Raman spectroscopy, 226, 319 protein structure from ultraviolet resonance Raman spectroscopy, 226, 374 single-crystal micro-Raman spectroscopy, 226, 397 nanosecond time-resolved resonance Raman spectroscopy, 226, 409 techniques for obtaining resonance Raman spectra of metalloproteins, 226, 431 Raman optical activity, 226, 470 surface-enhanced resonance Raman scattering, 226, 482 luminescence... 

These spectra, taken at variable temperatures and a small polarizing applied magnetic field, show a temperature-dependent transition for spinach ferredoxin. As the temperature is lowered, the effects of an internal magnetic field on the Mossbauer spectra become more distinct until they result at around 30 °K, in a spectrum which is characteristic of the low temperature data of the plant-type ferredoxins (Fig. 11). We attribute this transition in the spectra to spin-lattice relaxation effects. This conclusion is preferred over a spin-spin mechanism as the transition was identical for both the lyophilized and 10 mM aqueous solution samples. Thus, the variable temperature data for reduced spinach ferredoxin indicate that the electron-spin relaxation time is around 10-7 seconds at 50 °K. The temperature at which this transition in the Mossbauer spectra is half-complete is estimated to be the following spinach ferredoxin, 50 K parsley ferredoxin, 60 °K adrenodoxin, putidaredoxin, Clostridium. and Axotobacter iron-sulfur proteins, 100 °K. 

ESR observations were conducted at 9.5 GHz, using a Varian E-line ESR spectrometer with variable temperature capabilities from 90K-300K. The g value was determined using a Varian pitch standard with a g-value of 2.00302 + 0.00005. The integrated intensity was also calibrated to a Varian pitch standard. The parameters g, AH u, AH-p and the radical density were determined for each sample. Saturation measurements were made on a selected subset of samples. Low temperature runs at 125 K were made for all inertinite samples, as well as for selected samples of the other maceral types. Little temperature variation in g value, linewidth, or lineshape, was seen in any sample. The integrated intensity varied approximately as 1/T, suggesting Boltzmann polarization of the spins at lower temperatures. 

Pyridinethiol is in equilibrium with 2-pyridinethione and it is been previously thought that the thiol is more stable in nonpolar solvents while the thione is more stable in polar solvents. Recent variable temperature Fourier transform IR experiments and computational studies calculated at the B3LYP/6-311 level of theory indicate that the thione is more thermodynamically stable than the thiol in nonpolar solvents and that tautomerism occurs via the dimer <2002JOC9061> (Equation 82). 

The conformational equilibria of 2-substituted 1,3-dithianes 63 were determined by application of variable temperature 13C NMR spectroscopy (cf. Scheme 20) (99T359). The thermodynamic data, given in Table IX, provide strong evidence that the predominance of the axial conformer in 63 is enthalpic in origin more polar solvents stabilize the more polar equatorial conformation. Both ab initio and DFT calculations reproduce the experimental results (99T359). 

The temperature dependence of the spectral spin diffusion and crossrelaxation was examined by Mueller et a/.287,288 with spin- and spin-1 systems. They showed that the diffusion rate can be strongly temperature dependent if it is motionally driven. It is therefore, unreliable to discriminate spin diffusion and chemical exchange by variable-temperature measurement of 2D exchange spectra. Mueller et al. suggested that the dependence of the polarization transfer rate on the spectral difference of the relevant resonances should be measured in a single crystal to safely distinguish the two different polarization transfer processes (see also ref. 289). They also explained satisfactorily why the relaxation of the quadrupolar order is much faster than the Zeeman order. This... 

Variable-temperature NMR spectroscopy is a well-established technique in the study of conformational exchange kinetics in liquid amides. [7,58] Using DNMR spectroscopy, it is possible to measure the temperature dependence of interconversion rates in order to yield a complete set of kinetic parameters characterizing the internal rotation about the carbon-nitrogen bond. In the last several years, the development of gas-phase NMR methods has provided a valuable tool for the elucidation of the magnitude of intrinsic and environmental contributions associated with this conformational exchange. In order to characterize these contributions, a series of molecules with systematic variations in substituent size and polarity must be analyzed both in the liquid and gas phase. 

Clarke and Parker54 have reported a thermodynamic study of the tautomerization of 2-(J/V-cyclohexylimino)-l,3-diphenylpropane (47) to its Z- and E-enamine tautomers 48a and 48b in DMSO-d6 solution (Scheme 4). The equilibrium constants and the values of the thermodynamic parameters AH, AG and AS have been determined by variable-temperature NMR measurements. Polar solvents are found to favor enaminization, but have little effect on the E Z isomerization of the enamine tautomers. 

The chiral triflate complex (ebthi)Ti(OTf)2 has been synthesized by the reaction of the parent dichloride with AgOTf and used to catalyze the Diels-Alder reaction of cyclopentadiene with oxazolidinone-derived dienophiles. The level of asymmetric induction is dramatically affected by solvent polarity and this behavior can be partially explained with reference to the results of variable-temperature NMR studies.1665... 

These compounds are less aggregated in solution. For instance, 2,2-dibutyl-l,3,2-dioxastannolane, a polymer in the solid state, has been shown, by variable temperature Sn NMR spectroscopy, to be a mixture of dimers, trimers, and tetramers in solution, with dimers predominating at room temperature and above. This technique has also indicated that most carbohydrate-derived stannylene acetals are present predominantly as dimers in solution." " " Supporting evidence has been obtained from mass spectral studies and by comparison of solid-state NMR spectra with those of solutions." Some dibutylstannylene acetals derived from cw-diols contain an observable proportion of higher oligomers benzyl 4,6-0-benzylidene-2,3-0-dibutylstannylene-o -D-mannopyranoside is present as a mixture of a dimer and a trimer in chloroform-d at -60 °C but, in the less polar solvent toluene-dg, is mainly present as a tetramer at that temperature. ... 

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