Exchange experiments/spectroscopy

Protonated methanes and their homologues and derivatives are experimentally indicated in superacidic chemistry by hydrogen-deuterium exchange experiments, as well as by core electron (ESCA) spectroscopy of their frozen matrixes. Some of their derivatives could even be isolated as crystalline compounds. In recent years, Schmidbaur has pre-... 

Exchange correlation spectroscopy (E. COSY), a modified form of COSY, is useful for measuring coupling constants. The pulse sequence of the E. COSY experiment has a mixing pulse )3 of variable angle. A number of experiments with different values of /3 are recorded that eliminate the multiplet components of unconnected transitions and leave only the multiplet components for connected transitions. This simplified 2D plot can then be used to measure coupling constants. 

We have examined the X-ray results from various oriented curdlan gels and report our findings and calculations on a number of models. In addition we have investigated the behaviour of curdlan gels in a series of deuterium exchange experiments monitored with infra-red spectroscopy. 

Since both the stereochemical results and the exchange experiments are inconclusive regarding competition of the two reaction routes for the cis isomer, the dissection of the reaction into the contributions of the two routes requires the isolation or the estimation of the intermediate. Under suitable conditions, p-toluenesulphonylacetylene is the main product from bromo-(ll), while it is only detected by infrared spectroscopy during the reaction of chloro-(ll). Dissection of the overall substitution rate constant (kt) into contributions from elimination-addition ( ellm) and addition-elimination ( 8ub) is possible when the rate of the alkoxide-catalysed addition of alcohol to the intermediate acetylene (fcadd) and the concentrations of the latter during the reaction are known. Such analysis for cis-(ll) and (12) (Table 10) shows that at 0° the contributions of the two processes to the overall rate are nearly equal, but the importance of the elimination-addition route increases with the temperature, kum/fenih = 3 0 and 1-4 for (11) and (12) respectively at 25° (DiNunno... 

Multidimensional NMR experiments also provide additional information that is unavailable from onedimensional (ID) spectra even in the limit of high-resolution. For example, 2D 2H exchange experiments, which identify a particular change in molecular orientation between two evolution periods, have already been mentioned above.9 For dipolar-coupled nuclei, the combination of fast MAS with 2D multiple-quantum (MQ) spectroscopy achieves high resolution, while allowing the structural and dynamic information inherent to the dipolar couplings to be accessed. Specifically, using the homonuclear H- H double-... 

In velocity-change NMR the variables q and q 2 replace the axes ki and kz in Fig. 5.4.5, and ui and V2 replace r and r2 (Fig. 5.4.12). In a coordinate frame rotated by 45° the difference coordinate corresponds to acceleration a and the other to velocity v, so that this exchange experiment can be read as a velocity-acceleration correlation experiment. Following the coordinate transformations (5.4.21) for position-change spectroscopy, the following coordinate transformations apply for velocity-change spectroscopy. 

Historically, hydrogen exchange experiments (i.e., the replacement of one isotope of hydrogen bound to an O, N, or S atom in the protein interior by another isotope from the solvent water) provided some of the earliest evidence for the existence of conformational fluctuations in proteins. More recently, a wide range of experimental methods (such as fluorescence quenching and depolarization, nuclear magnetic resonance relaxation, infrared and Raman spectroscopy, and X-ray and inelastic neutron scattering) have been used to study the motions in proteins. However, it is primarily the application of theoretical methods, particularly molecular dynamics simulations, that have... 

Infra-red spectroscopy in combination with pyridine adsorption as well as sodium ion exchange experiments were applied to CoAPO -5 and -11 (samples 9 and 6) in order to monitor changes in the number of acidic sites during oxidation and reduction of framework cobalt. Our results (Fig. 7) clearly show a considerable increase in the number of Lewis and Bronsted acidic sites and in the cation exchange capacity upon reduction of Co(III) to Co(II). In particular, the cation exchange capacity is almost quantitatively in line with the amount of framework Co(II) as determined by means of UV-VIS spectroscopy (Fig.6). 

Solid state NMR spectroscopy was applied to measure the isotropic chemical shifts, chemical shift anisotropies and asymmetry parameters of three phosphorylated amino acids, O-phospho-L-serine, O-phospho-L-threonine and O-phospho-L-tyrosine. The CP buildup rates and longitudinal relaxation times of P and H were determined and compared with the values measured for a triphosphate bound to a crystalline protein. It was shown that the phosphorylated amino acids are well-suited model compounds, e.g. for the optimisation of experiments on crystalline proteins. In addition, from 2D exchange experiments on O-phospho-L-tyrosine the existence of an exchange between the two different conformations of the molecule was deduced. 

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