Fast-exchange process

Water exchange proceeds through a trigonal bipyramidal reactive intermediate [Li(H20)5]+, reached via a late transition state. In accordance with the above-mentioned experimental observation of a very fast exchange process, the activation barrier... 

Mechanistic details are very similar for DMSO and water exchange. The reaction proceeds through a distorted trigonal bipyramidal reactive intermediate [Li(DMSO)5]+ that is reached via a late transition state. The enthalpy profile (see Fig. 13) is in line with the experimentally observed very fast exchange process. The five-coordinate intermediate is computed to be 7.9kcalmol 1 less stable than [Li(DMS0)4]+ and free DMSO, while an overall activation barrier of only 8.4kcalmol 1 is computed. Obviously,... 

Salts of the various optically active aromatic acids and amines employed extensively as resolving agents frequently exhibit large nonequivalence (17,18,56-69). Since a fast-exchange process is... 

The ionophoric properties of 13 toward soft metal cations were evaluated by using the picrate extraction method. Metal ions like Ag" (91%), Tl" (38%) and Hg " (16%) were extracted efficiently with a maximum for Ag". Cu ", Co ", Zn ", Cd ", and ions were not extracted. The better ex-tractability of 13 toward silver(I) cation was attributed to the latter s high affinity for sulfur. A fast exchange process, on the NMR time scale, was observed by NMR between free and complexed 13. In presence of a twofold excess of Ag(I) salt, a 2 4 complex 132 (AgPic)4 was formed (see below) [70]. With this type of complex, the ionic radius of the metal ion is not concerned and the softness of the metal ion should be considered relatively to the soft... 

UV spectroscopy has been particularly informative about the oxepin-arene oxide ratio at room temperature (67AG(E)385) where NMR spectroscopy has been of little value (due to fast exchange processes). In order to use the UV method of analysis indane 8,9-oxide (the valence tautomer of oxepin 22) was assumed to be totally in the oxirane form while... 

ULTRASONIC RELAXATION STUDY OF FAST EXCHANGE PROCESSES IN MIXED MICELLE SYSTEMS OF ALCOHOL-DECYLTRIMETHYLAMMONIUM BROMIDE-WATER... 

Chemical relaxation methods are widely used to study the kinetics of fast chemical reactions in solution. In particular, ultrasonic absorption techniques have been used to investigate fast exchange processes with relaxation times in the range 0.3 xs to 0.3 ns. The longer end of this time range has become accessible in recent years through the use of cylindrical resonator methods [1] which have lowered the frequency range covered by fictional pulse echo methods. 

Fast exchange processes and thus, a high chromatographic efficiency of the separator column are ensured by the small size of the latex beads. 

Vq falls in between Va and Vb and can be used as a point of reference to discuss precession of the magnetic vectors of the protons at the two sites. At site A, the magnetic vector is imagined to process at a frequency Vq — Va at B, it processes in the opposite direction with a frequency Vb — Vq. Now the effect of the chemical exchange on the appearance of the NMR spectrum can be considered in greater detail (see fig. 7.18). Four representative situations are considered from a qualitative point of view, namely, very slow, moderately slow, moderately fast, and very fast exchange processes. 

Fig. 7.18 NMR spectra for chemical exchange between two S5mimetrical sites for the cases of (A) very slow, (B) moderately slow, (C) moderately fast, and (D) very fast exchange processes.

Kempf JG, Jung JY, Sampson NS, Loria JP (2003) Off-resonance TROSY (R1 rho -Rl) for quantitation of fast exchange processes in large proteins. J Am Chem Soc 125 12064—12065... 

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