Transition states, chemistry, hydration

Taube and Bray noted in 1940 that the standard potential for the F(g)/F couple is 4.04 V (310). Berdnikov and Bazhin obtained a solution-phase potential by use of a thermochemical cycle that involves estimating the free energy of hydration of the fluorine atom (45) this led to a calculated E° of 3.6 V for the F/F couple and a corresponding AfG° of 68 kJ/mol for F. The chemistry of F2 and HOF has been discussed in Thompson s review (315), but there is no evidence of fluorine-containing radicals in the reactions of these species. There does not seem to be any reported evidence for the existence of F2 in aqueous solution, although the species has been detected in irradiated crystals. In the reversible reaction of H with F to give e q and HF, it has been argued that HF" does not exist as an intermediate but only as a transition state (16). 

Preparation of peptide aldehydes has been reported in numerous papers because these compounds present two main interests. First, they can be used for further chemistry (formation of reduced bond, Wittig reactions, Ugation, etc.), and second, they have been found to be potential inhibitors of several classes of enzymes such as serine proteases (34,35), prohormone convertases (36), cysteinyl proteases (34,37,38), and aspartyl proteases (39,40). These inhibitory properties result from the tetrahedral hydrated C-terminus aldehydic function, which mimics the transition state of the substrate hydrolysis. 

There are many examples of second-order analyzers that are used in analytical chemistry including many hyphenated spectroscopic tools such as FTIR-TGA, IR-microscopy, as well as GC-MS, or even two-dimensional spectroscopic techniques. Another hyphenated technique that is being developed for the study of solid-state transitions in crystalline materials is dynamic vapor sorption coupled with NIR spectroscopy (DVS-NIR).26 DVS is a water sorption balance by which the weight of a sample is carefully monitored during exposure to defined temperature and humidity. It can be used to study the stability of materials, and in this case has been used to induce solid-state transitions in anhydrous theophylline. By interfacing an NIR spectrometer with a fiber-optic probe to the DVS, the transitions of the theophylline can be monitored spectroscopically. The DVS-NIR has proven to be a useful tool in the study of the solid-state transitions of theophylline. It has been used to identify a transition that exists in the conversion of the anhydrous form to the hydrate during the course of water sorption. 

The contribution of pulse radiolysis to general chemistry is very significant, and this is exemplified by the following studies of transition metal complexes. The reduction of tris(2,2 -bipyridine)ruthenium(III) ion by the hydrated electron was the first example of this type of reaction to show clearly the formation of a product in an electronically excited state [80] ... 

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