Cage reactions, description

CARBORANE. A cry stalline compound composed of boron, carbon, and hydrogen. It can be synthesized in various ways, chiefly by the reaction of a borane (penta-or deca-) with acetylene, either at high temperature in the gas phase or in the presence of a Lewis base. Alkylated derivatives have been prepared. Carborancs have different structural and chemical characteristics and should not be conTused with hydrocarbon derivatives or boron hydrides. The predominant structures arc the cage type, the nest type, and the web type, these terms being descriptive of the arrangement of atoms in the crystals. Active research on cargorane chemistry has been conducted under sponsorship of the U.S. Office of Naval Research, http //www.onr.navy.mil/... 

In order for two reactants A and B to react in a bimoiecuiar reaction, they need to be brought in the vicinity of each other. When dispersed in a fluid, this happens by diffusive motion, which is entirely different from the free motion in the gas phase. Once an encounter between two reactants takes place, they will usually stay together much longer than in a gas phase due to a cage effect of the surrounding fluid molecules. This allows for numerous exchanges of energy between reactants and fluid, and thereby for activation and deactivation of the reaction complex. A complicated interplay between diffusion rates and reaction rates may determine the overall reaction rate in a fluid. We shall study an example of how diffusive motion and chemical reactions are combined in a description of chemical reactions in solution. 

At the same time, intemive theoretical developments have been devoted to the microscopic description of a solvent cage around ground or excited states 25-31). An understanding of solvent motions during electron-transfer reactions requires the investigation of elementary steps in conjunction with the... 

A naive description of the main steps of a reactive event can be sketched two reactants approach each other in the reaction medium, and the result of such approach (collision) may or may not lead to the products, depending both on the molecular states and reciprocal orientation and the interactions with the medium. While liquid phase reactions are favourite in laboratories, solid matrices are also used as reaction media. Among solids, zeolites, with their arrays of molecular sized cages and channels, are used as reaction pots in many synthetic processes both in research laboratories and in large scale industrial plants. 

There are two distinct types of experiments with caged compounds to be considered here those concerned with the development and assessment of new compounds and those connected with specific applications to different physiological processes. We shall discuss the two problems in succession. Detailed descriptions of methods for assessing the rates and quantities of the photolytic formation of active substrates from different caged compounds are presented by Walker et al. (1988) and Milbum et al. (1989). Some aspects of the decomposition of npe-ATP will serve as an example for such investigations. The broad absorption band of l-(2-nitrophenyl)ethyl phosphate in the near UV, allows the use of a wavelength for photolysis which results in minimum interference with that used for monitoring the reaction. The rapid ( > 10 s ) initial formation of the presumed acinitro intermediates (with extinction coefficients of 6.3 x 10 cm and 9.1 X 10 M cm at 380 nm and 406 nm respectively) and the concomi-... 

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