Experimental Characterization of Reaction Intermediates Retardation

There are various ways of slowing down or even suppressing totally the reactions of a molecule. In principle, we may limit its collisions with other molecules by confining it to the gas phase and there minimizing its pressure we may minimize the thermal energy to which it has access by lowering its temperature or we may trap it with a suitable substrate. The best policy in practice may well be either simultaneously or in separate experiments to apply more than one of these restraints. 

A good example is the molecule SiO, a cosmic precursor to silicon oxide chemistry as we now know it on earth (22). In more recent times, too, this molecule has assumed considerable significance because of its relevance to oxidation reactions taking place at the surfaces of silicon wafers and to the creation of antireflection coatings on these and other solid-state devices. Unlike its more familiar counterpart carbon monoxide, SiO is normally quick to aggregate and disproportionate [Reaction (5)] at temperatures below 1000°C  

The molecule CIO is another reactive species that has attracted still more attention. It plays a significant role in the destruction of stratospheric ozone through catalytic cycles involving heterogeneous and homogeneous reactions, as in Eq. (6) (17)  

Current wisdom can be summarized in the flow chart of Fig. 3. More specifically, the CIO radical is involved in several possible kinetic mechanisms linking global release of chlorofluorocarbons (CFCs) to the Antarctic ozone loss during each austral spring (18). An accurate characterization of the spectroscopic and other properties of CIO is therefore vital, not least as a prelude to tracing the crucial correlation between CFCs, CIO, and 0 j. CIO is short-lived at the high molecular concentrations characteristic of the condensed phases but can be generated at low pressures in the gas phase, for example by the action of 

Chemical cycles affecting the formation and decay of chlorine oxide trace species in the earth s atmosphere (reproduced with permission from Wayne, R. P. Chemistry of Atmospheres, 2nd ed., p. 137, Clarendon Press Oxford, 1991). 

Stability is not a problem at high temperatures, though, and Reaction 

Solid boron nitride is a ceramic material of some consequence that can be formed at high temperatures by the reaction of boron atoms with N2 or NH3. A likely intermediary in its formation is another high-temperature molecule BN, first observed through its electronic emission spectrum nearly 60 years ago but the properties of which have emerged only slowly (14). Identification of the several low-lying electronic states of the molecule is a major problem and only very recently has it become clear that the ground state is not (as with the isoelectronic C2 molecule) but I1 (15). The A n—X Ili transition. 

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Reaction Intermediates Nerve Centers of Chemical Reactions Experimental Characterization of Reaction Intermediates Retardation... 

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