Elementary reactions of boron atoms with hydrocarbons - Toward the formation of organo-boron compounds

Abstract

This review article compiles recent studies on the reactivity of boron atoms with organic moleculesspredominantly hydrocarbonssleading to the formation of novel organoboron species, with the characterization of their formation mechanisms. The studies summarized here comprise kinetic and matrix isolation experiments, crossed molecular beam studies, and theoretical investigations of the underlying potential energy surfaces. The gas phase kinetics studies of atomic boron reactions provided unique information on the reaction rate. These data can be provided neither by matrix isolation studies nor by crossed beam experiments. Nevertheless, since in the kinetics investigations the rate constants were derived by observing the decay of the concentration of the boron atom reactant, kinetics studies cannot deliver important information on the reaction products, the intermediates involved, and, hence, the reaction mechanisms. On the one hand, the matrix isolation experiments have the unprecedented capability to observe the reaction intermediatessif they can be stabilized in the matrixsand sometimes the reaction products spectroscopically, predominantly via infrared spectroscopy. Since these studies were not conducted under single collision conditions, it is difficult, however, to untangle the underlying reaction mechanisms and extract information on the chemical dynamics. Likewise, since the boron atoms are generated via laser ablation, it is unknown to what extent he atoms are generated in their electronic ground or excited state. This is crucial, since the reactivity strongly depends on the electronic state. Finally, cage effects from the matrix can influence the outcome of the reaction, and the main reaction products and intermediates might be different from those observed under single collision conditions in the gas phase. On the other hand, the third experimental approach illustratedsthe crossed beam techniqueshas the advantage that both reactants can be prepared under well-defined experimental conditions in separate supersonic beams. Because of the large mean free path, achieved by operating at a very low pressure, the products are formed only at the collision center and then fly undisturbed toward the detector. This eliminates completely the possible wall effects of bulk experiments and the cage effect of matrix experiments. Further, the spin state of the reacting boron atoms can be controlled. Based on the mass-to-charge ratio of the observed product(s) and the energetics, the product isomer can be identified unambiguously. Mechanistical studies can be further expanded by utilizing (partially) deuterated reactants in those cases in which the hydrogen atom(s) of the secondary reactant are not chemically equivalent. In conclusion, the chemistry that controls the formation of organo-boron compounds in combustion chemistry, astrochemistry, weakly ionized plasmas, material sciences, and CVD processes has partially been unveiled. A more general conclusion is that, due to the advantages but also limitations of each of the experimental approaches presented here, only a combined effort including kinetics and matrix isolation experiments as well as crossed molecular beam and theoretical studies can allow us to understand the chemical reactivity of systems of increasing complexity, such as those described in this review. © 2010 American Chemical Society.