Bond energies of weakly bound molecules

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The rate of reaction of elementary bimolecular reactions in the gas phase is usually determined by the height of the energy barrier between reagents and products. Most reactions proceed exponentially faster as the temperature is raised according to the Arrhenius rate law. This is because the fraction of molecules with sufficient energy to surmount the barrier is determined by the Boltzmann distribution function. Some reactions however exhibit a negative temperature dependence in which the rate of reaction increases as the temperature decreases. This so-called non Arrhenius behaviour occurs when the reaction is barrierless. An interesting class of reactions show Arrhenius behaviour at high temperatures but also speed up at low temperatures. This behaviour is attributed to the existence of long range weakly bound complexes in the entrance channel due to van der Waals forces between the reagents, and an important class of reactions which often exhibit this behaviour are those between radicals and molecules. These reactions are of particular importance in the cold environments of the tropopause of the earth's atmosphere, the atmospheres of the gaseous planets and some of their moons, and in the gas clouds surrounding young stars in interstellar space. Recently it has become possible to trap these weakly bound species in the laboratory and to study their properties. An experiment carried out at the University of Pennsylvania by Marsha Lester and her co-workers measures the infrared absorption spectra of these complexes by exciting them with a pulse of infrared radiation and observing one of the resulting fragments as the complex is heated using laser induced fluorescence. By calculating the energy difference between the energy of the dissociating photon and the maximum internal energy observed in the photofragment they are able to deduce an upper bound for the bond dissociation energy of the complex. In the case of the hydrotrioxy radical, HO-OO, the value they so obtain is large enough that in the upper earth atmosphere about a quarter of the OH radicals would be expected to be complexed with molecular oxygen and if true would provoke a significant rethink of our understanding of the chemistry of the tropopause. Theory on the other hand predicts a bond dissociation energy for HO-OO that is about half the value suggested by the experiment. The reason for the discrepancy is most probably due to the assumption that the dissociation is barrierless and what is required is a direct measurement of the bond energy of the complex. We propose to do this using a technique called velocity map imaging. Even though we are fairly confident that we understand the reason for the apparent discrepency between theory and experiment in the case of the hydrotrioxy radical, our experiment would be the first direct measurement of the dissociation energy and would provide the essential data needed to assess the true atmospheric importance of the species. More generally we propose to study a number of other similar complexes and obtain accurate data on their dissociation dynamics by measuring properties such as the internal energy distribution in the photoproducts and correlations between their recoil velocity vectors and rotational angular momenta. The principal output of our research will be to provide data on the bond energies of these complexes which may be compared to quantum mechanical electronic structure calculations and fed into chemical kinetic models of the reactions of radicals and molecules.