Weakly-Bound Complexes and Short-Lived Intermediates
Many ab initio studies have proposed weakly-bound complexes as potentially crucial precursors to atmospheric reactions. If these pre-reactive complexes do indeed exist under atmospheric conditions, they could exert strong influence over the kinetics of associated reactions by altering barrier heights and/or affecting the energy partitioning of reaction products. The presence of a complex may also have a significant effect on the dynamics of the reaction by spatially directing the site of the reaction, either by steric direction or by favoring a reaction site by providing a pre-reactive potential energy well.
A supersonic jet expansion will allow for the cold preparation of these complexes, which should be detectable using IR CRDS [Paul et al., 1997]. Characterization will allow for the determination of binding energies and equilibrium constants, an important step in determination of atmospheric abundances [Vaida and Headrick, 2000]. Once the existence of a given complex is determined and its gas-phase spectroscopic signature is known, its formation may be examined under thermal conditions, especially at the low temperatures and high pressures of our HPF systems. If formation is found to be significant, kinetics of bimolecular reactions may be measured directly and the effects of complexation on reaction kinetics may be better understood.
For example, there is considerable theoretical [Zhu and Lin, 2002] and experimental [Nelander, 1997] [Aloisio et al., 2000] evidence that the hydroperoxy radical (HO2) will form a hydrogen-bonded complex with a water molecule, affecting the kinetics of HO2 reactions significantly. The water molecule may affect the steric direction of reactions, or in the case of reactions proceeding via reactive intermediates (HO2 + NO, HO2 + HO2), may serve as a “chaperone” molecule, facilitating stabilization of the intermediate. While little attention has been paid to RO2-H2O complexes, it is reasonable to assume that these too may affect RO2 + NO reaction kinetics or branching, and therefore the efficiency of the tropospheric HOX cycle. However, complexes of H2O and RO2 (or HO2) have not been observed in the gas phase, making assessment of the tropospheric importance of these complexes difficult. By detecting these complexes directly, estimating their atmospheric abundances, and measuring their effects on HO2 or RO2 kinetics, we may thus better understand the effect of water vapor on HOX cycling.
Other relevant complexes that we plan to investigate include O3-alkene, O3-H2O and nitric acid-H2O; detection of short-lived radical-molecule complexes such as OH-nitric acid or OH-alkene may also be possible. Detecting these complexes, and determining their role in reaction kinetics, constitutes an important step in our ability to model the chemistry of the troposphere and stratosphere. In addition, this experimental approach—preparation and detection of reactive species in a supersonic jet coupled with CRDS—will also be used for the spectroscopic characterization of the short-lived reaction intermediates discussed above, such as RO2, RO, HOONO, and ROONO.
Our HPF system, coupled with LIF detection of radicals, has been used to constrain the cate of the HO + NO2 reaction at temperatures and pressures relevant to the upper troposphere/lower stratosphere [Dransfield et al., 1999]. We plan to extend this technique to a number of simple OH-molecule reactions, including
Imprementation of a wider variety of spectroscopic techniques for the direct detection of radicals (ARF, CRDS) will allow us to also study the kinetics of reactions such as