Experimental and ab Initio Study of the HO_2·CH_3OH Complex: Thermodynamics and Kinetics of Formation
Abstract
Near-infrared spectroscopy was used to monitor HO_2 formed by pulsed laser photolysis of Cl_2−O_2−CH_3OH−N_2 mixtures. On the microsecond time scale, [HO_2] exhibited a time dependence consistent with a mechanism in which [HO_2] approached equilibrium via HO_2 + HO_3OH^M⇆_M HO_2·CH_3OH (3, −3). The equilibrium constant for reaction 3, Kp, was measured between 231 and 261 K at 50 and 100 Torr, leading to standard reaction enthalpy and entropy values (1 σ) of Δ_rH°_(246K) = −37.4 ± 4.8 kJ mol^(-1) and Δ_rS°_(246K) = −100 ± 19 J mol^(-1) K^(-1). The effective bimolecular rate constant, k_3, for formation of the HO_2·CH_3OH complex is 2.8^(+7.5)_(-2.0)·10^(-15)·exp[(1800 ± 500)/T] cm^3 molecule^(-1) s^(-1) at 100 Torr (1 σ). Ab initio calculations of the optimized structure and energetics of the HO_2·CH_3OH complex were performed at the CCSD(T)/6-311++G(3df,3pd)//MP2(full)/6-311++G(2df,2pd) level. The complex was found to have a strong hydrogen bond (D_e = 43.9 kJ mol^(-1)) with the hydrogen in HO_2 binding to the oxygen in CH_3OH. The calculated enthalpy for association is Δ_rH°_(245K) = −36.8 kJ mol^(-1). The potentials for the torsion about the O_2−H bond and for the hydrogen-bond stretch were computed and 1D vibrational levels determined. After explicitly accounting for these degrees of freedom, the calculated Third Law entropy of association is Δ_rS°_(245K) = −106 J mol^(-1) K^(-1). Both the calculated enthalpy and entropy of association are in reasonably good agreement with experiment. When combined with results from our previous study (Christensen et al. Geophys. Res. Lett. 2002, 29; doi:10.1029/2001GL014525), the rate coefficient for the reaction of HO_2 with the complex, HO_2 + HO_2·CH_3-OH, is determined to be (2.1 ± 0.7) × 10^(-11) cm^3 molecule^(-1) s^(-1). The results of the present work argue for a reinterpretation of the recent measurement of the HO_2 self-reaction rate constant by Stone and Rowley (Phys. Chem. Chem. Phys. 2005, 7, 2156). Significant complex concentrations are present at the high methanol concentrations used in that work and lead to a nonlinear methanol dependence of the apparent rate constant. This nonlinearity introduces substantial uncertainty in the extrapolation to zero methanol.
Additional Information
© 2006 American Chemical Society. Received 14 November 2005. Published online 19 April 2006. Published in print 1 June 2006. This research was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). This work was supported by the NASA Upper Atmosphere Research and Tropospheric Chemistry Programs and the NASA Graduate Student Research Program (GSRP). This research has also been supported in part by a grant from the U.S. Environmental Protection Agency National Center for Environmental Research's Science to Achieve Results (STAR) program, through grant R826236-01-0. It has not been subjected to any EPA review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. We would like to acknowledge the indispensable scientific advice of Barna László, Kyle Bayes, Brian Drouin, and Herbert Pickett, the vital technical support of Dave Natzic and Jürgen Linke, and all the exceptional work of Siamak Forouhar, Kamjou Mansour, and Sam Keo of the Microdevices Laboratory in the manufacture and testing of the diode laser. We thank David Rowley and Daniel Stone for helpful discussions of their data. Note Added in Proof: Very recently, rotational transitions of HO2·H2O have been observed (Suma et al. Science 2006, 311, 1278).Additional details
- Eprint ID
- 77970
- Resolver ID
- CaltechAUTHORS:20170606-131245100
- NASA/JPL/Caltech
- NASA Graduate Student Research Fellowship
- R826236-01-0
- Environmental Protection Agency (EPA)
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2017-06-07Created from EPrint's datestamp field
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2021-11-15Created from EPrint's last_modified field