Experimental and ab Initio Study of the HO₂·CH₃OH Complex:  Thermodynamics and Kinetics of Formation^†

Near-infrared spectroscopy was used to monitor HO₂ formed by pulsed laser photolysis of Cl₂−O₂−CH₃OH−N₂ mixtures. On the microsecond time scale, [HO₂] exhibited a time dependence consistent with a mechanism in which [HO₂] approached equilibrium via HO₂ + HO₂·CH₃OH (3, −3). The equilibrium constant for reaction 3, K_p, was measured between 231 and 261 K at 50 and 100 Torr, leading to standard reaction enthalpy and entropy values (1 σ) of Δ_r = −37.4 ± 4.8 kJ mol^-1 and Δ_r = −100 ± 19 J mol^-1 K^-1. The effective bimolecular rate constant, k₃, for formation of the HO₂·CH₃OH complex is ·10^-15·exp[(1800 ± 500)/T] cm³ molecule^-1 s^-1 at 100 Torr (1 σ). Ab initio calculations of the optimized structure and energetics of the HO₂·CH₃OH 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 HO₂ binding to the oxygen in CH₃OH. The calculated enthalpy for association is Δ_r = −36.8 kJ mol^-1. The potentials for the torsion about the O₂−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_r = −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 HO₂ with the complex, HO₂ + HO₂·CH₃OH, is determined to be (2.1 ± 0.7) × 10^-11 cm³ molecule^-1 s^-1. The results of the present work argue for a reinterpretation of the recent measurement of the HO₂ 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.