Christensen Lance E, Okumura Mitchio, Hansen Jaron C, Sander Stanley P, Francisco Joseph S
Arthur Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA.
J Phys Chem A. 2006 Jun 1;110(21):6948-59. doi: 10.1021/jp056579a.
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 sigma) of delta(r) = -37.4 +/- 4.8 kJ mol(-1) and delta(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 sigma). 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 delta(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 delta(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) x 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.
近红外光谱法用于监测由Cl2 - O2 - CH3OH - N2混合物的脉冲激光光解形成的HO2。在微秒时间尺度上，[HO2]表现出与一种机制一致的时间依赖性，即[HO2]通过HO2 + HO2·CH3OH (3, -3)达到平衡。反应3的平衡常数K(p)在231至261 K、50和100 Torr下进行了测量，得到标准反应焓和熵值(1σ)分别为Δr = -37.4 ± 4.8 kJ mol(-1)和Δr = -100 ± 19 J mol(-1) K(-1)。在100 Torr (1σ)下，形成HO2·CH3OH络合物的有效双分子速率常数k3为10(-15)·exp[(1800 ± 500)/T] cm3 molecule(-1) s(-1)。在CCSD(T)/6 - 311++G(3df,3pd)//MP2(full)/6 - 311++G(2df,2pd)水平上对HO2·CH3OH络合物的优化结构和能量进行了从头算。发现该络合物具有很强的氢键(D(e) = 43.9 kJ mol(-1))，HO2中的氢与CH3OH中的氧结合。计算得到的缔合焓为Δr = -36.8 kJ mol(-1)。计算了围绕O2 - H键的扭转势能和氢键伸缩势能，并确定了一维振动能级。在明确考虑这些自由度后，计算得到的第三定律缔合熵为Δr = -106 J mol(-1) K(-1)。计算得到的缔合焓和熵与实验结果都相当吻合。结合我们之前研究(Christensen等人，《地球物理研究快报》2002年，29卷；doi:10.1029/2001GL014525)的结果，确定HO2与该络合物反应HO2 + HO2·CH3OH的速率系数为(2.1 ± 0.7)×10(-11) cm3 molecule(-1) s(-1)。本工作的结果表明需要重新解释Stone和Rowley最近对HO2自反应速率常数的测量(《物理化学化学物理》2005年，7卷，2156页)。在该工作中使用的高甲醇浓度下存在显著的络合物浓度，导致表观速率常数对甲醇呈非线性依赖。这种非线性在外推至零甲醇时引入了很大的不确定性。
