OsO~+氧化活化氢分子气相反应机理的密度泛函理论计算

为了探寻OsO+与H2气相反应的机理,用密度泛函理论方法 UB3LYP,全优化了该反应的加成(氧化加成和[2+2]环加成)-消除、氢抽提-反弹,以及氧端插入等四种可能路径中所有可能的反应物、中间体、过渡态和产物在六重态、四重态和二重态等三个自旋态下的几何结构,计算了各种机理反应的势能面.结果表明,标题反应为自旋禁阻反应,反应起始自四重态,最终产物为六重态基态,整个反应放热21.0 kJ mol-1.因反应络合物相对于入口通道有太正Gibbs函数,氧端插入机理是高能的过程.其他三种机理都具有多(或二)态反应性(MSR或TSR).其中,两种加成-消除机理的最低能量路径都可能经由四重态-二重态-四重态-六重态的三次自旋翻转,抽提-反弹机理的最低能量路径可能经历由四重态-六重态的自旋翻转.抽提-反弹机理由势能面一路攀升的吸热氢抽提过程和几乎无能垒的强放热的反弹过程组成,所以按该机理反应在常温常压下难以发生.两种加成-消去机理的决速步(第二个H的迁移步)相同,虽然其位垒稍高,为156.9 kJ mol-1,但与其进程中前面的强放热步骤耦合,常温常压下该反应是可以发生的.其中,协同环加成步的位垒仅28.7 kJ mol-1,比第一个H的还原消去步的位垒低113.7 kJ mol-1,所以竞争的结果是,常温常压下[2+2]环加成-消去机理比氧化加成-消去机理在动力学上更有利.

Density Functional Investigation on the Reaction Mechanisms of Oxidative Activation of Dihydrogen by Osmium Oxide Cation in Gas Phase

Density functional calculations with UB3LYP functional and an extended ECP basis set are employed to calculate the geometries and energies for all possible reactants,intermediates,transition states and products on sextet,quartet and doublet surfaces in four pathways of addition(oxidative addition and [2+2] cycloaddition)-elimination,abstraction-rebound and oxene-insertion for investigating the mechanisms of oxidative activation of dihydrogen by osmium oxide cation.From the results calculated,the title reaction is spin-forbidden,which starts on the quartet surface and ends on sextet surface,the overall reaction is exothermic by 21.0 kJ mol-1.Oxene-insertion process is unfavorable thermodynamically due to more positive Gibbs free energy for the reactant complexes.The other three mechanisms proposed exhibit multiple-state-reactivity(MSR) or two-state-reactivity(TSR).Individually the surfaces in three spin states for the two addition-elimination pathways may cross over three times,while the sextet and quartet surfaces for abstraction-rebound may cross once,respectively.The abstraction-rebound mechanism starts on the H-abstraction process with uphill potential surfaces and high endothermicity,followed by a barrierless and highly exothermic rebound of H atom,thus it cannot take place at normal temperature.While the two addition-elimination processes have the same rate-determining step,where each barrier is about 156.9 kJ mol-1,which is a little higher than that for the usual reactions in liquid,however it is possible to take place due to coupling with the highly exothermic steps before.Furthermore,the concerted [2+2] cycloaddition step has a lower barrier of only 28.7 kJ mol-1,which is 113.7 kJ mol-1 lower than that for the step of the reductive elimination of the first hydride in oxidative addition-elimination process.Thus,[2+2] cycloaddition process is more favorable than oxidative addition process kinetically.