Departamento de Química Física, Facultad de Química, Universidad Complutense (Unidad Asociada CSIC), 28040 Madrid, Spain.
Phys Chem Chem Phys. 2012 Nov 14;14(42):14596-604. doi: 10.1039/c2cp42130e. Epub 2012 Sep 28.
Quantum mechanical (QM) and quasiclassical trajectory (QCT) calculations have been carried out for the exchange reactions of D and Mu (Mu = muonium) with hydrogen molecules in their ground and first vibrational states. In all the cases considered, the QM rate coefficients, k(T), are in very good agreement with the available experimental results. In particular, QM calculations on the most accurate potential energy surfaces (PESs) predict a rate coefficient for the Mu + H(2) (ν = 1) reaction which is very close to the preliminary estimate of its experimental value at 300 K. In contrast to the D + H(2) (ν = 0,1) and the Mu + H(2) (ν = 0) reactions, the QCT calculations for Mu + H(2) (ν = 1) predict a much smaller k(T) than that obtained with the accurate QM method. This behaviour is indicative of tunneling. The QM reaction probabilities and total reactive cross sections show that the total energy thresholds for the reactions of Mu with H(2) in ν = 0 and ν = 1 are very similar, whereas for the corresponding reaction with D the ν = 0 total energy threshold is about 0.3 eV lower than that for ν = 1. The results just mentioned can be explained by considering the vibrational adiabatic potentials along the minimum energy path. The threshold for the reaction of Mu with H(2) in both ν = 0 and ν = 1 states is the same and is given by the height of the ground vibrational adiabatic collinear potential, whereas for the D + H(2) reaction the adiabaticity is preserved and the threshold for the reaction in ν = 1 is very close to the height of the ν = 1 adiabatic collinear barrier. For Mu + H(2) (ν = 1) the reaction takes place by crossing from the ν = 1 to the ν = 0 adiabat, since the exit channel leading to MuH (ν = 1) is not energetically accessible. At the lowest possible energies, the non-adiabatic vibrational crossing implies a strong tunneling effect through the ν = 1 adiabatic barrier. Absence of tunneling in the classical calculations results in a threshold that coincides with the height of the ν = 1 adiabatic barrier. Most interestingly, the expected tunneling effect in the reaction of Mu with hydrogen molecules occurs for H(2) (ν = 1) but not for H(2) (ν = 0) where zero-point-energy effects clearly dominate.
已针对 D 和 Mu（Mu = muonium）与氢分子在基态和第一振动态的交换反应进行了量子力学（QM）和准经典轨迹（QCT）计算。在所考虑的所有情况下，QM 速率系数 k（T）与可用的实验结果非常吻合。特别是，在最精确的势能面（PES）上的 QM 计算预测了 Mu + H（2）（ν = 1）反应的速率系数，该值非常接近其在 300 K 下的实验值的初步估计值。与 D + H（2）（ν = 0，1）和 Mu + H（2）（ν = 0）反应相反，对于 Mu + H（2）（ν = 1）的 QCT 计算预测的 k（T）比用准确的 QM 方法获得的 k（T）小得多。这种行为表明存在隧道效应。QM 反应概率和总反应截面表明，Mu 与 H（2）在ν = 0 和ν = 1 下的反应的总能量阈值非常相似，而对于相应的与 D 的反应，ν = 0 的总能量阈值比ν = 1 低约 0.3 eV。可以通过考虑沿最小能量路径的振动绝热势能来解释刚才提到的结果。在ν = 0 和ν = 1 状态下 Mu 与 H（2）的反应的阈值相同，由基态振动绝热共线势能的高度给出，而对于 D + H（2）反应，绝热性得以保持，ν = 1 反应的阈值非常接近ν = 1 绝热共线势垒的高度。对于 Mu + H（2）（ν = 1），反应通过从ν = 1 到ν = 0 绝热的交叉来进行，因为通往 MuH（ν = 1）的出口通道在能量上无法到达。在可能的最低能量下，非绝热振动交叉意味着通过ν = 1 绝热势垒的强烈隧道效应。在经典计算中不存在隧道效应导致的阈值与ν = 1 绝热势垒的高度重合。最有趣的是，Mu 与氢分子的反应中预期的隧道效应发生在 H（2）（ν = 1）上，但不在 H（2）（ν = 0）上，其中零点能效应显然占主导地位。
