Hayes R L, Fattal E, Govind N, Carter E A
Contribution from the Department of Chemistry and Biochemistry, Box 951569, University of California, Los Angeles, California 90095-1569, USA.
J Am Chem Soc. 2001 Jan 31;123(4):641-57. doi: 10.1021/ja000907x.
We present complete active space self-consistent field (CASSCF) ab initio molecular dynamics (AIMD) simulations of the preparation of the metastable species vinylidene, and its subsequent, highly exothermic isomerization to acetylene, via electron removal from vinylidene anion (D(2)C=C(-) --> D(2)C=C: --> DC triple bond CD). After equilibrating vinylidene anion-d(2) at either 600 +/- 300 K (slightly below the isomerization barrier) or 1440 K +/- 720 K (just above the isomerization barrier), we remove an electron to form a vibrationally excited singlet vinylidene-d(2) and follow its dynamical evolution for 1.0 ps. Remarkably, we find that none of the vinylidenes equilibrated at 600 K and only 20% of the vinylidenes equilibrated at 1440 K isomerized, suggesting average lifetimes >1 ps for vibrationally excited vinylidene-d(2). Since the anion and neutral vinylidene are structurally similar, and yet extremely different geometrically from the isomerization transition state (TS), neutral vinylidene is not formed near the TS so that it must live until it has sufficient instantaneous kinetic energy in the correct vibrational mode(s). The origin of the delay is explained via both orbital rearrangement and intramolecular vibrational energy redistribution (IVR) effects. Unique signatures of the isomerization dynamics are revealed in the anharmonic vibrational frequencies extracted from the AIMD, which should be observable by ultrafast vibrational spectroscopy and in fact are consistent with currently available experimental spectra. Most interestingly, of those trajectories that did isomerize, every one of them violated conventional transition-state theory by recrossing back to vinylidene multiple times, against conventional notions that expect highly exothermic reactions to be irreversible. The dynamical motion responsible for the multiple barrier recrossings involves strong mode-coupling between the vinylidene CD(2) rock and a local acetylene DCC bend mode that has been recently observed experimentally. The multiple barrier recrossings can be used, via a generalized definition of lifetime, to reconcile extremely disparate experimental estimates of vinylidene's lifetime (differing by at least 6 orders of magnitude). Last, a caveat: These results are constrained by the approximations inherent in the simulation (classical nuclear motion, neglect of rotation-vibration coupling, and restriction to C(s) symmetry); refinement of these predictions may be necessary when more exact simulations someday become feasible.
我们展示了对亚稳态物种亚乙烯基制备过程的完全活性空间自洽场（CASSCF）从头算分子动力学（AIMD）模拟，以及随后通过从亚乙烯基阴离子中移除电子使其高度放热异构化为乙炔的过程（D₂C = C⁻ → D₂C = C: → DC≡CD）。在600 ± 300 K（略低于异构化势垒）或1440 K ± 720 K（略高于异构化势垒）下使亚乙烯基阴离子-d₂达到平衡后，我们移除一个电子以形成振动激发的单重态亚乙烯基-d₂，并跟踪其1.0 ps的动力学演化。值得注意的是，我们发现，在600 K达到平衡的亚乙烯基中没有一个发生异构化，而在1440 K达到平衡的亚乙烯基中只有20%发生异构化，这表明振动激发的亚乙烯基-d₂的平均寿命>1 ps。由于阴离子和中性亚乙烯基在结构上相似，但在几何形状上与异构化过渡态（TS）极为不同，中性亚乙烯基不是在过渡态附近形成的，因此它必须存活到在正确的振动模式中具有足够的瞬时动能。延迟的起源通过轨道重排和分子内振动能量重新分布（IVR）效应来解释。从AIMD中提取的非谐振动频率揭示了异构化动力学的独特特征，这应该可以通过超快振动光谱观察到，并且实际上与目前可用的实验光谱一致。最有趣的是，在那些确实发生异构化的轨迹中，每一个都违反了传统的过渡态理论，多次回穿过渡回亚乙烯基，这与预期高度放热反应不可逆的传统观念相悖。导致多次势垒穿越的动力学运动涉及亚乙烯基CD₂摇摆与最近实验观察到的局部乙炔DCC弯曲模式之间的强模式耦合。通过对寿命的广义定义，多次势垒穿越可用于协调对亚乙烯基寿命极为不同的实验估计（相差至少6个数量级）。最后需要提醒的是：这些结果受到模拟中固有的近似（经典核运动、忽略转动 - 振动耦合以及限制为Cₛ对称性）的限制；当更精确的模拟在未来某天变得可行时，可能需要对这些预测进行完善。
