Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia 30322, USA.
J Chem Phys. 2012 Jan 21;136(3):034305. doi: 10.1063/1.3666987.
We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated "reaction paths." The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm(-1). Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.
我们报告了与乙烯基自由基中 1,2 个 H 原子大振幅迁移到全局最小值相关的隧道分裂。这些是使用最近的全维从头算势能面（PES）[A. R. Sharma、B. J. Braams、S. Carter、B. C. Shepler 和 J. M. Bowman，J. Chem. Phys. 130(17), 174301 (2009)]和独立地直接计算的“反应路径”获得的。PES 是耦合簇单和双和微扰三重激发耦合簇单双三重（CCSD(T)）与增广相关一致三重 zeta 基组（aug-cc-pVTZ）的多维拟合。反应路径势能是从一系列 CCSD(T)/aug-cc-pVnTZ 计算中获得的，这些计算外推到完全基组极限。使用这些势能以及非常不同的 1D 哈密顿量，对这些 1,2-H 原子迁移的隧道分裂进行了近似 1D 计算。在很大的能量范围内计算了分裂，两种计算方法的结果非常吻合。尽管对于振动基态来说是可以忽略的缓慢（>1 s），但这项工作预测乙烯基中隧穿促进的 1,2 氢化物迁移动力学将随着内部振动激发而呈现指数增长，特别是在能量高于 E ≈ 7500 cm(-1)时达到亚微秒时间尺度的速率。最重要的是，这些结果开始阐明通过无离解的势垒进行量子异构化的可能作用，与传统的经典漫游图景竞争，后者在键离解极限下方的能量窗口要窄得多。此外，当在热能量的玻尔兹曼分布上进行积分时，这些微正则隧穿速率与 T > 1400 K 时 1,2 氢化物迁移动力学的亚微秒时间尺度一致。这些结果对于低压火焰燃烧模型以及最近从乙烯基自由基的高分辨率红外/微波光谱中观察到的核自旋统计混合具有潜在的相关性。
