Woon David E
Department of Chemistry, University of Illinois at Urbana-Champaign, Box 92-6, CLSL, 600 S. Mathews, Urbana, IL 61801, USA.
Phys Chem Chem Phys. 2015 Nov 21;17(43):28705-18. doi: 10.1039/c5cp03393d.
Icy grain mantles that accrete on refractory dust particles in the very cold interstellar medium or beyond the snow line in protoplanetary disks serve as minute incubators for heterogeneous chemistry. Ice mantle chemistry can differ significantly from the gas phase chemistry that occurs in these environments and is often richer. Modeling ices and their chemistry is a challenging task for quantum theoretical methods, but theory promises insight into these systems that is difficult to attain with experiments. Density functional theory (DFT) is predominately employed for modeling reactions in icy grain mantles due to its favorable scalability, but DFT has limitations that risk undercutting its reliability for this task. In this work, basic protocols are proposed for identifying the degree to which DFT methods are able to reproduce experimental or higher level theoretical results for the fundamental interactions upon which ice mantle chemistry depends, including both reactive interactions and non-reactive scaffolding interactions. The exemplar of this study is the reaction of C(+) with H2O, where substantial methodological differences are found in the prediction of gas phase relative energetics for stationary points (about 10 kcal mol(-1) for the C-O bond energy of the H2OC(+) intermediate), which in turn casts doubt about employing it to treat the C(+) + H2O reaction on an ice surface. However, careful explorations demonstrate that B3LYP with small correlation consistent basis sets performs in a sufficiently reliable manner to justify using it to identify plausible chemical pathways, where the dominant products were found to be neutral HOC and the CO(-) anion plus one and two H3O(+) cations, respectively. Predicted vibrational and electronic spectra are presented that would serve to verify or disconfirm the pathways; the latter were computed with time-dependent DFT. Conclusions are compared with those of a recent similar study by McBride and coworkers (J. Phys. Chem. A, 2014, 118, 6991).
冰冷的颗粒包层积聚在极冷的星际介质中或原行星盘雪线以外的难熔尘埃颗粒上，充当了异相化学的微小孵化器。冰包层化学与这些环境中发生的气相化学有显著不同，而且通常更丰富。对冰及其化学性质进行建模，对于量子理论方法来说是一项具有挑战性的任务，但理论有望深入了解这些用实验难以实现的系统。密度泛函理论（DFT）因其良好的可扩展性而主要用于模拟冰颗粒包层中的反应，但DFT存在局限性，可能会削弱其在这项任务中的可靠性。在这项工作中，提出了基本方案，以确定DFT方法能够在多大程度上重现冰包层化学所依赖的基本相互作用的实验结果或更高水平的理论结果，包括反应性相互作用和非反应性支架相互作用。本研究的范例是C(+)与H2O的反应，其中在预测驻点的气相相对能量方面发现了很大的方法差异（对于H2OC(+)中间体的C - O键能约为10 kcal mol(-1)），这反过来又让人怀疑用它来处理冰表面的C(+) + H2O反应是否可行。然而，仔细研究表明，具有小相关一致基组的B3LYP以足够可靠的方式运行，足以证明用它来识别合理的化学途径是合理的，其中主要产物分别被发现是中性HOC以及CO(-)阴离子加一个和两个H3O(+)阳离子。给出了预测的振动光谱和电子光谱，这些光谱将用于验证或否定这些途径；后者是用含时DFT计算的。将所得结论与McBride及其同事最近的一项类似研究（《物理化学杂志A》，2014年，118卷，6991页）的结论进行了比较。
