Proton relays in anomalous carbocations dictate spectroscopy, stability, and mechanisms: case studies on C 2 H 5 + and C 3 H 3 +
Proton relays in anomalous carbocations dictate spectroscopy, stability, and mechanisms: case studies on C 2 H 5 + and C 3 H 3 +
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异常碳阳离子中的质子中继决定了光谱、稳定性和机制：C 2 H 5 和 C 3 H 3 的案例研究
DOI：
10.1039/c7cp05577c
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Iyengar, Srinivasan S.
中科院分区：
文献类型：
--
作者：
Sager, LeeAnn M.;Iyengar, Srinivasan S.
我们提出了一个详细的分析异常碳阳离子：C2H5+和C3H3+。这项工作涉及（a）探测电子结构性质，（B）在一定内能范围内的从头算动力学模拟，（c）分析沿着选定的构象转变途径的降维势面，（d）从从头算动力学轨迹计算的动态平均振动光谱，以及（e）二维时频分析以探测构象动力学。主要调查结果如下：（i）如我们先前对C2H3+的研究所述，这些非经典碳正离子似乎通过离域核框架和“质子穿梭”而稳定。我们分析了这种核离域，并发现在C2H3+，C2H5+，和C3H3+的构象变化之间的关键相似之处。(ii)C2H5+的振动特征主要是由“桥质子”构象，但也显示出关键的贡献，从“经典”的配置，这是一个过渡态，在几乎所有的理论水平。这一结果通过二维时频分析得到进一步证实，并且与早期对实验光谱的解释不一致，其中接近经典区域的频率被认为是由杂质引起的。虽然这仍然是可能的，但我们的结果表明了一个涉及“经典”异构体的额外（也许更可能）解释。(iii)最后，在C3H3+的情况下，我们对实验结果的解释包括存在多个，即“环状”，“直链”和炔丙基，配置。质子穿梭和核离域，让人想起那些在C2H3+的情况下看到的，被认为是所有通过，并在我们所有的观察中具有关键作用。
We present a detailed analysis of the anomalous carbocations: C2H5+ and C3H3+. This work involves (a) probing electronic structural properties, (b) ab initio dynamics simulations over a range of internal energies, (c) analysis of reduced dimensional potential surfaces directed along selected conformational transition pathways, (d) dynamically averaged vibrational spectra computed from ab initio dynamics trajectories, and (e) two-dimensional time–frequency analysis to probe conformational dynamics. Key findings are as follows: (i) as noted in our previous study on C2H3+, it appears that these non-classical carbocations are stabilized by delocalized nuclear frameworks and “proton shuttles”. We analyze this nuclear delocalization and find critical parallels between conformational changes in C2H3+, C2H5+, and C3H3+. (ii) The vibrational signatures of C2H5+ are dominated by the “bridge-proton” conformation, but also show critical contributions from the “classical” configuration, which is a transition state at almost all levels of theory. This result is further substantiated through two-dimensional time–frequency analysis and is at odds with earlier explanations of the experimental spectra, where frequencies close to the classical region were thought to arise from an impurity. While this is still possible, our results here indicate an additional (perhaps more likely) explanation that involves the “classical” isomer. (iii) Finally, in the case of C3H3+ our explanation of the experimental result includes the presence of multiple, namely, “cyclic”, “straight”, and propargyl, configurations. Proton shuttles and nuclear delocalization, reminiscent of those seen in the case of C2H3+, were seen all through and have a critical role in all our observations.