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A guinea pig for conformer selectivity and mechanistic insights into dissociative ionization by photoelectron photoion coincidence: fluorocyclohexane.
Physical Chemistry Chemical Physics ( IF 3.3 ) Pub Date : 2020-01-14 , DOI: 10.1039/c9cp05617c
Xiangkun Wu 1 , Xiaoguo Zhou 2 , Patrick Hemberger 3 , Andras Bodi 3
Affiliation  

We studied fluorocyclohexane (C6H11F, FC6) by double imaging photoelectron photoion coincidence spectroscopy in the 9.90-13.90 eV photon energy range. The photoelectron spectrum can identify species isomer and, in this case, even conformer selectively. Ab initio results indicated that the axial conformer has two, close-lying cation electronic states. With the help of Franck-Condon simulations of the vibrational fine structure, we determined the origin of three transitions, (i) axial FC6 → axial FC6+ of C1 symmetry (X[combining tilde]+, A'' in CS), (ii) equatorial FC6 → equatorial FC6+ of C1 symmetry (X[combining tilde]+, A'' in CS), and (iii) axial FC6 → A' axial FC6+ of CS symmetry (Ã+) as 10.12 ± 0.01, 10.15 ± 0.01 and 10.15 ± 0.02 eV, respectively. At slightly higher energies, the FC6 cation starts fragmenting by HF loss (E0 = 10.60 eV), followed by sequential CH3 (E0 = 10.71 eV) or C2H4 (E0 = 11.06 eV) loss. Surprisingly, the methyl-loss step has an effective barrier of only 0.11 eV, and yet it is a slow process at threshold. Based on the statistical model, this is explained by isomerization and stabilization of the C6H10+ intermediate. The highest energy channel observed, vinyl fluoride (C2H3F) loss yielding C4H8+ appears in the breakdown diagram at 12 eV, which agrees with the computed threshold to cyclobutane cation formation. However, the model predicted a ca. 1 eV competitive shift for this parallel channel, i.e., an E0 = 11.23 eV. This led us to explore the potential energy surface to find a lower-lying fragmentation channel including H-transfer steps. Rate constant measurements and statistical modeling thus yield fundamental insights into the reaction mechanism beyond what is immediately seen in the mass spectra.

中文翻译:

豚鼠,用于构象选择性和通过光电子光子巧合解离电离的机械原理:氟代环己烷。

我们通过在9.90-13.90 eV光子能量范围内的双成像光电子重合能谱研究了氟环己烷(C6H11F,FC6)。光电子光谱可以识别物种异构体,在这种情况下甚至可以选择性地识别构象异构体。从头算结果表明,轴向构象异构体具有两个紧密的阳离子电子态。借助于弗兰克-康登对振动精细结构的模拟,我们确定了三个过渡点的起源,(i)C1对称性的轴向FC6→轴向FC6 +(CS中的X [合并波浪号] +,A''),(ii )C1对称的赤道FC6→赤道FC6 +(CS中的X [合并波浪号] +,A''),CS对称性(Ã+)的轴向FC6→A'轴向FC6 +为10.12±0.01,10.15±0.01和10.15±0.02 eV分别。在更高的能量下 FC6阳离子开始通过HF损失(E0 = 10.60 eV)碎裂,然后依次发生CH3(E0 = 10.71 eV)或C2H4(E0 = 11.06 eV)损失。令人惊讶的是,甲基损失步骤的有效势垒仅为0.11 eV,但在阈值下却是一个缓慢的过程。基于统计模型,这可以通过C6H10 +中间体的异构化和稳定化来解释。在12 eV的击穿图中,观察到的最高能量通道为氟乙烯(C2H3F)损失,产生C4H8 +,这与计算得出的环丁烷阳离子形成阈值一致。但是,该模型预测约 此并行通道的竞争位移为1 eV,即E0 = 11.23 eV。这导致我们探索势能表面,以找到一个较低水平的碎裂通道,包括H转移步骤。
更新日期:2020-01-14
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