Carrascosa Eduardo, Michaelsen Tim, Stei Martin, Bastian Björn, Meyer Jennifer, Mikosch Jochen, Wester Roland
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck , 6020 Innsbruck, Austria.
Max-Born-Institut für Nichtlineare Optik, 12489 Berlin, Germany.
J Phys Chem A. 2016 Jul 14;120(27):4711-9. doi: 10.1021/acs.jpca.5b11181. Epub 2016 Feb 5.
Ion-molecule reactions of the type X(-) + CH3Y are commonly assumed to produce Y(-) through bimolecular nucleophilic substitution (SN2). Beyond this reaction, additional reaction products have been observed throughout the last decades and have been ascribed to different entrance channel geometries differing from the commonly assumed collinear approach. We have performed a crossed beam velocity map imaging experiment on the F(-) + CH3I reaction at different relative collision energies between 0.4 and 2.9 eV. We find three additional channels competing with nucleophilic substitution at high energies. Experimental branching ratios and angle- and energy differential cross sections are presented for each product channel. The proton transfer product CH2I(-) is the main reaction channel, which competes with nucleophilic substitution up to 2.9 eV relative collision energy. At this level, the second additional channel, the formation of IF(-) via halogen abstraction, becomes more efficient. In addition, we present the first evidence for an FHI product ion. This FHI product ion is present only for a narrow range of collision energies, indicating possible dissociation at high energies. All three products show a similar trend with respect to their velocity- and scattering angle distributions, with isotropic scattering and forward scattering of the product ions occurring at low and high energies, respectively. Reactions leading to all three reaction channels present a considerable amount of energy partitioning in product internal excitation. The internally excited fraction shows a collision energy dependence only for CH2I(-). A similar trend is observed for the isoelectronic OH(-) + CH3I system. The comparison of our experimental data at 1.55 eV collision energy with a recent theoretical calculation for the same system shows a slightly higher fraction of internal excitation than predicted, which is, however, compatible within the experimental accuracy.
X(-) + CH3Y 类型的离子 - 分子反应通常被认为通过双分子亲核取代（SN2）生成 Y(-)。在该反应之外，在过去几十年中还观察到了其他反应产物，这些产物被归因于与通常假定的共线方法不同的不同入口通道几何结构。我们在 0.4 至 2.9 eV 的不同相对碰撞能量下对 F(-) + CH3I 反应进行了交叉束速度映射成像实验。我们发现在高能量下有三个额外的通道与亲核取代竞争。给出了每个产物通道的实验分支比以及角度和能量微分截面。质子转移产物 CH2I(-) 是主要反应通道，在高达 2.9 eV 的相对碰撞能量下与亲核取代竞争。在此能量水平下，第二个额外通道，即通过卤素提取形成 IF(-)，变得更加高效。此外，我们首次给出了 FHI 产物离子的证据。这种 FHI 产物离子仅在狭窄的碰撞能量范围内存在，表明在高能量下可能发生解离。所有三种产物在其速度和散射角分布方面都呈现出相似的趋势，产物离子的各向同性散射和前向散射分别发生在低能量和高能量下。导致所有三个反应通道的反应在产物内部激发中都存在相当数量的能量分配。内部激发分数仅对 CH2I(-) 表现出碰撞能量依赖性。对于等电子的 OH(-) + CH3I 系统也观察到了类似的趋势。我们在 1.55 eV 碰撞能量下的实验数据与同一系统最近的理论计算结果的比较表明，内部激发分数略高于预测值，不过在实验精度范围内是相符的。
