Mensa-Bonsu Golda, Tozer David J, Verlet Jan R R
Department of Chemistry, Durham University, Durham DH1 3LE, UK.
Phys Chem Chem Phys. 2019 Jul 3;21(26):13977-13985. doi: 10.1039/c8cp06593d.
Photodetachment and 2D photoelectron spectra of the mass-selected I-·CF3I complex are presented together with electronic structure calculations. Calculations show that the I- is located at the iodine side of CF3I. Vertical and adiabatic detachment energies were measured at 4.03 and approximately 3.8 eV, respectively. The photoelectron spectra and molecular orbitals show a significant covalent bonding character in the cluster. The presence of electronic excited states is observed. Below threshold, iodide is generated which can be assigned to the photoexcitation of degenerate charge-transfer bands from the off-axis p-orbitals localised on iodide. Near the onset of two spin-orbit thresholds, bright excited states are seen in the experiment and calculations. Excitation of these leads to the formation of slow electrons. The spectroscopy of I-·CF3I is compared to the well-studied I-·CH3I cluster, a pre-reaction complex in the text-book I- + CH3I SN2 reaction. Despite the reversed stereodynamics (i.e. inversion of the CX3 between X = H and F) of the SN2 reaction, striking similarities are seen. Both complexes possess charge transfer excited states near their respective vertical detachment energies and exhibit vibrational structure in their photoelectron spectra. The strong binding is consistent with observations in crossed molecular beam studies and molecular dynamics simulations that suggest that iodine as a leaving group in an SN2 reaction affects the reaction dynamics.
本文展示了质量选择的I⁻·CF₃I络合物的光剥离和二维光电子能谱，并进行了电子结构计算。计算表明，I⁻位于CF₃I的碘侧。垂直剥离能和绝热剥离能分别测得为4.03 eV和约3.8 eV。光电子能谱和分子轨道表明该团簇具有显著的共价键特征。观察到了电子激发态的存在。在阈值以下，会产生碘化物，这可归因于碘化物上离轴p轨道的简并电荷转移带的光激发。在两个自旋轨道阈值的起始附近，实验和计算中都观察到了明亮的激发态。对这些激发态的激发会导致慢电子的形成。将I⁻·CF₃I的光谱与研究充分的I⁻·CH₃I团簇进行了比较，I⁻·CH₃I团簇是教科书式I⁻ + CH₃I SN2反应中的预反应络合物。尽管SN2反应的立体动力学相反（即X = H和F时CX₃的反转），但仍有显著的相似之处。两种络合物在各自的垂直剥离能附近都具有电荷转移激发态，并且在其光电子能谱中都表现出振动结构。这种强结合与交叉分子束研究和分子动力学模拟中的观察结果一致，这些研究表明碘作为SN2反应中的离去基团会影响反应动力学。
