Department of Chemistry, Purdue University , West Lafayette, Indiana 47907-2084, United States.
J Phys Chem A. 2013 Dec 19;117(50):13465-80. doi: 10.1021/jp406945u. Epub 2013 Sep 9.
The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-cooled conditions. Two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence, and dispersed fluorescence spectra were obtained for the D0-D1 electronic transition of this prototypical resonance-stabilized radical in which the methyl group is immediately adjacent to the primary radical site. Extensive Franck-Condon activity in hindered rotor levels was observed in the excitation spectrum, reflecting a reorientation of the methyl group upon electronic excitation. Dispersed fluorescence spectra from the set of internal rotor levels are combined with the excitation spectrum to obtain a global fit of the barrier heights and angular change of the methyl group in both D0 and D1 states. The best-fit methyl rotor potential in the ground electronic state (D0) is a flat-topped 3-fold potential (V3" = 151 cm(-1), V6" = 34 cm(-1)) while the D1 state has a lower barrier (V3' = 72 cm(-1), V6' = 15 cm(-1)) with Δφ = ± π/3, π, consistent with a reorientation of the methyl group upon electronic excitation. The ground state results are compared with calculations carried out at the DFT B3LYP level of theory using the 6-311+G(d,p) basis set, and a variety of excited state calculations are carried out to compare against experiment. The preferred geometry of the methyl rotor in the ground state is anti, which switches to syn in the D1 state and in the cation. The calculations uncover a subtle combination of effects that contribute to the shift in orientation and change in barrier in the excited state relative to ground state. Steric interaction favors the anti conformation, while hyperconjugation is greater in the syn orientation. The presence of a second excited state close by D1 is postulated to influence the methyl rotor properties. A resonant ion-dip infrared (RIDIR) spectrum in the alkyl and aromatic CH stretch regions was also recorded, probing in a complementary way the state-dependent conformation of α-MeBz. Using a scheme in which infrared depletion occurs between excitation and ionization steps of the 2C-R2PI process, analogous infrared spectra in D1 were also obtained, probing the response of the CH stretch fundamentals to electronic excitation. A reduced-dimension Wilson G-matrix model was implemented to simulate and interpret the observed infrared results. Finally, photoionization efficiency scans were carried out to determine the adiabatic ionization threshold of α-MeBz (IP = 6.835 ± 0.002 eV) and provide thresholds for ionization out of specific internal rotor levels, which report on the methyl rotor barrier in the cation state.
已在喷射冷却条件下研究了 α-甲基苄基自由基（α-MeBz）的态依赖光谱。获得了该原型共振稳定自由基的 D0-D1 电子跃迁的双色共振双光子电离（2C-R2PI）、激光诱导荧光和分散荧光光谱，其中甲基基团紧邻主要自由基位点。在激发光谱中观察到受阻转子能级的广泛 Franck-Condon 活性，这反映了电子激发时甲基基团的重新取向。从一组内部转子能级获得的分散荧光光谱与激发光谱相结合，以获得 D0 和 D1 态下甲基基团的势垒高度和角度变化的全局拟合。在基态电子态（D0）中，最佳拟合的甲基转子势是平顶的 3 重势（V3" = 151 cm(-1)，V6" = 34 cm(-1))，而 D1 态的势垒较低（V3' = 72 cm(-1)，V6' = 15 cm(-1))，φ = ±π/3，π，与电子激发时甲基基团的重新取向一致。将实验结果与使用 6-311+G(d,p)基组在 DFT B3LYP 水平上进行的计算进行了比较，并进行了各种激发态计算以与实验进行比较。在基态中，甲基转子的优选几何形状为反式，在 D1 态和阳离子中变为顺式。计算揭示了一些微妙的组合效应，这些效应导致与基态相比，激发态中取向和势垒的变化。立体相互作用有利于反式构象，而超共轭在顺式构象中更大。假定附近存在第二个激发态会影响甲基转子的性质。还记录了烷基和芳族 CH 伸缩区域的共振离子陷 IRIDIR 光谱，以互补的方式探测 α-MeBz 的状态依赖构象。使用方案，其中在 2C-R2PI 过程的激发和电离步骤之间发生红外耗竭，也获得了 D1 中的类似红外光谱，探测 CH 伸缩基频对电子激发的响应。实施了简化的 Wilson G-矩阵模型来模拟和解释观察到的红外结果。最后，进行光电离效率扫描以确定 α-MeBz 的绝热电离阈值（IP = 6.835 ± 0.002 eV），并提供特定内部转子能级出射的电离阈值，这报告了阳离子态下甲基转子势垒。
