Nakamura Masaaki, Palazzetti Federico, Tsai Po-Yu, Yang Shiun-Jr, Lin King-Chuen, Kasai Toshio, Che Dock-Chil, Lombardi Andrea, Aquilanti Vincenzo
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan.
Phys Chem Chem Phys. 2019 Jul 3;21(26):14164-14172. doi: 10.1039/c8cp04270e.
Molecular orientation techniques are becoming available in the study of elementary chemical processes, in order to highlight those structural and dynamical properties that would be concealed by random rotational motions. Recently successful orientation was achieved for asymmetric-top and chiral molecules of much larger complexity than hitherto. In this work, we report and discuss the correlation between the vectors' photofragment recoil velocity v, transition dipole moment μ, and permanent dipole moment d in a dissociation experiment on hexapole oriented 2-bromobutane, photoinitiated by a linearly polarized laser. The sliced ion images of the Br*(2P1/2) and Br(2P3/2) photofragments were acquired at 234.0 and 254.1 nm, respectively, by a (2 + 1) resonance-enhanced multiphoton ionization technique. A detailed analysis of the sliced ion images obtained at a tilting angle 45° of laser polarization provides information on the correlation of the three vectors, which are confined by two polar angles α and χ and one azimuthal angle φμd in the recoil frame. The sliced ion images of Br fragments eliminated individually from the enantiomers at 254.1 nm yield an asymmetric factor close to zero; for this reason the photofragment angular distributions do not show significant differences. The elimination of the Br* fragment at 234.0 nm is mainly correlated with a parallel transition, giving rise to a large anisotropy parameter of 1.85, and thus can be considered as a single state excitation. The resulting recoil frame angles are optimized to 163° ± 8° and 164° ± 1° for α and χ, respectively, whereas φμd is approaching 0° for the best fit. Since for the present molecule, the three vectors have an only slight spatial arrangement, the photofragment angular distributions of the two enantiomers do not show appreciable differences. Theoretical and computational simulations provide us the basis to state that oriented enantiomers can be discriminated on-the-fly in photodissociation processes even initiated by non-circularly polarized light, provided that the three vectors encountered above have specific three-dimensional arrangements. The fact that Br fragment elimination involves a multi-potential dissociation carries uncertainties in theoretical estimates of the vector direction. Therefore, this work represents a preliminary but significant step on the road to chiral discrimination on-the-fly, which is shown to be best propitiated in molecules where vectors are far from having degenerate mutual angular directions.
分子取向技术在基础化学过程研究中逐渐可用，以便突出那些会被随机旋转运动所掩盖的结构和动力学性质。最近，对于比以往复杂得多的不对称陀螺分子和手性分子，成功实现了取向。在这项工作中，我们报告并讨论了在由线偏振激光光引发的六极取向2-溴丁烷的解离实验中，矢量的光碎片反冲速度v、跃迁偶极矩μ和永久偶极矩d之间的相关性。分别通过（2 + 1）共振增强多光子电离技术，在234.0和254.1 nm处获取了Br*(2P1/2)和Br(2P3/2)光碎片的切片离子图像。对在激光偏振倾斜角45°下获得的切片离子图像进行详细分析，提供了关于三个矢量相关性的信息，这三个矢量在反冲框架中由两个极角α和χ以及一个方位角φμd限定。在254.1 nm处从对映体中单独消除的Br碎片的切片离子图像产生的不对称因子接近零；因此，光碎片角分布没有显示出显著差异。在234.0 nm处消除Br*碎片主要与平行跃迁相关，产生了1.85的大各向异性参数，因此可以被视为单态激发。对于α和χ，所得反冲框架角分别优化为163°±8°和164°±1°，而对于最佳拟合，φμd接近0°。由于对于当前分子，这三个矢量只有轻微的空间排列，两种对映体的光碎片角分布没有显示出明显差异。理论和计算模拟为我们提供了依据，表明即使由非圆偏振光引发，在光解离过程中也可以即时区分取向的对映体，前提是上述三个矢量具有特定的三维排列。Br碎片消除涉及多势解离这一事实在矢量方向的理论估计中带来了不确定性。因此，这项工作代表了即时手性区分道路上的初步但重要的一步，结果表明在矢量相互角方向远非简并的分子中最有利于实现这一点。
