Department of Chemistry, University of Warwick, Library Road, Coventry, CV4 7AL, UK.
Phys Chem Chem Phys. 2012 Oct 14;14(38):13415-28. doi: 10.1039/c2cp42289a.
Deactivation of excited electronic states through coupling to dissociative (1)πσ* states in heteroaromatic systems has received considerable attention in recent years, particularly as a mechanism that contributes to the ultraviolet (UV) photostability of numerous aromatic biomolecules and their chromophores. Recent studies have expanded upon this work to look at more complex species, which involves understanding competing dynamics on two different (1)πσ* potential energy surfaces (PESs) localized on different heteroatom hydride coordinates (O-H and N-H bonds) within the same molecule. In a similar spirit, the work presented here utilizes ultrafast time-resolved velocity map ion imaging to study competing dissociation pathways along (1)πσ* PESs in mequinol (p-methoxyphenol), localized at O-H and O-CH(3) bonds yielding H atoms or CH(3) radicals, respectively, over an excitation wavelength range of 298-238 nm and at 200 nm. H atom elimination is found to be operative via either tunneling under a conical intersection (CI) (298 ≥ λ ≥ 280 nm) or ultrafast internal conversion through appropriate CIs (λ ≤ 245 nm), both of which provide mechanisms for coupling onto the dissociative state associated with the O-H bond. In the intermediate wavelength range of 280 ≥ λ ≥ 245 nm, mediated H atom elimination is not observed. In contrast, we find that state driven CH(3) radical elimination is only observed in the excitation range 264 ≥ λ ≥ 238 nm. Interpretation of these experimental results is guided by: (i) high level complete active space with second order perturbation theory (CASPT2) calculations, which provide 1-D potential energy cuts of the ground and low lying singlet excited electronic states along the O-H and O-CH(3) bond coordinates; and (ii) calculated excitation energies using CASPT2 and the equation-of-motion coupled cluster with singles and doubles excitations (EOM-CCSD) formalism. From these comprehensive studies, we find that the dynamics along the O-H coordinate generally mimic H atom elimination previously observed in phenol, whereas O-CH(3) bond fission in mequinol appears to present notably different behavior to the CH(3) elimination dynamics previously observed in anisole (methoxybenzene).
近年来，通过与杂芳环体系中离解的（1）πσ态耦合来使激发电子态失活的现象引起了相当大的关注，特别是作为许多芳香生物分子及其发色团的紫外（UV）光稳定性的贡献机制。最近的研究扩展了这项工作，以研究更复杂的物种，这涉及到理解同一分子中不同杂原子氢化物坐标（O-H 和 N-H 键）上两个不同（1）πσ势能面（PES）上的竞争动力学。本着类似的精神，这里呈现的工作利用超快时间分辨速度映射离子成像来研究在甲氧基苯酚（对甲氧基苯酚）中沿着（1）πσ* PES 的竞争解离途径，该分子的 O-H 和 O-CH（3）键分别定位在 H 原子或 CH（3）自由基上，激发波长范围为 298-238nm，在 200nm 处。发现 H 原子消除是通过在锥形交叉（CI）下的隧道（298≥λ≥280nm）或通过适当的 CI 的超快内转换来实现的（λ≤245nm），这两种方法都为与 O-H 键相关的离解态提供了耦合机制。在 280≥λ≥245nm 的中间波长范围内，未观察到中介 H 原子消除。相比之下，我们发现只有在 264≥λ≥238nm 的激发范围内才观察到状态驱动的 CH（3）自由基消除。这些实验结果的解释由以下几个方面指导：（i）高水准的完全活性空间二阶微扰理论（CASPT2）计算，该计算沿 O-H 和 O-CH（3）键坐标提供了基态和低能单重激发电子态的 1-D 势能切割；（ii）使用 CASPT2 和单重和双激发的运动方程耦合簇（EOM-CCSD）公式计算激发能。通过这些全面的研究，我们发现 O-H 坐标上的动力学通常模拟了先前在苯酚中观察到的 H 原子消除，而甲氧基苯酚中的 O-CH（3）键断裂似乎表现出与先前在茴香醚（甲氧基苯）中观察到的 CH（3）消除动力学明显不同的行为。
