Bin Mohd Yusof Muhammad Shafiq, Song Hongwei, Debnath Tushar, Lowe Bethany, Yang Minghui, Loh Zhi-Heng
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore.
State Key Laboratory for Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071, China.
Phys Chem Chem Phys. 2022 May 25;24(20):12236-12248. doi: 10.1039/d2cp00505k.
Proton transfer (PT) reactions are fundamental to numerous chemical and biological processes. While sub-picosecond PT involving electronically excited states has been extensively studied, little is known about ultrafast PT triggered by photoionization. Here, we employ femtosecond optical pump-probe spectroscopy and quantum dynamics calculations to investigate the ultrafast proton transfer dynamics of the aqueous phenol radical cation (PhOH˙). Analysis of the vibrational wave packet dynamics reveals unusually short dephasing times of 0.18 ± 0.02 ps and 0.16 ± 0.02 ps for the PhOH˙ O-H wag and bend frequencies, respectively, suggestive of ultrafast PT occurring on the ∼0.1 ps timescale. The reduced potential energy surface obtained from calculations shows that PT is barrierless when it is coupled to the intermolecular hindered translation between PhOH˙ and the proton-acceptor water molecule. Quantum dynamics calculations yield a lifetime of 193 fs for PhOH˙, in good agreement with the experimental results and consistent with the PT reaction being mediated by the intermolecular O⋯O stretch. These results suggest that photoionization can be harnessed to produce photoacids that undergo ultrafast PT. In addition, they also show that PT can serve as an ultrafast deactivation channel for limiting the oxidative damage potential of radical cations.
质子转移（PT）反应是众多化学和生物过程的基础。虽然涉及电子激发态的亚皮秒级质子转移已得到广泛研究，但对于光电离引发的超快质子转移却知之甚少。在此，我们采用飞秒光泵浦 - 探测光谱和量子动力学计算来研究苯酚自由基阳离子（PhOH˙）在水溶液中的超快质子转移动力学。对振动波包动力学的分析表明，对于PhOH˙的O - H摇摆和弯曲频率，其退相时间异常短，分别为0.18±0.02皮秒和0.16±0.02皮秒，这表明在约0.1皮秒的时间尺度上发生了超快质子转移。从计算中获得的简化势能面表明，当质子转移与PhOH˙和质子受体水分子之间的分子间受阻平移耦合时，质子转移是无障碍的。量子动力学计算得出PhOH˙的寿命为193飞秒，与实验结果吻合良好，并且与质子转移反应由分子间O⋯O伸缩介导一致。这些结果表明，光电离可用于产生经历超快质子转移的光酸。此外，它们还表明质子转移可以作为一种超快失活通道，用于限制自由基阳离子的氧化损伤潜力。
