Mancini John S, Samanta Amit K, Bowman Joel M, Reisler Hanna
Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University , Atlanta, Georgia 30322, United States.
J Phys Chem A. 2014 Sep 18;118(37):8402-10. doi: 10.1021/jp5015753. Epub 2014 Feb 28.
The breaking of hydrogen bonds in molecular systems has profound effects on liquids, e.g., water, biomolecules, e.g., DNA, etc., and so it is no exaggeration to assert the importance of these bonds to living systems. However, despite years of extensive research on hydrogen bonds, many of the details of how these bonds break and the corresponding energy redistribution processes remain poorly understood. Here we report extensive experimental and theoretical insights into the breakup of two or three hydrogen bonds in the dissociation of a paradigm system of a hydrogen-bonded network, the ring HCl trimer. Experimental state-to-state vibrational predissociation dynamics of the trimer following vibrational excitation were studied by using velocity map imaging and resonance-enhanced multiphoton ionization, providing dissociation energies and product state distributions for the trimer's breakup into three separate monomers or into dimer + monomer. Accompanying the experiments are high-level calculations using diffusion Monte Carlo and quasiclassical simulations, whose results validate the experimental ones and further elucidate energy distributions in the products. The calculations make use of a new, highly accurate potential energy surface. Simulations indicate that the dissociation mechanism requires the excitation to first relax into low-frequency motions of the trimer, resulting in the breaking of a single hydrogen bond. This allows the system to explore a critical van der Waals minimum region from which dissociation occurs readily to monomer + dimer.
分子体系中氢键的断裂对液体（如水）、生物分子（如DNA等）有着深远影响，因此断言这些键对生命系统的重要性并不为过。然而，尽管对氢键进行了多年广泛研究，但这些键如何断裂以及相应的能量重新分布过程的许多细节仍知之甚少。在此，我们报告了对氢键网络范例体系——环状HCl三聚体解离过程中两个或三个氢键断裂的广泛实验和理论见解。通过使用速度映射成像和共振增强多光子电离研究了三聚体在振动激发后的实验态-态振动预解离动力学，提供了三聚体分解为三个独立单体或二聚体+单体时的解离能和产物态分布。与实验相伴的是使用扩散蒙特卡罗和准经典模拟的高级计算，其结果验证了实验结果并进一步阐明了产物中的能量分布。计算使用了一个新的、高度精确的势能面。模拟表明，解离机制要求激发首先弛豫到三聚体的低频运动，导致单个氢键断裂。这使系统能够探索一个关键的范德华极小区域，从该区域很容易解离为单体+二聚体。
