Departamento de Química Inorgánica Analítica y Química-Física e INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, 1428 Buenos Aires, Argentina.
J Chem Phys. 2013 Nov 7;139(17):174315. doi: 10.1063/1.4827935.
We use ring-polymer-molecular-dynamics (RPMD) techniques and the semi-empirical q-TIP4P/F water model to investigate the relationship between hydrogen bond connectivity and the characteristics of nuclear position fluctuations, including explicit incorporation of quantum effects, for the energetically low lying isomers of the prototype cluster [H2O]8 at T = 50 K and at 150 K. Our results reveal that tunneling and zero-point energy effects lead to sensible increments in the magnitudes of the fluctuations of intra and intermolecular distances. The degree of proton spatial delocalization is found to map logically with the hydrogen-bond connectivity pattern of the cluster. Dangling hydrogen bonds exhibit the largest extent of spatial delocalization and participate in shorter intramolecular O-H bonds. Combined effects from quantum and polarization fluctuations on the resulting individual dipole moments are also examined. From the dynamical side, we analyze the characteristics of the infrared absorption spectrum. The incorporation of nuclear quantum fluctuations promotes red shifts and sensible broadening relative to the classical profile, bringing the simulation results in much more satisfactory agreement with direct experimental information in the mid and high frequency range of the stretching band. While RPMD predictions overestimate the peak position of the low frequency shoulder, the overall agreement with that reported using an accurate, parameterized, many-body potential is reasonable, and far superior to that one obtains by implementing a partially adiabatic centroid molecular dynamics approach. Quantum effects on the collective dynamics, as reported by instantaneous normal modes, are also discussed.
我们使用环聚合物分子动力学（RPMD）技术和半经验 q-TIP4P/F 水模型，研究了氢键连接性与核位置波动特征之间的关系，包括量子效应的显式包含，对于原型团簇 [H2O]8 在 T = 50 K 和 150 K 下的低能异构体。我们的结果表明，隧道和零点能效应对分子内和分子间距离波动幅度的增加有明显的影响。质子空间离域的程度与团簇的氢键连接模式逻辑上相关。悬空氢键表现出最大程度的空间离域，并参与较短的分子内 O-H 键。还研究了量子和极化波动对产生的单个偶极矩的综合影响。从动力学方面，我们分析了红外吸收光谱的特征。核量子波动的结合相对于经典轮廓促进了红移和明显的展宽，使模拟结果在中高频拉伸带与直接实验信息更吻合。虽然 RPMD 预测高估了低频肩峰的位置，但与使用准确的、参数化的多体势报告的结果相比，总体上的一致性是合理的，并且远优于通过实施部分绝热质心分子动力学方法获得的结果。即时正则模式报告的量子对集体动力学的影响也进行了讨论。
