Valley Nicholas A, Blower Patrick G, Wood Suzannah R, Plath Kathryn L, McWilliams Laura E, Richmond Geraldine L
Department of Chemistry, University of Oregon , 1253 University of Oregon, Eugene, Oregon 97403, United States.
J Phys Chem A. 2014 Jul 3;118(26):4778-89. doi: 10.1021/jp501498h. Epub 2014 Jun 23.
The behavior of complex interfacial systems is central to an ever-increasing number of applications. Vibrational sum frequency (VSF) spectroscopy is a powerful technique for obtaining surface specific structural information. The coherent nature of VSF that provides surface specificity, however, also creates difficulty in spectral interpretation especially as the system complexity increases. Computations of VSF spectra shed light on the molecular level source of the experimental VSF signal, allowing for the analysis of more complicated systems. Unfortunately, the majority of calculations of VSF spectra look at the response of the solvent or of rigid molecules and therefore often poorly reflect the experimental environment of most VSF spectroscopic measurements. In this work, flexible solute molecules at interfaces are investigated by doubling down, obtaining and comparing experimental and theoretical spectra, to determine a more accurate computational treatment. The surface behavior and VSF spectra of glutaric acid and adipic acid at the air/water interface are determined experimentally and calculated using a combination of classical molecular dynamics and density functional theory. Both diacids are found to be surface active. At high concentrations, glutaric acid forms dimers altering its VSF response and acidic properties. Calculated VSF spectra are found to be sensitive to vibrational mode frequencies, with ordering and spacing affecting relative intensities, as well as molecular conformation. A proper description requires consideration of multiple conformers and anharmonic effects on the molecular vibrational energies.
复杂界面系统的行为对于越来越多的应用至关重要。振动和频（VSF）光谱学是获取表面特定结构信息的有力技术。然而，VSF的相干性质虽能提供表面特异性，但在光谱解释方面也造成了困难，尤其是随着系统复杂性增加。VSF光谱的计算揭示了实验VSF信号的分子层面来源，有助于分析更复杂的系统。不幸的是，大多数VSF光谱计算着眼于溶剂或刚性分子的响应，因此往往不能很好地反映大多数VSF光谱测量的实验环境。在这项工作中，通过加倍努力、获取并比较实验光谱和理论光谱，研究了界面处的柔性溶质分子，以确定更准确的计算方法。实验测定了戊二酸和己二酸在空气/水界面的表面行为和VSF光谱，并结合经典分子动力学和密度泛函理论进行了计算。发现这两种二元酸都具有表面活性。在高浓度下，戊二酸形成二聚体，改变了其VSF响应和酸性性质。计算得到的VSF光谱对振动模式频率敏感，排列和间距会影响相对强度以及分子构象。恰当的描述需要考虑多种构象以及分子振动能量上的非谐效应。
