Spectoscopic Studies of Hydrogen Bonding in Size-and Conformation-Selected Biomolecule-(water)n Clusters

普渡大学的Timothy Zwier得到了实验物理化学计划的支持，该计划研究芳香氨基酸-(水)n簇、模型DNA碱-(水)n簇和模型DNA碱基对中存在的氢键网络。将采用多种实验方法，包括共振双光子电离(R2PI)、UV-UV和IR-UV烧孔光谱、共振离子浸渍红外光谱(RIDIRS)和空腔衰荡光谱(CRDS)。还将进行从头计算，以便进行比较，并帮助进行光谱分配。从水二聚体中孤立的单一氢键到液态水和冰中错综复杂的氢键网络，需要很长的一步。其中一个原因是氢键具有很强的协同性。用两两相加的分子间势对氢键的定量描述甚至不能解释小气相团簇的最基本性质，更不用说体相凝聚相了。单个氢键的强度敏感地取决于氢键网络中氢键的数目、强度和取向。这种氢键网络的实验研究充满了困难，很大程度上是因为随着簇大小，样品中存在的物种数量也会增加，从而掩盖了单一物种所需的光谱。对更好的实验探头的需求是这些研究的主要动机。

Timothy Zwier of Purdue University is supported by the Experimental Physical Chemistry Program to study hydrogen bonding networks present in aromatic amino acid-(water)n clusters, in model DNA base-(water)n clusters, and in model DNA base pairs. A variety of experimental methods will be exploited including resonant two-photon ionization (R2PI), UV-UV and IR-UV hole-burning spectroscopies, resonant ion-dip infrared spectroscopy (RIDIRS) and cavity ring-down spectroscopy (CRDS). Ab initio calculations will also be carried out for comparison purposes and to aid in spectral assignment. It is a long step from an isolated single hydrogen bond in the water dimer to the intricate and complex hydrogen-bonded networks in liquid water and ice. One of the reasons is that H-bonds are strongly cooperative. Quantitative descriptions of the H-bonding using pairwise-additive intermolecular potentials fail to account for even the most basic properties of small gas-phase clusters, much less the bulk condensed phase. The strength of individual hydrogen bonds depends sensitively on the number, strength and orientation of the hydrogen bonds in the H-bonded network. The experimental study of such H-bonded networks is fraught with difficulty, largely because as the cluster size grows, so does the number of species present in the sample, masking the desired spectrum of a single species. The need for better experimental probes is the primary motivation for these studies.