Gas Phase NMR Studies of Conformational Kinetics

在化学系物理化学计划的这个项目中，加州大学戴维斯分校化学系的Nancy S.True教授将继续进行一项研究计划，以确定气相中简单构象过程的动力学是否受到限制的分子内振动能量重新分配(IVR)的显著影响。为此，将在有助于解决这一问题的实验条件下确定衰减曲线(即单分子速率常数与压力的关系)。超过阈值能量的振动态的能量、对称性和非谐耦合常数的计算将被用来建立临界激发的分子中IVR过程的模型。这些模型将用于采用松弛矩阵方法的衰落计算中，该方法明确包括IVR和Decay之间的竞争。这些研究首次尝试将实验确定的构象过程的衰减形状与包含对IVR过程的描述的动力学模型相关联。实验衰减曲线也将与使用标准RRKM理论计算的曲线进行比较，以测试其在过渡态化学中的有效性。这项工作中使用的实验技术包括高压气相核磁共振。亚硝酸甲酯、环己烷和四氟化硫，以及涉及酰胺和取代乙烷的内旋转动力学、四甲基硅基环戊二烯中的三甲基硅基旋转、流动无机分子中的假旋转和不饱和六元环中的环反转的过程。分子内振动能量的重新分配是化学反应动力学和平衡理论的核心。本提出了几种理论来解释这种再分配过程，其中最突出的是由莱斯、拉姆斯·珀格、卡塞尔和马库斯提出的理论，简称RRKM理论。这一理论是基于能量流的统计模型，最近的光谱实验表明，并不是所有的分子内能量重新分布都可以用它来解释。在这个项目中进行的研究，使用不同的实验技术，将提供额外的数据，这些数据将有助于描绘RRKM理论的适用范围。

In this project in the Physical Chemistry Program of the Chemistry Division, Prof. Nancy S. True of the Chemistry Department of the University of California at Davis will pursue a research program to determine if the kinetics of simple conformational processes in the gas phase is significantly influenced by restricted intramolecular vibrational energy redistribution (IVR). To this end the fall-off curves (i.e. unimolecular rate constants vs. pressure) will be determined under experimental conditions that will help address this question. Calculation of the energies, symmetries, and anharmonic coupling constants for vibrational states above the threshold energy will be used to develop models for the IVR process in the critically energized molecules. These models will be used in fall-off calculations employing a relaxation matrix approach which explicitly includes competition between IVR and decay. The studies are a first attempt to relate the experimentally determined shape of the fall-off for conformational processes to a kinetic model containing a description of the IVR process. Experimental fall-off curves will also be compared to those calculated using standard RRKM theory in order to test its realm of validity in transition state chemistry. The experimental techniques to be used in this work include high pressure gas phase NMR. Methyl nitrite, cyclohexane, and sulfur tetrafluoride, as well as processes involving the internal rotation dynamics in amides and substituted ethanes, trimethylsilyl group rotation in tetramethylsilylcyclopentadiene, pseudorotation in fluxional inorganic molecules and ring inversion in unsaturated six-membered rings will be investigated. The intramolecular redistribution of vibrational energy is at the heart of theories of chemical reaction dynamics and equilibrium. Several theories have ben proposed to account for such redistribution processes, the most prominent of which is that proposed by Rice, Rams perger, Kassel, and Marcus, which is known under the acronym RRKM theory. This theory is based on a statistical model of energy flow and recent spectroscopic experiments have indicated that not all intramolecular energy redistribution can be accounted for by it. The research performed in this project, using different experimental techniques, will provide additional data which will help delineate the range of applicability of the RRKM theory.