Diagonal and off-diagonal anharmonicity in hydrogen bonds:Direct detection by infrared spectroscopy in the gas phase

化学主要是关于原子的键断裂和重排，而光谱学和分子动力学在很大程度上仍然集中在谐振子上，不允许这样的过程。在这个项目中，我们将在三个实验方向上摆脱气相氢键的谐波图：通过研究氢键供体拉伸振动的泛音，通过表征与氢键方向正交的振动运动，以及通过研究CC多键的亲电方法。主要目标是在高频拉伸和低频振动坐标系下氢键的实验非调和常数，以及OH对接到C=C双键所引起的微妙影响。除谱移外，由于振动电荷位错引起的较不明显的强度效应也应加以解决。该领域缺乏实验数据的原因包括过渡矩不利、多维耦合、光源微弱和检测条件困难。我们最近的工作表明，这些限制现在可以克服几个模型系统。因此，人们可能希望以定量的方式来理解可能是最简单的有机氢键，即两个甲醇分子之间的氢键。关于氢键中施主振动的变化，实验与现有理论之间有50%的差异有待解释。如果成功，该项目将为氢键振动动力学提供理论方法，并提供详细的实验基准，使他们能够准确判断潜在的势能超表面。反之，理论结果可以促进本项目计划的实验研究。对于40多年来计算的光谱红移，可能会出现定量的新观点。

Chemistry is mostly about bond breaking and rearrangement of atoms, whereas spectroscopy and molecular dynamics still focus to a large extent on the harmonic oscillator which does not allow for such processes. In this project, we will move away from the harmonic picture for hydrogen bonds in the gas phase in three experimental directions: by studying overtones of hydrogen bond donor stretching vibrations, by characterizing librational motions orthogonal to the hydrogen bond direction, and by investigating the electrophilic approach to CC multiple bonds. Essential goals are the poorly known experimental anharmonicity constants in hydrogen bonds along high frequency stretching and low frequency librational coordinates as well as the subtle effects induced by OH docking to C=C double bonds. Besides spectral shifts, the less accessible pronounced intensity effects due to vibrational charge dislocations shall be addressed. Reasons for a lack of experimental data in this field include unfavorable transition moments, multidimensional couplings, feeble light sources and difficult detection conditions. Our recent work indicates that these limitations can now be overcome for several model systems. One may thus hope to understand in a quantitative way the perhaps simplest organic hydrogen bond, the one between two methanol molecules. A 50% discrepancy between experiment and state of the art theory for the change of the donor vibration in the hydrogen bond waits to be explained. If successful, the project will provide theoretical approaches to the hydrogen bond vibrational dynamics with detailed experimental benchmarks, allowing them to accurately judge the underlying potential energy hypersurfaces. Vice versa, theoretical results can foster the experimental studies planned in this project. A quantitatively new perspective on more than 40 years of calculated spectral red shifts could emerge.