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仍然在很大程度上关注不允许这种过程的谐振子。在这个项目中，我们将远离谐波图片在气相中的氢键在三个实验方向：通过研究的泛音氢键供体伸缩振动，通过表征正交于氢键方向的librational运动，并通过调查亲电的方法CC多重键。基本目标是鲜为人知的实验非谐常数氢键沿着高频伸缩和低频Librational坐标，以及由OH对接到C=C双键引起的微妙影响。 除了光谱位移，由于振动电荷位错的不太容易明显的强度效应应加以解决。这方面的实验数据缺乏的原因包括不利的过渡时刻，多维耦合，弱光源和困难的检测条件。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了解可能是最简单的有机氢键，即两个甲醇分子之间的氢键。对于氢键中供体振动的变化，实验与现有理论之间存在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曲面。反之亦然，理论结果可以促进本项目中计划的实验研究。一个定量的新观点超过40年的计算光谱红移可能会出现。

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.