Hanna Gabriel, Geva Eitan
Department of Chemistry and FOCUS center, University of Michigan, Ann Arbor, MI 48109-1055, USA.
J Phys Chem B. 2008 Oct 16;112(41):12991-3004. doi: 10.1021/jp804120v. Epub 2008 Sep 18.
The effect of the commonly employed Condon and second-order cumulant approximations on one- and two-dimensional infrared spectra is examined in the case of a vibrational mode which is strongly coupled to its environment. The analysis is performed within the context of the hydrogen stretch of a moderately strong hydrogen-bonded complex dissolved in a dipolar liquid. The IR spectra are calculated using an adiabatic mixed quantum-classical approach that treats the hydrogen quantum-mechanically, while the remaining degrees of freedom are treated classically. While the cumulant and Condon treatments are seen to produce extremely broad and rather structureless spectra, the non-Condon spectra are found to consist of several relatively narrow bands that can be traced back to subsets of bath configurations with large transition dipole moments. Thus, although the cumulant and Condon approximations can capture some general qualitative spectral trends and are able to reproduce some highly averaged quantities such as the photon-echo peak shift, they fail to reproduce many important features of the spectra. We show that the great sensitivity of the transition dipole moment to the bath configuration provides new means for decongesting the spectra, probing statistically unfavorable bath configurations, and obtaining unique information regarding the dynamics of individual subsets of bath configurations and of the rates of transitions between them.
在一个与环境强烈耦合的振动模式的情况下，研究了常用的康登近似和二阶累积量近似对一维和二维红外光谱的影响。分析是在溶解于偶极液体中的中等强度氢键复合物的氢伸缩振动的背景下进行的。红外光谱是使用绝热混合量子 - 经典方法计算的，该方法对氢进行量子力学处理，而其余自由度则进行经典处理。虽然累积量处理和康登处理会产生极其宽泛且几乎无结构的光谱，但发现非康登光谱由几个相对较窄的带组成，这些带可追溯到具有大跃迁偶极矩的浴构型子集。因此，尽管累积量近似和康登近似可以捕捉一些一般的定性光谱趋势，并且能够重现一些高度平均的量，如光子回波峰位移，但它们无法重现光谱的许多重要特征。我们表明，跃迁偶极矩对浴构型的高度敏感性为解卷积光谱、探测统计上不利的浴构型以及获取有关浴构型各个子集的动力学及其之间跃迁速率的独特信息提供了新方法。
