Vibrational coherence transfer illuminates dark modes in models of the FeFe hydrogenase active site
Vibrational coherence transfer illuminates dark modes in models of the FeFe hydrogenase active site
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DOI：
10.1063/1.5111016
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发表时间：
2019-08-07
影响因子：
4.4
通讯作者：
Kubarych, Kevin J.
中科院分区：
文献类型：
--
作者：
Eckert, Peter A.;Kubarych, Kevin J.
在Redfield理论的概念框架内，光响应函数来源于系统密度算符的动态演化，其中非么正弛豫编码在Redfield弛豫超算符中。在传统的方法中，所谓的长期近似忽略了引起不同相干之间以及相干和布居之间的跃迁的项。其基本原理是，与占主导地位的人口弛豫和相干性退相贡献相比，这些非世俗项是很小的。由于二维红外(2D-IR)光谱对粒子数弛豫和转移路径有重要的贡献，因此分离非长期弛豫信号是一项具有挑战性的工作。本文报道了作为[FeFe]氢酶H簇亚单位的小分子模型的三种二硫代铁六羰基络合物中，偶然的振动能量结构使得在烷烃溶液中的振动相干转移具有直接和明显的特征。这一发现有望从分子上详细了解振动相干转移过程的机制，这要归功于基于这些研究得很好的双铁化合物的化学模数合成衍生物的简便性。除了在分子光谱学中描述相干传递的基本需要外，我们还在这组分子中发现了非长期动力学的一个实用工具：确定IR非活动模式的频率的能力。在2D-IR测量的等待时间内产生的相干性转变为涉及单个暗CO伸展模的相干性，该相干性调制了2D光谱中的一些峰值幅度，揭示了其瞬时激发。
Within the conceptual framework of Redfield theory, the optical response function arises from the dynamical evolution of the system's density operator, where nonunitary relaxation is encoded in the Redfield relaxation superoperator. In the conventional approach, the so-called secular approximation neglects terms that induce transitions between distinct coherences and among coherences and populations. The rationale is that these nonsecular terms are small in comparison to the far more dominant population relaxation and coherence dephasing contributions. Since two-dimensional infrared (2D-IR) spectroscopy has significant contributions arising from population relaxation and transfer pathways, it can be challenging to isolate signatures of the nonsecular relaxation. We report here that in three diiron dithiolate hexacarbonyl complexes that serve as small-molecule models of the [FeFe] hydrogenase H-cluster subsite, a fortuitous vibrational energy structure enables direct and clear signatures of vibrational coherence transfer in alkane solution. This finding holds promise towards developing a molecularly detailed understanding of the mechanism of vibrational coherence transfer processes, thanks to the ease of synthesizing derivatives based on the chemical modularity of these well studied diiron compounds. In addition to the fundamental need to characterize coherence transfer in molecular spectroscopy, we find in this set of molecules a practical utility for the nonsecular dynamics: the ability to determine the frequency of an IR-inactive mode. A coherence generated during the waiting time of the 2D-IR measurement transfers to a coherence involving the single dark CO stretching mode, which modulates some peak amplitudes in the 2D spectrum, revealing its transient excitation.