Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, USA.
J Chem Phys. 2009 Oct 21;131(15):154508. doi: 10.1063/1.3246862.
Vibrational energy transport in transition metal complexes involves stages where energy crosses relatively weak coordination bonds between a coordinated metal atom and the ligands. Understanding the energy transport rules on a molecular level is fundamentally important; it is also essential in relation to a recently proposed structural method, the relaxation-assisted two-dimensional infrared (RA 2DIR) technique, where the vibrational population transport time across the molecule of interest is linked to the transport distance. In this study we report on the energy transport across coordination bonds in tetraethylammonium bis(maleonitriledithiolate)iron(III)nitrosyl complex, studied using dual-frequency RA 2DIR spectroscopy. Three mode pairs, C[triple bond]N and N=O, N=O and C[triple bond]N, and N=O and C-C, were interrogated. All three cross-peaks show substantial amplification due to vibrational energy transport from the initially excited mode toward the "probed" mode, including a record amplification of 27-fold observed for the C[triple bond]N/N=O cross-peak. A ninefold amplification measured for the N=O/C[triple bond]N cross-peak, where the "probed" CN mode has higher frequency than the initially excited NO, proves unequivocally that the excitation of the "probed" mode via energy transport is not essential for observing stronger cross-peaks and that lower frequency modes serve as the energy accepting modes. A simple modeling of the energy transport is presented highlighting the role of a spatial overlap of the interacting modes. The observed strong cross-peak amplifications and a correlation between the energy transport time and the intermode distance, the distance between atom pairs on which vibrational excitations predominantly reside, demonstrate an applicability of the RA 2DIR method for structural interrogation of transition metal complexes.
振动能量在过渡金属配合物中的输运涉及到能量穿过配位金属原子与配体之间相对较弱的配位键的阶段。从分子水平上理解能量输运规则是至关重要的；这对于最近提出的结构方法——弛豫辅助二维红外（RA 2DIR）技术也很重要，其中跨感兴趣分子的振动布居输运时间与输运距离相关联。在这项研究中，我们报告了使用双频 RA 2DIR 光谱研究四乙基铵双（马来二腈二硫醇）铁（III）亚硝酰配合物中配位键的能量输运情况。我们研究了三个模式对，C≡N 和 N=O、N=O 和 C≡N，以及 N=O 和 C-C。所有三个交叉峰都由于振动能量从初始激发模式向“探测”模式的输运而显示出显著的放大，包括对于 C≡N/N=O 交叉峰观察到的 27 倍的记录放大。对于 N=O/C≡N 交叉峰测量到的九倍放大，其中“探测”的 CN 模式的频率高于初始激发的 NO，这毫不含糊地证明了通过能量输运激发“探测”模式对于观察更强的交叉峰不是必需的，并且较低频率的模式充当能量接受模式。提出了一个简单的能量输运模型，突出了相互作用模式的空间重叠的作用。观察到的强交叉峰放大以及能量输运时间与互模距离之间的相关性，即振动激发主要存在的原子对之间的距离，证明了 RA 2DIR 方法对于过渡金属配合物的结构探测的适用性。
