Department of Chemistry, Washington University in St. Louis, St. Louis, Missouri, United States 63130.
J Phys Chem B. 2022 May 19;126(19):3551-3562. doi: 10.1021/acs.jpcb.2c00793. Epub 2022 May 10.
Acetylacetone (AcAc) has proven to be a fruitful but highly challenging model system for the experimental and computational interrogation of strong intramolecular hydrogen bonds. Key questions remain, however, regarding the identity of the minimum-energy structure of AcAc and the dynamics of intramolecular proton transfer. Here, we investigate the OH/OD stretch and bend regions of the enol tautomer of AcAc and its deuterated isotopologue with transient absorption and 2D IR spectroscopy. The OH bend region reveals a single dominant diagonal transition near 1625 cm with intense cross peaks to lower-frequency modes, demonstrating highly mixed fingerprint transitions that contain OH bend character. The anharmonic coupling of the OH bend results in a highly elongated OH bend excited-state absorption transition that indicates a large manifold of OH bend overtone/combination bands in the OH stretch region that leads to strong bend-stretch Fermi resonance interactions. The OH and OD stretch regions consist of broad ground-state bleach signals, but there is no clear evidence of ω excited-state absorptions due to rapid population relaxation arising from strong intramolecular coupling to bending, fingerprint, and low-frequency H-bond modes. Orientational relaxation dynamics persist for timescales longer than the vibrational lifetimes, with polarization anisotropy components decaying within approximately 2 and 10 periods of the O-O oscillation for the OH and OD stretch, respectively. The significant isotopic dependence of the orientational dynamics is discussed in the context of intramolecular mode coupling, diffusional processes, and contributions from proton/deuteron transfer dynamics.
乙酰丙酮 (AcAc) 已被证明是一个富有成效但极具挑战性的模型体系，可用于实验和计算研究强分子内氢键。然而，关于 AcAc 的最低能量结构和分子内质子转移动力学的一些关键问题仍然存在。在这里，我们使用瞬态吸收和二维红外光谱研究了 AcAc 的烯醇互变异构体及其氘代同位素的 OH/OD 伸缩和弯曲区域。OH 弯曲区域在 1625cm 附近显示出一个单一的主导对角跃迁，与低频模式具有强烈的交叉峰，表明高度混合的指纹跃迁包含 OH 弯曲特征。OH 弯曲的非谐耦合导致 OH 弯曲激发态吸收跃迁非常伸长，这表明在 OH 伸缩区域存在大量 OH 弯曲泛频/组合带，导致强弯曲-伸缩费米共振相互作用。OH 和 OD 伸缩区域由宽的基态漂白信号组成，但由于与弯曲、指纹和低频氢键模式的强烈分子内耦合导致的快速粒子数弛豫，没有明显的 ω 激发态吸收的证据。取向弛豫动力学持续时间长于振动寿命，对于 OH 和 OD 伸缩，分别约为 O-O 振动的 2 和 10 个周期内，偏振各向异性分量衰减。在分子内模式耦合、扩散过程以及质子/氘转移动力学贡献的背景下，讨论了取向动力学的显著同位素依赖性。
