Department of Chemistry, Stanford University, Stanford, California 94305, United States.
J Phys Chem B. 2022 Aug 11;126(31):5876-5886. doi: 10.1021/acs.jpcb.2c03095. Epub 2022 Jul 28.
The ability to exploit carbonyl groups to measure electric fields in enzymes and other complex reactive environments by using the vibrational Stark effect has inspired growing interest in how these fields can be measured, tuned, and ultimately designed. Previous studies have concentrated on the role of the solvent in tuning the fields exerted on the solute. Here, we explore instead the role of the solute electronic structure in modifying the local solvent organization and electric field exerted on the solute. By measuring the infrared absorption spectra of amide-containing molecules, as prototypical peptides, and contrasting them with non-amide carbonyls in a wide range of solvents, we show that these solutes experience notable differences in their frequency shifts in polar solvents. Using vibrational Stark spectroscopy and molecular dynamics simulations, we demonstrate that while some of these differences can be rationalized by using the distinct intrinsic Stark tuning rates of the solutes, the larger frequency shifts for amides and dimethylurea primarily result from the larger solvent electric fields experienced by their carbonyl groups. These larger fields arise due to their stronger -π conjugation, which results in larger C═O bond dipole moments that further induce substantial solvent organization. Using electronic structure calculations, we decompose the electric fields into contributions from solvent molecules that are in the first solvation shell and those from the bulk and show that both of these contributions are significant and become larger with enhanced conjugation in solutes. These results show that structural modifications of a solute can be used to tune both the solvent organization and electrostatic environment, indicating the importance of a solute-centric paradigm in modulating and designing the electrostatic environment in condensed-phase chemical processes.
利用振动斯塔克效应将羰基基团用于测量酶和其他复杂反应环境中的电场的能力激发了人们对如何测量、调节和最终设计这些场的浓厚兴趣。以前的研究集中在溶剂在调节对溶质施加的场方面的作用。在这里，我们转而探索溶质电子结构在改变局部溶剂组织和对溶质施加的电场方面的作用。通过测量包含酰胺的分子（如典型的肽）的红外吸收光谱，并将其与在各种溶剂中的非酰胺羰基进行对比，我们表明这些溶质在极性溶剂中经历了显著不同的频率位移。使用振动斯塔克光谱和分子动力学模拟，我们证明，尽管其中一些差异可以用溶质的独特固有斯塔克调谐率来合理化，但酰胺和二甲基脲的较大频率位移主要是由于其羰基基团所经历的更大溶剂电场。这些更大的场源于它们更强的 -π 共轭，这导致 C═O 键偶极矩更大，从而进一步引起大量溶剂组织。使用电子结构计算，我们将电场分解为处于第一溶剂化壳层的溶剂分子的贡献和来自体相的贡献，并表明这两个贡献都是显著的，并且随着溶质中共轭的增强而变得更大。这些结果表明，溶质的结构修饰可用于调节溶剂组织和静电环境，表明在调节和设计凝聚相化学过程中的静电环境时，以溶质为中心的范例的重要性。
