Taccone Martín I, Thomas Daniel A, Ober Katja, Gewinner Sandy, Schöllkopf Wieland, Meijer Gerard, von Helden Gert
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.
Department of Chemistry, University of Rhode Island, 140 Flagg Rd., Kingston, Rhode Island 02881, USA.
Phys Chem Chem Phys. 2023 Apr 12;25(15):10907-10916. doi: 10.1039/d2cp05409d.
Hydrogen bonding interactions are essential in the structural stabilization and physicochemical properties of complex molecular systems, and carboxylic acid functional groups are common participants in these motifs. Consequently, the neutral formic acid (FA) dimer has been extensively investigated in the past, as it represents a useful model system to investigate proton donor-acceptor interactions. The analogous deprotonated dimers, in which two carboxylate groups are bound by a single proton, have also served as informative model systems. In these complexes, the position of the shared proton is mainly determined by the proton affinity of the carboxylate units. However, very little is known about the nature of the hydrogen bonding interactions in systems containing more than two carboxylate units. Here we report a study on the deprotonated (anionic) FA trimer. IR spectra are recorded in the 400-2000 cm spectral range by means of vibrational action spectroscopy of FA trimer ions embedded in helium nanodroplets. Characterization of the gas-phase conformer and assignment of the vibrational features is achieved by comparing the experimental results with electronic structure calculations. To assist in the assignments, the H and O FA trimer anion isotopologues are also measured under the same experimental conditions. Comparison between the experimental and computed spectra, especially the observed shifts in spectral line positions upon isotopic substitution of the exchangeable protons, suggests that the prevalent conformer, under the experimental conditions, exhibits a planar structure that resembles the crystalline structure of formic acid.
氢键相互作用对于复杂分子体系的结构稳定性和物理化学性质至关重要，羧酸官能团是这些结构单元中的常见参与者。因此，中性甲酸（FA）二聚体在过去已被广泛研究，因为它是研究质子供体 - 受体相互作用的有用模型体系。类似的去质子化二聚体，其中两个羧酸根基团由单个质子连接，也已作为有价值的模型体系。在这些配合物中，共享质子的位置主要由羧酸根单元的质子亲和力决定。然而，对于含有两个以上羧酸根单元的体系中氢键相互作用的本质知之甚少。在此，我们报告一项关于去质子化（阴离子）FA三聚体的研究。通过对嵌入氦纳米液滴中的FA三聚体离子进行振动作用光谱，在400 - 2000 cm光谱范围内记录红外光谱。通过将实验结果与电子结构计算进行比较，实现气相构象异构体的表征和振动特征的归属。为辅助归属，还在相同实验条件下测量了H和O FA三聚体阴离子同位素异构体。实验光谱与计算光谱之间的比较，特别是在可交换质子的同位素取代后观察到的谱线位置的移动，表明在实验条件下，普遍存在的构象异构体呈现出类似于甲酸晶体结构的平面结构。
