CQC, Department of Chemistry, University of Coimbra, Coimbra 3004-535, Portugal.
Department of Physics, University of Jammu, Jammu 180001, India.
J Phys Chem A. 2020 Dec 10;124(49):10277-10287. doi: 10.1021/acs.jpca.0c09141. Epub 2020 Nov 27.
The conformational behavior of carboxylic acids has attracted considerable attention, as it can be used as a gateway for the study of more complex phenomena. Here, we present an experimental and computational study of pyrrole-2-carboxylic acid (PCA) conformational space and the vibrational characterization of the compound by infrared spectroscopy. The possibility of promoting conformational transformations using selective vibrational excitation of the 2ν(OH) and 2ν(NH) stretching overtones is explored. Two conformers, exhibiting the configuration of the COOH group (O═C-O-H dihedral angle near 0°) and differing by the orientation of the carboxylic group with respect to the pyrrole ring (i.e., showing either a or a NCC═O arrangement), were found to coexist initially for the compound isolated in a cryogenic nitrogen matrix, in an 86:14 ratio, and were characterized by infrared spectroscopy. A third conformer, with the COOH group in the configuration, was produced, , by narrowband near-infrared (NIR) excitation of the most stable PCA form (with a NCC═O moiety). The photogenerated PCA conformer was found to decay back to the most stable PCA form, by H-atom quantum mechanical tunneling, with a characteristic half-life time of ∼10 min in the nitrogen matrix at 10 K. Tunneling rates were theoretically estimated and compared for the observed isomerization of pyrrole-2-carboxylic acid and for the structurally similar furan-2-carboxylic acid. This comparison showcases the effect of small modifications in the potential energy surface and the implications of quantum tunneling for the stability of short-living species.
羧酸的构象行为引起了相当大的关注，因为它可以作为研究更复杂现象的入口。在这里，我们通过红外光谱对吡咯-2-羧酸（PCA）构象空间和化合物的振动特性进行了实验和计算研究。探索了通过选择性激发 2ν（OH）和 2ν（NH）伸缩泛频来促进构象转变的可能性。最初发现两种构象体共存，它们都具有 COOH 基团的 构型（O═C-O-H 二面角接近 0°），但羧酸基团相对于吡咯环的取向不同（即，表现出 或 NCC═O 排列），在低温氮基质中，化合物以 86:14 的比例分离，并用红外光谱进行了表征。第三种构象体，COOH 基团处于 构型，是通过最稳定 PCA 形式（具有 NCC═O 部分）的窄带近红外（NIR）激发产生的。通过 H 原子量子力学隧穿，光生成的 PCA 构象体被发现会回到最稳定的 PCA 形式，在 10 K 的氮基质中，特征半衰期约为 10 分钟。理论上估计并比较了观察到的吡咯-2-羧酸和结构相似的呋喃-2-羧酸的异构化的隧穿速率。这种比较展示了在势能表面的微小修改以及量子隧穿对短寿命物种稳定性的影响。
