CQC, Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal.
J Chem Phys. 2017 Mar 28;146(12):124305. doi: 10.1063/1.4978794.
DFT(B3LYP)/6-311++G(d,p) calculations on 2-fluorobenzoic acid (2FBA) show that the molecule has four conformers: two low energy conformers (forms I and II) with the carboxylic acid group assuming the cis configuration (O=C-O-H dihedral equal to 0°) and two higher-energy conformers (III, IV) with a trans carboxylic group configuration. Isolation of 2FBA monomers in argon or nitrogen matrices allows for the efficient trapping of both low-energy conformers. Narrowband selective near-IR(NIR) excitation of the 2νOH mode of I in both argon and N matrices leads to its efficient conversion into conformer III, which is stabilized by an intramolecular O-H···F interaction. On the other hand, upon identical selective vibrational excitation of II no changes could be noticed in the argon matrix spectra, while experiments carried out on N matrices showed conversion of II into III. In conformer IV (the expected direct product resulting from NIR excitation of II), the stabilizing O-H···F interaction existing in III is replaced by an O-H···H repulsive interaction, which leads to a barrier separating this form from II that is about one third of that separating III from I. Under these circumstances, once formed by vibrational excitation of II, conformer IV can easily convert to the reactant species by fast tunneling, justifying the apparent inefficiency of the II→IV conversion upon vibrational excitation of II in an argon matrix. On the other hand, the stabilization of the initially formed conformer IV by the N matrix (due to OH···N interactions) allows this species to survive long enough to allow the IV→III over-the-barrier conversion to be competitive with the IV→IItunneling, justifying the observed net conversion of the NIR excited conformer II into form III in N matrix. These results demonstrate as the intramolecular local topology may decisively influence the intrinsic kinetic stability of different conformers of the same molecule. They are also a clear additional evidence of the stabilization of higher-energy trans conformers of carboxylic acids by the N matrix medium that has been found before for other molecules of the same family.
使用 DFT(B3LYP)/6-311++G(d,p) 计算 2-氟苯甲酸（2FBA）表明，该分子有四个构象：两个低能构象（构象 I 和 II），其中羧酸基团呈顺式构象（O=C-O-H 二面角等于 0°），两个高能构象（构象 III 和 IV），其中羧酸基团呈反式构象。在氩气或氮气基质中分离 2FBA 单体可以有效地捕获两种低能构象。在氩气和氮气基质中，窄带选择性近红外（NIR）激发 I 中 2νOH 模式可以有效地将其转化为构象 III，这是通过分子内 O-H···F 相互作用稳定的。另一方面，在氩气基质中，对 I 的 2νOH 模式进行相同的选择性振动激发，在 II 的光谱中没有观察到变化，而在氮气基质中进行的实验表明 II 转化为 III。在构象 IV（通过 NIR 激发 II 直接生成的预期产物）中，存在于 III 中的稳定 O-H···F 相互作用被 O-H···H 排斥相互作用取代，这导致该构象与 II 之间的势垒约为 III 与 I 之间势垒的三分之一。在这种情况下，通过 II 的振动激发形成构象 IV 后，它可以通过快速隧道穿过势垒，很容易转化为反应物，这解释了在氩气基质中振动激发 II 时，II→IV 转化效率低的原因。另一方面，N 基质对初始形成的构象 IV 的稳定作用（由于 OH···N 相互作用）使其能够存在足够长的时间，使得 IV→III 过势垒转化与 IV→Itunnel 竞争，这解释了在 N 基质中观察到的 NIR 激发的构象 II 向构象 III 的净转化。这些结果表明，分子内局部拓扑结构可能会对同一分子的不同构象的固有动力学稳定性产生决定性影响。它们也是以前在同一族其他分子中发现的羧酸高能反式构象由 N 基质稳定的又一明确证据。
