Institute for Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany.
J Am Chem Soc. 2010 Nov 17;132(45):15902-4. doi: 10.1021/ja107531y. Epub 2010 Oct 22.
We present the first generation and spectroscopic identification of the higher-lying E conformer of the simplest aromatic carboxylic acid, benzoic acid (1a), as its O-deuterated isotopologue (E)-d(1)-1a using matrix-isolation techniques; the parent (E)-1a could not be observed because of fast H-tunneling to the more stable conformer (Z)-1a. Even deuterated (E)-d(1)-1a converts quickly back to (Z)-d(1)-1a through D-tunneling with a half-life (τ) of ∼12 min in Ar at 11 K. Tunneling computations using an Eckart barrier in conjunction with a CCSD(T)/cc-pVTZ//MP2/cc-pVDZ + ZPVE intrinsic reaction path revealed that τ of (E)-1a is only ∼10(-5) min, in marked contrast to those of simple aliphatic acids, which are in the range of minutes. The electronic substituent effects on D-tunneling in para-substituted benzoic acid derivatives (p-X-PhCOOD, d(1)-1) were systematically studied in Ar matrices at 11 K to derive the first Hammett relationships for atom tunneling. σ-Electron donors (X = alkyl) increase the half-life of d(1)-1, while σ-acceptor/π-donor groups (X = OD, NH(2), halogen) and to an even greater extent a σ-/π-acceptor group (X = NO(2)) decrease τ. The latter finding is in line with the smaller E-to-Z reaction barriers and narrower reaction widths for the isomerization. Tunneling substituent constants (σ(t)) for this conformational isomerization were derived experimentally and computationally.
我们首次通过基质隔离技术，展示了最简单芳香羧酸苯甲酸（1a）的高能 E 构象的第一代和光谱鉴定，其为 O-氘代同位素物（E）-d(1)-1a；由于快速的 H 隧道跃迁到更稳定的构象（Z）-1a，无法观察到母体（E）-1a。即使是氘代（E）-d(1)-1a 也会通过 D 隧道迅速回落到（Z）-d(1)-1a，在 11 K 的 Ar 中半衰期（τ）约为 12 分钟。使用 Eckart 势垒结合 CCSD(T)/cc-pVTZ//MP2/cc-pVDZ + ZPVE 内禀反应路径的隧道计算表明，（E）-1a 的 τ 仅约为 10(-5)分钟，与简单脂肪族酸形成鲜明对比，其范围为分钟。在 11 K 的 Ar 基质中，系统研究了对取代苯甲酸衍生物（p-X-PhCOOD，d(1)-1）中 D 隧道电子取代基效应，以得出原子隧道的第一个哈米特关系。σ-电子给体（X = 烷基）增加了 d(1)-1 的半衰期，而 σ-受体/π-供体基团（X = OD、NH(2)、卤素），甚至更大程度上的 σ-/π-受体基团（X = NO(2))，会减少 τ。后一种发现与异构体化的较小 E-Z 反应势垒和更窄的反应宽度一致。通过实验和计算推导出了这种构象异构体化的隧道取代常数（σ(t)）。
