Department of Chemistry, University of California-San Diego , La Jolla, California 92093-0358, United States.
J Am Chem Soc. 2014 Mar 19;136(11):4355-62. doi: 10.1021/ja500174y. Epub 2014 Mar 5.
The symmetry of the hydrogen bond in hydrogen cyclohexene-1,2-dicarboxylate monoanion was determined in chloroform using the NMR method of isotopic perturbation. As the temperature decreases, the (18)O-induced (13)C chemical-shift separations increase not only at carboxyl carbons but also at ipso (alkene) carbons. The magnitude of the ipso increase is consistent with an (18)O isotope effect on carboxylic acid acidity. Therefore it is concluded that this monoanion is a mixture of tautomers in rapid equilibrium, rather than a single symmetric structure in which a chemical-shift separation arises from coupling between a desymmetrizing vibration and anharmonic isotope-dependent vibrations, which is expected to show the opposite temperature dependence.
利用同位素扰动的 NMR 方法，在氯仿中确定了氢环己烯-1,2-二羧酸单阴离子中氢键的对称性。随着温度的降低，不仅在羧基碳原子上，而且在 ipso（烯烃）碳原子上，(18)O 诱导的 (13)C 化学位移分离增加。这种 ipso 增加的幅度与羧酸酸度的 (18)O 同位素效应一致。因此，可以得出结论，这种单阴离子是快速平衡的互变异构体混合物，而不是单一的对称结构，其中化学位移分离来自去对称化振动和非谐同位素依赖振动之间的耦合，预计会表现出相反的温度依赖性。
