Perrin, Charles L. Burke, Kathryn D. Variable-Temperature Study of Hydrogen-Bond Symmetry in Cyclohexene-1,2-dicarboxylate Monoanion in Chloroform‑<i>d</i> 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 <sup>18</sup>O-induced <sup>13</sup>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 <sup>18</sup>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. temperature dependence;symmetry;cyclohexene;perturbation;carboxyl carbons;Chloroform;anharmonic;desymmetrizing vibration;equilibrium;mixture;chloroform;Monoanion;separation;tautomer;temperature decreases;carboxylic acid acidity;Symmetry;18 O isotope effect;NMR method;alkene;monoanion;hydrogen bond;magnitude;Cyclohexene 2014-03-19
    https://acs.figshare.com/articles/journal_contribution/Variable_Temperature_Study_of_Hydrogen_Bond_Symmetry_in_Cyclohexene_1_2_dicarboxylate_Monoanion_in_Chloroform_i_d_i_/2313667
10.1021/ja500174y.s002