Hydrogen Bond Geometries and Proton Tautomerism of Homoconjugated Anions of Carboxylic Acids Studied via H/D Isotope Effects on C-13 NMR Chemical Shifts

作者：Guo Jing; Tolstoy Peter M^*; Koeppe Benjamin; Golubev Nikolai S; Denisov Gleb S; Smirnov Sergei N; Limbach Hans Heinrich

来源：Journal of Physical Chemistry A, 2012, 116(46): 11180-11188.

DOI：10.1021/jp304943h

Ten formally symmetric anionic OHO hydrogen bonded complexes, modeling ASp/Glu amino acid side chain interactions in nonaqueous environment (CDF3/CDF2Cl solution, 200-110 K) have been studied by H-1, H-2, and C-13 NMR spectroscopy, i.e. intermolecularly H-bonded homoconjugated anions of acetic, chloro- acetic, dichloroacetic, trifluoroacetic, trimethylacetic, and isobutyric acids, and intramolecularly H-bonded hydrogen succinate, hydrogen rac-dimethylsuccinate, hydrogen maleate, and hydrogen phthalate. In particular, primary H/D isotope effects on the hydrogen bond proton signals as well as secondary H/D isotope effects, on the C-13 signals of the carboxylic groups are reported and analyzed. We demonstrate that in most of the studied systems there is a degenerate proton tautomerism between O-H center dot center dot center dot O- and O-center dot center dot center dot H-O structures which is fast in the NMR time scale. The stronger is the proton donating ability of the acid, the shorter and more symmetric are the H-bonds in each tautomer of the homoconjugate. For the maleate and phthalate anions exhibiting intramolecular hydrogen bonds, evidence for symmetric single well potentials is obtained. We propose a correlation between H/D isotope effects on carboxylic Carbon chemical shifts and the proton transfer coordinate, q(1) = 1/2(r(OH) - r(HO)), which allows us to estimate the desired OHO hydrogen bond geometries from the observed C-13 NMR parameters, taking into account the degenerate proton tautomerism.

• 出版日期2012-11-22