TY - JOUR

T1 - Hydrogen bond geometries and proton tautomerism of homoconjugated anions of carboxylic acids studied via H/D isotope effects on 13 C NMR chemical shifts

AU - Guo, Jing

AU - Tolstoy, Peter M.

AU - Koeppe, Benjamin

AU - Golubev, Nikolai S.

AU - Denisov, Gleb S.

AU - Smirnov, Sergei N.

AU - Limbach, Hans Heinrich

PY - 2012/11/26

Y1 - 2012/11/26

N2 - Ten formally symmetric anionic OHO hydrogen bonded complexes, modeling Asp/Glu amino acid side chain interactions in nonaqueous environment (CDF 3 /CDF 2 Cl solution, 200-110 K) have been studied by 1 H, 2 H, and 13 C NMR spectroscopy, i.e. intermolecularly H-bonded homoconjugated anions of acetic, chloroacetic, 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 13 C 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•••O - and O - •••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 13 C NMR parameters, taking into account the degenerate proton tautomerism.

AB - Ten formally symmetric anionic OHO hydrogen bonded complexes, modeling Asp/Glu amino acid side chain interactions in nonaqueous environment (CDF 3 /CDF 2 Cl solution, 200-110 K) have been studied by 1 H, 2 H, and 13 C NMR spectroscopy, i.e. intermolecularly H-bonded homoconjugated anions of acetic, chloroacetic, 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 13 C 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•••O - and O - •••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 13 C NMR parameters, taking into account the degenerate proton tautomerism.

UR - http://www.scopus.com/inward/record.url?scp=84870008949&partnerID=8YFLogxK

U2 - 10.1021/jp304943h

DO - 10.1021/jp304943h

M3 - Article

C2 - 22738093

VL - 116

SP - 11180

EP - 11188

JO - Journal of Physical Chemistry B

JF - Journal of Physical Chemistry B

SN - 1520-6106

IS - 46

ER -