Hydrogen bond geometries and proton tautomerism of homoconjugated anions of carboxylic acids studied via H/D isotope effects on 13C NMR chemical shifts.
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) = ½(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.
Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany.
Jing
J
Guo
Peter M
PM
Tolstoy
Benjamin
B
Koeppe
Nikolai S
NS
Golubev
Gleb S
GS
Denisov
Sergei N
SN
Smirnov
Hans-Heinrich
HH
Limbach
D000838:Anions / Q000737:chemistry; D002247:Carbon Isotopes; D002264:Carboxylic Acids / Q000737:chemistry*; D041961:Deuterium Exchange Measurement; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy / Q000592:standards; D015394:Molecular Structure; D011522:Protons; D012015:Reference Standards
D000838:Anions; D002247:Carbon Isotopes; D002264:Carboxylic Acids; D011522:Protons