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

Abstract

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.

Analysis of Proton NMR in Hydrogen Bonds in Terms of Lone‐Pair and Bond Orbital Contributions

“…Overall, the ordering of D XHX from highest to lowest correlates well with the ordering of the proton shielding from highest to lowest, and with the ordering of the chemical shifts from lowest to highest, both for the equilibrium and for the effective structures. The present calculations confirm the correlation of D XHX with the proton shifts noted previously by Limbach et al 24. 27, 29…”

“…An interesting related system is 2‐hydroxyacetophenone, where a CC bond is in conjugation with an H‐bonding carbonyl group but also part of an aromatic 6‐membered ring; the chemical shift of the OH proton is 12 ppm 23. A very large proton shift exceeding 20 ppm has been found for the internal H‐bond of the hydrogen succinate anion (HSA), a partially deprotonated dicarboxylic acid, and related systems 24. Large proton shifts of 12 ppm and above are also found for intermolecular hydrogen bonds, such as for O⋅⋅⋅H in carboxylic acid dimers or N⋅⋅⋅H in DNA base pairs 25.…”

Accurate ab initio calculations of O–H⋯O and O–H⋯−O proton chemical shifts: towards elucidation of the nature of the hydrogen bond and prediction of hydrogen bond distances

“…Furthermore, the anions 36 to 43 seem to present a separate class of compounds with an intercept of linear regression nearly parallel to that of compounds 5 to 35. 79,86 Natural bond orbital analysisthe nature of hydrogen bonding Table 2 represents the natural bond orbital (NBO) analysis of the compounds of Scheme 1 that has been carried out at the B3LYP/6-31+G(d) and M06-2X/6-31+G(d) level of theory. 84 In contrast, an excellent linear correlation of computed δ(OH, ppm) vs. computed (O)H⋯O, r (O)H⋯O , hydrogen bond distances of the form: with a slope of −20.49 ppm Å −1 was also obtained with minimization of the structures at the B3LYP/6-311+Gd level of theory (Fig.…”

Structure assignment and H/D-exchange behavior of several glycosylated polyphenols

“…47 In NMR spectroscopy, the IE terminology refers to differences in chemical shift for nuclei that are directly attached to the isotope or that are neighbors to the site of isotope substitution. In NMR spectroscopy, IEs have been generally reported for different classes of nuclei and substances, 48 have been used to describe conformational changes 49 and hydrogen-bonded systems, 50 and have been applied successfully to the structural description of small proteins in solution and in solid-state. 48 The absence of H/D-exchange at H6 and H8 for 5 and the progressing exchange of H6 and H8 in 5 is shown in Figure 7.…”