Carboxylic acids hydrogen bonding distance

That such a delicate balance exists between tautomers is not an obvious prediction based solely on ApK s. The 10 pK unit difference in 9 benzoate favors the amidine-carboxylic acid form by 0.6 eV. However, a simple electrostatic calculation [123] for a positive and negative charge at a salt bridge distance of 3.8 A translates into a stabilization energy of-0.50 eV in the solvent THF, which nearly offsets the stabilization of the amidine-carboxylic acid tautomer derived from the ApfQ. Electron-rich carboxylates such as benzoate are sufficiently basic that the amidine-carboxylic acid hydrogen bond interaction prevails while the interface retains its ionic nature for more acidic carboxylic acids and various sulfonic acids. 

Figure 1. Solid state NMR chemical shifts of carboxylic acids and imidazoles are plotted against the hydrogen bonding distance a) data for carboxylic acids (data from reference (2021)), circles are compounds with COO—H COO motif, ApKa 0, and crosses are compounds with COO—H - COOH motif, Ap a > 15 b) data for imidazoles (data from Table I), circles present cationic imidazoles (Im+—H- COO , ApA a 3), and crosses are neutral imidazoles (Im COO , ApKa 10).

Figure 2.10 SAMT active site. The distance between salicylic acid and SAH in the crystal structure is 3 A. Hydrogen bonding distances for Gln25, Trpl51, and the carboxyl moiety of salicylic acid is 3.1 A and 2.8 A, respectively.

These hydrogen-bond distances in ice provide a useful standard with which to compare the range of O-H- -O distances observed in other types of crystal structures. For example when water molecules accept hydrogen bonds from the carboxylic acid -OH donors, i.e., 0=C0H- -OwH2, the bonds are stronger than in ice. In the reverse situation, when the OH donors of the water molecules bond to carboxylic acid C=0 acceptors, the bonds are significantly Weaker (see Thble 7.5). 

Figure 5.8 Hydrogen bond distances (DIST1) plotted against number of observations for the carboxylic acid dimer query described in Figure 5.7.

Figure 5.9 Plot of the hydrogen bond distances (DIST1 and DIST2) against each other for the carboxylic acid dimers found in the search of the Cambridge Crystallographic Database.

H and C. Thus the amide force field could be used on carboxylic acids without any modification But how could the hydrogen bond energy be larger and the hydrogen bond distance shorter Simply be-... 

---