FTIR and 13C NMR studies of hydrogen bonds and proton transfer in complexes of N-dodecyl-N,N-dimethylamine oxide with 26 phenols, carboxylic and mineral acids in solution.

Abstract

Abstract Carbon-13 NMR spectra of N-dodecyl-N,N-dimethylamine oxide (DDAO) and its 26 hydrogen bonded complexes in deuteriated chloroform have been studied in relation to the variation of their aqueous pKa (AH) values ranging from −10 to 15. Correlation of the chemical shifts with pKa(AH) yields typical titration curves fitted by the equation: y = (Amax + Amin 10(C ± DX))/ (1 + 10(C ± DX)) where Amax and Amin are maximum (due to a molecular complex, B…HA) and minimum (due to a polar complex, B+H..A−) values of chemical shifts,and C and D are constants. The pKa values at the inflection point vary modestly with the carbon atom used in the calculation (from pKa = 4.55 to 4.87). A single carbonyl band in the second-derivative FTIR spectra of equimolar mixtures DDAO with four acetic acids in dichloromethane implies that hydrogen bonds are described by a strongly asymmetrical quasi-single minimum potential and that the tautomeric equilibrium, B…HA ⇌ B+H…A−, is excluded in the investigated complexes. Some equimolar complexes interact with a second molecule of DDAO to form homoconjugated complex, RCOO−(B.H.B)+. This interaction is controlled by pKa(AH). The equimolar complexes interact also with a second molecule of acid, and the second hydrogen bond causes only a minor effect on the first hydrogen bond. Relation between the acid-base titration curves and shape of potential energy function is discussed.