Complex formation and proton transfer in the polarizable hydrogen bonds of two N,N′-dioxides + substituted phenol systems as a function of the pKa of the phenols. An FTIR study

Abstract

Complex formation, and proton transfer processes in the polarizable hydrogen bonds formed, have been studied with 1,8-bis(dimethylaminomethyl)naphthalene N,N′-dioxide (1)+ R-phenol and with 1,2-bis(dimethylaminomethyl)benzene N,N′-dioxide (2)+ R-phenol systems in acetonitrile solutions as a function of the pK_a of the substituted phenols. In the first family of systems the complex formation is not complete but increases with decreasing pK_a of the phenols, whereas in the second one it is always complete.

In both complexes an OH ⋯ ON ⇌ O^–⋯ H⁺ON equilibrium is observed. With decreasing pK_a of the phenols the left hand well of the double minimum proton potential is first raised. After formation of a strong shorter hydrogen bond with almost no barrier within the proton potential, which is on average largely symmetrical, the right hand well is lowered and finally the proton is localized at the N-oxide group. The proton polarizability of the hydrogen bond first increases and then decreases again. With compound 2, besides the heteroconjugated hydrogen bonds increasingly shorter homoconjugated intramolecular NO ⋯ H⁺⋯ ON hydrogen bonds are formed with decreasing pK_a of the phenols. In the systems with more acidic phenols only this species is present.