Broad single-minimum proton potential and proton polarizability of the hydrogen bonds in trifluoroacetic acid + pyridine-N-oxide systems as a function of donor and acceptor properties and environment. Infrared studies

Abstract

Trifluoroacetic acid + pyridine-N-oxide (1:1) systems have been studied in CCI₄ and acetonitrile as a function of the basicity of the N-oxide. In CCI₄ acid–base complexes are formed almost completely, whereas in acetonitrile the degree of complex formation increases with increasing basicity of the N-oxide, i.e. with increasing ΔpK_a(pK_a of the protonated N-oxide minus pK_a of the acid). Log K_a increases in proportion to ΔpK_a.

In the O^–⋯H⁺⋯ON bonds a single-minimum proton potential is present, and if the degree of symmetry of this potential is sufficiently large continua indicate that these hydrogen bonds show a large proton polarizability caused by the fluctuating proton. From the shift of v(CO) and from the intensity of the continuum, both as a function of ΔpK_a, the following conclusion is drawn: the broad single-minimum potential well shifts with increasing basicity of the N-oxide, i.e. with increasing ΔpK_a from the donor to the acceptor. This shift is larger in acetonitrile solutions than in CCI₄ solutions since in the polar solvent the interaction of the hydrogen-bond dipole with the reaction field induced by it in the solvent is larger.