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
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 ΔpKa(pKa of the protonated N-oxide minus pKa of the acid). Log Ka increases in proportion to ΔpKa.
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 ΔpKa, the following conclusion is drawn: the broad single-minimum potential well shifts with increasing basicity of the N-oxide, i.e. with increasing ΔpKa 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.