FTIR investigation of O···H···O hydrogen bonds with large proton polarizability in sulfonic acid–N-oxide systems in the middle and far-IR

Abstract

Sixteen 1:1 methanesulfonic acid–N-oxide systems were studied as a function of the basicity of the N-oxides. The observed IR continua demonstrate that a single minimum proton potential is shifted, with increasing basicity, from the acid to the N-oxide. The largest proton polarizability is attained with the system that shows the (on average) most symmetrical proton potential, as indicated by the maximum bathochromic shift of the IR continuum. This shift toward lower wavenumbers is largest with the methanesulfonic acid–3,5-dichloropyridine-N-oxide complex, for which the integrated absorbance of the IR continuum is also largest and the continuous absorption is nearly temperature independent. The significant intensity distribution at the symmetry point indicates very strong and short O···H···O hydrogen bonds. The proton transfer was studied via the SO-stretching vibration bands. The steady shift of the acid to the acid anion SO bands reflects a continuous proton transfer process within this family of systems. Study of the far-IR region shows that, especially in the most symmetrical cases, the IR continua extend down to 100 cm^-1 or less. Semi-empirical calculations show that the observed hydrogen-bond vibrations differ largely from hydrogen-bond stretching vibrations (ν_σ) and have complicated vibrational character (ν_HB). In contrast to the methanesulfonic acid–sulfoxide (phosphine oxide, arsine oxide) family of systems no broadening of this transition occurs. The position of the bands indicates, however, a significant trend of the force constant. It is shown that the system with the strongest hydrogen bonds is that with the largest bathochromic shift of the IR continuum, i.e. the system with the largest proton polarizability.