FT-IR Investigation of OH···N ⇌ O-···H+N Hydrogen Bonds with Large Proton Polarizability in Phosphinic Acid + N-Base Systems in the Middle and Far Infrared Region

Abstract

Sixteen 1:1 dimethylphosphinic acid + N-base systems were studied in acetonitrile−chloroform (1:2) solutions as a function of the basicity of the N-bases. The complexes were measured in the middle infrared (MIR) and far-infrared (FIR) region at 20 °C and at −40 °C. The observed IR continua demonstrate that in the systems with the weaker bases an asymmetrical double minimum proton potential with the deeper well at the acid site is present. With increasing basicity the well at the base site becomes deeper and deeper and the proton potentials obtain a more symmetrical shape. The largest proton polarizability is attained with the system that shows the (on the 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 dimethylphosphinic acid + triallylamine complex. The most intense integrated absorbance of the IR continuum is also observed in this system. The characteristic intensity distribution at the symmetry point indicates medium-strong and relatively long OH···N ⇌ O-···H+N hydrogen bonds. With further increasing basicity the proton potential becomes asymmetrical again, but now with the deeper well at the base site. The proton transfer was studied considering the PO stretching vibration bands. The simultaneous observation of the acid as well as the acid anion PO bands reflect a proton-transfer equilibrium between the nonpolar and polar structure. The far-IR region demonstrates that in the most symmetrical cases the IR continua extend down to about 100 cm-1. Moreover, in contrast to the results with other families of systems, it was for the first time possible to distinguish the nonpolar and polar structure in the far-IR region. Furthermore, the position of the hydrogen bond vibration indicates a significant trend of the force constant. The precise evaluation shows that the system with the (on the average) most symmetrical hydrogen bonds is achieved with a little weaker base than triallylamine, showing the largest bathochromic shift of the IR continuum.