Ten 1: 1 methanesulfonic acid-oxide systems are studied in acetonitrile-chloroform (2: 1) solutions in the middle-infrared (MIR) and far-infrared (FIR) regions at 20 and -40 "C as a function of the basicity of the oxide. An IR continuum demonstrates that first a proton potential with a relatively narrow single minimum at the acid is present. This minimum shifts with increasing basicity in the direction of the oxide and becomes much broader. The proton polarizability is largest with the most symmetrical systems. With a further increase of the basicity, the minimum approaches the base and becomes narrow again. This shift is studied considering the SO stretching vibration bands. Furthermore, it is concluded from the splitting of these bands with increasing polarity of the complexes that these complexes associate via dipole-dipole forces. This is confirmed by osmometric measurements. The observed hydrogen bond vibrations differ largely from hydrogen bond stretching vibrations, yo, and have very difficult vibration character. With the asymmetrical systems the bands of the hydrogen bond vibrations are relatively narrow. With the more or less symmetrical systems the continua, caused by the hydrogen bonds with large proton polarizabilities, extend in the FIR region and the hydrogen bond vibration broadens extremely. This broadening effect of the hydrogen bond vibration should be explained by theoretical studies.
Twelve carboxylic acid-pyridine systems have been studied in chloroform solutions in the mid-IR and the far-lR regions as a function of the pK, of the carboxylic acids at 25°C and at -40°C. If the acidity of the carboxylic acid is small a strongly asymmetrical double-minimum proton potential is present. With increasing acidity the hydrogen bonds become stronger, i.e. the bond length shortens and the proton potentials become more symmetrical in character. In these cases the barrier of the potentials becomes very low and the situation may be compared with a broad, flat, asymmetrical single minimum. Therefore, the continua in these complexes are very similar to those in systems with broad, flat single-minimum proton potentials. With increasing polarity of the complexes the hydrogen bonds lengthen again and the double minimum becomes still more symmetrical. The two proton-limiting structures OH. . .N % 0-. . .H+N can be distinguished by different carbonyl bands; the band of the polar structure is found at relatively high wavenumbers indicating that the barrier is low. Almost symmetrical double-minimum proton potentials are now present. With the most acidic system at -4O"C, the polar structure has a larger weight.In the far-IR spectra two hydrogen-bond vibrations are usually found which are not, however, caused by the two proton-limiting structures. One of these bands has more stretching and the other more librational character. As a rule, the second vibration, in which the whole carboxylic acid as well as the whole pyridine molecule perform a rotational motion against each other, is the less intense one. In the cases of the strongest hydrogen bonds at 25 "C, a slight broadening of the band with stretching character is observed.
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 h 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.
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