FAR-INFRARED INVESTIGATION OF THE PROTON-TRANSFER WITH SUBSTITUTED PHENOL+N-MONOOXIDES AND N,N'-DIOXIDES AS A FUNCTION OF THE PKA OF THE PHENOLS

The hydrogen bond stretching vibration, nu(sigma), in the far-infrared region is studied for substituted phenols with trimethylamine N-oxide (TMAO) as well as with 1,2-bis((dimethylamino)methyl)benzene N,N'-dioxide (DMAMBO) solutions in a 2:1 mixture of chloroform and acetonitrile as a function of the pK(a) of the phenols. It is found and explained that the position of this vibration is only slightly dependent on the mass of the substituted phenols. In the case of the phenol + TMAO systems, nu(sigma) first shifts with decreasing pK(a) of the phenols toward higher wavenumbers showing a maximum with the 3,4-dinitrophenol + TMAO system and shifts then again toward smaller wavenumbers. The position of this band reflects the strength of these hydrogen bonds. Always only a single hydrogen bond stretching vibration is observed. With the systems with more asymmetrical double minimum proton potentials, the ground state of the proton is not split; i.e., only one of the two proton limiting structures OH...ON reversible O-...H+ON is realized if the system is in the ground state. In the most symmetrical system, O-...H+...ON bonds are built; hence also only one hydrogen bond stretching vibration is expected. In the cases in which the double minimum proton potentials are almost largely symmetrical, the lowest level is split. Both proton limiting structures are realized in the ground state. Hence the result that only one hydrogen bond stretching vibration is found in these cases demonstrates that the proton fluctuates with a frequency larger than 10(12) s-1 within these hydrogen bonds. In the case of the phenol + DMAMBO systems, the nature of the hydrogen bonds is equivalent to those in the phenol + TMAO systems. But with decreasing pK(a) of the phenols beginning with the 4-cyanophenol + DMAMBO system intramolecular homoconjugated NO...H+...ON hydrogen bonds are also formed. If the pK(a) of the phenols decreases further, with the 3,4-dinitrophenol + DMAMBO systems only such homoconjugated hydrogen bonds are found and the phenolates are present as free ions. This formation of homoconjugated hydrogen bonds can be explained by sterical reasons.