Far-infrared investigation of the proton transfer with substituted phenol + N-mono- and N,N-dioxides as a function of the pKa of the phenols

Abstract

The hydrogen bond stretching vibration, vu, 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 Nfl-dioxide (DMAMBO) solutions in a 2:l mixture of chloroform and acetonitrile as a function of the pKa 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, vu first shifts with decreasing pKa 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; Le., only one of the two proton limiting structures OH-ON * 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 lo'* s-I 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 pKa of the phenols beginning with the 4-cyanophenol + DMAMBO system intramolecular homoconjugated NO-H+-ON hydrogen bonds are also formed. If the pK, 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.

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

“…19 In good agreement with this spectral behaviour, the continuous absorption in system 5 is nearly temperature independent. In summary we can state that the most interesting system is the MSAÈ35DC1PyO complex (5) in which the highest symmetry of the proton potential and the largest proton polarizability are reached.…”

NMR studies of proton transfer in 1∶1 tris(trimethoxyphenyl)phosphine oxide–phenol complexes

“…White crystals were exposed to high vacuum for several hours to fully dehydrate the material. Elemental analysis and 1 H, 13 C and 31 P NMR spectra of the product in solution showed TMPPO was free from impurities.…”

“…First, these adducts were examined in solution by means of 1 H and 31 P NMR spectroscopy to verify the presence of a hydrogen-transfer process. Then, the complexes were studied in the solid state by means of 13 C NMR, according to the following treatment.…”

Photophysics and Homoconjugation Effect of the Acid‐base Complex between 2,4,6‐Trimethoxy‐2‐styrylquinoline and Protic Acids

“…7 Zundel and co-workers reported that the proton transfer process in the HB complexes between phenol and amines are decided by their pKa differences. 8 Mizutani has mentioned the effect of temperature on the phenol-amine complexes. 5 The influence of solvent polarity on acid-base behavior has been the focus of many studies.…”