PROTON POTENTIAL AS A FUNCTION OF THE PKA OF PHENOL IN INTERMOLECULAR PHENOL TRIMETHYLAMINE N-OXIDE HYDROGEN-BONDS

Fourteen 1:1 R-substituted phenol-trimethylamine N-oxide solutions in CH2Cl2 were studied by FTIR spectroscopy as a function of the acidity of phenol. With all complexes, phenol-N-oxide hydrogen bonds are formed completely. With the less acidic system, the proton is present in a single minimum at the phenol. The hydrogen bonds show no proton polarizability. With increasing acidity, a double-minimum proton potential is observed in the OH...ON half arrow right over half arrow left O-...H+ON bonds whereby with increasing acidity this double minimum becomes more and more symmetrical. In the case of the 3,4-dinitrophenol-trimethylamine N-oxide complex, the hydrogen bonds become much stronger and shorter and the double minimum has changed to a double minimum with a very low barrier or to a broad flat single minimum. If the acidity of the phenols increases still further, again a double-minimum proton potential is found whereby the deeper well is present at the acceptor and becomes deeper and deeper with increasing acidity. Finally, the proton is localized at the acceptor. A single minimum with the well at the acceptor is now present. The reasons for these changes of the protons potential are discussed in detail.