PROTON TRANSFER IN P-NITROPHENOL-TRIETHYLAMINE SYSTEM IN APROTIC SOLVENTS

The interaction of p-nitrophenol with triethylamine in various aprotic solvents was studied quantitatively by means of electronic absorption spectroscopy, with special regard to proton transfer. The acid-base interaction leads to different types of complexes, depending upon the dielectric constant of the solvents. In 1,2-dichloroethane as the solvent, proton transfer takes place from the nitrophenol to the amine to yield a hydrogen-bonded ion pair, which is in equilibrium with a simple hydrogen-bonded complex. The enthalpy change determined from the spectral data indicates that the ion pair is more stable by 3.1 kcal mole in the ground state. Schematic double-minimum potential curves are given for the proton in the excited state as well as in the ground state of the acid-base system. A calculation was made to estimate the solvation energy of the complexes on the basis of the reaction field theory. The result suggests that in a medium of dielectric constant unity the ion pair should be less stable by 5.0 kcal/mole in the ground state, but more stable by 7.8 kcal mole in the excited state. In acetonitrile, only a proton-transferred species was observed which was assigned to a solvent-separated ion pair. © 1969.