Theoretical study of the ionization of phenol-water and phenol-ammonia hydrogen-bonded complexes

The ionization of phenol - water and phenol - ammonia complexes have been determined both using ab initio methods that include electron correlation and the hybrid three-parameter B3LYP density functional method. The most stable structure of phenol - water cation corresponds to the C6H5OH+ - H2O non-proton-transferred complex. However, for the phenol - ammonia cation the calculations indicate that the only minimum on the potential energy surface corresponds to the C6H5O - NH4+ proton transferred form. The computed B3LYP adiabatic ionization potentials for C6H5OH - H2O and C6H5OH - NH3 have been determined to be 7.65 and 7.33 eV, respectively. The results obtained indicate that, for the neutral H-bonded systems, the B3LYP density functional method yields very similar results to those obtained with the ab initio MP2 or MCPF methods. However, for the ionized radical cations, B3LYP results compare much better with experiment and to the MCPF method than UMP2. The unsealed B3LYP vibrational frequencies are in very good agreement with the known experimental data.