A gradient-corrected density functional and MP2 study of phenol-ammonia and phenol-ammonia(+) hydrogen-bonded complexes

A gradient-corrected DFT and MP2 study of phenol-ammonia neutral and cationic dimers were carried out. Various combinations of exchange and correlation functionals were employed within the DFT approach: B3LYP, MPW1PW91, PBE1PBE, and the modified B3LYP(M) [J. Mol. Struct. 416 (1997) 1]. The BSSE-corrected PES of the neutral dimer was also explored at B3LYP, B3LYP(M) and MP2/6-31++G(2d,p) levels. A single minimum was located on the neutral complex PES corresponding to the hydrogen-bonded non-proton-transferred form, while the only minimum on cationic complex PES corresponds to a phenoxyl-ammonium proton-transferred form, regardless on the level of theory. Dissociation energies corrected for BSSE and including the relaxation energy terms are reported for both complexes. The calculated vertical IE values at all DFT levels of theory are in excellent agreement with the experimental data. While the anharmonic vibrational analysis based on all ID DFT vibrational potentials overestimate the nu(OH) mode frequency shift upon complexation, the value obtained from the ID MP2 potential for the BSSE-corrected MP2/6-31++G(2d,p) PES is in excellent agreement with the experimental data (457 vs. 469 cm(-1)). Various contributions to the overall anharmonic nu(OH) frequency shifts are considered, as well as the BSSE influence which was found to be very small. As revealed by the CFP ID nu(OH) DFT and MP2 vibrational potentials, the electrostatic interaction alone can not account neither for the substantial red shift of this mode upon hydrogen bonding, and especially for the substantial intensity enhancement. (C) 2002 Elsevier Science B.V. All rights reserved.