The π-type hydrogen bond with triple C-C bond acting as a proton-acceptor. A gradient-corrected hybrid HF-DFT and MP2 study of the phenol-acetylene dimer in the neutral S0 ground state

The π-hydrogen bonded minimum on the S0 ground state potential energy hypersurface (PES) of the neutral phenol-acetylene dimer is located and analyzed in details. Three DFT approaches - B3LYP, mPW1PW91, and PBE1PBE/6-31++G(d,p) along with the MP2/6-31++G(d,p) level of theory were used for the presented analyses. Both the standard and the counterpoise-corrected PESs of the studied dimer were explored. Besides the weak hydrogen-bonding interaction of a π-type, also purely electrostatic interaction of a dipole-quadrupole type between the monomeric units is responsible for the stabilization of this global minimum on the studied PESs. The computed counterpoise-corrected interaction and dissociation energies at all levels of theory are in excellent agreement with the estimations based on experimental spectroscopic data. Anharmonic OH stretching frequencies for the free phenol and the π-hydrogen-bonded phenol-acetylene complex were computed on the basis of 1D DFT and MP2 OH stretching potentials. While all DFT levels significantly overestimate the experimentally measured ν(OH) vibrational frequency shift, the corresponding MP2 value is in excellent agreement with the experimental data (70.2 vs. 68 cm−1). On the other hand, when the frequency shifts are computed within the harmonic approximation, all DFT levels are in fortuitous good agreement with the experiment, due to cancellation of errors. Passing to the counterpoise-corrected PESs has a little influence on the calculated harmonic and anharmonic vibrational frequency shifts of the phenol OH stretching mode. According to the charge-field perturbation analyses of the 1D OH stretching potentials, the purely electrostatic interaction of phenol with the proton-accepting acetylene molecule governs only a small portion of the overall OH vibrational frequency shift, while the role of electrostatics with this respect is more important at MP2 level of theory. However, all levels of theory lead to a conclusion that the dynamical changes of charge-transfer interaction in the course of OH stretching are the main factor governing the IR intensity enhancement of this vibrational transition upon π-hydrogen bonding, while the electrostatic interaction is of a second-order importance with respect to this. According to the second-order perturbation theory analysis of the Fock matrix (or its Kohn-Sham analog) within the NBO basis and the NBO deletion analyses, the charge-transfer between the monomeric units within the dimer is essentially one-directional (occurring from acetylene to phenol), and predominantly of a π → σ* type. The energetic effect of this interaction was estimated as well. The existence of another, much less stable minimum on the explored PESs, predicted on the basis of mainly electrostatic arguments, was also confirmed.