Unimolecular Dissociation and Association Followed by Ensuing Dissociation Kinetics of Phenol-Dimer: A Prototype of Hydrogen Bonded Complex.

On-the-fly chemical dynamics simulations are carried out to study the unimolecular dissociation and the association followed by the ensuing dissociation of the phenol-dimer, a prototype of a hydrogen-bonded complex. All simulations are carried out at three different temperatures of 1000, 1500, and 2000 K. The PM7 semiempirical method, which provides reasonably good minimum energies and the corresponding geometries of the dimer, is used to calculate the potential energy gradient, which is required for integrating Hamilton's equations of motion. The results are compared with benzene dimer (Bz2) and benzene- hexachlorobenzene (Bz-HCB), studied in the past to understand the effect of hydrogen bonding interactions present in phenol-dimer and absent in other weakly bound dimers. The unimolecular dissociation rate constants and the association probabilities, association cross sections, and association rate constants are calculated, reported, and compared. The activation energies and frequency factors are also calculated. In general, among Bz2, Bz-HCB, and phenol-dimer, phenol-dimer showed the slowest rate of unimolecular dissociation, whereas it has the highest rate of association. The ensuing dissociation rate is also the slowest in the phenol-dimer, confirming a high dimer lifetime.