Simulation of Optical Spectra and Photolysis of Phenolic Compounds

Phenol enters the environment due to combustion of plants and biomass, as well as through anthropogenic emissions. Phenolic compounds can act as precursors to organic aerosols; they adversely affect human health and reduce atmospheric visibility. The static absorption spectra of phenol and its volatile substituted compounds (p-cresol and vanillin) presented in this work were obtained and analyzed using computer simulations based on quantum mechanical molecular dynamics. The optical spectra were averaged over excited instantaneous molecular conformers fluctuating along system’s evolution trajectories. The photodynamic dissipation of electronically excited states of the molecules is studied by generating trajectories in nonadiabatic molecular dynamics. Changes in the electronic structure of phenol, p-cresol, and vanillin, along with the crossing points and dissociation of potential energy surfaces, are obtained using the mixed-reference spin-flip approach within the time-dependent density functional theory. The O–H bond breaking in the hydroxyl group followed by deprotonation causes minor structural deformations in the molecules under study. It is shown that, upon excitation, dissociation of the hydroxyl group occurs via an electronic transition to the σ–MO localized on the elongated O–H bond. The results of the work can be used in ecology of the environment for developing devices based on photochemistry for monitoring and purification from phenolic contaminations.