p-cresol quantum-classical photodynamics, photostatic UV, IR and Raman spectra

Abstract

A joint theoretical and experimental study of photoinduced physicochemical and thermodynamic properties of p-cresol was performed using conventional quantum-mechanical approaches and hybrid quantum-classical methodologies combined with the experimental optical absorption and fluorescent spectroscopy. The CCSD, CC2, MP2 and DFT methods as well as the EoM-CCSD, ADC(2)-MP2 and TD-DFT extensions to electron excitations were involved for optimizations and vertical electron transitions in the isolated p-cresol molecule and continuum implicit solvent model. The absorption and fluorescence spectra of the p-cresol in water were measured experimentally to be used as a reference. Static optical spectra were obtained as the statistically averaged electronic states of instantaneous vibrational conformers fluctuating on quantum-classical molecular dynamical trajectories due to combinations of classical forces with quantum gradient and embedding electrostatic potential fitting with point atomic charges. The excited dissipative crossing potential energy surfaces were defined through conical intersection searches and non-adiabatic molecular dynamics simulations using mixed-reference spin-flip with trajectory surface hopping for photodynamic propagation. Conical intersection points were considered for direct transitions from the lowest excited state to the ground state, as well as for two-stage transitions from the second excited state through a lower-lying level to the unexcited electronic structure. The electronic types of excited states at their intersections and key points on photodynamic trajectories were revealed using electron density differences and Dyson’s molecular orbitals based on the extended Koopmans' theorem, whereas canonical molecular orbitals were applied for conventional quantum-mechanical methods. Swift O-H deprotonation through the |π→${\sigma }_{\text{OH}}^{*}$〉 transition was demonstrated using both conical intersection and photodynamic simulations.