Quantum-Chemical Simulation of the Solvent Effect on Spontaneous Emission of Singlet Oxygen

Abstract

A molecular simulation of the solvent effect on radiative rate constant k_r of singlet oxygen is carried out. This study included a search for the most probable conformations of the complexes of molecules of singlet oxygen and ten solvents and calculation of dipole moments M of transitions a¹Δ_g–b¹Σ_g⁺ (M_a–b) and a¹Δ_g–X³Σ_g^-(M_a–X) of the oxygen molecule for them. Averaging of M_a–b by conformations, taking into account the probability of their formation for complexes without atoms with a large atomic number (Cl, S), yields values that, as a rule, correlate well with the behavior of k_r in the experiment. Taking into account the possibility of decreasing the distance (compared to equilibrium) between molecules in a collision complex at room temperature made it possible to achieve satisfactory agreement of the calculated and experimental data also for complexes with CCl₄, C₂Cl₄, and CS₂. The obtained data indicate that a number of factors affect k_r. The correlation of k_r with molecular polarizability in a number of cases is due, on the one hand, to its effect on the strength of dispersion interactions in the complex and, on the other hand, to the fact that it to some extent reflects the position of the upper filled orbitals of the solvent molecule. Both factors affect the degree of mixing of the π orbitals of the singlet oxygen molecule with the orbitals of the solvent molecule, which, as was found earlier, facilitates the activation of the a¹Δ_g–b¹Σ_g⁺ transition and the borrowing of its intensity by the a¹Δ_g–X³Σ_g^- transition.