Quantum-chemical simulation of the solvent effect on spontaneous emission of singlet oxygen

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 $a^{1}\Delta_{g}$ – $b^{1}\Sigma^{+}_{g}$ $(M_{a-b})$ и $a^{1}\Delta_{g}$ – $X^{3}\Sigma_{g}^{-}$ $(M_{a-X})$ 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 $(\mathrm{Cl},\mathrm{S})$, 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 $\mathrm{CCl}_{4}$, $\mathrm{C}_{2}\mathrm{Cl}_{4}$, and $\mathrm{CS}_{2}$. 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 $\pi$ orbitals of the singlet oxygen molecule with the orbitals of the solvent molecule, which, as was found earlier, facilitates the activation of the $a^{1}\Delta_{g}-b^{1}\Sigma_{g}^{+}$ transition and the borrowing of its intensity by the $a^{1}\Delta_{g}-X^{3}\Sigma_{g}^{-}$ transition.