The role of triplet states in the long wavelength absorption region of bromine nitrate

A theoretical study of the low-lying singlet and triplet electronic states of $BrONO2$ is presented. Calculations of excitation energies and oscillator strengths are reported using excited-state coupled cluster response methods, as well as the complete active space self-consistent field method with the full Breit–Pauli spin-orbit operator. The calculations predict that there is only one singlet state for $BrONO2,$ the $Ã 1A″$ state, that is accessible at wavelengths longer than 300 nm. At energies below the first singlet state, i.e., λ>330 nm, the calculations reveal two triplet states with significant oscillator strength. Therefore, we propose that the origin of absorption in the long wavelength region from 350 to 500 nm, responsible for the majority of atmospheric photolysis, is due to transitions to triplet states and not singlet states. A comparison of the reported benchmark coupled cluster calculations (CCSD) with the results of (1) configuration interaction with all single substitutions and a perturbative correction for the double substitutions [CIS(D)] and (2) time-dependent density-functional (TDDF) calculations is provided. For the lowest energy excitations, CIS(D) calculations provide quantitative agreement with the CCSD results, while TDDF calculations yield qualitative agreement.