Excited states of bromites are true challenges for theoretical chemistry. It’s time to face them.
When methyl hypobromite (CH3OBr) is excited, it quickly dissociates producing a Br radical. Simple to say, difficult to show. The quantum chemical description of this dissociation process is extremely challenging.
In a project led by Ljiljana Stojanović and in collaboration with our colleagues from the King Abdulaziz University, we have been working on the description of the excited states of this molecule . After building a comprehensive benchmark including MR-CISD, CCSD(T), CCSD, CC2, ADC(2), and TDDFT, we figured out that none of these methods could fully describe CH3OBr.
The single-reference methods (CCs, ADC, and TDDFT) failed to provide reasonable dissociation limits. MR-CISD, on its turn, provided nice dissociation curves, but delivered a poor description of the bright σσ* state.
Moreover, the presence of Br, a relatively heavy atom, leads to non-negligible spin-orbit coupling effects. For instance, the intensity of the first absorption band if spin-orbit couplings are not considered is wrong by a factor two compared to the experiments.
In spite of all these challenges, we have been able to provide a full description of the absorption spectrum and dissociation of methyl hypobromite. As shown in the figure above, the molecule strongly absorbs above 6.6 eV due to σσ* and Rydberg states. It also has a very weak band due to nσ* transitions at 4.5 eV.
When we multiply this spectrum by the solar irradiance at the Earth’s surface, we get the action spectrum, which shows that this weak nσ* band is the main responsible for the photochemistry of CH3OBr in the environment.
 L. Stojanović, G. Pereira Rodrigues, S. G. Aziz, R. H. Hilal, and M. Barbatti, Photochemistry of methyl hypobromite (CH3OBr): excited states and photoabsorption spectrum, RSC Adv., doi:10.1039/C5RA18578E (2015).