Département de chimie moléculaire

Singlet oxygen (1O2) comes in two flavors—namely the dominant lower-energy a1Δg state and the higher-energy shorter-lived b1Σ+g state—and plays a key role in many photochemical and photobiological reactions. For this reason, and because of the large size of the systems treated, many papers have appeared with density-functional theory (DFT) treatments of the reactions of 1O2 with different chemical species. The present work serves as a reminder that the common assumption that it is enough to fix the spin multiplicity as unity is not enough to insure a correct treatment of singlet oxygen. We review the correct group theoretical treatment of the three lowest energy electronic states of O2 which, in the case of 1O2 is often so badly explained in the relevant photochemical literature that the explanation borders on being incorrect and prevents, rather than encourages, a correct treatment of this interesting and important photochemical species. We then show how many electronic structure programs, such as a freely downloadable and personal-computer compatible Linux version of deMon2k, may be used, together with the multiplet sum method (MSM), to obtain a more accurate estimation of the potential energy curves (PECs) of the two 1O2 states. Applications of the MSM DFT method to 1O2 appear to be extremely rare as we were only able to find one correct application of the DFT MSM (or rather a very similar approach) to 1O2 in our literature search. Here we treat both the a1Δg and b1Σ+g state with a wide variety of density-functional approximations (DFAs). Various strengths and weaknesses of different DFAs emerge through our application of the MSM method. In particular, the quality of the a1Δg excitation energy reflects how well functionals are able to describe the spin-flip energy in DFT while the quality of the b1Σ+g excitation energy reflects how well functionals are able to describe the spin-pairing energy in DFT. Finally, we note that improvements in DFT-based excited-state methods will be needed to describe the full PECs of 1O2 including both the equilibrium bond lengths and dissociation behavior.