Theoretical Characterization of the Gas‐Phase O₃⋅⋅⋅HO Hydrogen‐Bonded Complex

Abstract

We report a theoretical study on two gas-phase hydrogen-bonded complexes formed between ozone and hydroxyl radical that have relevance to atmospheric chemistry. This study was carried out by using CASSCF, CASPT2, QCISD, and CCSD(T) theoretical approaches in conjunction with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. Both complexes have a planar structure and differ from each other in the orientation of the electronic density of the unpaired electron associated with the HO radical moiety. Our calculations predict their stabilities to be 0.87 and 0.67 kcal mol(-1), respectively, at 0 K and show the importance of anharmonic effects in computing the red shift of the HO stretch originating from the hydrogen-bonding interaction. We also report two transition states involving the movement of the HO moiety on the potential energy surfaces of these hydrogen-bonded complexes.

The Gas‐Phase Reaction between O3 and HO Radical: A Theoretical Study

“…The same kind of calculation has been already performed on the pre-reactive hydrogen-bonded complex (C1). [20] Along the reaction path, we found the possible existence of the HO 4 polyoxide intermediate. This radical was first investigated at the CASSCF level of theory and in a further stage by performing a numerical optimisation [34,35] at the MRCI level of theory.…”

“…We recently reported the pre-reactive O 3 ···HO hydrogenbonded complex, [20] and we refer the reader to that study for its discussion. In that work, we concluded that the best level of theory to describe this pre-reactive hydrogen-bonded complex is CCSD(T)//QCISD, because it can properly account for the dynamic correlation effects needed to describe the hydrogen-bond interaction, whereas the wavefunction is still reliable regarding its possible multireferential character.…”

“…In calculating K eq , we have considered the partition functions computed at the CASSCF level of theory, the best energy values discussed in reference [20] and listed in Table 1, set s = 2, as the hydrogen atom of hydroxyl radical can form a hydrogen-bonded complex with each of the two terminal oxygen atoms of ozone. Both k TS1 and k TS2 have been calculated using the canonical variational transition state theory [Eq.…”

“…The discrepancies between our calculated and the experimental rate constants in the pertinent temperature range may be addressed mainly by two points. The first is the difficulty of very accurately computing the stability of the pre-reactive hydrogen-bonded complex, as pointed out in reference [20]. The second can be assigned to the difficulties in obtaining an accurate description of the whole potential energy surface because of the very complex electronic features, as exemplified, for instance, by the different multireferential character of the wavefunction along the potential energy surface and also in the theoretical difficulties in properly describing the HO 4 radical intermediate (see above).…”

“…[18] Also very recently, Engdahl and Nelander [19] were able to observe experimentally a pre-reactive complex formed between ozone and hydroxyl radical that was characterised theoretically as a hydrogen-bonded complex in a more recent study reported by us. [20] The existence of this prereactive hydrogen-bonded complex may be of great importance for the full understanding of the kinetic behaviour, and therefore we have performed a theoretical study on the mechanism of Equation (1) employing high-level ab initio methods. Herein we focus, in a first stage, on the reaction mechanism after the formation of the pre-reactive hydrogen-bonded complex and analyse the reliability of the multireference-and single-determinant-based methods in the interaction between ozone, with its strong biradical character, and a radical (hydroxyl).…”