The HO(3) and HO(4) polyoxide radicals have attracted some attention due to their potential role in ozone chemistry. Experimentally, the geometrical structure of HO(3) is known whereas that of HO(4) is not. Moreover, the existence of the latter radical has been questioned. Theoretical calculations on the two species have been reported before, showing important structural differences depending on the computational level. Both radicals present an unusually long OO bond (around 1.7-1.8 A) that can be associated with an intricate interaction between HO, or HO(2), with O(2). The nature of such interaction is investigated in detail using large scale ab initio methods (CASSCF, CASPT2, MRCI, QCISD, CCSD(T)) and density functional techniques (B3LYP) in connection with extended basis sets. Stabilization enthalpies at 298 K with respect to HO (or HO(2)) and O(2) have been computed amounting to -3.21 kcal mol(-1) for HO(3) (trans conformation) and 11.33 kcal mol(-1) for HO(4) (cis conformation). The corresponding formation enthalpies are 6.12 and 11.83 kcal mol(-1). The trans conformation of HO(4) is less stable than the cis one by 6.17 kcal mol(-1). Transition states for HO(4) dissociation and for cis/trans conversion are also described.

We report a theoretical study on the reaction of ozone with hydroxyl radical, which is important in the chemistry of the atmosphere and in particular participates in stratospheric ozone destruction. The reaction is a complex process that involves, in the first stage, a pre-reactive hydrogen-bonded complex (C1), which is formed previous to two transition states (TS1 and TS2) involving the addition of the hydroxyl radical to ozone, and leads to the formation of HO4 polyoxide radical before the release of the products HO2 and O2. The reaction is computed to be exothermic by 42.72 kcal mol(-1), which compares quite well with the experimental estimate, and the energy barriers of TS1 and TS2 with respect to C1 are computed to be 1.80 and 2.26 kcal mol(-1) at 0 K. A kinetic study based on the variational transition state theory (VTST) predicts a rate constant, at 298 K, of 7.37 x 10(-14) cm3 molecule(-1) s(-1), compared to the experimentally recommended value of 7.25 x 10(-14) cm3 molecule(-1) s(-1).

We report a theoretical study on the gas-phase hydrogen-bonded complexes formed between ozone and hydroperoxyl radical, which are of interest in atmospheric chemistry. We have employed CASSCF, CASPT2, QCISD, and CCSD(T) theoretical approaches employing 6-311+G(2df,2p) and aug-cc-pVTZ basis sets, and we have found three complexes whose stabilities are computed to be 2.02, 1.19, and 1.34 kcal/mol, respectively, at 0 K. In addition, we have also found three transition states connecting these complexes that lie below the energy of the separate reactants. To help for possible experimental identification of these hydrogen-bonded complexes, we report also the computed harmonic vibrational frequencies along with the frequency shifts of the complexes, relative to the monomers, and the computed rotational constants.

The reaction mechanism of carbonyl oxide with hydroxyl radical was investigated by using CASSCF, B3LYP, QCISD, CASPT2, and CCSD(T) theoretical approaches with the 6-311+G(d,p), 6-311+G(2df, 2p), and aug-cc-pVTZ basis sets. This reaction involves the formation of H2CO + HO2 radical in a process that is computed to be exothermic by 57 kcal/mol. However, the reaction mechanism is very complex and begins with the formation of a pre-reactive hydrogen-bonded complex and follows by the addition of HO radical to the carbon atom of H2COO, forming the intermediate peroxy-radical H2C(OO)OH before producing formaldehyde and hydroperoxy radical. Our calculations predict that both the pre-reactive hydrogen-bonded complex and the transition state of the addition process lie energetically below the enthalpy of the separate reactants (DeltaH(298K) = -6.1 and -2.5 kcal/mol, respectively) and the formation of the H2C(OO)OH adduct is exothermic by about 74 kcal/mol. Beyond this addition process, further reaction mechanisms have also been investigated, which involve the abstraction of a hydrogen of carbonyl oxide by HO radical, but the computed activation barriers suggest that they will not contribute to the gas-phase reaction of H2COO + HO.

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.