The lowest singlet and triplet potential energy surfaces of formaldehyde carbonyl oxide (1) and acetaldehyde carbonyl oxide (2) have been investigated in the regions concerning the most relevant unimolecular reactions by means of CASSCF and MRDCI ab initio quantum-chemical calculations. The questions related to the mechanism of O-atom loss from carbonyl oxides, as well as the competition between the cyclization to dioxirane and the tautomerization to hydroperoxide in methyl-substituted carbonyl oxides are addressed in this investigation. The theoretical predictions are consistent with experimental findings obtained from stopped-flow studies of the gas-phase ozonation of both trans-butene and tetramethylethylene. An unexpected result is that the most reasonable pathway for O-atom loss from “hot” singlet carbonyl oxides 1 and 2 involves internal rotation about the CO bond axis, followed by intersystem crossing to the lowest triplet state and subsequent scission of the OO bond.

The reactions of twelve carbonyl oxides or Criegee intermediates with the water monomer and with the water dimer have been investigated employing high level theoretical methods. The study includes all possible carbonyl oxides arising from the isoprene ozonolysis and the methyl and dimethyl carbonyl oxides that originated from the reaction of ozone with several hydrocarbons. These reactions have great significance in the chemistry of the atmosphere because Criegee intermediates have recently been identified as important oxidants in the troposphere and as precursors of secondary organic aerosols. Moreover, water vapor is one of the most abundant trace gases in the atmosphere and the water dimer can trigger the atmospheric decomposition of Criegee intermediates. Our calculations show that the nature and position of the substituents in carbonyl oxides play a very important role in the reactivity of these species with both the water monomer and the water dimer. This fact results in differences in rate constants of up to six orders of magnitude depending on the carbonyl oxide. In this work we have defined an effective rate constant (keff) for the atmospheric reaction of carbonyl oxides with water vapor, which depends on the temperature and on the relative humidity as well. With this keff we show that the water dimer, despite its low tropospheric concentration, enhances the atmospheric reactivity of Criegee intermediates, but its effect changes with the nature of carbonyl oxide, ranging between 59 and 295 times in the most favorable case (syn-methyl carbonyl oxide), and between 1.4 and 3 times only in the most unfavorable case.

High-level ab initio electronic structure calculations have been carried out with respect to the intermolecular hydrogen-transfer reaction HCOOH+.OH-->HCOO.+H(2)O and the intramolecular hydrogen-transfer reaction .OOCH2OH-->HOOCH(2)O.. In both cases we found that the hydrogen atom transfer can take place via two different transition structures. The lowest energy transition structure involves a proton transfer coupled to an electron transfer from the ROH species to the radical, whereas the higher energy transition structure corresponds to the conventional radical hydrogen atom abstraction. An analysis of the atomic spin population, computed within the framework of the topological theory of atoms in molecules, suggests that the triplet repulsion between the unpaired electrons located on the oxygen atoms that undergo hydrogen exchange must be much higher in the transition structure for the radical hydrogen abstraction than that for the proton-coupled electron-transfer mechanism. It is suggested that, in the gas phase, hydrogen atom transfer from the OH group to oxygen-centered radicals occurs by the proton-coupled electron-transfer mechanism when this pathway is accessible.

The mechanism of the CH3O• + O2 reaction in the gas phase leading to CH2O + HO2 • was studied by using high-level quantum mechanical electronic structure calculations. The CASSCF method with the 6-311G(d,p) basis set was employed for geometry optimization of 15 stationary points on the ground-state potential energy reaction surface and computing their harmonic vibrational frequencies. These stationary points were confirmed by subsequent geometry optimizations and vibrational frequencies calculations by using the CISD and QCISD methods with the 6-31G(d) and 6-311G(d,p) basis sets. Relative energies were calculated at the CCSD(T) level of theory with extended basis sets up to cc-pVTZ at the CASSCF/6-311G(d,p)-optimized geometries. In contrast to a recent theoretical study predicting an addition/elimination mechanism forming the trioxy radical CH3OOO• as intermediate, the oxidation of CH3O• by O2 is found to occur by a direct H atom transfer mechanism through a ringlike transition structure of C s symmetry. This transition structure shows an intermolecular noncovalent O···O bonding interaction, which lowers its potential energy with respect to that of a noncyclic transition structure by about 8 kcal/mol. The 1,4 H atom transfer in CH3OOO• is not accompanied by HO2 • elimination but leads to the trioxomethyl radical •CH2OOOH via a puckered ringlike transition structure, lying 50.6 kcal/mol above the energy of the reactants. The direct H atom transfer pathway is predicted to occur with an Arrhenius activation energy of 2.8 kcal/mol and a preexponential factor of 3.5733 × 10-14 molecule cm3 s-1 at 298 K. Inclusion of quantum mechanical tunneling correction to the rate constant computed with these parameters leads to a rate constant of 2.7 × 10-15 molecule-1 cm3 s-1 at 298 K, in good agreement with the experimental value of 1.9 × 10-15 molecule-1 cm3 s-1.

The thermal decomposition of the phenoxyl radical (1) to form CO plus C5H5' , a key reaction in the hightemperature oxidation of benzene, has been studied using ab initio quantum mechanical electronic structure methods. The complete active space (CAS) SCF method was used for geometry optimization of 10 stationary points on the ground-state potential energy reaction surface and computing their harmonic vibrational frequencies. Subsequent calculations using the multireference second-order perturbation theory based on a CASSCF reference function (CASPT2) with 6-3 lG(d,p) basis set established the energetics along the two altemative reaction paths proposed by Benson and co-workers. The energetics were further corrected for zero point vibrational energy at the CASSCF level of theory. The present study predicts the decomposition of 1 to occur preferably through an electrocyclic cyclization mechanism involving the formation of the 6-oxobicyclo[3.1 .O]hex-3-en-2-y1 radical (2) as intermediate, rather than by a ring-opening process leading to the (3~)-6-0~0-1,3,5-hexatrien-1-yl radical (4) intermediate. In contrast to early reported experimental kinetic data, the preexponential factor of the thermal Arrhenius expression of the rate constant for the unimolecular decomposition of 1 is predicted to have a normal value (A > 10'3.5 s-I).