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.

Some symmetrical Ar3C and unsymmetrical (C6C15)3-xAr,C radicals with different chlorine substitution patterns (x = 1, 2; Ar: 2H-C6HC14, 3H,5H-C6H2C14, 4H-C6HC14, C6H5) are prepared. X-ray crystal structures for most of them have been obtained at room temperature. The general conformations are conditioned by the great volume of the chlorine atoms in the ortho positions resulting in unsymmetrical, propellerlike conformations. Experimental evidence of the steric shielding of the trivalent carbon atom, as well as the practical nonexistence of the so-called buttressing effect, is given. The steric shielding is in correlation with the observed stabilities. The magnetic susceptibilities of the radicals are discussed and related to molecular packing and spin densities in terms of McConnell's theory, when antiferromagnetism is observed. The g-factors, hyperfine coupling constants (hcc) of IH and I3C nuclei, and line widths of the radicals are determined at several temperatures by ESR experiments in solid-state and isotropic solutions. Different conformational dynamic behaviors are traced to the number of chlorine atoms in ortho positions. Temperature-dependent line widths are explained by the relative contribution of the modulation of the dipolar hyperfine interaction through molecular tumbling and chlorine nuclear quadrupolar relaxation mechanism. The hcc values have been calculated according to the INDO method with the experimental X-ray geometries and compared with the experimental values; a g o d agreement is obtained. The assignment of the I3C satellites to bridgehead and ortho positions is confirmed IntroductionIn the course of our studies on derivatives of perchlorotri-

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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.