The H, CH 2 CHO, CO, and OH products of the reaction of O( 3 P) atom with alkenes were studied by laserinduced fluorescence (LIF) under single-collision conditions. The average kinetic energies of the H atoms were 10-12 kcal/mol. The CO and OH rotational state populations were characterized by near room temperature Boltzmann distributions. The relative LIF intensities of the various products provide vivid proof of the following mechanism for the reaction of O( 3 P) atoms with molecules of the form R′RCdCH 2 , where R′ and R are H or an alkyl group. The O atom attaches itself to the less substituted carbon atom forming a triplet ketocarbene. There is a barrier to the release of an H atom, and the rate of release must compete with the rate of intersystem crossing. If an H atom is not released, following the intersystem crossing an H atom migrates to the adjacent C atom forming an energized aldehyde, R′RCH-CHdO. The aldehyde dissociates unimolecularly forming the pair of radicals R′ • and the substituted vinoxy • RCH-CHdO or the pair R′RCH and HCO. Some of the latter have enough internal energy to dissociate to H and CO. In a side reaction O( 3 P) abstracts H atoms but only from allylic C-H bonds. The most remarkable observation is that chemical reactions that do not involve the side chains such as release of H atoms or breakup of HCO depend sensitively on the length of these chains.

Ab initio calculations have been performed to determine the structure and energies of the ground and first excited electronic states of bromine-and iodine-containing singlet carbenes. Effective core potential basis sets augmented with polarization functions were utilized at the CASSCF, CASPT2, and CISD levels of theory.Validation of the effective core potential basis sets for the ground and excited states of the singlet carbenes was carried out by comparison with previous results from all-electron basis set calculations. As was the case in previous studies of chlorine-and fluorine-containing halocarbenes, the bromine-and iodine-containing singlet carbenes are characterized by small bond angles in their ground states, ranging from 100°to 112°, and dramatically larger bond angles in their first excited states, ranging from 125°to 132°. This increase is due to the promotion of an electron from a carbon lone pair orbital coplanar with the carbon-halogen bonds to a carbon p-type orbital perpendicular to the bonds. Adiabatic transition energies for transitions from the ground to first excited state for the singlet carbenes determined at the CASPT2(18,12) and CISD levels range from 21 277 to 10 870 cm -1 and are in excellent agreement with experimental measurements where comparisons are available.

The wavelength 205.14 nm is absorbed by many hydrogen-containing molecules, which then dissociate to form hydrogen atoms. These in turn can absorb two more 205.14 nm photons and reach the 3s or the 3d state. They can absorb a third photon and form hydrogen ions or decay to the ground state by stepwise fluorescence, first emitting the Balmer α line at 656.2 nm (3s or 3d→2p)and then the Lyman α line (2p→1s)at 121.6 nm. Thus the hydrogen atom kinetic energy can be probed in three different ways. This method broadens greatly the possibilities of investigating photodissociations leading to hydrogen atom products. It has the advantage of simplicity and the disadvantages of a single-color experiment. The method is tested with a molecule that has been extensively investigated, H2S, and then applied to three other molecules, formic acid (HCO2H), vinyl radical (C2H3), and allyl radical (C3H5). H2S has a perpendicular transition with a large release of kinetic energy. Studies of the latter molecules lead to the conclusion that formyl, carboxyl, vinyl, and allyl radicals absorb 205.1 nm light and release hydrogen atoms with a large fraction of the available energy. The dissociation pathways of formic acid are clarified.

The reaction of O(3P) atoms with CH3 radicals is shown to produce CO (in addition to the major product CH2O) which is detected by laser induced fluorescence. The rotational and vibrational temperatures of the CO product are about 2000 K. The results are explained by the assumption that the reaction takes place mainly by an indirect mechanism in which a methoxyl radical is formed and then dissociates unimolecularly.

The HCO product of the reaction of O(3P) with ethene has been detected by cavity ring-down spectroscopy using its A−X transition. For propene a somewhat smaller yield of HCO was obtained but the overall rate constant is much larger. The yield of HCO in this reaction is quite small (∼0.05). Moreover, a large number of other alkenes were tried with negative results. The failure of the 1,2 H atom shift followed by breaking the 1,2 bond implies that the unimolecular decomposition has found a more favorable channel. The proposed mechanism is as follows. For an alkene of the form RCH2CHCH2 the first step is attachment of the O(3P) to the terminal carbon atom, C1. Then, intersystem crossing occurs and finally a H atom shifts from C3 to C2 and not from C1 to C2. In this way a molecule of formaldehyde and an alkene shorter by one carbon atom are formed.