The photofragment ion imaging technique is used to measure the velocity distributions of atomic chlorine, methyl, and carbon monoxide fragments generated in the photodissociation of acetyl chloride at 236 nm.The fragments are selectively ionized by (2 + 1) multiphoton ionization and projected onto a two-dimensional position-sensitive detector to obtain speed and angular distributions. The C1 images display anisotropic angular distributions, characteristic of a prompt, impulsive dissociation of the C-C1 bond. A fraction of the CH3C0, produced as a primary photoproduct, subsequently decomposes to form CH3 and CO. The CH3 and CO images are isotropic, suggesting that rotation of the acetyl radical intermediate occurs prior to the secondary dissociation. The internal state distribution of CO is probed using ( 2 + 1) multiphoton ionization via the B I F state near 230 nm. The rotational state distribution of CO extends to S' = 30, while no vibrational excitation is observed. The transition state structure of the CH3CO intermediate, leading to dissociation into CH3 and CO, is computed via ab initio quantum mechanical methods. The barrier for CH3CO dissociation is theoretically predicted at 19.1 kcal/mol at the MP2kc-pVTZ level of theory. The theoretically predicted dissociation mechanism and barrier agree well with the measured internal state distributions and velocities of the CH3 and CO secondary fragments.

On the role of conical intersections of two potential energy surfaces of the same symmetry in photodissociation. I. CH3SH→CH3S+H and CH3+SH

ABSTRACT:We have studied 4d transition metal monoboride, monocarbide, mononitride, monoxide, and monofluorides using density functional method at B3LYP/ LanL2Dz level. The lowest spin state, relative stability, bond length, atomic charges, electron affinity, ionization potential, binding energy, and vibrational frequencies for these dimers are obtained. The cation and anion of these dimers are also studied. The properties of these dimers are compared. It was found that the ionization potentials for these dimers are much higher than the electron affinities of these dimers. The range of electron affinities is widest for 4d transition metal monocarbides and is narrow for 4d transition metal mononitrides. The range of ionization potential is widest for 4d transition metal monoxides and is narrow for 4d transition metal monocarbides.

Results are reported on the 193 and 248 nm photolysis of iodoethane, specifically with respect to H-atom production. Experiments using selectively deuterated iodoethanes, ICD2CH3 and ICH2CD3, reveal that at 193 nm the carbon–hydrogen bond cleavage is not carbon-atom specific. However, following photolysis at 248 nm, it is clear that C–H (or C–D) bond dissociation occurs preferentially at the β carbon atom.

Experiments involving two photolysis lasers and one probe laser demonstrate that 248 nm excimer laser radiation will induce C–H bond cleavage preferentially at the β position in the ethyl radical. To facilitate carbon site labeling, selectively deuterated chloroethanes (ClCH2CD3 and ClCD2CH3) are used as precursor compounds. Two-photon ionization via resonance with the Lyman-α transition is used to detect H (or D) atoms. An initial 193 nm photolysis pulse serves to cleave the C–Cl bond in ClCH2CH3, while a second pulse at 248 nm dramatically enhances H-atom production. Experiments on ClCH2CD3 and ClCD2CH3 clearly show that this enhancement occurs preferentially through carbon–hydrogen bond cleavage at the β carbon site. It is apparent that 248 nm photon absorption by the ethyl radical is an important step in the overall mechanism.