Vibrational state distribution and relaxation of vinoxy radicals

Abstract

The vinoxy radical •CH 2 CHO is a product of the reaction of O( 3 P) atoms with terminal alkenes and can also be made by photodissociation of an alkyl vinyl ether. In either case it is formed in a vibrationally excited state. The nascent radical displays a rich electronic spectrum to the red of its X→B band origin consisting of bands originating from vibrationally excited states. Some transitions, true ''hot bands,'' terminate on the vibrationless B state; others, sequence bands, terminate on vibrationally excited B states. The spectra become unobservably weak at a certain energy. The difference between that energy and the energy of the band origin is roughly the maximum vibrational energy in the radical. This is 5600 cm Ϫ1 for the vinoxy produced by photodissociation of ethyl vinyl ether at 193 nm and 3200 cm Ϫ1 for the product of the reaction of O( 3 P) with ethylene, propene, 1-butene, and 1-pentene. There is a remarkable cooling of the vibrations as the hydrocarbon chain lengthens. The average vibrational energy of the vinoxy product of the reaction O( 3 P) with ethylene, propene, 1-butene, and 1-pentene is 2100, 1800, 1570, and 1180 cm Ϫ1 , respectively. This cooling implies that the reaction complex lives long enough for internal vibrational relaxation to occur. The average vibrational energy in the reaction-produced vinoxy is small, which implies that there is considerable kinetic energy. The time dependence of the intensity of the hot bands measures the relaxation rates of different energies, some of which are the energies of a single vibrational state. The ground-state population increases monotonically to an asymptote. The population of most states grows with time and then decays. The growth is due to a cascading from upper states. The populations of the highest energy states decay monotonically; the still higher energy states are almost unpopulated. These results prove that the relaxation proceeds stepwise. The magnitude of the step, ϳ200-300 cm Ϫ1 , can be inferred from the growth rate of the ground-state population.

A Hierarchical and Comparative Kinetic Modeling Study of C1 − C2 Hydrocarbon and Oxygenated Fuels

“…In the ground state, vinoxy radical can exist in two mesomeric structures (i.e., one radical in a unique state that cannot be represented by a Lewis form with localized orbitals; the dominant structure is a matter of probability), with the radical site located on the oxygen as in the case of the vinoxy radical (CH 2 CH) or on the primary carbon as in the case of the methylene formyl radical (H 2 CHO). It has been shown that greater electron‐spin density is present on the methylene formyl radical , indicating that this is the dominant structure. The H 2 CHO form has been included in this mechanism, but it is convention to refer to this as vinoxy radical.…”

Slow photoelectron velocity-map imaging spectroscopy of the vinoxide anion

“…Brock and Rohlfing 27 reported LIF, dispersed fluorescence, and twocolor resonant four-wave mixing spectra, identifying the nine lowest-frequency vibrational modes of the vinoxy radical in its ground and second excited states. Photofragment yield spectroscopy of the vinoxy radical measured by Osborn et al 6 showed that the B state predissociates into CH 3 +CO and H + CH 2 CO, motivating subsequent experimental 7,29,30 and theoretical 8,9,20,31 studies that further explored the dissociation pathways of the vinoxy radical.…”

“…23,24 Structural information is also gleaned from the extensive laser-induced fluorescence ͑LIF͒ studies of the B ← X band. [25][26][27][28][29] The first LIF experiments, carried out by Inoue et al 25 and DiMauro et al, 26 identified the main vibrations of the carbon-oxygen backbone, an additional C-H bending mode, and determined backbone geometries from rotationally resolved spectra. Brock and Rohlfing 27 reported LIF, dispersed fluorescence, and twocolor resonant four-wave mixing spectra, identifying the nine lowest-frequency vibrational modes of the vinoxy radical in its ground and second excited states.…”

Multiresonant Spectroscopy and the High-Resolution Threshold Photoionization of Combustion Free Radicals

“…52 Considerable spectroscopic work on this system has thoroughly characterized the ground state and the brightly fluorescent B 2 A" state. [53][54][55][56][57][58] This data provides the information necessary to benchmark ab initio calculations and develop diagnostics in order to probe the reaction dynamics of this radical.…”