Vibrational state distribution and relaxation of vinoxy radicals

Abstract

The vinoxy radical ⋅CH2CHO is a product of the reaction of O(3P) 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−1 for the vinoxy produced by photodissociation of ethyl vinyl ether at 193 nm and 3200 cm−1 for the product of the reaction of O(3P) 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(3P) with ethylene, propene, 1-butene, and 1-pentene is 2100, 1800, 1570, and 1180 cm−1, 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, ∼200–300 cm−1, can be inferred from the growth rate of the ground-state population.