Excess Energy and Structural Dependence of the Rate of Energy Redistribution during the Photodissociation of Iodotoluenes

Article abstract of DOI:10.1021/j100064a013

The ortho (2-), meta (3-), and para (4-) iodotoluenes (C7H7I) are photodissociated at 266 and 304 nm and studied with state-selective one-dimensional photofragment translational spectroscopy.Angular, velocity, and translational energy distributions are obtained for the ground state I(²P_3/2) and spin-orbit excited state I^*(²P_1/2) iodine atoms produced upon photodissociation.As has been observed in iodobenzene, the ground-state I channel observed in each isomer of iodotoluene exhibits both a prompt alkyl iodide-like dissociation channel following a parallel excitation to the alkyl iodide ³Q₀(n,?^*) repulsive state and curve crossing to the ¹Q₁(n,?^*) state, and a slower, indirect dissociation channel following a competitive excitation to aromatic charge-transfer (?,?^*) predissociative excited states at both 266 and 304 nm.The I^* channel observed at both 266 and 304 nm for each isomer results from a prompt dissociation resulting from parallel absorption to an alkyl iodide type ³Q₀(n,?^*) state.The rapid I and I^* dissociative channels observed for each isomer are found to exhibit strong dependence on the excess excitation.This is discussed in terms of a strong coupling between the dissociation coordinate and the more dense vibrational states of the toluene ring.Dissociation times and rates of internal energy redistribution (IER) from the slow dissociative channel (β) are estimated for each isomer.For 2-iodotoluene, the rate of IER for the slow I channel increases from 20 kcal/mol*ps at 304 nm to 30 kcal/mol*ps at 266 nm, while the rates of IER for 3- and 4-iodotoluene (16 kcal/mol*ps and 15 kcal/mol*ps, respectively) remain unchanged as the photon energy is increased.The difference in the observed excess energy dependence of the rates of IER in these isomers is discussed in terms of the complex coupling schemes between the optical doorway states and the n,?^* repulsive states that produce the iodine atoms, monitored in the experiment.