Reactions of O, H, and Cl atoms with highly vibrationally excited HCN: Using product states to determine mechanisms

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Abstract
Oxygen, hydrogen, and chlorine atoms react with vibrationally excited HCN to produce CN and OH, H2, or HCl, respectively. The experiments presented here use direct vibrational overtone excitation to prepare states of HCN having four quanta of C–H stretching excitation [(004) state] or three quanta of C≡N stretching and two quanta of C–H stretching excitation [(302) state] and laser-induced fluorescence to determine the rotational and vibrational states of the CN product. We find that the reaction of HCN with O produces CN having little vibrational and rotational energy, with 85% of the CN in v=0, 12% in v=1, and 3% in v=2. The CN from the reaction of H with HCN is slightly more energetic, with 77% in v=0, 17% in v=1, and 6% in v=2. By contrast, the reaction of Cl with HCN produces CN with a considerable amount of excitation, about 30% is in v=1 and at least 10% is in v=2, depending on the initial vibrational state of the HCN reactant. The enhanced excitation of the CN product of the reaction with Cl reflects the contribution of a different mechanism. We conclude that the O-atom reaction forms CN exclusively by a direct abstraction reaction, the H-atom reaction produces CN primarily by direct reaction at the collisional energies of our experiment, and the Cl-atom reaction forms CN by the dissociation of an intermediate complex in addition to the direct abstraction reaction.

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