We have determined the branching ratio for the reaction of hydrogen atoms and HOD with either the O–H bond excited or the O–D bond excited. In both cases, the initially excited bond reacts preferentially. Excitation of the third O–H stretching overtone, 4νOH, favors breaking the O–H bond by a factor of ∼200, and excitation of the fourth O–D stretching overtone, 5νOD, favors breaking the O–D bond by a factor of ∼220. Thus vibrational excitation can control the H+HOD reaction to produce either product almost exclusively. A simple model using the calculated wave function for each state and the measured reaction cross section for a particular vibrational excitation predicts the high selectivity observed for the two reactions.
The measured OH product state distribution from the C1 + H20 (104)-) -HC1 + OH reaction reveals an energy disposal pattern that is similar to that in the analogous H i -H20 (104)-) -H2 + OH reaction. Of the 9700 cm-' available energy, approximately 6% appears in internal OH excitation, approximately 21% in product translation, and the remainder (about 73%) appears as internal HCl excitation. The result that most of the available energy appears as internal energy of the newly formed diatomic molecule supports a spectator picture in which the old bond does not participate in the reaction. We measure the bond-selective branching ratio for the C1 i-HOD ( 4~0~) reaction and find that initial excitation of the 0 -H bond favors the HCl + OD product channel by at least a factor of 40. Measurements of the relative reactivity of water molecules prepared in the 104)-, 104)+ and 103)-12) local mode vibrational states suggest that all three vibrational modes enhance the reactivity by approximately the same factor.
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.
Previous studies of the hydrogen abstraction from vibrationally excited H2O and HCN by various atoms have probed the vibrational and rotational energy of the product containing the surviving bond to assess the energy disposal and determine the mechanism of the reaction. Estimating the relative translational energy of the products from the Doppler broadening of the probe transitions has allowed the inference of the internal energy of the unobserved product containing the new bond using conservation of energy. The experiments presented here directly measure the vibrational and rotational energy of both the OH product (containing the new bond) and OD product (containing the old bond) from the reaction of O atoms with HOD having four quanta of O–H stretching excitation (4νOH). All of the OH products are vibrationally excited, being formed almost exclusively in ν=2. Nearly all of the OD products are vibrationally unexcited, with 93% in v=0 and only 7% in v=1. The results are consistent with a spectator picture of the reaction in which the new bond receives most of the available energy.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.