Vibrational state control of bimolecular reactions

The influence of rotation and vibration on the reactivity and the dynamics of the reaction X + HCN(nu(1), nu(2), nu(3), J)-->HX + CN(upsilon, J) with X = H, Cl has been studied. The HCN molecule is prepared in a specific rovibrational level by IR/VIS overtone excitation in the wavelength region 6500-18000 cm(-1). The H atoms are generated by laser photolysis of CH3SH at 266 Mn, the Cl atoms are formed in the photodissociation of Cl-2 at 355 nm. The CN products are probed quantum state specifically by laser-induced fluorescence (LIF). For low rotational states of HCN, the reactivity of Cl and H is independent of the initial rotational state. However, an enhancement in reactivity of the Cl + HCN reaction is observed when the time of rotation becomes comparable to the passing time of the Cl atom. The reaction of Cl as well as of the H atom with HCN shows strong mode specific behavior, implying a simple direct reaction mechanism, which is also supported from Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. An increase in CH stretch vibration increases both the reaction rate and the CN product vibration. Channeling energy in CN stretch vibration has only a minor effect on the reactivity and the CN product vibration even decreases. Trajectory calculations of the H + HCN system agree with the experimental results. The dependence of reaction rates on reactant approach geometry is investigated by preparing aligned reactants using linear polarized light. The CN signal is markedly influenced by the prepared alignments (steric effect). The experimental results suggest that the reaction of hydrogen and chlorine atoms with vibrationally excited HCN proceeds mainly via a collinear transition state, but the cone of acceptance is larger for chlorine atoms. (C) 1998 American Institute of Physics.