Bond and mode selectivity in the reaction of atomic chlorine with vibrationally excited CH2D2

The title reaction is investigated by co-expanding a mixture of Cl-2 and CH2D2 into a vacuum chamber and initiating the reaction by photolyzing Cl-2 with linearly polarized 355 nm light. Excitation of the first C-H overtone of CH2D2 leads to a preference for hydrogen abstraction over deuterium abstraction by at least a factor of 20, whereas excitation of the first C-D overtone of CH2D2 reverses this preference by at least a factor of 10. Reactions with CH2D2 prepared in a local mode containing two quanta in one C-H oscillator \2000>(-) or in a local mode containing one quantum each in two C-H oscillators \1100> lead to products with significantly different rotational, vibrational, and angular distributions, although the vibrational energy for each mode is nearly identical. The Cl+CH2D2\2000>(-) reaction yields methyl radical products primarily in their ground state, whereas the Cl+CH2D2\1100> reaction yields methyl radical products that are C-H stretch excited. The HCl(v=1) rotational distribution from the Cl+CH2D2\2000>(-) reaction is significantly hotter than the HCl(v=1) rotational distribution from the Cl+CH2D2\1100> reaction, and the HCl(v=1) differential cross-section (DCS) of the Cl+CH2D2\2000>(-) reaction is more broadly side scattered than the HCl(v=1) DCS of the Cl+CH2D2\1100> reaction. The results can be explained by a simple spectator model and by noting that the \2000>(-) mode leads to a wider cone of acceptance for the reaction than the \1100> mode. These measurements represent the first example of mode selectivity observed in a differential cross section, and they demonstrate that vibrational excitation can be used to direct the reaction pathway of the Cl+CH2D2 reaction. (C) 2004 American Institute of Physics.