Stretching the bonds of chemical reactions.

摘要

The effects of reagent stretch-excitation on the gas-phase Cl+CH 4 → HCl+CH3 reaction are examined experimentally using a photoinitiated reaction technique in which the reagents are prepared by direct infrared absorption or stimulated Raman pumping and the products are state-selectively ionized and detected in a time-of-flight mass spectrometer. Our measurements indicate that the rovibrational distributions and state-selected differential cross sections of the HCl and CH3 products from the symmetric and antisymmetric stretch excited reactions are indistinguishable. This result suggests that the mechanisms of these nearly isoenergetic vibrationally excited reactions are similar, despite theoretical and experimental measurements that suggest the symmetric stretch enhances the reaction rate more than the antisymmetric stretch. Furthermore, we have demonstrated mode- and bond-selectivity in the Cl+CH2D2 reaction.{09}Excitation of the first C-H overtone of CH2D2 leads to a preference for hydrogen excitation of the first C-D overtone of CH2D2 reverses this preference by at least a factor of 10. Reactions with CH2D 2 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. 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. Our results indicate that reagent stretch-excitation enhances the reaction rate by localizing energy along the reaction coordinate, thereby widening the cone of acceptance and allowing reactive collisions at high impact parameter. The chlorine atom appears to react with a single C-H oscillator, while the methyl radical is largely a spectator throughout the course of the reaction. Moreover, our results are consistent with a model in which the impact parameter governs the angular distributions of the product.