Effects of reagent translational and vibrational energy on the dynamics of endothermic reactions

The endothermic reactions Br + CH3I yields CH3 + IBr (delta H sub zero = 13 kcal/mole) and Br + CF3I yields CF3 + IBr (delta H sub zero = 11 kcal/mole) were studied by the crossed molecular beams method. Detailed center of mass contour maps of the IBr product flux as a function of recoil velocity and scattering angle are derived. For both systems it is found that the IBr product is sharply backward scattered with respect to the incident Br direction, and that most of the available energy goes into product translation. Vibrational enhancement of the Br + CF3I reaction was investigated by using the infrared multiphoton absorption process to prepare highly vibrationally excited CF3I. At a collision energy of 31 kcal/mole (several times the barrier height), reagent vibrational energy appears to be less effective than an equivalent amount of (additional) translational energy in promoting reaction. More forward scattered IBr is produced in reactions of Br with vibrationally hot CF3I.