Molecular Beam Kinetics: Reactions of K Atoms with Alkyl Iodides

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Abstract
Angular distributions of reactively scattered KI have been measured for several K+RI reactions, with R=CH3, C2H5, n‐ and i‐C3H7, n‐, i‐, s‐, and t‐C4H9, n‐C5H11, and n‐C7H15. The reaction cross sections are roughly ∼30 Å2 and the activation energies are undetectably small, <̃0.5 kcal/mole, for the whole series of reactions. The laboratory angular distributions are strongly anisotropic. A qualitative analysis which requires only the conservation laws for energy and momentum shows that these reactions proceed predominantly via a “rebound” mechanism: An observer stationed at the center of mass would see most of the KI recoil into the backward hemisphere (and the R group forward) with respect to the incoming K beam. The kinematic analysis also indicates that for K + CH3I roughly half (60% ± 20%) of the energy of reaction (∼15 ± 5 out of ∼25 kcal/mole) appears as internal excitation of the products, probably mainly in vibrational excitation. As the size of the R group is increased, the backward peaking of the KI appears to become less pronounced; also, the energy released into relative translational motion of the products decreases and the internal excitation increases. The angular distributions of the K atoms scattered without reaction resemble those for scattering from unreactive molecules of similar size, except for some falloff in intensity at wide angles. The customary “rainbow” analysis (only rigorous for atom‐atom scattering) yields 0.5 ± 0.15 kcal/mole for the depth of the van der Waals potential well, with no noticeable variation with the size of the R group.