Evidence for orbital-specific electron transfer to oriented haloform molecules

Beams of hyperthermal K atoms cross beams of the oriented haloforms CF₃H, CCl₃H, and CBr₃H, and transfer of an electron mainly produces K⁺ and the X^- halide ion which are detected in coincidence. As expected, the steric asymmetry of CCl₃H and CBr₃H is very small and the halogen end is more reactive. However, even though there are three potentially reactive centers on each molecule, the F^- ion yield in CF3H is strongly dependent on orientation. At energies close to the threshold for ion-pair formation (∼5.5 eV), H-end attack is more reactive to form F^-. As the energy is increased, the more productive end switches, and F-end attack dominates the reactivity. In CF₃H near threshold the electron is apparently transferred to the σ*_CH antibonding orbital, and small signals are observed from electrons and CF₃^- ions, indicating "activation" of this orbital. In CCl₃H and CBr₃H the steric asymmetry is very small, and signals from free electrons and CX₃^- ions are barely detectable, indicating that the σ*_CH antibonding orbital is not activated. The electron is apparently transferred to the σ_cx* orbital which is believed to be the LUMO. At very low energies the proximity of the incipient ions probably determines whether salt molecules or ions are formed.