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 σCX* 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.