Evidence for orbital-specific electron transfer to oriented haloform molecules.

Beams of hyperthermal K atoms cross beams of the oriented haloforms CF(3)H, CCl(3)H, and CBr(3)H, and transfer of an electron mainly produces K(+) and the X(-) halide ion which are detected in coincidence. As expected, the steric asymmetry of CCl(3)H and CBr(3)H 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 CF(3)H is strongly dependent on orientation. At energies close to the threshold for ion-pair formation ( approximately 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 CF(3)H near threshold the electron is apparently transferred to the sigma(CH) antibonding orbital, and small signals are observed from electrons and CF(3)(-) ions, indicating "activation" of this orbital. In CCl(3)H and CBr(3)H the steric asymmetry is very small, and signals from free electrons and CX(3)(-) ions are barely detectable, indicating that the sigma(CH) antibonding orbital is not activated. The electron is apparently transferred to the sigma(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.