Evidence for Orbital-Specific Electron Transfer to Oriented Haloform Molecules

Abstract

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.

Reversed Electron Apportionment in Mesolytic Cleavage: The Reduction of Benzyl Halides by SmI2

“…Namely, the triply charged samarium cation may electrophilically assist, through electrostatic stabilization, the expulsion of the leaving group. Molecular beam studies show that the electron donor may use a front side approach to the substrate,17 unlike the anti‐approach of a nucleophile in the classical S N 2 reaction. This is explained as resulting from an electrostatic stabilization of the product.…”

Dramatic Steric Behavior in Electron Transfer from Various Donors to CF3Br

“…Comparison to Other Molecules. Figure 12 compares the Fsteric asymmetry for potassium reacting with CF 3 Br and CF 3 H. 52 The steric asymmetry for Ffor each of these compounds is very similar, except that CF 3 Br is moved to higher energy by about 3 eV. (We regard the kink in the CF 3 Br curve near 15 eV as a meaningless artifact of the fit.)…”

Quantification of the Interaction of SmI2 with Substrates and Ligands

“…[19] At the same time, these reactions are known to be of the innersphere electron transfer type due to the crucial need for electrophilica ssistance by the samariumc ation to the expulsion of the chloride anion. [20] Moreover,m olecular beam studies show that the electron donor may use af ront side approacht ot he substrate, [21] unlike the anti-approach of an ucleophile in the classical S N 2r eaction. The above analysiso ft he equilibrium constantss uggests that the reaction takes place in two steps.…”