Frontside versus backside reactivity in electron transfer to oriented tert butyl bromide and methyl bromide

Electron transfer collisions between beams of neutral K atoms and neutral alkyl bromide ~R–Br! molecules (R5CH3 ,t-C4H9) are observed by detecting positive and negative ions in coincidence for energies *4 eV, the minimum energy for overcoming the Coulomb attraction between ions. The molecules are state selected by a hexapole electric field and oriented prior to the electron transfer. The steric asymmetry for both molecules above '6 eV shows that ‘‘frontside,’’ or Br end attack, is favored to form Br, with t-C4H9Br being more asymmetric than CH3Br. The asymmetry maximizes near 5 eV and as the energy decreases, apparently changes sign to favor ‘‘backside,’’ or alkyl-end attack. Free electrons ~and K! are detected from t-C4H9Br and show a similar change in preferred orientation: at low energies alkyl end attack is favored, and at high energies Br end is favored. These observations suggest that the electron is transferred into different orbitals with different spatial distributions as the energy is varied. Steric factors are evaluated from the experimental data. The steric factor for t-C4H9Br is generally smaller than for CH3Br and above about 5 eV, both increase with energy in Arrhenius-type dependence. The apparent ‘‘steric activation energy’’ is '2.2 eV for CH3Br and 3.9 eV for t-C4H9Br. © 2002 American Institute of Physics. @DOI: 10.1063/1.1496477#