TITLE

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

AUTHOR(S)
Brooks, Philip R.; Harris, Sean A.
PUB. DATE
September 2002
SOURCE
Journal of Chemical Physics;9/1/2002, Vol. 117 Issue 9, p4220
SOURCE TYPE
Academic Journal
DOC. TYPE
Article
ABSTRACT
Electron transfer collisions between beams of neutral K atoms and neutral alkyl bromide (R-Br) molecules (R = CH[sub 3] ,t-C[sub 4]H[sub 9]) 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[sup -], with t-C[sub 4]H[sub 9]Br being more asymmetric than CH[sub 3]Br. 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[sup +]) are detected from t-C[sub 4]H[sub 9]Br 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-C[sub 4]H[sub 9]Br is generally smaller than for CH[sub 3]Br and above about 5 eV, both increase with energy in Arrhenius-type dependence. The apparent "steric activation energy" is ≈2.2 eV for CH[sub 3]Br and 3.9 eV for t-C[sub 4]H[sub 9]Br.
ACCESSION #
7273234

 

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