Beams of oriented molecules have been used to directly study geometrical requirements in chemical reactions. These studies have shown that reactivity is much greater in some orientations than others and demonstrated the existence of steric effects. For some reactions portions of the orientation results are in good accord with traditional views of steric hindrance, but for others it is clear that our chemical intuition needs recalibrating. Indeed, the information gained from simultaneously orienting the reactants and observing the scattering angle of the products may lead to new insights about the detailed mechanism of certain reactions. Further work must be done to extend the scope and detail of the studies described here. More detailed information is needed on the CH(3)I reaction and the CF(3)I reaction. The effects of alkyl groups of various sizes and alkali metals of various sizes are of interest. In addition, reactions where a long-lived complex is formed should be studied to see if orientation is important. Finally, it would be of interest to apply the technique to the sort of reactions that led to our interest in the first place: the S(N)2 displacements in alkyl halides where the fascinating Walden inversion occurs.

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Electron transfer collisions between beams of neutral K atoms and neutral alkyl bromide ͑R-Br͒ molecules (RϭCH 3 ,t-C 4 H 9) are observed by detecting positive and negative ions in coincidence for energies տ4 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 Ϸ6 eV shows that ''frontside,'' or Br end attack, is favored to form Br Ϫ , with t-C 4 H 9 Br being more asymmetric than CH 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 ϩ ͒ are detected from t-C 4 H 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 4 H 9 Br is generally smaller than for CH 3 Br and above about 5 eV, both increase with energy in Arrhenius-type dependence. The apparent ''steric activation energy'' is Ϸ2.2 eV for CH 3 Br and 3.9 eV for t-C 4 H 9 Br.

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Articles you may be interested inEntanglement of polar symmetric top molecules as candidate qubits J. Chem. Phys. 135, 154102 (2011); 10.1063/1.3649949 Steric effect in the scattering of hexapoleoriented beams of symmetrictop molecules by graphite(0001) J. Chem. Phys. 93, 7387 (1990); 10.1063/1.459414Theory of oriented symmetrictop molecule beams: Precession, degree of orientation, and photofragmentation of rotationally stateselected molecules Symmetric-top molecules exist in various orientations in even very weak electric fields, in contrast to diatomic molecules which are oriented only in fields so large as to be impractical. For symmetric tops these orientations can be separated in an inhomogeneous electric field, and calculations are presented for the separating properties of a hexapole field. The transmission and the final distributions of velocity and orientations have been calculated for several different molecules over a range of conditions. Experimental transmission measurements were made and are in good agreement with calculations. By fitting experimental points to calculated transmission curves dipole moments in good agreement with literature values have been determin,ed. The molecules separated by the hexapole field are experimentally shown to make adiabatic transitions into a homogeneous electric fiehi, providing evidence that the molecules can be oriented in the laboratory reference frame.

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Previous experiments have suggested that different negative ions are formed by electron transfer to different ends of a molecule. To investigate this possibility, a crossed molecular beam apparatus has been constructed to mass-analyze the ions produced in collisions between fast K atoms and oriented molecules. Initial studies are reported on ion formation in collisions of unoriented SF6 and oriented CH3Br. For lab energies ≈ 5−30 eV, Br- is the only ion observed from CH3Br, and its formation is favored by attack at the Br-end of CH3Br. The Br and CH3 ends have the same energetic threshold for forming Br-. SF5 -, SF6 -, and F- ions are observed from SF6 and O2 - from O2. These ions are formed over a range of energies unlike those formed by electron attachment and suggest that the nascent negative ion can be stabilized by the accompanying positive K+.

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