Dramatic Steric Behavior in Electron Transfer from Various Donors to CF₃Br

Abstract

Different alkali metal atoms are observed to donate electrons to CF(3)Br molecules that are oriented in space. For collision energies high enough to overcome the Coulomb attraction, a positive ion/negative ion pair is observed and mass-analyzed using coincident time-of-flight mass spectroscopy. The alkali metal cation and various negative ions are observed. The most abundant negative ion is the bromide ion, Br(-), formed preferentially by attack at the Br end of the molecule. The steric asymmetry to produce Br(-) is almost identical for all of the alkali metal donors. Fluoride ions are formed in smaller abundance and reflect completely different behavior among the donors. Sodium and potassium have high thresholds and prefer the CF(3) end of the molecule, whereas cesium prefers the Br end of the molecule. Sodium and potassium apparently interact with the transient CF(3)Br(-) molecular negative ion while it is in the process of decomposing.

State-Specific Reactions of Cu+(1S, 3D) with CH3X and CF3X (X = Cl, Br, I): Exploring the Influence of Dipole Orientation on Association and C–X Bond Activation

“…Because CuCF 3 + production is likely precluded by unfavorable thermochemistry, the inefficiency of the Cu + ( 3 D)/CF 3 Cl system can be explained on the basis that the orientation of the CF 3 Cl dipole is unfavorable, requiring a reorientation of the reaction complex for a productive encounter. Notably, such an orientation dependence has been observed previously in reactions of the methane analogues examined here with a number of alkali metal atoms, where an oriented beam of the neutral reactant was crossed with a beam of the alkali metal to yield the metal halide and either the CH 3 or CF 3 radical. − A steric (orientation) dependence on reaction efficiency was observed in all cases such that the end of the molecule with the labile halogen (Cl, Br, or I) was observed to be more reactive at low interaction energy regardless of the orientation of the dipole. The observation of a steric dependence carries with it the possibility of certain mechanistic implications for these methane analogues.…”

“…Specifically, the harpooning mechanism (or a modified version thereof) has been invoked to explain the behavior of these systems in the oriented beam experiments. [61][62][63][64]66 A variation described as "structured harpooning" has likewise been postulated as the mechanism in fluorine abstraction reactions from fluorocarbons by lanthanide and calcium cations at lower interaction energies than those extant in beam experiments. 68,69 In our case, the formidable energetic requirement presented by the second ionization energy for copper would seem to make any sort of long-range electron transfer unlikely.…”

How Orientation of Small Molecules Affects Chemical Reaction

“…For the reactions M＋CF 3 Br, where M＝alkali atoms Na, K, and Cs, the crossing ranges from §3.5 Å to §5 Å, but this has a major eŠect since the interaction scales exponentially with distance. In our studies with this series 35) we found that if the donor were Cs (r C §5 Å) the electron seemed to be simply transferred to the positive, Br-end, of the molecule. The other two alkali metals transfer the electron at shorter range and give orders of magnitude diŠerent yields of electrons and F -, depending on collision energy.…”

Multidimensional steric effect for the XeBr∗ (B, C) formation in the oriented Xe∗(P32, MJ=2)+oriented CF3Br reaction

“…3 Contrary to this expectation, we can observe a large steric effect for the XeBr ‫ء‬ formation such as the KBr formation in the K + CF 3 Br reaction ͓r c = 4.2 Å, IE ͑K͒ = 4.35 eV, and EA ͑CF 3 Br͒ = 0.91 eV͔ in which the large reactive asymmetry on molecular orientation ͑Br-end is about threefold more reactive than the CF 3 -end͒ has been reported. 8 Because the basic idea that a long-range electron jump will exhibit less of a steric effect than a short range jump has been supported by Harland and Brooks, 11 it seems likely that the observed large steric effect is the perverse effect of the genuine harpoon mechanism with the large crossing distance r c .…”

“…The RgX ‫ء‬ formation is known to proceed via the "harpoon" mechanism [22][23][24][25][26][27] such as the metal halide MX formation in the reaction of alkali-metal atoms M, for which steric effects have been widely studied using the oriented molecules. [1][2][3][4][5][6][7][8][9][10][11][12][13][16][17][18] For the KI formation in the K + CF 3 I reaction system, it has been reported that the molecular orientation exerts an influence on only the angular distribution of KI with the comparable reaction probability at the two ends. 3 In contrast, for the KBr formation in the K + CF 3 Br reaction, the reactive scattering of KBr as a function of orientation of CF 3 Br with respect to incident K has been reported over range of scattering angles.…”