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40986-AC6
Gas Phase Electron Transfer Thresholds
Electrons bind nuclei together into molecules and chemical reactions are a consequence of rearrangement of electrons around the nuclei involved. The transfer of electrons is thus fundamental to chemistry, but since few reactions produce charged particles under normal conditions, the participation of the electron is only rarely evident, such as in reactions in electric cells. By colliding molecular beams of neutral molecules at slightly elevated energies we are able to study electron-transfer reactions because the electron transfer forms an ion pair and the energy is high enough that both ions can be detected using coincidence time-of-flight mass spectrometry. Gaseous negative ions are more difficult to study than solvated negative ions and we are able to identify the negative ions. This crossed beam method also allows us to use molecules that have been oriented prior to reaction so that the effect of molecular orientation can be measured.
Our orientation studies were initially confined to simple sigma-bonded molecules such as CH3Br and more recent studies have been expanded to study pi-bonded molecules such as CH3CN. (Everyone's standard pi-bonded molecule, benzene, can't be oriented using our techniques.) We have studied electron transfer collisions between K atoms and oriented CH3CN and find that the dominant product is the CN- ion with CH2CN- formed in about 1% yield. The electron (at the lowest energies) must enter the molecular LUMO which is the pi antibonding orbital, but in order to break the CH3-CN bond the electron must find its way into the CH3-CN antibonding sigma* orbital. A Pi-Sigma surface crossing must be encountered to form CN-, and this is enormously facilitated by the nearby nascent K+ ion. In stark contrast, other experiments show that free electrons readily enter the pi* orbital but do NOT readily break the CH3-CN bond and in those experiments CN- is the minor product.
The effects of orientation on the CN- and CH2CN- products are completely different. The asymmetry for CN- is energy-independent and nearly zero, characteristic of sideways collisions. In contrast, CH2CN- production is favored by CH3-end collisions and the steric asymmetry becomes very large at low energies suggesting that at threshold the ion is formed only at that end.
Chlorine substitution on CH3CN giving CCl3CN changes the LUMO to one with C-Cl sigma character and the dominant ion is Cl-, slightly favored at the CCl3 end. (The steric behavior is reminiscent of that of CCl3H studied earlier.) For this molecule, very small yields of the parent ion CCl3CN- were detected and although too small for definitive conclusions regarding the steric requirements, we were able to measure the electron affinity of the parent CCl3CN to be 0.30±0.13 eV. Small yields of fragment ions CCl2CN- and CClCN- are also observed with electron affinities estimated to be 2.06±0.73 and 2.16±0.30 eV. Again, the signals are too small for steric measurements.
