Charge transfer from neutral perfluorocarbons to various cations: long-range versus short-range reaction mechanisms

Abstract

The bimolecular reactions of the high recombination energy cations Ar + , F + and Ne + with four fully saturated (CF 4 , C 2 F 6 , C 3 F 8 and n-C 4 F 10 ) and three unsaturated (C 2 F 4 , C 3 F 6 and 2-C 4 F 8 ) Phys. Chem. 100 (1996) 17166), are compared with those determined from the threshold photoelectron-photoion coincidence spectra of the PFCs at the recombination energies of the reagent cations. This comparison provides information that helps to interpret the dynamics of charge-transfer, and whether it occurs via a long-range or a short-range mechanism. Energy resonance and goodFranck-Condon factors connecting the ground electronic state of a reactant neutral molecule to one of its ionic states, at the recombination of the reagent cation, are generally considered to be sufficient for long-range charge-transfer to occur. However, the results from this study imply that good Franck-Condon factors are not critical in determining the efficiency of a long-range chargetransfer. Instead, the results suggest that, in addition to the requirement for energy resonance, the electron taking part in the charge-transfer process must be removed from a molecular orbital which is unshielded from the approaching reagent cation. This enables the cation to exert an influence on the electron at large impact parameters.

Selected Ion Flow Tube Study of the Reactions between Gas Phase Cations and CHCl2F, CHClF2, and CH2ClF

“…4,32 This is further supported by the Cl lone pair and C-Cl σ-bonding orbital character evident in all three neutrals over the energy region that corresponds to the recombination energies of these reagent ions. 4 It is interesting to note that the product branching ratios resulting from reactions with atomic and molecular Genuine long-range charge transfer, where substantial overlap between the ground state and an accessible ionic state in the neutral molecule exists at the recombination energy of the reagent ion plus an agreement between TPEPICO and SIFT branching ratios, 3 is observed in about ten out of a total of fifty-five reactions studied. The only results which notably conflict with those of the TPEPICO study, rather than display systematic differences, are the F + + CHCl 2 F, Ne + + CHClF 2 and, to a lesser extent, Ne + + CH 2 ClF.…”

“…9 Briefly, each reagent ion of interest was produced in a high pressure electron impact ion source containing an appropriate source gas (H 2 O for H 3 O + , SF 6 for SF x + (where x = 1-5), CF 4 for CF 2 Ar for Ar + , Ne for Ne + ). The O 2 + ions were produced using a mixture comprising a 4:1 ratio of O 2 and N 2 in order to reduce the chances of filament burnout in the ion source.…”

A Selected Ion Flow Tube Study of the Reactions of Several Cations with the Group 6B Hexafluorides SF6, SeF6, and TeF6

“…Differences observed in the product ion branching ratios may indicate that a short-range ion-molecule reaction involving an intimate encounter, rather than a longrange electron jump, has occurred. The recent studies of the charge-transfer reactions of CCl 4 and SF 6 , 6 and of several saturated and unsaturated perfluorocarbons, 23 show how the comparison of TPEPICO and flow tube data can lead to a better fundamental understanding of the reactions. Photoionisation studies are conducted at much lower pressures than flow-tube investigations of charge-transfer reactions.…”

“…7,8 This means that for the reactions involving CO + and N + , dissociative charge transfer (if it occurs) can only do so by a short-range rather than a long-range mechanism. 4,6,12,23,[26][27][28][29][30] A long-range mechanism requires energy resonance with non-zero Franck-Condon factors and no distortion of the potential energy curves, with all the available energy going into fragmentation. For a short-range charge transfer, an intimate complex is formed within which chemical …”

“…However, this is not conclusive in itself because other factors, such as the type of molecular orbital from which the electron is ejected, can inhibit longrange charge transfer. 23 In these circumstances, comparisons between TPEPICO and ion-molecule branching ratios are useful to decide if long-range charge transfer is occurring. For the F + reaction, two product ions are observed, SF 5 + (90%) and SF 3 + (10%).…”