Collision-induced fragmentation of gaseous negative ions and the rates of proton transfer reactions

Abstract

The collision-induced dissociation reactions of the negative ions CH 3OCOCH −1 2, CF 3COCH − 2, C 6H 5CH 2CH(CH 3O − and C 6H 5CH 2C(CH 3) 2O − have been studied as a function of collision energy over the range of centre-of-mass collision energy from about 1 eV to 20 eV. If we identify the starting negative ion as RHY − (R = CH 3O, CF 3, C 6H 5CH 2; HY = CH 2CO, CH 3CHO, CH 3COCH 3), fragmentation occurs as follows: ▪ In all cases formation of RH+Y − is favoured thermochemically and is the most important reaction at low collision (and internal) energies. With increasing collision energy (and increasing internal energy of the fragmenting ion) the yield of R − increases. The extent of this increase correlates with the efficiency of the proton transfer reaction R − + HY → RH + Y −, the rate constants for which have been measured separately; when the efficiency is high the yield of R − in the CID experiment is low. The CID and kinetic results are discussed in terms of the magnitude of the energy barrier and the entropy of activation for the proton transfer reaction R − + HY → RH + Y −.