Energy transfer in collisional activation. Energy dependence of the fragmentation of n-alkylbenzene molecular ions

Abstract

n-Alkylbenzene molecular ions, C 6H 5CH 2R +·, show two major fragmentation routes, reactions (1) and (2). C 6H 5CH 2R +· → C 7H 7 ++R · → C 7H 8 + +(R-H) Using charge exchange mass spectrometry, the C 7H 7 +/C 7H 8 +· ratio was found to increase from 0 at ∼ 2 eV internal energy to values ranging from ∼ 8 (R = C 3H 7) to ∼ 2 (R = C 7H 15) at ∼ 7 eV internal energy of the fragmenting C 6H 5CH 2R +· ions. The low energy (tandem quadrupole) collision-induced dissociation (CID) of C 6H 5CH 2R +· ions show increasing C 7H 7 +/C 7H 8 +· ratios with increasing ion kinetic energy but reach plateau values, dependent on the identity of R, at ∼ 60 eV collision energy using N 2 and Ar collision gases. From a comparison of the CID and charge exchange results, the average kinetic energy transformed into internal energy on collision is evaluated. The energy transfer efficiency is high (33–87%) and increases with increasing degrees of vibrational freedom in the ion. These results are consistent with the formation of a collision complex between the ion and the neutral species in which the relative translational energy is converted into vibrational energy of the complex. The high energy (8 keV) CID of the alkylbenzene molecular ions shows that the average energy transfer is in the range 4.5–6.7 eV, again increasing with increasing size of the alkylbenzene.