Examination of Sputtered Ion Mechanisms Leading to the Formation of C7H7+ during Surface Induced Dissociation (SID) Tandem Mass Spectrometry (MS/MS) of Benzene Molecular Cations

Abstract

Recent work reported in the literature has shown new evidence of a sputtered ion mechanism for associative ion surface reactions occurring during surface induced dissociation (SID) tandem mass spectrometry (MS/MS). In the context of a sputtered ion mechanism, we have studied selected routes to the formation of C7H7+ during the low-energy collision (ca. 30 eV) of benzene molecular ions with surfaces covered with a hydrocarbon overlayer. A key approach utilized in this work is the use of gas phase ion molecule chemistry to model reactive ion surface collisions. Benzene, 2H6-labeled benzene, and 13C6-labeled benzene have been examined by SID and collision activated dissociation (CAD). The CAD experiments have been used to investigate the fragmentation of hydrocarbon adducts of benzene and labeled benzene formed by methane and isobutane chemical ionization (CI) as models of those ions which may be formed via a sputtered ion mechanism during SID. In addition, the thermodynamics of many of the possible sputtered ion routes to the formation of C7H7+ have been compared using experimental heats of formation as well as total energies and zero-point vibrational energies (ZPVE) obtained by ab initio (MP2 6-31G*//HF 6-31G*) calculations. While there are several seemingly viable sputtered ion routes to the formation of C7H7+ during SID of benzene molecular ions, the combination of thermodynamic considerations and experimental results suggest that a likely sputtered ion route involves the reaction of neutralized benzene ions with C3H5+ ions (sputtered from the hydrocarbon overlayer) followed by the loss of ethene.