Fragmentation and charge transfer in gas-phase complexes of divalent metal ions with acetonitrile Alexandre A. Shvartsburg a, * , Jon G. Wilkes a , Jackson O. Lay a , K.W. Michael Siu b a Division of Chemistry, National Center for Toxicological Research, HFT-233, 3900 NCTR Road, Jeerson, AR 72079, USA b Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ont. Canada M3J 1P3 Received 23 August 2001; in ®nal form 1 October 2001 Abstract The development of electrospray has enabled generation of gas-phase multiply charged metal ion complexes with various solvent molecules. These species exhibit rich fragmentation chemistry, involving competition among neutral ligand loss, ligand cleavage, and dissociative electron and proton transfer. Acetonitrile is a common aprotic solvent. Here we present a comprehensive MS/MS study on acetonitrile complexes of divalent metal cations. We measured the critical sizes below which dissociation channels other than the trivial neutral evaporation become operative and min- imum sizes at which dications remain stable against charge reduction. For all sizes between the two, low-energy fragmentation patterns have been elucidated in detail. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Solvation of ions in liquids has been at the core of physical chemistry since its inception [1]. Lately, the focus has shifted to ®nite systems, which can be conveniently studied by mass spectrometry [2]. This allows one to investigate the process one step at a time while making a connection to high-level theoretical modeling. Solvation of metal ions has been particularly topical, especially in view of the critical function that their coordination has in many proteins. Another aspect is that metalated organic and biological molecules in the gas phase often dissociate along pathways that materially dier from those of protonated molecules [3,4]. This is of advantage in protein identi®cation using peptide sequencing by tandem mass spectrometry MS/MS) [5]. This technology is presently devel- oped for automated sequencing. These factors have precipitated a substantial interest in metal ion±ligand complexes. However, most early work was limited to singly charged systems because solvated multiply charged metal ions had not been experimentally accessible until recently. Indeed, the ionization potentials IPs) of most common organic ligands are in the range of 8±12 eV, while the second IPs IP2s) of metals 21 December 2001 Chemical Physics Letters 350 2001) 216±224 www.elsevier.com/locate/cplett * Corresponding author. Fax: +1-870-543-7686. E-mail address: ashvartsburg@nctr.fda.gov A.A. Shvarts- burg). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0009-261401)01278-7