Ionic species are ubiquitous in a wide variety of media (planetary atmospheres, interstellar clouds, plasmas, flames, and of course organic and inorganic solutions) and moreover are important reactive species in biological systems and many synthetic procedures. Ions can be generated by many different techniques, are readily manipulated by means of electrical and magnetic fields, and can be detected with very high sensitivity. As a consequence of these favorable characteristics, ions have played a prominent role in fundamental studies of intermolecular interactions, collision dynamics, and reaction mechanisms, as well as in practical analytical methodology. My research interests center on fundamental aspects of the gas-phase chemistry of organic ions and molecules, with emphasis on thermochemistry and structures of ions and cluster ions and on the detailed mechanisms and dynamics of bimolecular and unimolecular reactions of ions.
We employ a combination of experimental studies and theoretical calculations to study ion-molecule reactions. The experiments are carried out in a custom-designed guided ion beam apparatus. which is a special kind of mass spectrometer that is ideally suited to investigations of reaction dynamics and kinetics. In addition to reaction kinetics, we can study photoinduced processes such as dissociation and electron detachment by using a laser to excite the ions within the octopole region of' the guided beam apparatus. In many cases, we can further augment the information yielded by the experimental results by using statistical phase space theory in conjunction with molecular orbital theory to model the results. This approach provides information about the ion structures, reaction mechanisms, and the potential energy surfaces that govern the reactions.
This research program can be divided into three distinct but related areas within gas-phase ion chemistry: bimolecular and unimolecular ion reaction dynamics, cluster ions and highly charged ions, and photodissociation and photodetachment spectroscopy. Research in the first area is directed toward the elucidation of mechanisms, transition states. and intermediates for fundamental organic reactions such a bimolecular nucleophilic substitution (SN2) reactions, hydride and proton transfers, and elimination reactions. Studies of cluster ions are motivated in part by the fact that these species can constitute models for ion-molecule reaction intermediates. but also because by examining the chemistry of cluster ions we gain insight to molecular interactions and effects of stepwise solvation. Highly charged ions have found practical applications recently in mass spectrometric analysis of high molecular weight species such as polypeptides and polymers. Moreover, ions with two or more charges may be better models for catalytic sites of transition metal organometallic species than the singly charged, coordinatively unsaturated metal ions favored in the past. These ions can be produced in the gas phase by ionspray or electrospray ionization. Finally, photodissociation and photodetachment spectroscopy can provide insight to ion structures and accurate spectral data and bond energies.