Gas-phase reactions of organic anions as studied by the flowing afterglow technique

Abstract

It is as futile to try and understand reactions in solution by studying them in the gas phase as to try to reach Paradise by way of Kensal Green.W. John Albery2 When ionic reactions occur in solution the solvent molecules play an important, often dominant, role. In order to investigate the reactions themselves, unobscured by the effects of solvation, an increasing number of studies of gas-phase reactions between ions and molecules are being carried out. The study of organic cations in the gas phase has a long and distinguished history dating from the earliest days of mass spectrometry.3 Only more recently have instrumental developments made it possible to generate and examine, as to their structure and reactivity, a wide variety of organic anions in the gas phase. The newest of these techniques, at least in its applications to organic chemistry, is the flowing afterglow (FA) and its offspring, the selected ion flow tube (SIFT). Experimental TechniqueThe flowing afterglow technique was developed in 1963 by Ferguson, Fehsenfeld, and Schmeltekopf* of the National Oceanic and Atmospheric Administration laboratories in Boulder, CO. Since that time they and their co-workers have exploited the technique to provide a wealth of data on atmospheric ion chemistry and physics while continuing to expand and refine the experimental capabilities.5The heart of our instrument,6 as shown in Figure 1, is a stainless steel flow tube (100 x 7.6 cm i.d.) in which ions are generated and allowed to react and a differentially pumped mass spectrometer system for mass Charles H. DePuy was born in Detroit, MI, in 1927. He received his B.S. degree from Berkeley and his Ph.D. degree from Yale University in 1953 where he studied with William Doering. After postdoctoral work with D. J. Cram at UCLA, he joined the faculty at Iowa State University. He moved to the University of Colorado in 1963. Before entering the field of gas-phase Ion chemistry, he carried out studies in a number of areas of physical organic chemistry Inclutflng cyclopropane and organometalUc chemistry, the chemistry of bimolecular and pyrolytic elimination reactions, and cyclopentane chemistry. Veronica M. Bierbaum was born in Bethlehem, PA, in 1948. She received her B.A. degree from the Catholic University of America (1970) and her Ph.D. degree in physical chemistry from the University of Pittsburgh (1974) under the direction of Frederick Kaufman. She is currently a Senior Research Associate In the Department of Chemistry at the University of Colorado. In addition to the work described In this paper, she is active in studying product state distributions of Ion-molecule reactions by infrared chemiluminescence and laser induced fluorescence.

Gas Phase Ion-Molecule Chemistry of Carbon, Nitrogen and Oxygen Compounds.

“…4 We were able to show that the rates of reaction of the two halides differed profoundly in the way they responded to nucleophiles, and concluded that silanes did, indeed, react by way of a pentacovalent intermediate.…”

Applications of Fourier transform mass spectrometry

“…Reactions of the selected ion can then be monitored without complication from reactions of other ions. In an experiment analogous to a flowing afterglow selected-ion flow tube (SIFT) experiment (42), Kleingeld and Nibbering (43) studied the gas-phase reaction of protonated methanol ions with neutral methanol molecules using naturally occurring lsO isotope labeling. Two mechanisms have been proposed for the formation of protonated dimethyl ether ion:…”

Vibrationally induced charge transfer in a bimolecular model complex in vacuo

“…In contrast, gas phase reactions can exhibit potential energy wells corresponding to entrance and exit channel complexes, in particular for reactions involving ions (see, e.g., Refs. [1][2][3]. Experiments on massselected clusters in vacuo are therefore well-suited for studying the fundamental physics of chemical reactions, starting from entrance channel complexes.…”

“…This dissociative swave capture reaction 21 is strongly exothermic and the energies involved have been well-characterized. 21,22 Depending on the time scale of electron transfer vs. intramolecular vibrational relaxation to a "bath" of solvent, solvation of the complex ion with Ar atoms can be used to quench electron transfer (see Figure 1), resulting instead in evaporation of the weakly bound Ar atoms from the complex CH 3 This feature of Ar solvation can be used to gain some information on the barrier height for electron transfer. 18,23,24 In the present work, we demonstrate vibrationally mediated electron transfer from a nitromethane anion to a methyl iodide molecule in a bimolecular complex.…”