12 Organic gas-phase ion chemistry

Abstract

Gas-phase ion chemistry is a useful approach for the investigation of physical organic chemistry and the study of reactivity, structure, and thermochemical properties of ionic and neutral organic substrates. This review summarizes the physical organic chemistry studies utilizing gas-phase ion chemistry reported in the past year. Among the more significant advances reported in the past year included a report of the reactivity of p-benzyne by Kenttämaa and co-workers, a comparison of the S_N2 reaction in the gas-phase and in solution by Brauman and co-workers, and a full characterization of the structure and energetics of cyanocarbene. Information learned from these gas-phase ion studies includes: 1) p-benzyne undergoes radical reactions but also reacts as a weak electrophile; 2) unlike what is found in solution, steric bulk does not significantly affect the barriers for S_N2 reactions of halogenated nitriles; and 3) HCCN is a “floppy” molecule with a triplet ground state and a singlet excited state that is ca. 50 kJ mol⁻¹ higher in energy.

Papers included in this review were selected by considering contributions to the standard organic, physical, and mass spectrometry journals. The selected articles are those that address the issue of reactivity and structure of organic molecules. Our emphasis in this work is on the chemistry that is observed, and information regarding the instrumental technique is only provided in situations where it is critical for understanding the results. The majority of the studies were carried out by using Fourier transform-ion cyclotron resonance (FT-ICR), quadrupole ion trap, or flowing afterglow techniques. We have generally not included papers regarding ionization fragmentation patterns, unless the work was part of broader studies. Studies of organometallic ions were generally not included if the chemistry was dominated by the metal center. Moreover, computational studies were also not included unless they provided significant insight into specific experimental results.

It was with some trepidation that we restrict the number of biological studies included in this paper. Recent advances in mass spectrometric capabilities have led to numerous fundamental studies of the properties of biologically relevant molecules. In this work, we generally have only included the studies of the simplest biological molecules, amino acids, nucleic acids, and monosaccharides. It is clear that many more future mass spectrometric studies will likely involve biological substrates. As such, it is anticipated that future versions of this report may need to be expanded to include more biologically relevant material.

The articles summarized in this review are organized into broad classifications: reactivity, reactive species, spectroscopy, and thermochemistry. As has been traditionally the case with this report, we separate the reactivity studies into multiple sections including one specifically addressing substitution reactions and one addressing stereoselective reactivity and stereochemical analysis.