Molecular Probes of Cation-Arene Interactions
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The three phases of matter are gas phase, liquid (or condensed), and solid state. In recent years, the stunning advances in computational studies have led some to refer to it in a jocular sense as the fourth: “the computational state.” Some of the studies discussed in this article concern gas phase results determined by mass spectrometry and by calculation. Additionally, data have been derived from solution and solid-state studies. Important inferences can be drawn from computations, work undertaken in the gas phase, in solution, and in the solid state. Significant as all of these methods are, only X-ray crystallographic data permit unambiguous analysis and comparison of structural information. The emphasis of this article is therefore on the evidence for cation–pi interactions that is available from solid-state structures.