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The bridge connecting gas-phase and solution chemistries.

Mass spectrometry (MS) has long served as the gas-phase nursery for exotic ions of ephemeral existence and reactions of ephemeral occurrence in solution. These examples include CH5 , the first ever pentavalent form of carbon, which was later formed in a superacid solution by Olah and Schlosberg; and many elusive ionized molecules such as the prototype carbonyl ylide in its “excentric” distonic ion form, CH2 O CH2, with an apparently bizarre separation of charge and spin sites. MS has also served as the gas-phase medium in which to form key ions and hence probe reaction mechanisms and establish intrinsic reactivity orders. Gaseous NO2 , for instance, was used to settle the mechanism of electrophilic aromatic nitration, a chemical canon that was enthusiastically debated over for decades. Intrinsic physicochemical properties and catalysis, most particularly directed to the activation of inert C H bonds, have also been extensively investigated in gas-phase reactions of (organo)metallic ions. MS was born, however, with a “genetic defect”, that is, “blindness” to neutral species. But an elegant strategy was elaborated, and distant charge sites (negative or positive) were used as charged tags to allow MS to manipulate otherwise neutral species such as radicals. However, skepticism obscured the beauty of gas-phase ion chemistry because the high vacuum was considered by many as an eccentric environment with no bridge to the “real world” of solutions. But John Fenn came to the rescue with his Nobel Prize winning electrospray ionization (ESI) technique. To paraphrase Fenn: “ESI made the power and elegance of MS applicable to both gas-phase and solution ions”. ESI was revolutionary since it brought MS down to the “real world” of solutions by “fishing” solution ions directly into the gas phase. ESI-MS provides continuous snapshots of the dynamic ionic composition of reaction solutions, and ESI-MS “ion fishing” investigations of reaction mechanisms are therefore free from the gas-phase skepticism; the MS bridge is still being used since ions are transferred to the gas phase for MS analysis but they are fished from the reaction medium exactly at the moment of action, and stay mostly undisturbed in the gas phase during their flight to the MS detector. But ESI is also blind to neutral species. To again overcome this limitation, the elegant concept of charged tags developed in the gas phase was brought into solution to facilitate the “flight to the moon”, as Scheme 1 exemplifies. In his Nobel lecture, Fenn summarized ESI as “wings for molecular elephants”; similarly, the combination of ESI and charged tags can be viewed as “charged wings for the flying fish of reaction intermediates” (Figure 1). The gas phase continued to play a major role since the gaseous intermediates can still be investigated by MS to access intrinsic reactivities and functions in the specific reaction step they were fished from. ESI has also allowed