Benzylium versus Tropylium Ion Dichotomy: Vibrational Spectroscopy of Gaseous C8H9+ Ions

Direct Evidence of the Benzylium and Tropylium Cations as the Two Long-Lived Isomers of C7 H7. Chemphyschem 2018;19:3182-3185. [PMID: 30238585 PMCID: PMC6420061 DOI: 10.1002/cphc.201800744]

Disentangling the isomeric structure of C7 H7 + is a longstanding experimental issue. We report here the full mid-infrared vibrational spectrum of C7 H7 + tagged with Ne obtained with infrared-predissociation spectroscopy at 10 K. Saturation depletion measurements were used to assign the contribution of benzylium and tropylium isomers and demonstrate that no other isomer is involved. Recorded spectral features compare well with density functional theory calculations. This opens perspectives for a better understanding and control of the formation paths leading to either tropylium or benzylium ions.

Why Are Vinyl Cations Sluggish Electrophiles?

The kinetics of the reactions of the vinyl cations 2 [Ph₂C═C⁺-(4-MeO-C₆H₄)] and 3 [Me₂C═C⁺-(4-MeO-C₆H₄)] (generated by laser flash photolysis) with diverse nucleophiles (e.g., pyrroles, halide ions, and solvents containing variable amounts of water or alcohol) have been determined photometrically. It was found that the reactivity order of the nucleophiles toward these vinyl cations is the same as that toward diarylcarbenium ions (benzhydrylium ions). However, the reaction rates of vinyl cations are affected only half as much by variation of the nucleophiles as those of the benzhydrylium ions. For that reason, the relative reactivities of vinyl cations and benzhydrylium ions depend strongly on the nature of the nucleophiles. It is shown that vinyl cations 2 and 3 react, respectively, 227 and 14 times more slowly with trifluoroethanol than the parent benzhydrylium ion (Ph)₂CH⁺, even though in solvolysis reactions (80% aqueous ethanol at 25 °C) the vinyl bromides leading to 2 and 3 ionize much more slowly (half-lives 1.15 yrs and 33 days) than (Ph)₂CH-Br (half-life 23 s). The origin of this counterintuitive phenomenon was investigated by high-level MO calculations. We report that vinyl cations are not exceptionally high energy intermediates, and that high intrinsic barriers for the sp² ⇌ sp rehybridizations account for the general phenomenon that vinyl cations are formed slowly by solvolytic cleavage of vinyl derivatives, and are also consumed slowly by reactions with nucleophiles.