Energy dependence of the fragmentation of haloanisole molecular ions

Gas-Phase Reactivity of Quinoline-Based Singlet Oxenium Cations

Isomeric quinolyloxenium cations were generated in the gas phase in an ion trap mass spectrometer to explore their reactions. The structures of some products were identified via collision-activated dissociation experiments involving model compounds to demonstrate that they have the expected heavy atom connectivity. The lack of radical reactions suggests that the cations have closed-shell singlet electronic ground states. Calculations (CASPT2/CASSCF(16,14)/cc-pVTZ//CASSCF(16,14)/cc-pVTZ) predict that their closed-shell singlet (1A') ground states are lower in energy by ca. 25 kcal mol-1 than their lowest-lying excited states. All cations are reactive toward dimethyl disulfide, dimethyl sulfide, and allyl iodide and most toward water and moderately reactive toward cyclohexane, reflecting their strongly electrophilic nature. They form adducts with nucleophiles in exothermic reactions (ca. 50 kcal mol-1 for dimethyl sulfide) that can fragment or be stabilized via IR emission. Most water adducts spontaneously isomerize to lower-energy tautomers. The nucleophiles preferentially add to those carbon atoms in the benzene ring that have the greatest positive charge (but not the carbonyl carbon). The cations react with cyclohexane via hydride abstraction by the oxygen atom. This is the only reaction that initially involves the oxygen atom and hence reflects the formally positively charged, monovalent oxygen atom in these cations.

Direct Detection of the Open-Shell Singlet Phenyloxenium Ion: An Atom-Centered Diradical Reacts as an Electrophile

A new photoprecursor to the phenyloxenium ion, 4-methoxyphenoxypyridinium tetrafluoroborate, was investigated using trapping studies, product analysis, computational investigations, and laser flash photolysis experiments ranging from the femtosecond to the millisecond time scale. These experiments allowed us to trace the complete arc of the photophysics and photochemistry of this photoprecursor beginning with the initially populated excited states to its sequential formation of transient intermediates and ultimate formation of stable photoproducts. We find that the excited state of the photoprecursor undergoes heterolysis to generate the phenyloxenium ion in ∼2 ps but surprisingly generates the ion in its open-shell singlet diradical configuration (¹A₂), permitting an unexpected look at the reactivity of an atom-centered open-shell singlet diradical. The open-shell phenyloxenium ion (¹A₂) has a much shorter lifetime (τ ∼ 0.2 ns) in acetonitrile than the previously observed closed-shell singlet (¹A₁) phenyloxenium ion (τ ∼ 5 ns). Remarkably, despite possessing no empty valence orbitals, this open-shell singlet oxenium ion behaves as an even more powerful electrophile than the closed-shell singlet oxenium ion, undergoing solvent trapping by weakly nucleophilic solvents such as water and acetonitrile or externally added nucleophiles (e.g., azide) rather than engaging in typical diradical chemistry, such as H atom abstraction, which we have previously observed for a triplet oxenium ion. In acetonitrile, the open-shell singlet oxenium ion is trapped to generate ortho and para Ritter intermediates, one of which (para) is directly observed as a longer-lived species (τ ∼ 0.1 ms) in time-resolved resonance Raman experiments. The Ritter intermediates are ultimately trapped by either the 4-methoxypyridine leaving group (in the case of para addition) or trapped internally via an essentially barrierless rearrangement (in the case of ortho addition) to generate a cyclized product. The expectation that singlet diradicals react similarly to triplet or uncoupled diradicals needs to be reconsidered, as a recent study by Perrin and Reyes-Rodríguez (J. Am. Chem. Soc. 2014, 136, 15263) suggested the unsettling possibility that singlet p-benzyne could suffer nucleophilic attack to generate a naked phenyl anion. Now, this study provides direct spectroscopic observation of this phenomenon, with an atom-centered open-shell singlet diradical reacting as a powerful electrophile. To the question of whether a nucleophile can attack a singly occupied molecular orbital, the answer is apparently yes, at least if another partially occupied orbital is available to avoid violation of the rules of valence.