Quinoline Triradicals: A Reactivity Study

Abstract

The gas-phase reactivities of several protonated quinoline-based σ-type (carbon-centered) mono-, bi-, and triradicals toward dimethyl disulfide (DMDS) were studied by using a linear quadrupole ion trap mass spectrometer. The mono- and biradicals produce abundant thiomethyl abstraction products and small amounts of DMDS radical cation, as expected. Surprisingly, all triradicals produce very abundant DMDS radical cations. A single-step mechanism involving electron transfer from DMDS to the triradicals is highly unlikely because the (experimental) adiabatic ionization energy of DMDS is almost 3 eV greater than the (calculated) adiabatic electron affinities of the triradicals. The unexpected reactivity can be explained based on an unprecedented two-step mechanism wherein the protonated triradical first transfers a proton to DMDS, which is then followed by hydrogen atom abstraction from the protonated sulfur atom in DMDS by the radical site in the benzene ring of the deprotonated triradical to generate the conventional DMDS radical cation and a neutral biradical. Quantum chemical calculations as well as examination of deuterated and methylated triradicals provide support for this mechanism. The proton affinities of the neutral triradicals (and DMDS) influence the first step of the reaction while the vertical electron affinities and spin–spin coupling of the neutral triradicals influence the second step. The calculated total reaction exothermicities for the triradicals studied range from 27.6 up to 29.9 kcal mol–1.

Spin‐Spin Coupling Controls the Gas‐Phase Reactivity of Aromatic σ‐Type Triradicals

“…The precursors 1 b-4 b used to generate triradicals 1-4 were synthesized as described previously. [27,33,35,44] The precursors 5 b and 6 b used to generate triradicals 5 and 6 were synthesized and characterized as described in the Supporting Information.…”

“…The radicals were confirmed to have the correct number of radical sites based on the number of H atoms that they abstracted from cyclohexane, as reported previously. [12,26,33,35,38,39,41] Branching ratios of primary reactions were determined by dividing the abundance of each primary product ion by the sum of the abundances of all product ions at short reaction times. The reproducibility of the branching ratios was better than � 10 %.…”

“…[26,33,34] Related tri-and tetraradicals, with three or four electrons in nonbonding molecular orbitals that are similar in energy, contain several low-lying electronic states, which suggests that they might have interesting chemical properties. [33,35,36] However, while the reactivity of aromatic σ-type mono-and biradicals has been fairly thoroughly studied, [24,26,27,30,34] this is not true for the related tri-and tetraradicals. [33,[35][36][37][38][39][40][41] The bestunderstood systems are based on the pyridinium skeleton and therefore contain three or four strongly coupled unpaired electrons.…”

“…[33,35,36] However, while the reactivity of aromatic σ-type mono-and biradicals has been fairly thoroughly studied, [24,26,27,30,34] this is not true for the related tri-and tetraradicals. [33,[35][36][37][38][39][40][41] The bestunderstood systems are based on the pyridinium skeleton and therefore contain three or four strongly coupled unpaired electrons. Examination of the reactivity of the 2,4,6-tridehydropyridinium cation revealed that this species is best described as a reactive monoradical with a substantially less reactive, strongly coupled biradical moiety (meta-pyridyne).…”

Graphene‐Like Covalent Organic Framework with a Wide Band Gap Synthesized On Surface via Stepwise Reactions

“…The variation and reaction of this step are consistent with the on-surface C À H bond activation reported previously. [28] According to our DFT simulations, this molecular layer is physisorbed at a vertical distance of 3.35 on Au(111), as the charge transfer between the COF layer with the substrate is negligible ( Figure S4). The band structure of this t-COF is calculated by DFT with the HSE06 functional, revealing a direct band gap of 3.41 eV (Figure 4 d).…”