Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals

Abstract

Coupling of the radical sites in isomeric benzynes is known to hinder their radical reactivity. In order to determine how far apart the radical sites must be for them not to interact, the gasphase reactivity of several isomeric protonated (iso)quinoline-and acridine-based biradicals was examined. All the (iso)quinoliniumbased biradicals were found to react slower than the related monoradicals with similar vertical electron affinities (i.e., similar polar effects). In sharp contrast, the acridinium-based biradicals, most with the radical sites farther apart than in the (iso)quinolinium-based systems, showed greater reactivities than the relevant monoradicals with similar vertical electron affinities. The greater distances between the two radical sites in these biradicals lead to very little or no spin−spin coupling, and no suppression of radical reactivity was observed. Therefore, the radical sites can still interact if they are located on adjacent benzene rings and only after being separated further than that does no coupling occur. The most reactive radical site of each biradical was experimentally determined to be the one predicted to be more reactive based on the monoradical reactivity data. Therefore, the calculated vertical electron affinities of relevant monoradicals can be used to predict which radical site is most reactive in the biradicals.

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

“…The vertical electron affinities (EA v ) and singlet triplet splittings ( ~ES-T ) were calculated as described in the literature. [27,33] The EA v values for triradicals were calculated for the radical site that, based on calculations, gains most electron density upon addition of an electron to the triradical. The distortion energy values (ΔE 2.3 Å ) were calculated at the UBLYP/ cc-pVDZ level of theory.…”

“…Previous studies have demonstrated that the reactivity of biradicals of the type discussed above are controlled by several parameters. [25][26][27] The calculated vertical electron affinity (EA v ) of the radical site has been reported to be a significant reactivity controlling factor for polar (charged and uncharged) mono-and biradicals. [25][26][27][28][29] A high EA v stabilizes an important polar resonance structure of the transition states of atom abstraction reactions (i. e., H atom abstraction) and therefore lowers the transition state energy.…”

“…[25][26][27] The calculated vertical electron affinity (EA v ) of the radical site has been reported to be a significant reactivity controlling factor for polar (charged and uncharged) mono-and biradicals. [25][26][27][28][29] A high EA v stabilizes an important polar resonance structure of the transition states of atom abstraction reactions (i. e., H atom abstraction) and therefore lowers the transition state energy. [15,18] On the other hand, the magnitude of the singlet-triplet splitting ( ~ES-T ) of singlet biradicals has been reported to be related to extra energy required to partially uncouple the biradical electrons in transition states of radical reactions.…”

“…[15,18] On the other hand, the magnitude of the singlet-triplet splitting ( ~ES-T ) of singlet biradicals has been reported to be related to extra energy required to partially uncouple the biradical electrons in transition states of radical reactions. [26,27,[30][31][32] An additional reactivity-controlling factor that only applies to meta-benzyne analogs is the distortion energy (ΔE 2.3 Å) or the energy required for the biradical to reach the dehydrocarbon atom separation (DAS) that they have in their transition states of radical reactions. [26,33,34] This DAS is approximately 2.3 Å for H atom abstraction reactions.…”

“…[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.…”