Spin–Spin Coupling Between Two meta‐Benzyne Moieties In a Quinolinium Tetraradical Cation Increases Their Reactivities. Issue 17 (25th February 2019) - Kotha, Raghavendhar R.
Abstract: The reactivity of a carbon‐centered σ, σ, σ, σ‐type singlet‐ground‐state tetraradical containing two meta ‐benzyne moieties was examined in the gas phase. Surprisingly, the tetraradical showed higher reactivity than its individual meta ‐benzyne counterparts. The reactivity of meta ‐benzynes is controlled by their (calculated) distortion energy Δ E 2.3, singlet–triplet spitting Δ E S–T, and electron affinity (EA2.3 ) of the meta ‐benzyne moiety at the transition state geometry for hydrogen‐atom abstraction reactions. The addition of a second meta ‐benzyne moiety to a meta ‐benzyne does not significantly change EA2.3 . However, Δ E 2.3 is substantially decreased for both meta ‐benzyne moieties in the tetraradical, and this explains their higher reactivities. The decrease in Δ E 2.3 for each meta ‐benzyne moiety in the tetraradical is rationalized by stabilizing spin–spin coupling between one radical site in each meta ‐benzyne moiety. Therefore, spin–spin coupling between the meta ‐benzyne moieties in this tetraradical increases its reactivity, whereas spin–spin coupling within each meta ‐benzyne moiety decreases its reactivity. Abstract : Radically different : A tetraradical containing two meta ‐benzyne moieties showed unexpectedly higher gas‐phase reactivity than its individual meta ‐benzyne counterparts. The figure shows the strongest spin–spin coupling (blue) and cross‐coupling (red) between the radical sites in the meta ‐benzyne moieties of the tetraradical. The latter is the likely reason for the second minimum on the potential‐energy surface of the tetraradical.