Orientational preference of long, multicenter bonds in radical anion dimers: a case study of π-[TCNB]2 (2-) and π-[TCNP]2 (2.).

The similar shape and electronic structure of the radical anions of 1,2,4,5-tetracyanopyrazine (TCNP) and 1,2,4,5-tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon-carbon bond in π-[TCNP]2 (2-) and in π-[TCNB]2 (2-) , in good accord with the Maximin Principle predictions. Instead, the two known structures of π-[TCNP]2 (2-) have a D2h (θ=0°) and a C2 (θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six-membered rings). The only known π-[TCNB]2 (2-) structure has a C2 (θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (Eint ) and the variation of the Eint components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the Eint (θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of Eint (θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the Eint (θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)(.-) ⋅⋅⋅cation(+) interactions are also included in the model aggregate are the experimental data reproduced computationally.