Orientational Preference of Long, Multicenter Bonds in Radical Anion Dimers: A Case Study of π-[TCNB]22- and π-[TCNP]22-

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 carboncarbon bond in -[TCNP](2)(2-) and in -[TCNB](2)(2-), in good accord with the Maximin Principle predictions. Instead, the two known structures of -[TCNP](2)(2-) have a D-2h(=0 degrees) and a C-2(=30 degrees) orientation ( 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 -[TCNB](2)(2-) structure has a C-2(=60 degrees) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with of the interaction energy (E-int) and the variation of the E-int components. It was found that: 1)a long, multicenter bond exists for all orientations; 2)the E-int() angular dependence is similar in both dimers; 3)for all orientations the electrostatic component dominates the value of E-int(), although the dispersion and bonding components also play a relevant role; and 4)the Maximin Principle curve reproduces well the shape of the E-int() 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.