Electronic structure 1,3,5-triaminobenxene trication and related triradicals: Doublet versus quartet ground state

Quantum chemical calculations have been carried out to determine the electronic ground state of the parent 1,3,5-triaminobenzene trication triradical (TAB(2)(9)(9)(8)(9)(7)(9)(2)(8)(8)(7)(7)(2)(6)(6)(3)(3+, C6H9N3+3) containing a six-membered 3 benzene ring coupled with three exocyclic amino NH)(+ groups, each containing an unpaired electron, as the simplest model for high-spin polyarylamine polycations. Related triradicals, including the 1,3,5-trimethylenebenzene (TMB, C)(H)() and its nitrogen derivatives such as the monocation C)(H)(N+, the dication C)(H)(N2+)(, and the neutral C)(H)(N, C)(H)(N)(, and C)(H)(N) (systems containing NH groups, have also been considered. Results obtained using the CASSCF [multiconfigurational complete active space (SCF-self-consistent field)] method, with active spaces ranging from (9e / 9o) to (15e / 12o), followed by second-order perturbation theory [CASPT2 and MS-CASPT2 (MS-multistate)] with polarized 6-311G(d,p) and natural orbital (ANO-L) basis sets reveal the following: (i) both TAB3+ and TMB (D)() have a quartet 4A)(, ground state with doublet-quartet 2B)(-4A)(3h)(1)(1)(1) (energy gaps of 8.0 2.0 and 12.4 2.0 kcal/mol, respectively; (ii) in the neutral N series, the quartet state remains the electronic ground state, irrespective of the number of N atoms, but each with slightly reduced gap, 11 kcal/mol for C)(H)(N (4A), 10 kcal/mol for C)(H)(N)(8)(8)(7)(7)(2) ((4A)(), and 9 kcal/mol for C)(H)(N)(2)(6)(6)(3) ((4A)(); and (iii) the ground state of monoamino cation and diamino dication is a low-spin doublet state (2B)(2)(1) (for C)(H)(N+ and 2A)(8)(9)(2) (for C)(H)(N2+)(7)(9)(2)) and lying 2 well below the corresponding quartet state by 10 and 12 kcal/mol, respectively. In the monocationic and dicationic amino systems, a slight preference is found for the low-spin state, apparently violating Hund's rule. This effect is due to the splitting of the orbital energies and the presence of the positive charge whose delocalization strongly modifies the electronic distribution and some structural features. In the latter cations, the positive charge basically pushes impaired electrons onto the ring forming a kind of distonic radical cations and thus gives a preference for a low-spin state. 2005 American Institute of Physics.