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The 5-Dehydro- m -xylylene Triradical and Its Nitrogen and Phosphorus Derivatives: Open-Shell Doublet versus Quartet Ground State

2004; American Chemical Society; Volume: 108; Issue: 40 Linguagem: Inglês

10.1021/jp047768g

1520-5215

Hue Minh Thi Nguyen, Gopinadhanpillai Gopakumar, Jozef Peeters, Minh Tho Nguyen,

Free Radicals and Antioxidants

Quantum chemical calculations have been applied to investigations of the electronic structure of the parent 5-dehydro-m-xylylene (DMX, or 5-dehydro-1,3-quinodimethane, C8H7) triradical containing a six-membered-ring radical coupled with two exocyclic CH2 groups situated in the meta position, each containing an unpaired electron and its 4,6-dinitrogen (DMX-N) and 4,6-diphosphorus (DMX-P) derivatives. The purpose of the study is to determine the identity of their electronic ground states. Our results obtained using state-averaged complete active space self-consistent-field (CASSCF) followed by second-order multistate multiconfiguration perturbation theories MS-CASPT2 and MR-QDPT in conjunction with large ANO-L and the 6-311G(d,p) basis set reveal the following: (i) DMX has a three-open-shell (σ1π1π1) doublet 2B2 ground state with a 4B2−2B2 energy gap in the range 1−3 kcal/mol, and (ii) the ground state of both DMX-N and DMX-P triradicals is also the doublet 2B2 being below the 4B2 state by 1 and 2 kcal/mol, respectively. In the triradicals considered, both doublet and quartet states are nearly degenerate but have a slight preference for the low-spin state, apparently violating Hund's rule. Protonation at C5 of DMX giving the MX•+ radical cation modifies the electronic landscape, the one-open-shell doublet 2B1 being the MX•+ ground state. The electron affinities (EAs), ionization energies (IEs), and proton affinities (PAs) are computed for the triradicals. For DMX: EA = 1.1 eV, IE = 7.50 eV, and PA = 401 kcal/mol. For DMX-N: EA = 2.3 eV, IE = 8.20 eV, and PA = 233 kcal/mol. For DMX-P: EA = 4.5 eV, IE = 8.98 eV, and PA = 214 kcal/mol. Comparable data for the anions follows. PA = 401, 367, and 371 kcal/mol for DMX-, DMX-N-, and DMX-P-, respectively.