The 5-Dehydro-m-xylylene Triradical and Its Nitrogen and Phosphorus Derivatives: Open-Shell Doublet versus Quartet Ground State Hue Minh Thi Nguyen, ²,‡ G. Gopakumar, ² Jozef Peeters, ² and Minh Tho Nguyen* ,² Department of Chemistry, UniVersity of LeuVen, Celestijnenlaan 200F, B-3001 LeuVen, Belgium, and Faculty of Chemistry, UniVersity of Education, Hanoi, Vietnam ReceiVed: May 24, 2004; In Final Form: July 25, 2004 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, C 8 H 7 ) triradical containing a six-membered- ring radical coupled with two exocyclic CH 2 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 (σ 1 π 1 π 1 ) doublet 2 B 2 ground state with a 4 B 2 - 2 B 2 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 2 B 2 being below the 4 B 2 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 2 B 1 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. 1. Introduction Reactive intermediates form a fascinating world, owing not only to their key role in chemical transformations, but also to their intrinsically bewildering nature and deceptively com- plicated electronic structure and molecular properties. 1 Of transient intermediates, diradicals in which two unpaired electrons occupy two nearly degenerate orbitals, and the partially filled orbitals reside on two different atomic centers, are relatively well characterized. 2 Derived from a two-electrons- in-two-orbitals distribution, the electronic ground state of a diradical could in general be a triplet or an open-shell or closed- shell singlet state. Non-Kekule ´ hydrocarbon diradicals (Scheme 1) such as trimethylenemethane (TMM) and m-xylylene (MX) (also named m-benzoquinodimethane) exhibit a triplet ground state, 3 whereas tetramethyleneethane (TME) and tetramethylenebenzene (TMB) feature a singlet ground state. 4 When two open-shell centers are linked by a phenyl moiety such as in p-phenyl-bis-carbene (p-PhX, X ) CH) and its nitrene (p-PhX, X ) N) and phosphinidene (p-PhX, X ) P) analogues, the open-shell singlet becomes also the ground state 5 (for the notations, cf. Scheme 1). However, in all these cases, the energy gaps between both multiplicity manifolds are small, amounting to only a few kilocalories per mole. In a sense, singlet ground-state diradicals violate both the Aufbau principle and Hund’s rule that govern the electron occupancy of molecular orbitals. Recently, Slipchenko et al. 6 were able to generate, using mass spectrometric techniques, the gas-phase 5-dehydro-m- xylylene (DMX, Scheme 1), which is formally a hydrocarbon triradical (C 8 H 7 ). Using quantum chemical calculations with a spin-flip technique SF-CCSD/6-311G(d,p), 7 these authors derived an energy gap of 3.7 kcal/mol (0.16 eV) in favor of the three-open-shell doublet 2 B 2 state over the corresponding quartet 4 B 2 state. The SF technique is based on coupled-cluster reference wave functions in which the single and double excitation operators involve the flip of the spin of one electron. Accordingly, the open-shell doublet state arises from an occupation of three unpaired electrons in three orbitals having comparable energy but different symmetry, 2 B 2 , (a 1 ) 1 (b 1 ) 1 (a 2 ) 1 . While the a 1 orbital corresponds to an in-plane σ orbital at the C5 position, both b 1 and a 2 orbitals contain π components. Such a preference for low-spin structure could be understood by the fact that, in MX, where two (π 1 π 1 ) electrons interact, a ferromagnetic triplet state is preferred. In DMX, the existence of unpaired σ and π electrons leads to a more dominant antiferromagnetic coupling. Even though low-spin open-shell states often occur in transition-metal complexes, DMX represents a nice and rare example of a hydrocarbon triradical having a three-open-shell (σ 1 π 1 π 1 ) electron configuration and a doublet ground state. 8 Thus, with the emergence of low-spin states in a multispin configuration, Hund’s rule is once more broken down! 6 * Corresponding author. E-mail: minh.nguyen@chem.kuleuven.ac.be. Fax: 32-16-32 7992. ² University of Leuven. ‡ University of Education. 8411 J. Phys. Chem. A 2004, 108, 8411-8418 10.1021/jp047768g CCC: $27.50 © 2004 American Chemical Society Published on Web 09/15/2004