Methodologies for Computational Studies of Quininoidal Diiminediyls:  Biradical vs Dinitrene Behavior

Abstract

Density functional and post Hartree-Fock ab initio computations were carried out on the lowest singlet, triplet, and quintet states of 1,4-phenylenedinitrene, biphenyl-4,4′-dinitrene, (E)-stilbene-4,4′-dinitrene, and (E,E)-1,4-bis(4-nitrenophenyl)-1,3-butadiene, and (E,E,E)-1,6-bis(4-nitrenophenyl)-1,3,5-hexatriene. Neardegenerate singlet and triplet quinonoidal ground states were found for all systems using CASSCF methodology, with a slight favoring of the singlet, in accord with experimental results. The aromatic quintet dinitrene states lie much higher in energy. Restricted B3LYP hybrid density functional theory (DFT) methods give artifactually high biradical singlet state energies relative to the triplet biradical states, but unrestricted (mixed-state) B3LYP methods correctly give singlet energies that lie somewhat below the triplet state energies, as well as giving geometric results that compare well to the best CASSCF results we could achieve for these biradical states. Appropriate guidelines for selecting CASSCF versus DFT procedures in such cases are suggested in light of comparisons of computed to experimental results. IntroductionThrough understanding exchange interaction in open-shell molecules, one may hope not only to understand molecular bonding better, but also to predict and to control ground-state multiplicity. 1 The p,p′-dinitrenes epitomize the interplay between π-bond formation and ring aromatization. Studies of these have occurred at intervals since the ground-breaking work of Reiser 2 and Trozzolo et al., 3 but a rekindling of interest has occurred during the past decade. For example, 1,4-phenylenedinitrene, 1, has been investigated by UV-vis, 2 cryogenic ESR, 3-5 and recently by Fourier transform infrared (FTIR) 6 spectroscopy. There have also been recent studies of biphenyl-4,4′-dinitrene, 2, 5a,d,7a-c and (E)-stilbene-4,4′-dinitrene, 3. 7c-d,8 While spectroscopic investigations have given critical structural and reactivity information about these unusual systems, considerable uncertainty remains concerning the precise natures of their various electronic states.Computational chemistry can assist with understanding electronic structure and bonding features in systems where experiment cannot give definitive answers. The p,p′-dinitrenes are excellent targets for such studies, given that they can exhibit multiple electronic states that are not all readily studied by experimental methods. In this article, we describe computational studies of quinonoidal dinitrenes by post-Hartree-Fock and density functional methods. We show that both complete active space self-consistent field (CASSCF) and density functional theory (DFT) give computational results that are in accord with experimental findings, and explore some of the limitations of these methods in reproducing experiment. BackgroundThe series of dinitrenes 1-5srepresented as quinonoidal diiminediyls due to their ground state natures described

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