Triradicals

Abstract

Consistent with the definition of diradicals, triradicals arespecies in which three electrons occupy three (nearly) degenerate orbitals, resulting in close-lying quartet and doublet states. The same concepts and rules that can be used to predict and rationalize the ground state multiplicity of diradicals also apply to triradicals, but the greater number of states in triradicals generally leads to more complex electronic structures. Most experimentally accessible triradicals are based onorganic π-systems; therefore triradicals are classified according to the σ/π symmetry of the nominally nonbonding molecular orbitals (NBMOs). The tridehydrobenzenes are prototypal σσσ triradicals with doublet ground states and significant doublet-quartet energy splittings. In all three isomers, the ordering of electronic states depends critically on the distance between the m-radical centers, which makes computational studies of these systems demanding. The experimental IR spectrum of matrix-isolated 1,2,3-tridehydrobenzene led to a revision of the previous ground state assignment based on computations. This work demonstrates the close interplay between experiment and theory in this realm of reactive intermediate chemistry. 1,3,5-Tridehydrobenzene can be isolated as its trifluoro derivative. The stabilization of dehydrophenyl nitrenes, typical members of the σσπ family of triradicals, also requires ortho-fluorination. Because of their quartet ground states, derivatives of 2-dehydrophenyl nitrene and 4-dehydrophenyl nitrene could be studied using IR or EPR spectroscopy. The zero-field splitting parameters of these systems provide direct evidence for the contribution of carbenoid resonance structures to the resonance hybrid of the high-spin systems. According to computations, the through-bond coupling of the in-plane electrons thermodynamically stabilizes the doublet ground states of m-dehydrophenyl nitrenes. But for the same reasons, these systems are prone to ring-opening reactions, which make them difficult to isolate. Remarkably, the m-phenylene unit leads to strongly antiferromagnetic coupling in σσπ triradicals, while o- or p-coupling results in high-spin systems. The more common all-π systems show the opposite pattern because the latter connectivity naturally results in closed-shell arrangements. Within the family of σππ triradicals, we could characterize 2-dehydro-m-xylylene and 4-dehydro-m-xylylene by EPR spectroscopy, whereas the 5-isomer features a doublet ground state. This observation is readily rationalized considering the nodal characteristics of the NBMOs involved and by simple spin polarization models. 1,3,5-Trimethylenebenzene strongly prefers ferromagnetic coupling and features a robust quartet ground state. We have synthesized this πππ triradical in cryogenic matrices and characterized it by IR and EPR spectroscopy. Interestingly, the triradical is photochemically much more stable than m-xylylene, a diradical that shows fascinating rearrangements upon irradiation in cryogenic matrices.

Exploring Redox States, Doping and Ordering of Electroactive Star‐Shaped Oligo(aniline)s

“…The singlet dication can be ruled out with a very high level of confidence as the major doped state of TDPB , as its simulated UV/Vis/NIR maximum of 923 nm is 150 nm higher than the experimental value. Other possibilities that cannot be ruled out include a triplet dicationic diradical, and doublet and quartet tricationic triradical species42 (calculated λ max values of 755, 750 and 751 nm, respectively; see Supporting Information for further details). These results clearly show that TDPB forms species containing unpaired electrons when oxidised samples are exposed to acids.…”

Theory and practice of uncommon molecular electronic configurations

“…Moreover, Coulomb interactions of the unpaired electrons with the paired (bonding) ones are generally weaker in open‐shell singlets compared to triplets and are ascribed qualitatively to instantaneous correlation effects, in particular the dynamic spin polarization (Scheme ). A damped spin wave so formed generates ferromagnetic coupling between the spins separated by an odd number of sites, and antiferromagnetic between those separated by an even number of sites, thus yielding the topology rules of spin coupling . Within the spin‐Hamiltonian VB theory, Ovchinnikov's qualitative rule dictates that the most spin‐alternant determinant is expected to have the lowest energy and the highest contribution .…”

“…The dominant product of electron attachment to DMX is a rare triplet carbanion . Topologically different isomers of DMX II and III with nondisjoint NBMOs have high‐spin (quartet) ground states (Figure ) . These simple polyradicals can also be thought of as coupling units that sustain certain spin alignment depending on topology, as seen in Schlenk's and Chichibabin's hydrocarbons, or in isomers of dinitroxide IV (Figure ) …”

“…A damped spin wave so formed generates ferromagnetic coupling between the spins separated by an odd number of sites, and antiferromagnetic between those separated by an even number of sites, 22 thus yielding the topology rules of spin coupling. 23,24 Within the spin-Hamiltonian VB theory, Ovchinnikov's qualitative rule dictates that the most spin-alternant determinant is expected to have the lowest energy and the highest contribution. 25 Crucially, all of these rules are not the direct consequences of quantum mechanics and only mimic the dynamic and static correlation effects with the help of simplifying assumptions, thus inevitably having limited scope.…”

The Antiferromagnetic Spin Coupling in Non‐Kekulé Acenes—Impressive Polyradical Character Revealed by High‐Level Multireference Methods

“…The orbitals to be used in these MR calculations were based on the CASSCF approach. In particular, CAS (6,6), CAS (10,10), and CAS (14,11) were used in these CASSCF calculations. In the notation CAS(m,n), m is the number of electrons and n is the number of orbitals.…”