Triradicals

Michael Winkler

doi:10.1021/ar4000218

Triradicals

2013-07-03

28

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AI

DOI：10.1021/ar4000218

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En 中文

Consistent with the definition of diradicals, triradicals are species in which three electrons occupy three (nearly) degenerate orbitals, resulting in dose-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 on organic pi-systems; therefore triradicals are classified according to the sigma/pi symmetry of the nominally nonbonding molecular orbitals (NBMOs). The tridehydrobenzenes are prototypal sigma sigma sigma 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 in-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 computation This work demonstrates the dose interplay between experiment and theory in this realm of reactive intemediate chemistry. 1,3,5-Tridehydrobenzene can be isolated as its trifluoro derivative. The stabilization of dehydrophenyl nitrenes, typical members of the sigma sigma pi family of triradicals, also requires Who-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 at m-phenylene unit leads to strongly antiferromagnetic coupling in sigma sigma pi triradicals, while or p-coupling results in high-spin systems. The more common all-pi systems show the opposite pattern because the latter connectivity naturally results in dosed-shell arrangements. Within the family of sigma pi pi 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 pi pi pi 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.

Keyword:

RESONANCE SPECTROSCOPIC CHARACTERIZATION

MATRIX-ISOLATION

GROUND-STATE

META-BENZYNE

PHOTOELECTRON-SPECTROSCOPY

ORGANIC DIRADICALS

M-XYLYLENE

QUARTET

CHEMISTRY

1,3,5-TRIMETHYLENEBENZENE

Triradicals

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