Triplet ground state electron spin resonance

Electron spin resonance (ESR) studies of the urazole-bridged 1,3 diradicals 64 derived from the azoalkanes 63 confirm a triplet ground state for these species. The nearly zero symmetry parameter, that is, Elhc= 0.0004 0.0001 cm-1, for the triplet diradical 64 of the diphenyl azoalkane 63 establishes a planar conformation <1995JOC308, 1997JA10673>. 

Most stable ground-state molecules contain closed-shell electron configurations with a completely filled valence shell in which all molecular orbitals are doubly occupied or empty. Radicals, on the other hand, have an odd number of electrons and are therefore paramagnetic species. Electron paramagnetic resonance (EPR), sometimes called electron spin resonance (ESR), is a spectroscopic technique used to study species with one or more unpaired electrons, such as those found in free radicals, triplets (in the solid phase) and some inorganic complexes of transition-metal ions. 

On the other hand, the cyanomethylene H—C—C=N triplet as well as a number of odd alternant methylene compounds, propargylene and its homologues, H— —C=CR (R = H, CH3, C6H5), and H—C— C=CC=CR [R = CH3, C(CH3)3, C6Hs] have been predicted to be linear on the basis of the zero-field splitting parameters from electron spin resonance (ESR) experiments (15). The linear nature of the first named H—C—C=N triplet carbene has been confirmed by a micro-wave study in the gas phase. However, if the potential well in which the linear configuration lies is shallow, the ground state may not show the effects of nonlinearity (142). 

A two-electron oxidation of allenes (1) (A = S, NMe) has been found to yield carbenes (2) (Scheme 1) dimerization and reaction with (2,2,6,6-tetraamethyl-piperidin-l-yl)oxyl (TEMPO) support a triplet ground state as predicted by computations. Triplet diphenylcarbenes bearing bulky substituents at the para positions have been generated and studied in rigid matrixes at low temperatures by electron spin resonance (ESR) and UV-Vis spectroscopy as well as by laser flash photolysis (LFP) in solution at room temperature. Their reactivity upon LFP was shown to be dominated by dimerization unless triplet carbene quenchers (such as oxygen and cyclohexadiene) were present. 

Buchwalter, S. L., and Closs, G. L. "Electron Spin Resonance and CIDNP Studies on 1,3-Cyclo-pentadiyls. A Localized 1,3 Carbon Biradical System with a Triplet Ground State. Tunneling in Carbon-Carbon Bond Formation." J. Am. Chem. Soc., 101,4688-4694 (1979). 

Cox, A., Kemp, T. J., Payne, D. R., Symons, M. C. R., and Pinot de Moira, R, Electron spin resonance characterization of ground state triplet aryl cations substituted at the 4-position by dialkylamino group, /. Am. Chem. Soc., 100, 4779, 1978. 

First, try to draw resonance contributors for both ground state and triplet anthrone. Then display a spin density surface for the triplet state of anthrone. (Note that the spin density surface shows the location of both unpaired electrons, one of which may be in a 7t orbital and one of which may be in a o orbital.) Where are the two unpaired electrons Are they localized or delocalized Given that spin delocalization generally leads to stabilization, would you expect the triplet state of anthrone to be stable ... 

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