5‐Dehydro‐1,3‐quinodimethane: A Hydrocarbon with an Open‐Shell Doublet Ground State

Abstract

Theory predicts an open‐shell doublet ground state for the 5‐dehydro‐1,3‐quinodimethane triradical (see picture), with three low‐spin coupled electrons in three singly occupied molecular orbitals. The bond dissociation enthalpy for formation of the triradical from meta‐xylylene, measured by MS indicates an interaction of (1±4) kcal mol−1 between the unpaired electrons in the σ and π systems in the triradical.

EPR spectroscopic and computational characterization of the 2‐dehydro‐m‐xylylene and 4‐dehydro‐m‐xylylene triradicals

“…According to isodesmic equations, the interaction energy between the σ radical electron and the π electrons of the m ‐xylylene framework in 20 is small, but weakly stabilizing (exp. : 1 ± 4 kcal/mol, SF‐CCSD: 2.9 kcal/mol) . According to computations (EOM‐SF‐CCSD/6‐31 G(d)), the 4 B 2 ground state of 2‐DMX 18 is 10.6 kcal/mol more stable than the open‐shell doublet 2 B 2 .…”

“…Interestingly, the p ‐phenylene bridged systems 14 and 15 feature high‐spin ground states, whereas doublet ground states are determined for m ‐phenylene linked triradicals 16 and 17 . Triradicals of this type that avoid the presence of heteroatoms and of complex structural entities are obtained by introducing a σ radical center into m ‐xylylene 1 (or, alternatively, by replacing a m ‐hydrogen atom in diradicals 7–9 by a methylene group), leading to dehydro‐ m ‐xylylenes 18–20 (Chart ) …”

“…Among the three dehydro‐ m ‐xylylene (DMX) isomers, 5‐DMX 20 has received by far the most attention yet. Wenthold, Krylov, and coworkers determined the heat of formation of 20 to 141 ± 5 kcal/mol using mass spectrometric methods in the gas phase . The ground state of this DMX isomer is an open‐shell doublet ( 2 B 2 ) that is 3.7 kcal/mol more stable than the 4 B 2 quartet state (SF‐CCSD/6‐311 G(d,p)).…”

Magnetism in hydro‐ and dehydrogenated benzene

“…In some of these molecules the failure of Hund's rule has been demonstrated [7]. Experimental and theoretical evidence has been presented indicating that DMX was the first example of an organic triradical with an open-shell doublet ground-state [5,6]. This work was followed by a thorough investigation of triradicals that confirmed the existence of doublet ground states and predicted that of quartet ground states, depending upon the nature of the triradical's molecular orbitals [6].…”

“…On the other hand, combining two radical modules with different spins has allowed to obtain organic polymers with ferro-or antiferromagnetic ordering [4]. Research on molecules containing polyradicals goes back to the early nineties [3,5,6], and has produced a variety of interesting results such as, for example, the synthesis of high spin organic molecules. In some of these molecules the failure of Hund's rule has been demonstrated [7].…”