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

Abstract

The isomeric 2‐dehydro‐ and 4‐dehydro‐1,3‐benzoquinodimethanes, 18 and 19, were generated by irradiation of the corresponding triiodo compounds in cryogenic argon matrices and characterized by electron paramagnetic resonance spectroscopy. In agreement with multireference computations, both systems possess quartet ground states, whereas in the isomeric 5‐dehydro‐m‐xylylene 20 the 2B2 doublet state lies energetically below the 4B2 state, so that no characteristic quartet signals could be observed for this triradical under similar experimental conditions. The best estimates for the adiabatic doublet–quartet energy splittings (CAS(7,7)‐AQCC/cc‐pVTZ//CAS(9,9)‐RS2c/cc‐pVTZ) of 18–20 are 10.4, 7.7, and −1.3 kcal/mol, respectively. The measured zero‐field splitting parameters of 18 and 19 are discussed in terms of the contributions of carbenoid resonance structures (spin polarization of the π‐system) to the resonance hybrid of the title triradicals. Copyright © 2011 John Wiley & Sons, Ltd.

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 .…”

“…For example, such π n polyradicals as m ‐xylylene (MX) and 1,3,5‐trimethylenebenzene (TriMB) have nondisjoint NBMOs with strong ferromagnetic spin coupling and resulting triplet ground states (Figure ). Among the α, n ‐didehydrotoluenes (DHT), ortho‐ ( n = 2) and para‐ ( n = 4) isomers have triplet ground states (due to large coefficients of the benzylic π‐SOMO in these positions and the consequential preference for ferromagnetic coupling with the orthogonal σ‐SOMO), while the meta‐isomer ( n = 3) is an open‐shell singlet (Figure ) . Similarly, 5‐dehydro‐1,3‐quinodimethane (DMX) is a triradical with an open‐shell doublet ground state because it combines parallel spin alignment between two benzylic π‐SOMOs of MX with their antiparallel coupling to a ring σ‐SOMO of α,3‐DHT .…”

“…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 ) …”

“…Within the configuration itself the ordering of orbitals by energies affects their occupation, which in turn determines properties of the reactive, frontier orbitals . The orbital configuration is colloquially thought to conform to the aufbau principle : a maximum of two electrons are put into orbitals in the order of increasing orbital energy, i.e., the lowest‐energy orbitals are filled before electrons are placed in higher‐energy orbitals,—leading to a low‐spin state . The aufbau (from German for ‘building up’) principle was formulated by Niels Bohr in 1923 on the basis of an empirical Madelung's rule .…”

“…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.…”