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

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 (2)B(2) doublet state lies energetically below the (4)B(2) 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 pi-system) to the resonance hybrid of the title triradicals. Copyright (C) 2011 John Wiley & Sons, Ltd.