Ground-State Multiplicities of 3,4-Dimethylenefuran and 3,4-Dimethylenethiophene. Experimental Tests of ab Initio and Semiempirical Theories of Heteroatom-Bridged Disjoint Biradicals

The structure and spin state of 3,4-dimethylenefuran, 3,4-dimethylenethiophene, and 3,4-dimethylenepyrrole are explored by AM1-CI and other computational methods. In agreement with previous semiempirical and ab initio calculations, the present results predict that these species should have singlet ground states. The experimental study of low-temperature preparations of the first two members of this series is in accord with the predictions. These molecules both are intensely purple compounds (⁁_max 560 and 572 nm, ɛ = 5.3 × 10³and 5.2 × 10³ M^-1 cm^-1, respectively). A new photobleaching reaction of 3,4-dimethylenefuran gives 2-(1-cyclopropenyl)-2-propen-1-al.