Is TME a Ground-State Singlet or Triplet

Controversy arose almost from the start in this story of the ground state of TME. Dowd, in 1986, prepared TME by photolysis of 4,5-bis(methylene)-3,4,5,6-tetra-hydropyridazine 35 in a methyltetrahydrofuran glass at 10 K. The ESR spectrum 

1 kcal mol lower in energy than the D2i singlet however, it is 0.5 kcal mol lower than the singlet at the geometry of the triplet. Given the low temperature of the experimental observation of triplet TME, their computations support the observation of a triplet. These results, however, conflict with the ESR experiment, which indicates a 2 triplet conformation. 

Over the next 3 years, three computational attempts at reconciling these contradictory results appeared. The first, by Filatov and Shaik, utilized a REKS/6-311G calculation of singlet and triplet TMEs. The singlet minimum 

Finally, Pittner and coworkers examined the PESs of singlet and triplet TMEs using two multireference coupled cluster methods (two-determinant CCSD and multireference Brillouin-Wigner CCSD) with the cc-pVDZ basis set. With both methods, the singlet is always below the triplet for all values of t. The smallest singlet-triplet gap, 1.3 kcal moE, occurs at an angle r = 45° and Ihis is reduced when a near-cc-pVTZ basis set is used. 

Related to TME is TMM 34. The lale of TMM also entails a synergistic interplay of theory and experiment similar to that of TME. This story is nicely retold in the review article of Lineberger and Borden. Instead of reviewing this TMM story, we present the case of the analogous oxyallyl diradical (OXA) 40. This diradical has been proposed as an intermediate in the ring opening of cyclopropanone and other reactions. 

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