Tetramethyleneethane (TME) is a triplet non-KekulC molecule that is currently the source of a difference between theory and experiment. The best theory places the energy of the triplet about 1 kcal/mol below the singlet at an angle of twist of 50° about the central carbon4arbon bond. However, the triplet ESR spectrum shows no splitting between the x-and y-lines, indicating that the zero field parameter, E, is 0 or very small. If it is the former, that would be consistent with a D u configured TME. In order to explore this point by removing proton hyperfine coupling, TME-ds was prepared. Again, no splitting was observed between the xand y-lines for TME-dg. This sets an upper limit for E of approximately 0.001 cm-l.

The semilocalized diradical, 4-methylenecyclopentane-1,3-diyl, 18 has been prepared by photolysis of 2,3-diaza-5-methylenebicyclo[2.2.l]hept-2-ene (15). The adduct of cyclopentadiene and di-t-butyl azodicarboxylate was carboxylated with carbon monoxide and palladium chloride in methanol yielding the diester 8 and the chloro ester 9. The latter was reduced with tri-n-butyltin hydride to the ester 10, then with lithium borohydride to the corresponding carbinol. Formation of the mesylate was followed by potassium tertbutoxide promoted elimination to the exocyclic methylene derivative 13. Trifluoracetic acid treatment removed the protecting tert-butyloxycarbonyl groups yielding the unstable hydrazine, which underwent spontaneous air oxidation to the desired azo compound 15.The diradical 18 was shown to be a ground-state triplet by Curie-Weiss Law analysis; 18 has zero field parameters D/hc=0.0591 cm-1 and E/hc=0.00254 cm-1. The latter value was obtained from the fully deuterated diradical18-de.The diradical 18 is converted by photolysis at 340-400 nm to the trimethylenemethane derivative, 2-methylenecyclopentane-1,3-diyl, 16.

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