CONFORMATION OF A GEMINAL DICYCLOPROPYL-SUBSTITUTED TRIMETHYLENEMETHANE [2-(DICYCLOPROPYLMETHYLIDENE)CYCLOPENTAN-1,3-DIYL]

Triplet 2-(dicyclopropylmethylidene)cyclopentane CP2TMM, has a Delta m (s) = 2 ESR transition revealing hyperfine splitting consistent with c oupling to two equivalent alpha hydrogens and five equivalent beta hyd rogens and one other hydrogen with a small hyperfine coupling constant , This is consistent only with a conformation in which one cyclopropan e ring conjugates with the pi triplet so that the other cyclopropane i s twisted allowing the tertiary cyclopropyl hydrogen to hyperconjugate . Geometry optimization of the triplet state of CP2TMM was carried out using the 3-21G basis set with a UHF wave function and while the sing let state was optimized using a GVB perfect pairing wave function, The most stable conformation of each spin state is a C-2-symmetric struct ure providing a 10 kcal/mol singlet-triplet energy gap favoring the tr iplet, This gap is reproduced at the 6-31G level using the 3-21G geom etries, However, this structure cannot reproduce the hyperfine splitti ng observed; 1 kcal/mol higher in energy is an asymmetrical conformati on of triplet CP2TMM which can account for the hyperfine interactions.