Conformation of a Geminal Dicyclopropyl-Substituted Trimethylenemethane Triplet [2-(Dicyclopropylmethylidene)cyclopentane-1,3-diyl

Abstract

Triplet 2-(dicyclopropylmethylidene)cyclopentane-1,3-diyl, CP2TMM, has a Deltam(s) = 2 ESR transition revealing hyperfine splitting consistent with coupling to two equivalent alpha hydrogens and five equivalent beta hydrogens and one other hydrogen with a small hyperfine coupling constant. This is consistent only with a conformation in which one cyclopropane ring conjugates with the pi triplet so that the other cyclopropane is 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 singlet state was optimized using a GVB perfect pairing wave function. The most stable conformation of each spin state is a C(2)-symmetric structure providing a 10 kcal/mol singlet-triplet energy gap favoring the triplet. This gap is reproduced at the 6-31G level using the 3-21G geometries. However, this structure cannot reproduce the hyperfine splitting observed; 1 kcal/mol higher in energy is an asymmetrical conformation of triplet CP2TMM which can account for the hyperfine interactions.