An ESR study on trimethylenemethane radical cation

The electronic structure of trimethylenemethane (TMM) radical cation was studied by ESR spectroscopy and ab-initio MO calculations. The 4.2 K ESR spectrum of TMM+ consists of a septet with isotropic 1H hf splitting of 0.93 mT (6 H) in a halocarbon matrix. No appreciable ESR line-shape change was observed between 4 K and 125 K. The ab-initio MO calculations resulted in the TMM+ with a 2A2 state in distorted C2ν structure, in which the unpaired electron mainly resides in the pz orbitals of the two equivalent terminal carbons. The spectra were explained by an intramolecular dynamics among three energetically equivalent C2ν structures to average the structural distortion out giving an apparent D3h structure even at a low temperature of 4.2 K. (author)