An ESR study of trimethylenemethane radical cation

The electronic structure of trimethylenemethane (TMM) radical cation has been investigated by ESR spectroscopy and ab initio MO calculations. The 4.2 K ESR spectrum of TMM+ consists of a septet with isotropic 1H hf splittings of 0.93 mT (6 H) in a CF2ClCF2Cl matrix. The ESR lineshape did not change appreciably between 4 and 125 K. Ab initio MO calculations predict that TMM+ is a 2A2 state in a distorted C2v structure, in which the unpaired electron mainly resides in the pz orbitals of the two equivalent terminal carbons. The spectra were explained by intramolecular dynamics between the three energetically equivalent C2v structures to average the structural distortion giving an apparent D3h structure even at a low temperature of 4.2 K.