A single crystal EPR study of ground state triplet trimethylenemethane

Abstract

Trimethylenemethane, a ground state triplet, has been generated in a single crystal of methylenecyclopropane by γ irradiation at 77 K and studied by the electron paramagnetic resonance technique in the temperature range 4.2–106 K. From the observed anisotropy of the EPR spectra, the zero-field splitting constants can be determined as D=0.0248 cm−1 and ‖E‖?0.0003 cm−1, and the principal values of the proton hyperfine coupling tensor as −14, −38, and −26 MHz with an isotropic coupling of −26 MHz at 77 K. Computer simulations of some of the spectra were performed using a computer program developed for this investigation. Both the position and the intensity of the spectral lines were reproduced with a good result. The simulations gave a positive sign for the zero-field splitting constant D. The zero-field splitting is temperature dependent above ca. 20 K, probably due to an oscillating motion of the molecular plane. The hyperfine structure changes above ca. 80 K and becomes isotropic at 100 K. This change is attributed to an anisotropic rotation about the threefold symmetry axis of the molecule.