ENDOR and EPR studies of benzene radical cations in halocarbon matrices: the static Jahn–Teller distortion of the monomer and geometry of the dimer cation

ENDOR investigations of the benzene radical cation in a CFCl3 matrix at 30 K gave evidence for a static Jahn–Teller distortion with the unpaired electron density predominantly in the 2b2g orbital. In this distortion the equivalence of the six protons is lost, with the major spin densities on H1 and H4. From ENDOR measurements the isotropic and dipolar couplings were accurately measured (aiso=−25.31 MHz (H1 and H4) and aiso=−5.43 MHz (H2, H3, H5 and H6)) which agree with those reported earlier from EPR measurements and theoretical calculations. (C6H6)2+., formed by warming in CFCl3 at 140 K and initially present in CF3CCl3, was characterised with ENDOR and assigned to a π-electron complex. The intermolecular distance was estimated from the measured dipolar hyperfine coupling to be in the range 2–3 Å.