ESR and optical spectra of C2− in various ion pairs trapped at 4°K in solid argon

C 2 − was prepared by photolysis of acetylene with and without the presence of Li, Na, K, or Cs atoms and trapped in argon at 4°K. With no alkali metal present, the optical bands at 5208 Å corresponding to the C2− gas-phase absorption at 5416 Å were obtained, but no ESR spectrum was observed. The countercation is not known in this case. In situ photolysis of metal-acetylene-argon matrices yielded new optical progressions at 4771 Å (Li), 4905 Å (Na), 5011 Å (K), and 5002 Å (Cs). ESR spectra were also observed exhibiting small (∼4 G) metal hfs and orthorhombic g tensors. 13C substitution confirmed that equivalent carbon atoms were present and that the ion pairs had C2ν symmetry. For Na+C2−, g1 = 2.0015(6), g2 = 2.0036(6), and g3 = 2.0124(30), where axis 2 is perpendicular to the plane of the ion pair and axis 3 is in the direction of the C–C bond. The g shifts resulting from distortion of the Σg+2 ground state of C2− correlate well with those expected for a 2A1 ground state for the ion pairs.