On the spectrum and structure of the isolated O4- anion in solid argon

Laser ablation of metal targets in front of condensing Ar/O-2 mixtures produced the strong antisymmetric fundamental of isolated O-4(-) at 953.5+/-0.3 cm(-1) in solid argon from the capture of ablated electrons by O-2 followed by the addition of another O-2 molecule. The sharp sextet mixed isotopic spectrum suggests a structure with two equivalent O-2 subunits and equivalent atomic positions in each subunit. DFT/UB3LYP calculations reveal doublet trans and rectangular structures with the latter more stable by 8.8 kcal/mol. Of more importance, the mixed oxygen isotopic spectrum calculated for the rectangular structure matches the observed spectrum much better than values calculated for the trans structure; thus, it is concluded that isolated O-4(-) probably has an approximately rectangular structure. It is further suggested that the earlier observed alkali metal M+O4- species have rectangular pyramidal structures and that the sharp 954+/-1 cm(-1) bands attributed to a different structural isomer are in fact due to the isolated anion owing to photoionization of the alkali metal atoms by radiation from the Nernst glower source of the grating spectrophotometer. (C) 1998 American Institute of Physics. [S0021-9606(98)02315-0].