Are there two different geometric isomers of the O=C=N=C=O cation?

Re-examination of the O=C=N=C=O+ cation reported previously by Eberlin et a l. (J. Chem. Soc., Perkin Trans. 2 1997, 2347) yielded a single local minim um for the C-2v bent structure (1a). The D-infinityh linear structure (1b) reported previously as a local minimum was found to be a second-order stati onary point that, upon slight geometry perturbation, collapsed to a C-2v tr ansition state (1c) for degenerate inversion of the N-C-N angle in 1a. The ion enthalpy of formation was obtained from the enthalpies of three reactio ns as DeltaH(f,298)(1a) = 718 +/- 6 kJ mol(-1).