Rotation-vibration energy levels of the isomers CCN+ and CNC+ have been determined in the following way: A large number of points in the minimum region of the 1Σ+ electronic ground state surfaces of the two ions have been computed by single reference state complete single and double excitations configuration interaction (CI-SD) calculations using canonical SCF orbitals as a molecular orbital basis. The effect of certain higher excitations has also been taken into account by using the Davidson estimate (CI-SDQ). A force constant expansion has been fitted by least squares to the ab initio points, and the resulting potential expression has been used to calculate rotational and vibrational constants using the perturbation theory expressions. In order to assess the reliability of these calculations and, in particular, the flexibility of the Gaussian basis sets used, identical calculations have been performed for the isoelectronic C3 radical. For C3 the rotational and vibrational energy levels obtained by the perturbational approach are compared to those calculated using the nonrigid bender Hamiltonian. The nonrigid bender results at the CI-SDQ level are found to be in good agreement with the available experimental data. From these CI-SDQ calculations the equilibrium structure of the ground state of C3 is obtained as bent (αe = 162°, re = 1.290 A ̊). Dipole moment functions and vibrational transition moments have also been calculated for all three molecules. The transition moment for the ν3 fundamental of C3 is found to be very large (0.44 Debye). © 1984.