Ab initio studies on the structures, vertical electron detachment energies, and fragmentation energies of C_nN^- clusters

In a recently published report on the time-of-flight mass spectrometry studies and ab initio RHF/3-2IG calculations of the C_nN^- clusters (n=1 to 13), Wang, Huang, Liu, and Zhang claimed that all the C_nN^- clusters (n=1 to 13) should be linear chains. In the present work, we report the new results obtained from a series of ab initio calculations on the ground states of the C_nN^- cluster anions. The results calculated at different approximation levels reveal that the geometries of the C_nN^- determined at the levels without using the polarization functions is qualitatively incorrect, and that the diffuse functions of the two atoms at the two end points of the C_nN^- chains and the higher-order electron correlation are important in evaluating the vertical electron detachment energies (VDEs) of the C_nN^- anions. It is concluded from the calculated results that for the singlet ground states, only the linear structures of CN^-, C₂N^-, C₃N^-, and C₅N^- are stable, and the linear structures of the other C_nN^- (n=4 and 6 to 13) are not at the local minima on the potential energy surfaces. The VDEs and fragmentation energies of C_nN^- (n= 1 to 7) in their ground states are evaluated with the MP4SDTQ method at the MP2 geometry using 6-31G(d) except for two end atoms of each chain, to which a set of diffuse sp functions are augmented. The fragmentation energies are also evaluated with the QCISD(T) method using the same basis to verify the convergence of the perturbational results. The calculated VDEs and fragmentation energies all show that the C_nN^- clusters with odd n are more stable than those with even n, which is consistent with the observed even-odd alternation of the TOP signal intensities.