Several structures and electronic states of the C2N and CN2 molecules have been studied using complete active space SCF (CASSCF), multireference configuration interaction (MRCI ), and coupled cluster (CCSD( T) ) methods. Both molecules are very stable. Our best computed total atomization energies 10, are 288.6f 2 kcal/mol for CN2, and 294.1& 2 kcal/mol for GIN. The CNC and CCN structures for C2N are nearly isoenergetic. CNN( 311) lies about 30 kcal/mol above NCN( 'I&), but has a high barrier towards interconversion and is therefore observed experimentally. Computed harmonic frequencies for CNN are sensitive to the correlation treatment: they are reproduced well using multireference methods as well as the CCSD (T ) method. High spin contamination has a detrimental effect on computed harmonic frequencies at the CCSD( T) level. 1. Introduetlon though the microwave spectrum could not be observed directly [ 81 due to the low computed dipole Considerable interest has been aroused recently by moment of CCN [ 9,lO 1. The infrared fundamentals thin solid films of boron carbide and of ternary {B, in argon matrix CNN were reported in 1966 by C, N} systems [ 11. Their precursors are the general Milligan and Jacox [ 111 as 393, 1241, and 2847 clusters B,C&N,,, which form the subject of a system-cm-'. The improbably high final frequency was atic theoretical investigation by our group. Through questioned in several theoretical papers [ 12-14 ] and an interaction between theory and experiment, the finally shown to be an overtone of a 14 19 cm-' fun-BC2 molecule has recently been characterized [ 21, damental in the Fourier transform laser induced fluand high-level ab initio calculations have been perorescence study of Wurfel et al. [ 15 1. This latter paformed on the BIG and BCN clusters. Among the per was followed by a density functional study [ 16 J, triatomic {B, C, N} species, this leaves only the C2N where it was shown that the dipole moment has a and CN2 molecules, which have been the subject of shallow first and a steep second derivative, which acseveral experimental studies. CCN and CNC were counts for the nonobservance of the fundamental and first observed by Merer and Travis in 1965 [ 31 and the observance of the overtone (see also Ref. [ 17 ] ) . 1966 [ 41, respectively. High-resolution gas-phase in-In contrast, relatively little data are available on NCN. frared work has since been performed on CCN (Ref. Its matrix infrared spectrum was first published in [ 51 and references therein); additionally, the rota-1965 by Milligan et al. [ 181, who found the two intional fine structure has been resolved [ 6,7], alfrared-active fundamentals at 423 and 1475 cm-'; a