AB-INITIO STUDY OF THE SPECTROSCOPY AND THERMOCHEMISTRY OF THE C2N AND CN2 MOLECULES

Several structures and electronic states of the C2N and CN2 molecules have been studied using complete active space SCF (CASSCF), multirefer ence configuration interaction (MRCI), and coupled cluster (CCSD(T)) m ethods. Both molecules are very stable. Our best computed total atomiz ation energies SIGMAD(e) are 288.6 +/- 2 kcal/mol for CN2, and 294.1 /- 2 kcal/mol for C2N. The CNC and CCN structures for C2N are nearly i soenergetic. CNN(3PI) lies about 30 kcal/mol above NCN(3PI(g)), but ha s a high barrier towards interconversion and is therefore observed exp erimentally. Computed harmonic frequencies for CNN are sensitive to th e 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) lev el.