The geometry, vibrational frequencies, thermochemistry, quadrupole moments and electronic structure of C2Na2: Comparison with C2Li2, C2H2, C2F2 and C2Cl2

The electronic structure of Na2C2 is studied using ab initio electronic structure methods and is compared to the companion molecule Li2C2. Both the linear D∞h and planar structures are minima on the ground state potential surface with the planar D2h conformation being the lowest energy form, similar to Li2C2. At the CCSD(t) level the planar form is more stable that the linear by 11.2 kcal/mol as compared with 7.34 kcal/mol for Li2C2. Both molecules are significantly ionic. The vibrational frequencies, atomization energy at 0 K, D0, and the standard enthalpy of formation, ΔH0f are calculated and compared to those of Li2C2 as well as HCCH, FCCF and ClCCCl. We find D0 and ΛH0f to be 331.1 and 84.92 kcal/mol for Li2C2 and 298.3 and 93.25 kcal/mol for Na2C2. We calibrate these by calculating the same quantities for HCCH, FCCF and ClCCCl.