Electronic Structures and Potential Energy Curves for the Low-Lying States of the CN Radical

At 11 internuclear separations ab initio calculations have been carried out on those 59 molecular states of CN which dissociate to atomic limits up to ${}^1{\mathrm{DC\hspace{0.17em}+\hspace{0.17em}}}^2\mathrm{DN}$ . Four electrons are held frozen in carbon and nitrogen $\text{1s}$ Hartree–Fock atomic orbitals, and a full configuration interaction is carried out for the remaining nine electrons using optimized $\text{2s}$ and $\text{2p}$ Slater‐type orbitals on each atom. The ${}^2\Pi$ calculations, which include 486 configurations, are the most complicated. Eighteen significantly bound states ${\mathrm{(D}}_e\text{\hspace{0.17em}≥\hspace{0.17em}0.84}\mathrm{eV})$ were obtained, nine of which have been observed spectroscopically. With the exception of the third ${}^2\Pi$ state, the theoretical ordering of states agrees with experiment. Three of the states never observed experimentally (${}^4{\Sigma }^{+}{,}^4\Pi$ and ${}^4\Delta$ ) lie below all but three of the known states of CN. Calculated spectroscopic constants are compared with experiment. The potential curves show many interesting features, including potential maxima in the ${}^4{\Sigma }^{-}{,}^2{\Sigma }^{-}{\mathrm{,\; J}}^2\Delta$ , and ${}^2\Pi$ IV bound states. By performing natural orbital analyses, dominant molecular orbital configurations have been unambiguously assigned to the lowest 18 bound states. The second, third, and fourth ${}^2\Pi$ states experience numerous avoided crossings among themselves, and natural orbitals have been used to follow the changes in electron configuration as a function of internuclear separation. The question of the approximate validity of the quantum numbers $g$ and $u$ is discussed.