A theoretical study of the geometrical structure and energy inversion barriers of the cyanopnictogens NC-XY₂ (X = N or P; Y = H, F, Cl or Br)

The structural properties and inversion barriers of the pnictogens (NC-XY₂; X = N or P, Y = H, F, Cl or Br) were obtained through ab initio SCF and CI techniques using several basis sets. The geometries of the ground- and transition-state structures were fully optimized at the SCF level, and CISD calculations were performed on the SCF optimum geometries. All molecules have a pyramidal ground state of symmetry ¹A'(C_s), and have a bent molecular spine, the angle NCX deviating from linearity by about 5°. For the phosphorous halocyanamides (NC-PX₂) two transition structures were detected, the traditional trigonal planar structure and a T-shaped one, both of C_2v symmetry. An attempt was made to rationalize our findings by employing the state characteristics of the CN and XY₂ fragments.