π-Electronic Structure of the HCN Molecule

The HCN molecule is treated as a system of four π electrons moving in a suitable potential field. Its lower excited states are calculated, using both the molecular orbital method with configuration interaction and the Heitler-London method including ionic-homopolar resonance. Slater atomic orbitals with effective charges 2δ=3.18 for the C atom and 3.85 for the N atom are employed. The calculated energy levels with the 1Σ+ ground state as zero are 3.53 ev (3Σ+), 5.60 ev (1Σ—), 7.71 ev (3Δ), etc. The effect of configuration interaction is remarkable, amounting to several electron volts, and ionic terms have almost the same weight as homopolar terms.