A b i n i t i o study of cyanogen: The X̃ 1Σ+g, ã 3Σ+u, B̃ 1Δu, and C̃ 1Πu states

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Abstract
An ab initio multiconfiguration Hartree–Fock (MCHF) study of the X̃ 1Σ+g, ã 3Σ+u, B̃ 1Δu, and C̃ 1Πu states of cyanogen (C2N2) is presented. Equilibrium structures, harmonic vibrational frequencies, excitation energies, and dissociation energies have been calculated and found to be in semiquantitative agreement with available experimental data. This study confirms the π → π* character of C2N2 (ã 3Σ+u) and C2N2 (B̃ 1Δu), and the n → π* character of C2N2 (C̃ 1Πu) near their equilibrium structures. Configuration mixing among the n and π electrons is needed to correct the broken symmetry description given by the Hartree–Fock (HF) method, and leads to a qualitatively correct representation of these states. For C2N2 (ã 3Σ+u) and C2N2 (B̃ 1Δu), the harmonic force constants show a strong through‐bond interaction which is due to the dominance of the charge transfer configuration +NCCN−. For C2N2 (C̃ 1Πu) the trans‐bending vibrational mode and the cis‐bending mode lead to different Renner–Teller splittings: the lower 1A’ state most likely has a slightly trans‐bent minimum energy structure. The calculated harmonic stretching frequencies of C2N2 (C̃ 1Πu) are ν1 (sym. CN str.)=2145 cm−1, ν2 (CC str.)=989 cm−1, and ν3 (asym. CN str.)=1789 cm−1.