Ab initio exploration of low-lying electronic states of linear and bent MNX+ (M = Ca, Sr, Ba, Ra; X = O, S, Se, Te, Po) and their origins

High-level multireference and coupled cluster quantum calculations were employed to analyze low-lying electronic states of linear-MNX⁺ and side-bonded-M[NX]⁺ (M = Ca, Sr, Ba, Ra; X = O, S, Se, Te, Po) species. Their full potential energy curves (PECs), dissociation energies (D_es), geometric parameters, excitation energies (T_es), and harmonic vibrational frequencies (ω_es) are reported. The first three chemically bound electronic states of MNX⁺ and M[NX]⁺ are ³∑⁻, ¹Δ, ¹∑⁺ and ³A″, ¹A′, ¹A″, respectively. The ³∑⁻, ¹Δ, ¹∑⁺ of MNX⁺ originate from the M⁺(²D) + NX(²Π) fragments, whereas the ³A″, ¹A′, ¹A″ states of M[NX]⁺ dissociate to M⁺(²S) + NX(²Π) as a result of avoided crossings. The MNX⁺ and M[NX]⁺ are real minima on the potential energy surface and their interconversions are possible. The M²⁺NX⁻/M²⁺[NX]⁻ ionic structure is an accurate representation for their low-lying electronic states. The D_es of MNX⁺ species were found to depend on the dipole moment (μ) of the corresponding NX ligands and a linear relationship between these two parameters was observed.