The valence isoelectronic molecules CCO, CNN, SiCO, and SiNN in their triplet ground states: Theoretical predictions of structures and infrared spectra

Abstract

We have completed theoretical studies on the 14‐valence–electron molecules CCO, CNN, SiCO, and SiNN in their ³Σ⁻ linear ground states. The theoretical methods that we employed were ab initio self‐consistent field (SCF), configuration interaction including single and double excitations from the dominant configuration (CISD), and complete active space (CASSCF). The extent of bonding of the C or Si atom with the CO or N₂ ligand is significantly different in each case. The strongest bond is formed between the C and N atoms in CNN and the weakest between the Si and N atoms in SiNN. Only for CNN does the resultant geometry vary significantly with the theoretical method. Our theoretical harmonic frequencies are compared to the experimental infrared spectra reported for these molecules in matrix isolation studies. The theoretical and experimental results are easily brought into harmony for CCO and SiCO, but not for CNN and SiNN.