A theoretical study of FeCN in the 6Δ electronic ground state

The three-dimensional potential energy and dipole moment surfaces for the electronic ground state 6Δ of FeCN have been computed at the MR-SDCI + Q + Erel/[Roos ANO (Fe), aug-cc-pVQZ (C, N)] level of theory, where MR-SDCI means 'multi-reference single and double excitation configuration interaction' and ANO means 'atomic natural orbital'. Based on these potential energy and dipole moment surfaces, the spectroscopic parameters, rovibronic energies, structural parameters, vibrational transition moments, and the wavenumbers and intensities of selected rotation-vibration transitions have been calculated. The equilibrium structure is linear with re(Fe-C) = 2.048 Å and re(C-N) = 1.168 Å, and the zero-point averaged structure is bent with 〈r(Fe-C)〉0 = 2.082 Å, 〈r(C-N)〉0 = 1.172 Å, and 〈∠(Fe-C-N)〉0 = 170(5)°. At all the MR-SDCI + Q and the size-extensive multi-reference averaged quadratic coupled-cluster (MR-AQCC) levels of theory, with and without relativistic correction Erel, that were employed in the present work, 6Δ FeCN is predicted to be slightly more stable than 6Δ FeNC. For example, the energy difference between the two isomers is approximately 150 cm−1 at the highest level of theory employed, MR-AQCC + Erel/[Roos ANO (Fe), aug-cc-pVQZ (C, N)] with zero-point energy correction. The electronic structure of 6Δ FeCN has also been compared with that of 6Δ FeNC. At present, no experimental spectroscopic data are available for 6Δ FeCN. It is hoped that the present work will stimulate experimental investigations of this molecule.