A solid-state density functional theory investigation of the effect of metal substitution (Metal = Mn, Cd, Co) on the terahertz spectra of isomorphous molecular metal 5-(4-pyridyl)tetrazolato complexes

The crystal structure and experimental terahertz spectroscopy of an isomorphous series [Mn(C6H4N5)2(H2O)4]⋅2H2O (Mn-4PT), [Co(C6H4N5)2(H2O)4]⋅2H2O (Co-4PT), and [Cd(C6H4N5)2(H2O)4]⋅2H2O (Cd-4PT) were compared using solid-state density functional theory (DFT) simulations. The effect of the central metal atom was investigated to determine the influence on the low energy lattice and molecular vibrations exhibited in the region from 10 to 100 cm−1, known as the terahertz (THz) region. Using solid-state DFT the normal modes of these THz vibrations were determined and it was shown that the mass and size of the metal center has a large effect in this region. Each complex exhibited common vibrational modes involving whole ligand motion around the central metal atom. These vibrations were found to shift to lower frequencies with a drastic mass increase; however, this trend is reversed with the smaller mass change between the manganese and cobalt due to the stronger cobalt–nitrogen bond compared to the manganese–nitrogen bond.