Probing the SO_2 Adsorption Mechanism in Hofmann Clathrates Via Inelastic Neutron Scattering and Density Functional Theory Calculations

The adsorption mechanism of SO2 in the Hofmann-like coordination polymer Fe(pz)[Pt(CN)4] is studied using inelastic neutron scattering and density functional theory calculations. We find that the most important spectral change upon gas adsorption is the blueshift of the low energy peak found at 100 cm-1, a feature that is fully confirmed by the computed neutron-weighted phonon density of states. Our calculations suggest that the origin of this change is twofold: i) an increase in the force constant of the cyanide out-of-plane movement due to the binding of the gas onto the Pt(CN)4 plane, and ii) the hampered rotation of the pyrazine due to steric hindrance. The high energy region of the neutron scattering data whose spectral weight is dominated by the internal vibrations of the pyrazine is negligibly affected by the presence of the gas as expected from a physisorption type of binding.