Assignment of the solid state spectra of the group VI hexacarbonyls by inelastic neutron scattering spectroscopy

Structure and Spectroscopy of Iron Pentacarbonyl, Fe(CO)5

We have re-investigated the structure and vibrational spectroscopy of the iconic molecule iron pentacarbonyl, Fe(CO)₅, in the solid state by neutron scattering methods. In addition to the known C2/c structure, we find that Fe(CO)₅ undergoes a displacive ferroelastic phase transition at 105 K to a P1̅ structure. We propose that this is a result of certain intermolecular contacts becoming shorter than the sum of the van der Waals radii, resulting in an increased contribution of electrostatic repulsion to these interactions; this is manifested as a strain that breaks the symmetry of the crystal. Evaluation of the strain in a triclinic crystal required a description of the spontaneous strain in terms of a second-rank tensor, something that is feasible with high-precision powder diffraction data but practically very difficult using strain gauges on a single crystal of such low symmetry. The use of neutron vibrational spectroscopy (which is not subject to selection rules) has allowed the observation of all the fundamentals below 700 cm^–1 for the first time. This has resulted in the re-assignment of several of the modes. Surprisingly, density functional theory calculations that were carried out to support the spectral assignments provided a poor description of the spectra.

Computational and Spectroscopic Studies of Carbon Disulfide

The vibrational spectroscopy of CS₂ has been investigated many times in all three phases. However, there is still some ambiguity about the location of two of the modes in the solid state. The aim of this work was to locate all of the modes by inelastic neutron scattering (INS) spectroscopy, (which has no selection rules), and to use periodic density functional theory to provide a complete and unambiguous assignment of all the modes in the solid state. A comparison of the observed and calculated INS spectra shows generally good agreement. All four of the ν₂ bending mode components are calculated to fall within 14 cm⁻¹. Inspection of the spectrum shows that there are no bands close to the intense feature at 390 cm⁻¹ (assigned to ν₂); this very strongly indicates that the A_u mode is within the envelope of the 390 cm⁻¹ band. Based on a simulation of the band shape of the 390 cm⁻¹ feature, the most likely position of the optically forbidden component of the ν₂ bending mode is 393 ± 2 cm⁻¹. The calculations show that the optically inactive A_u translational mode is strongly dispersed, so it does not result in a single feature in the INS spectrum.