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

The solid state vibrational spectra of M(CO)(6), (M = Cr, Mo, W) in the region below 800 cm(-1) have been assigned by a combination of infrared, Raman and the first reported inelastic neutron scattering (INS) spectra from homoleptic metal carbonyls. This region comprises of the lattice modes, the OC-M-CO deformations, the M-CO bends and the M-C stretches. Three modes that are forbidden in both the infrared and Raman spectra of the parent O-h symmetry gas phase molecule occur in this region. The absence of selection rules for INS spectroscopy means that all three modes are clearly seen for the first time, all previous work has relied on overtone and combination modes. Periodic density functional theory calculations of the complete orthorhombic structure support the assignments.