An investigation of adsorbed species and related model compounds by inelastic neutron scattering spectroscopy

Abstract

The theory of Inelastic Neutron Scattering (INS) Spectroscopy is outlined, and the spectrometers employed in the study of low frequency (< 1500cm(^-1)) vibrations of hydrogen-containing molecules described. Square planar platinum-ethylene complexes of the general form trans-PtCl(_2)(C(_2)H(_4)).L and cis-PtCl(_2)C(_2)H(_4)).L (where L is a Ligand) have been investigated by INS spectroscopy, and assignments made of their spectra below 850cm(^-1). In all cases, a frequency was found representing the torsion of the ethylene group about an axis projected from the platinum atom through the centre of the C=C band, and the barrier to this rotation calculated. INS active modes of Co(_2)(CO)(_6).C(_2)H(_2) (O-1450cm(_1)) have been tentatively, identified, and a value for the barrier to the rotation of the acetylene molecule about an axis running through the mid-points of the Co-Co and C=C bonds derived. INS spectra of C(_2)H(_4) and C(_2)D(_4), adsorbed at two overpressures onto silver exchanged type A (Ag(_12)A) zeolite have been measured. On this basis of deuteration shifts and relative intensities, all three torsional modesand two of the three translational modes expected for the ethylene moleculerelative to a silver ion have been assigned. In addition, several weak spectroscopic features appeared to indicate the presence of a secondless populated adsorption site. C(_2)H(_2) and C(_2)D(_2) adsorbed onto Ag(_12)A zeolite at one overpressure have been studied by INS spectroscopy. All six predicted acetylene-surface modes were observed, the presence of the torsional vibration of the acetylene molecule about an axis running parallel to the C=C bond indicating the adsorbed molecule to be non-linear. Finally , INS spectra of the complexes Cr(C(_6)H(_6))(_2), Cr(C(_6)H(_6)(_2)I, C(_6)H)(_6)Cr(CO)(_3) and C(_6)H(_6)Mn(CO)(_3) Br have been recorded in the region 0-900(^1). Assignments have been attempted and, wherever possible, the barrier to the rotation of the benzene ligand (s) about the six - fold axis calculated.