Multinuclear Magnetic Resonance Investigation of Crystalline Alkali Molybdates.

A variety of crystalline alkali molybdate phases are characterized by (23)Na, (133)Cs, and (95)Mo magic-angle-spinning nuclear magnetic resonance (MAS NMR) to provide spectroscopic handles for studies of devitrification products in borosilicate nuclear waste glasses. The NMR parameters obtained from line-shape simulations are plotted as a function of various structural parameters to discern trends that may prove useful in the determination of unknown phases. These are applied to Cs3Na(MoO4)2, the most common precipitate found in cesium- and molybdenum-bearing model nuclear waste glasses, the crystal structure of which has not yet been determined, to provide structural constraints that may guide the refinement of powder X-ray diffraction data.

Local and average structure in zinc cyanide: toward an understanding of the atomistic origin of negative thermal expansion.

Neutron diffraction at 11.4 and 295 K and solid-state (67)Zn NMR are used to determine both the local and the average structures in the disordered, negative thermal expansion (NTE) material, Zn(CN)2. Solid-state NMR not only confirms that there is head-to-tail disorder of the C≡N groups present in the solid, but yields information about the relative abundances of the different Zn(CN)4­n(NC)n tetrahedral species, which do not follow a simple binomial distribution. The Zn(CN)4 and Zn(NC)4 species occur with much lower probabilities than are predicted by binomial theory, supporting the conclusion that they are of higher energy than the other local arrangements. The lowest energy arrangement is Zn(CN)2(NC)2. The use of total neutron diffraction at 11.4 K, with analysis of both the Bragg diffraction and the derived total correlation function, yields the first experimental determination of the individual Zn­N and Zn­C bond lengths as 1.969(2) and 2.030(2) Å, respectively. The very small difference in bond lengths, of ~0.06 Å, means that it is impossible to obtain these bond lengths using Bragg diffraction in isolation. Total neutron diffraction also provides information on both the average and the local atomic displacements responsible for NTE in Zn(CN)2. The principal motions giving rise to NTE are shown to be those in which the carbon and nitrogen atoms within individual Zn­C≡N­Zn linkages are displaced to the same side of the Zn···Zn axis. Displacements of the carbon and nitrogen atoms to opposite sides of the Zn···Zn axis, suggested previously in X-ray studies as being responsible for NTE behavior, in fact make negligible contributions at temperatures up to 295 K.

Characterisation of heterogeneous molybdate and chromate phase assemblages in model nuclear waste glasses by multinuclear magnetic resonance spectroscopy.

A series of sodium borosilicate glasses containing cesium, molybdenum, and chromium was prepared to investigate the partitioning of chromium amongst the glass and phase-separated crystalline molybdates. The precipitates were examined by (133)Cs, (23)Na, and (95)Mo MAS NMR, revealing a phase assemblage consisting of Na(2)MoO(4), Na(2)MoO(4)·2H(2)O, Cs(2)MoO(4), Cs(2)CrO(4), CsNaMoO(4)·2H(2)O, and Cs(3)Na(MoO(4))(2). (133)Cs MAS NMR indicates random substitution of Cr into the Mo sites of Cs(3)Na(MoO(4))(2) and provides a quantitative assessment of Cr incorporation. The sample compositions were verified by various analytical techniques and highlight the centrality of NMR in the identification and quantification of heterogeneous crystalline composites, including sensitivity to cationic substitution. The observation and facile interconversion of hydrated phases invites careful consideration of these materials for nuclear waste disposal.

Characterising lone-pair activity of lead(II) by 207Pb solid-state NMR spectroscopy: coordination polymers of [N(CN)2]- and [Au(CN)2]- with terpyridine ancillary ligands.

A series of lead(II) coordination polymers containing [N(CN)(2)](-) (DCA) or [Au(CN)(2)](-) bridging ligands and substituted terpyridine (terpy) ancillary ligands ([Pb(DCA)(2)] (1), [Pb(terpy)(DCA)(2)] (2), [Pb(terpy){Au(CN)(2)}(2)] (3), [Pb(4'-chloro-terpy){Au(CN)(2)}(2)] (4) and [Pb(4'-bromo-terpy)(µ-OH(2))(0.5){Au(CN)(2)}(2)] (5)) was spectroscopically examined by solid-state (207)Pb MAS NMR spectroscopy in order to characterise the structural and electronic changes associated with lead(II) lone-pair activity. Two new compounds, 2 and [Pb(4'-hydroxy-terpy){Au(CN)(2)}(2)] (6), were prepared and structurally characterised. The series displays contrasting coordination environments, bridging ligands with differing basicities and structural and electronic effects that occur with various substitutions on the terpyridine ligand (for the [Au(CN)(2)](-) polymers). (207)Pb NMR spectra show an increase in both isotropic chemical shift and span (Ω) with increasing ligand basicity (from δ(iso) = -3090 ppm and Ω = 389 ppm for 1 (the least basic) to δ(iso) = -1553 ppm and Ω = 2238 ppm for 3 (the most basic)). The trends observed in (207)Pb NMR data correlate with the coordination sphere anisotropy through comparison and quantification of the Pb-N bond lengths about the lead centre. Density functional theory calculations confirm that the more basic ligands result in greater p-orbital character and show a strong correlation to the (207)Pb NMR chemical shift parameters. Preliminary trends suggest that (207)Pb NMR chemical shift anisotropy relates to the measured birefringence, given the established correlations with structure and lone-pair activity.