Isotopomeric polymorphism in a "doubly-polymorphic" multi-component molecular crystal

Isotopomeric polymorphism is observed in complexes of isonicotinamide with oxalic acid, highly unusual here in that each isotopic complex is itself polymorphic, a situation of “double polymorphism”. The four polymorphic forms exhibit different degrees of hydron transfer.</p

Joint Experimental and Computational 17O and 1H Solid State NMR Study of Ba2In2O4(OH)2 Structure and Dynamics.

A structural characterization of the hydrated form of the brownmillerite-type phase Ba2In2O5, Ba2In2O4(OH)2, is reported using experimental multinuclear NMR spectroscopy and density functional theory (DFT) energy and GIPAW NMR calculations. When the oxygen ions from H2O fill the inherent O vacancies of the brownmillerite structure, one of the water protons remains in the same layer (O3) while the second proton is located in the neighboring layer (O2) in sites with partial occupancies, as previously demonstrated by Jayaraman et al. (Solid State Ionics2004, 170, 25-32) using X-ray and neutron studies. Calculations of possible proton arrangements within the partially occupied layer of Ba2In2O4(OH)2 yield a set of low energy structures; GIPAW NMR calculations on these configurations yield 1H and 17O chemical shifts and peak intensity ratios, which are then used to help assign the experimental MAS NMR spectra. Three distinct 1H resonances in a 2:1:1 ratio are obtained experimentally, the most intense resonance being assigned to the proton in the O3 layer. The two weaker signals are due to O2 layer protons, one set hydrogen bonding to the O3 layer and the other hydrogen bonding alternately toward the O3 and O1 layers. 1H magnetization exchange experiments reveal that all three resonances originate from protons in the same crystallographic phase, the protons exchanging with each other above approximately 150 °C. Three distinct types of oxygen atoms are evident from the DFT GIPAW calculations bare oxygens (O), oxygens directly bonded to a proton (H-donor O), and oxygen ions that are hydrogen bonded to a proton (H-acceptor O). The 17O calculated shifts and quadrupolar parameters are used to assign the experimental spectra, the assignments being confirmed by 1H-17O double resonance experiments.This work was supported in part by Grants DMR050612 and CHE0714183 from the National Science Foundation and Grant DESC0001284 from the Department of Energy (supporting Y.- L.L. and D.M.), by an Advanced Fellowship from the EU-ERC (C.P.G.), and by the EPSRC (D.S.M.). F.B. thanks the EU Marie Curie actions FP7 for an International Incoming fellowship (Grant No. 275212) and Clare Hall, University of Cambridge, for a Research Fellowship.This is the final version of the article. It first appeared from ACS Publications via http://dx.doi.org/10.1021/acs.chemmater.5b0032

Joint Experimental and Computational O-17 and H-1 Solid State NMR Study of Ba2In2O4(OH)(2) Structure and Dynamics

This is the final version of the article. It first appeared from ACS Publications via http://dx.doi.org/10.1021/acs.chemmater.5b00328A structural characterization of the hydrated form of the brownmillerite-type phase Ba2In2O5, Ba2In2O4(OH)2, is reported using experimental multinuclear NMR spectroscopy and density functional theory (DFT) energy and GIPAW NMR calculations. When the oxygen ions from H2O fill the inherent O vacancies of the brownmillerite structure, one of the water protons remains in the same layer (O3) while the second proton is located in the neighboring layer (O2) in sites with partial occupancies, as previously demonstrated by Jayaraman et al. ( Solid State Ionics 2004, 170, 25?32) using X-ray and neutron studies. Calculations of possible proton arrangements within the partially occupied layer of Ba2In2O4(OH)2 yield a set of low energy structures; GIPAW NMR calculations on these configurations yield 1H and 17O chemical shifts and peak intensity ratios, which are then used to help assign the experimental MAS NMR spectra. Three distinct 1H resonances in a 2:1:1 ratio are obtained experimentally, the most intense resonance being assigned to the proton in the O3 layer. The two weaker signals are due to O2 layer protons, one set hydrogen bonding to the O3 layer and the other hydrogen bonding alternately toward the O3 and O1 layers. 1H magnetization exchange experiments reveal that all three resonances originate from protons in the same crystallographic phase, the protons exchanging with each other above approximately 150 ?C. Three distinct types of oxygen atoms are evident from the DFT GIPAW calculations bare oxygens (O), oxygens directly bonded to a proton (H-donor O), and oxygen ions that are hydrogen bonded to a proton (H-acceptor O). The 17O calculated shifts and quadrupolar parameters are used to assign the experimental spectra, the assignments being confirmed by 1H?17O double resonance experiments.This work was supported in part by Grants DMR050612 and CHE0714183 from the National Science Foundation and Grant DESC0001284 from the Department of Energy (supporting Y.- L.L. and D.M.), by an Advanced Fellowship from the EU-ERC (C.P.G.), and by the EPSRC (D.S.M.). F.B. thanks the EU Marie Curie actions FP7 for an International Incoming fellowship (Grant No. 275212) and Clare Hall, University of Cambridge, for a Research Fellowship

Isotopomeric polymorphism in a "doubly-polymorphic" multi-component molecular crystal

The deuterated molecular complexes of isonicotinamide with oxalic acid crystallise in two polymorphs, which are found to be distinct from the two polymorphs of the hydrogenous complexes previously reported. This phenomenon is known as isotopomeric polymorphism, is rarely observed in molecular materials and in particular the presence of multiple polymorphic forms of each isotopic material observed here appears to be unprecedented. The four polymorphic forms are found to exhibit different degrees of hydron transfer. Unlike the hydrogenous forms, the deuterated polymorphs do not show short, strong hydrogen bonding between the acid and the pyridine base. Periodic electronic structure calculations establish an energy scale for the polymorphism in these isotopomeric polymorphs.</p

First principles calculation of a large variation in dielectric tensor through the spin crossover in the CsFe[Cr(CN)6] Prussian blue analogue

The dielectric response of spin-crossover (SCO) materials is a key property facilitating their use in next-generation information processing technologies. Solid state hybrid density functional theory calculations show that the temperature-induced and strongly hysteretic SCO transition in the Cs+Fe2+[Cr3+(CN−)6] Prussian blue analogue (PBA) is associated with a large change (Δ) in both the static, Δɛ0(HS − LS), and high frequency, Δɛ∞(HS − LS) dielectric constants. The SCO-induced variation in CsFe[Cr(CN)6] is significantly greater than the experimental Δɛ values observed previously in other SCO materials. The phonon contribution, Δɛphon(HS − LS), determined within a lattice dynamics approach, dominates over the clamped nuclei term, Δɛ∞(HS − LS), and is in turn dominated by the low-frequency translational motions of Cs+ cations within the cubic voids of the Fe[Cr(CN)6]− framework. The Cs+ translational modes couple strongly to the large unit cell volume change occurring through the SCO transition. PBAs and associated metal-organic frameworks emerge as a potentially fruitful class of materials in which to search for SCO transitions associated with large changes in dielectric response and other macroscopic properties

Ferromagnetism and spin transitions in prussian blue: a solid-state hybrid functional study

The exchange interactions underlying the weak ferromagnetism of the prototypical mixed valence compound prussian blue (represented by K+Fe3+ [Fe2+ (CN)6]) and a Cr3+ substituted analog are investigated within a series of solid-state hybrid density functional calculations. Weights of Hartree–Fock (HF) exchange in the range from 30% to 100% are used. The magnetic order in these compounds is shown to be dominated by the coupling of nearest-neighbor high spin (HS) Fe3+-Fe3+ and Cr3+-Cr3+pairs, and not by the delocalization of spin along the ⋯NCFe2+CN⋯pathways as previously proposed. The functional containing 35% HF exchange yields an estimated critical temperature for spin ordering and a magnitude for weak Fe2+ spin polarization in good agreement with experimental data. The energies of the Fe2+ low spin (LS)→HS (t62g→t42ge2g) and Fe3+ HS→LS (t32ge2g→t52g) excitations are determined and compared with the results obtained previously in a range of Fe2+ coordination compounds. It is concluded that the accurate description of these transitions requires the use of a weight of HF exchange below the stable limit of 35% attained in the current study. However, the trends in the present results indicate that the Fe3+ HS→LS excitation is lower in energy than the Fe2+ LS→HS, and also that there is no reasonable prospect of a temperature-induced spin crossover in either compound