ORCID Profile
0000-0002-4538-6782
Current Organisation
University of St Andrews
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Publisher: American Chemical Society (ACS)
Date: 24-01-2019
DOI: 10.1021/JACS.8B11519
Publisher: American Chemical Society (ACS)
Date: 10-05-2006
DOI: 10.1021/JP060844Q
Abstract: The yttrium local environment in the series of pyrochlores Y2Ti2-xSnxO7 was studied using 89Y NMR. Oxides with the pyrochlore structure exhibit a range of interesting physical and chemical properties, resulting in many technological applications, including the encapsulation of lanthanide- and actinide-bearing radioactive waste. The use of the nonradioactive Y3+ cation provides a sensitive probe for any changes in the local structure and ordering with solid solution composition, through 89Y (I = 1/2) NMR. We confirm that a single pyrochlore phase is formed over the entire compositional range, with Y found only on the eight-coordinated A site. A significant (approximately 15 ppm) chemical shift is observed for each Sn substituted into the Y second neighbor coordination environment. The spectral signal intensities of the possible combinations of Sn/Ti neighbors match those predicted statistically assuming a random distribution of Sn4+/Ti4+ on the six-coordinated pyrochlore B site.
Publisher: Royal Society of Chemistry (RSC)
Date: 2015
DOI: 10.1039/C4CP05827E
Abstract: The use of solid-state NMR spectroscopy to provide new insight into the composition and atomic-scale disorder in pyrochlore-based ceramics produces a very different picture to that obtained using other techniques.
Publisher: American Chemical Society (ACS)
Date: 10-2007
DOI: 10.1021/JA074428A
Abstract: The 29Si and 17O NMR parameters of six polymorphs of MgSiO3 were determined through a combination of high-resolution solid-state NMR and first-principles gauge including projector augmented wave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO3 is an important component of the Earth's mantle that undergoes structural changes as a function of pressure and temperature. For the lower pressure polymorphs (ortho-, clino-, and protoenstatite), all oxygen species in the 17O high-resolution triple-quantum magic angle spinning (MAS) NMR spectra were resolved and assigned. These assignments differ from those tentatively suggested in previous work on the basis of empirical experimental correlations. The higher pressure polymorphs of MgSiO3 (majorite, akimotoite, and perovskite) are stabilized at pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the major constituent at depths >660 km. We present the first 17O NMR data for these materials and confirm previous 29Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments allows us to resolve distinct oxygen species, and full assignments are suggested. The six polymorphs exhibit a wide variety of structure types, providing an ideal opportunity to consider the variation of NMR parameters (both shielding and quadrupolar) with local structure, including changes in coordination number, local geometry (bond distances and angles), and bonding. For ex le, we find that, although there is a general correlation of increasing 17O chemical shift with increasing Si-O bond length, the shift observed also depends upon the exact coordination environment.
Publisher: Wiley
Date: 07-06-2013
Publisher: American Chemical Society (ACS)
Date: 19-10-2010
DOI: 10.1021/JA105347Q
Abstract: High-resolution (19)F magic angle spinning (MAS) NMR spectroscopy is used to study disorder and bonding in a crystalline solid. (19)F MAS NMR reveals four distinct F sites in a 50% fluorine-substituted deuterated hydrous magnesium silicate (clinohumite, 4Mg(2)SiO(4)·Mg(OD(1-x)F(x))(2) with x = 0.5), indicating extensive structural disorder. The four (19)F peaks can be assigned using density functional theory (DFT) calculations of NMR parameters for a number of structural models with a range of possible local F environments generated by F(-)/OH(-) substitution. These assignments are supported by two-dimensional (19)F double-quantum MAS NMR experiments that correlate F sites based on either spatial proximity (via dipolar couplings) or through-bond connectivity (via scalar, or J, couplings). The observation of (19)F-(19)F J couplings is unexpected as the fluorines coordinate Mg atoms and the Mg-F interaction is normally considered to be ionic in character (i.e., there is no formal F-Mg-F covalent bonding arrangement). However, DFT calculations predict significant (19)F-(19)F J couplings, and these are in good agreement with the splittings observed in a (19)F J-resolved MAS NMR experiment. The existence of these J couplings is discussed in relation to both the nature of bonding in the solid state and the occurrence of so-called "through-space" (19)F-(19)F J couplings in solution. Finally, we note that we have found similar structural disorder and spin-spin interactions in both synthetic and naturally occurring clinohumite s les.
Publisher: Royal Society of Chemistry (RSC)
Date: 2018
DOI: 10.1039/C8RA00596F
Abstract: Investigation of the conditions required for quantitative isotopic enrichment and the acquisition of quantitative 17 O NMR spectra of ceramic oxides.
Publisher: American Chemical Society (ACS)
Date: 24-09-2012
DOI: 10.1021/IC301627Y
Abstract: Twelve related monocation chalconium salts [{Nap(EPh)(E'Ph)Me}(+){CF(3)SO(3)}(-)] 2-4, [{Acenap(Br)(EPh)Me}{CF(3)SO(3)}(-)] 5-7, and [{Acenap(EPh)(E'Ph)Me}(+){CF(3)SO(3)}(-)] 8-13 have been prepared and structurally characterized. For their synthesis naphthalene compounds [Nap(EPh)(E'Ph)] (Nap = naphthalene-1,8-diyl E/E' = S, Se, Te) N2-N4 and associated acenaphthene derivatives [Acenap(X)(EPh)]/[Acenap(EPh)(E'Ph)] (Acenap = acenaphthene-5,6-diyl E/E' = S, Se, Te X = Br) A5-A13 were independently treated with a single molar equivalent of methyl trifluoromethanesulfonate (MeOTf). In addition, reaction of bis-tellurium compound A10 with 2 equiv of MeOTf afforded the doubly methylated dication salt [{Acenap(TePhMe)(2)}(2+){(CF(3)SO(3))(2)}(2-)}] 14. The distortion of the rigid naphthalene and acenaphthene backbone away from ideal was investigated in each case and correlated in general with the steric bulk of the interacting atoms located at the proximal peri positions. Naturally, introduction of the ethane linker in acenaphthene compounds increased the splay of the bay region compared with equivalent naphthalene derivatives resulting in greater peri distances. The conformation of the aromatic rings and subsequent location of p-type lone pairs has a significant impact on the geometry of the peri region, with anomalies in peri separations correlated to the ability of the frontier orbitals to take part in attractive or repulsive interactions. In all but one of the monocations a quasi-linear three-body C(Me)-E···Z (E = Te, Se, S Z = Br/E) fragment provides an attractive component for the E···Z interaction. Density functional studies confirmed these interactions and suggested the onset of formation of three-center, four-electron bonding under appropriate geometric conditions, becoming more prevalent as heavier congeners are introduced along the series. The increasingly large J values for Se-Se, Te-Se, and Te-Te coupling observed in the (77)Se and (125)Te NMR spectra for 1, 3, 4, 9, 10, and 13 give further evidence for the existence of a weakly attractive through-space interaction.
Publisher: Springer Science and Business Media LLC
Date: 25-01-2019
DOI: 10.1038/S41596-018-0114-6
Abstract: High-silica zeolites, some of the most important and widely used catalysts in industry, have potential for application across a wide range of traditional and emerging technologies. The many structural topologies of zeolites have a variety of potential uses, so a strong drive to create new zeolites exists. Here, we present a protocol, the assembly-disassembly-organization-reassembly (ADOR) process, for a relatively new method of preparing these important solids. It allows the synthesis of new high-silica zeolites (Si/Al >1,000), whose synthesis is considered infeasible with traditional (solvothermal) methods, offering new topologies that may find novel applications. We show how to identify the optimal conditions (e.g., duration of reaction, temperature, acidity) for ADOR, which is a complex process with different possible outcomes. Following the protocol will allow researchers to identify the different products that are possible from a reaction without recourse to repetitive and time-consuming trial and error. In developing the protocol, germanium-containing UTL zeolites were subjected to hydrolysis conditions using both water and hydrochloric acid as media, which provides an understanding of the effects of temperature and pH on the disassembly (D) and organization (O) steps of the process that define the potential products. S les were taken from the ongoing reaction periodically over a minimum of 8 h, and each s le was analyzed using powder X-ray diffraction to yield a time course for the reaction at each set of conditions selected s les were analyzed using transmission electron microscopy and solid-state NMR spectroscopy.
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/D0NJ02969F
Abstract: Hosting a BODIPY dye in a crystalline sponge bathochromically shifts luminescent emission.
Publisher: Royal Society of Chemistry (RSC)
Date: 2013
DOI: 10.1039/C3SC21892A
Publisher: Royal Society of Chemistry (RSC)
Date: 2023
DOI: 10.1039/D3SC00904A
Abstract: Application of pressure induces reactivity of pore-bound methanol with the secondary building unit of the metal–organic framework GUF-1(Sc), substituting coordinated bridging hydroxide ligands for bridging methoxides, in single crystals and in bulk.
Publisher: Springer Science and Business Media LLC
Date: 13-08-2018
DOI: 10.1038/S41557-018-0104-X
Abstract: Highly porous metal-organic frameworks (MOFs), which have undergone exciting developments over the past few decades, show promise for a wide range of applications. However, many studies indicate that they suffer from significant stability issues, especially with respect to their interactions with water, which severely limits their practical potential. Here we demonstrate how the presence of 'sacrificial' bonds in the coordination environment of its metal centres (referred to as hemilability) endows a dehydrated copper-based MOF with good hydrolytic stability. On exposure to water, in contrast to the indiscriminate breaking of coordination bonds that typically results in structure degradation, it is non-structural weak interactions between the MOF's copper paddlewheel clusters that are broken and the framework recovers its as-synthesized, hydrated structure. This MOF retained its structural integrity even after contact with water for one year, whereas HKUST-1, a compositionally similar material that lacks these sacrificial bonds, loses its crystallinity in less than a day under the same conditions.
Publisher: American Chemical Society (ACS)
Date: 06-09-2016
Publisher: Royal Society of Chemistry (RSC)
Date: 2011
DOI: 10.1039/C0CP01274B
Abstract: The local structure and cation disorder in Y(2)Ti(2-x)Sn(x)O(7) pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is studied using (119)Sn (I = 1/2) NMR spectroscopy. NMR provides an excellent probe of disorder, as it is sensitive to the atomic scale environment without the need for any long-range periodicity. However, the complex and overlapping spectral resonances that often result can be difficult to interpret. Here, we demonstrate how (119)Sn DFT calculations can be used to aid the spectral interpretation and assignment, confirming that Sn occupies only the six-coordinate pyrochlore B site, and that the Sn chemical shift is sensitive to the number of Sn/Ti on the neighbouring B sites. Although distinct resonances are resolved experimentally when the Ti content is low, there is significant spectral overlap for Ti-rich compositions. We establish that this is a result of two competing contributions to the Sn chemical shift an upfield shift resulting from the incorporation of the more polarizing Ti(4+) cation onto the neighbouring B sites, and a concomitant downfield shift arising from the decrease in unit cell size. Despite the considerably easier spectral acquisition, the lower resolution in the (119)Sn spectra hinders the extraction of the detailed structural information previously obtained using (89)Y NMR. However, the spectra we obtain are consistent with a random distribution of Sn/Ti on the pyrochlore B sites. Finally, we consider whether an equilibrium structure has been achieved by investigating materials that have been annealed for different durations.
Publisher: Wiley
Date: 15-02-2021
DOI: 10.1002/MRC.5140
Abstract: Characterising the local structures (e.g., the cation distribution) of mixed‐metal ceramics by NMR spectroscopy is often challenging owing to the unfavourable properties (low γ , large quadrupole moment and/or low abundance) of many metal nuclei. 17 O is an attractive option owing to the prevalence of oxygen within ceramics. The moderate γ and small quadrupole moment of 17 O mean that the greatest barrier to accessing the information available from this nucleus is isotopic enrichment. We explore the challenges of ensuring uniform isotopic enrichment with 17 O 2 (g) for the pyrochlore solid solutions, Y 2 Sn x Ti 2– x O 7 , La 2 Sn x Zr 2– x O 7 and La 2 Sn x Hf 2– x O 7 , demonstrating that high enrichment temperatures (900 °C for 12 hr) are required. In addition, for sites with very high symmetry (such as the tetrahedral OY4 and OLa4 sites with C Q ≈ 0 present here), we demonstrate that quantitative 17 O NMR spectra require correction for the differing contributions from the centreband of the satellite transitions, which can be as high as a factor of ~3.89. It is common to use first‐principles calculations to aid in interpreting NMR spectra of disordered solids. Here, we use an ensemble modelling approach to ensure that all possible cation arrangements are modelled in the minimum possible number of calculations. By combining uniform isotopic enrichment, quantitative NMR spectroscopy and a comprehensive computational approach, we are able to show that the cation distribution in Y 2 Sn x Ti 2– x O 7 is essentially random, whereas in La 2 Sn x Zr 2– x O 7 and La 2 Sn x Hf 2– x O 7 , OLa2SnZr and OLa2SnHf sites are slightly energetically disfavoured, leading to a weak preference for clustering of like cations.
Publisher: American Chemical Society (ACS)
Date: 09-07-2020
Publisher: Elsevier BV
Date: 09-2019
DOI: 10.1016/J.SSNMR.2019.05.005
Abstract: NMR spectroscopy of paramagnetic materials (pNMR) has the potential to provide great structural insight, but many challenges remain in interpreting the spectra in detail. This work presents a study of a series of structurally analogous metal-organic frameworks (MOFs) based on 5-substituted isophthalate linkers and Cu(II) paddlewheel dimers, of interest owing to their "crumple zone" structural rearrangement on dehydration/rehydration.
Publisher: American Chemical Society (ACS)
Date: 05-12-2008
DOI: 10.1021/JP808651X
Publisher: American Chemical Society (ACS)
Date: 20-02-2019
DOI: 10.1021/JACS.9B00643
Publisher: American Chemical Society (ACS)
Date: 18-02-2015
DOI: 10.1021/IC503056Z
Abstract: A series of peri-substituted phosphorus-tellurium systems R'Te-Acenap-PR2 (R' = Ph, p-An, Nap, Mes, Tip Acenap = acenaphthene-5,6-diyl (-C12H8) R = (i)Pr, Ph) exhibiting large "through-space" spin-spin coupling constants and the "onset" of three-center four-electron type interactions is presented. The influence of the substituents at the phosphorus and tellurium atoms as well as their behavior upon oxidation (with S, Se) or metal-coordination (Pt, Au) is discussed using NMR spectroscopy, single-crystal X-ray diffraction, and advanced density functional theory studies including NBO, AIM, and ELI-D analyses.
Publisher: American Chemical Society (ACS)
Date: 08-05-2014
DOI: 10.1021/OM500289B
Publisher: American Chemical Society (ACS)
Date: 05-05-2015
DOI: 10.1021/JACS.5B03353
Abstract: Solid-state NMR spectra of new P-Se heterocycles based on peri-substituted naphthalene motifs show the presence of unusual J couplings between Se and P. These couplings are between atoms in adjacent molecules and occur "through space", rather than through conventional covalent bonds. Experimental measurements are supported by relativistic DFT calculations, which confirm the presence of couplings between nonbonded atoms, and provide information on the pathway of the interaction. This observation improves the understanding of J couplings and offers insight into the factors that affect crystal packing in solids, for future synthetic exploitation.
Publisher: Royal Society of Chemistry (RSC)
Date: 2013
DOI: 10.1039/C3RA22704A
Publisher: Royal Society of Chemistry (RSC)
Date: 2014
DOI: 10.1039/C4DT00408F
Abstract: A series of eight 5-(TeAr)-6-(SePh)acenaphthenes (Ar = aryl) were prepared and structurally characterised by X-ray crystallography, solution and solid-state NMR spectroscopy and DFT/B3LYP calculations.
Publisher: Wiley
Date: 28-06-2018
Publisher: American Chemical Society (ACS)
Date: 20-08-2014
DOI: 10.1021/JO5015678
Abstract: The synthesis of rigid symmetric polyradical model systems with inter-spin distances between 1.4 and 4 nm and their room temperature continuous wave (CW) EPR spectra are reported. Conditions for attachment of the spin-label via esterification have been optimized on the direct synthesis of polyradicals from commercially available polyphenols and the carboxylic acid functionalized nitroxide TPC. A common synthetic protocol utilizing 4-hydroxy-4'-iodobiphenyl as a key building block has been used to synthesize an equilateral biradical and a triradical in only two steps from commercially available starting materials. The first synthesis of a tetraradical based upon an adamantane core bearing six equivalent nitroxide-nitroxide distances is also reported. These systems are very promising candidates for studying multi-spin effects in pulsed EPR distance measurements.
Publisher: Elsevier BV
Date: 09-2011
DOI: 10.1016/J.SSNMR.2011.08.004
Abstract: (25)Mg NMR parameters have been determined for two polymorphs of enstatite (MgSiO(3)), an important magnesium silicate phase present as a major component of the Earth's upper mantle. The crystal structures of both polymorphs contain two crystallographically distinct magnesium sites however, only a single resonance is observed in (25)Mg MAS NMR spectra recorded at 14.1 and 20.0 T. First-principles calculations performed on geometry-optimised crystal structures reveal that the quadrupolar interaction for the second site is expected to be very large, resulting in extensive broadening of the spectral resonance, explaining its apparent absence in the NMR spectrum. (25)Mg QCPMG NMR experiments employing variable offset cumulative spectroscopy (VOCS) are used to observe the broadened site and enable measurement of NMR parameters. The large difference in quadrupolar interaction between the two crystallographic magnesium sites is rationalised qualitatively in terms of the distortion of the local coordination environment as well as longer-range effects using a simple point charge model.
Publisher: Wiley
Date: 22-12-2014
Abstract: Three related series of peri-substituted bis(tellurides) bearing naphthalene, acenaphthene and acenaphthylene backbones (Nap/Acenap/Aceyl(TeY)2 (Nap = naphthalene-1,8-diyl N Acenap = acenaphthene-5,6-diyl A Aceyl = acenaphthylene-5,6-diyl Ay Y = Ph 1 Fp 2 Tol 3 An-p- 4 An-o- 5 Tp 6 Mes 7 Tip 8) have been synthesised and their solid-state structures determined by X-ray crystallography. Molecular conformations were classified as a function of the two C9-C-Te-C(Y) dihedral angles (θ) in the solid all members adopt AB or CCt configurations, with larger Te(aryl) moieties exclusively imposing the CCt variant. Exceptionally large J((125)Te,(125)Te) spin-spin coupling constants between 3289-3848 Hz were obtained for compounds substituted by bulky Te(aryl) groups, implying these species are locked in a CCt-type conformation. In contrast, compounds incorporating smaller Te(aryl) moieties are predicted to be rather dynamic in solution and afford much smaller J values (2050-2676 Hz), characteristic of greater populations of AB conformers with lower couplings. This conformational dependence of through-space coupling is supported by DFT calculations.
Publisher: Royal Society of Chemistry (RSC)
Date: 2016
DOI: 10.1039/C6CP01529H
Abstract: Ab initio random structure searching is employed to generate candidate structures of hydrous wadsleyite, predicting NMR parameters for experimental comparison.
Publisher: American Chemical Society (ACS)
Date: 07-02-2012
DOI: 10.1021/JP2105133
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Sharon Ashbrook.