Biomedical subjects

Stephen Wimperis

Publications and source records attributed to Stephen Wimperis.

12 recordsLinked to original sources

PubMed2006

Five-coordinate Pd(II) orthometallated triarylphosphite complexes.

The reaction of the orthopalladated triarylphosphite complexes [{Pd(mu-Cl){kappa(2)-P,C-P(OC(6)H(2)-2,4-R(2))(OC(6)H(3)-2,4-R(2))}(2)] (R = H, (t)Bu) with bis(2-diphenylphosphinoethyl)phenylphosphine leads to a five-coordinate palladium(II) (R = H) and a mixture containing four-and five-coordinate species (R = (t)Bu). The crystal structure of the five-coordinate species [Pd{kappa(2)-P,C-(P(OC(6)H(4))(OC(6)H(5))(2)}{bis(2-diphenylphosphinoethyl)phenylphosphine}][SbF(6)] is presented. This complex reacts with hydrogen peroxide or [AuCl(tht)] to give four-coordinate complexes in which the displaced phosphine residue is either oxidised or coordinated to gold chloride; this demonstrates that the five-coordinate complexes are labile in solution. By contrast, the reactions of the dimeric precursors with 1,1,1-tris(diphenylphosphinomethyl)ethane give four-coordinate complexes in the solid state, although evidence is presented that the smaller phosphite-containing system is five-coordinate at room temperature or higher in solution.

Journal Article↗

PubMed2006

Dynamics on the microsecond timescale in microporous aluminophosphate AlPO-14 as evidenced by 27Al MQMAS and STMAS NMR spectroscopy.

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an 27Al (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AlPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the 31P and 13C (both I = 1/2) MAS NMR spectra are also presented. A comparison of the 27Al MQMAS and STMAS NMR results show that, although calcined AlPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four Al sites in each material. Variable-temperature 27Al STMAS NMR studies of the two as-synthesized AlPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.

Journal Article↗

PubMed2006

Nuclear Overhauser effect (NOE) enhancement of 11B NMR spectra of borane adducts in the solid state.

A strong 11B {1H} nuclear Overhauser effect (NOE) enhancement can be observed in solid-state 11B NMR spectra of borane adducts, yielding fractional enhancements, fI{S} = (I - I0)/I0, of the magic angle spinning (MAS) NMR signal of up to 155%. This is an interesting and unusual observation as 11B (spin I = 3/2) is a quadrupolar nucleus and the corresponding NOE is completely absent in solution. More generally, it shows that the NOE may have a wider role to play in solid-state NMR studies of dynamics than has been envisaged hitherto.

Boranes↗

PubMed2005

Experimental observations of water-framework interactions in a hydrated microporous aluminum phosphate.

Differential scanning calorimetry of the hydrated, microporous aluminum phosphate AlPO-14 shows two distinct water losses between room temperature and 120 degrees C, indicating the presence of two types of water in the solid. Multiple-quantum magic angle spinning (MQMAS) (27)Al NMR shows that, while in dehydrated AlPO-14 all aluminum is found in tetrahedral sites, on hydration a significant proportion of the aluminum increases its coordination number to 6. This accounts for the presence of tightly bound water. The first detailed incoherent inelastic neutron scattering (IINS) studies of such a system give a spectrum with distinct and sharp librational bands for bound water, significantly different than seen in ice Ih. Using these data, and by consideration of the crystal structure of dehydrated AlPO-14, we propose a model for the hydrated material in which the tightly bound water bridges pairs of Lewis acidic framework aluminums in a dense region of the structure, while loosely bound water resides in the pores of the solid. Further IINS measurements using a high-incident neutron energy provide data that are in agreement with our model. We can detect two O-H stretching modes for bound water in hydrated AlPO-14, consistent with the model of two types of water present in the material, with the loosely bound water connected to neighboring water molecules by intermolecular hydrogen bonds.

Journal Article↗

PubMed2005

Separation of quadrupolar and chemical/paramagnetic shift interactions in two-dimensional 2H (I=1) nuclear magnetic resonance spectroscopy.

A novel two-dimensional (2)H (spin I=1) nuclear magnetic resonance technique is introduced for determination of both quadrupole and chemical/paramagnetic shift tensors and their relative orientation. The new method is based upon the well-known quadrupolar-echo experiment and is designed to refocus the quadrupolar interaction at the end of the t(1) evolution period while retaining the modulation introduced by the shift interaction. As a result, a projection of the resulting two-dimensional spectrum onto its F(1) dimension yields a shift anisotropy powder lineshape free from any quadrupolar broadening. The chemical/paramagnetic shifts appear in both F(1) and F(2) dimensions and are thus spread along a +1 frequency gradient; hence, a projection orthogonal to this gradient yields the pure quadrupolar powder lineshape, free from all shift interaction effects. The relative orientation of the quadrupole and shift tensors can be obtained by analysis of the full two-dimensional correlation lineshape. Unlike the well-known double-quantum experiment, the new method is, in principle, equally effective for all values of the quadrupolar splitting, including zero. The properties of the new technique are demonstrated using computer simulation and methods for the extraction of quadrupole and shift tensor parameters are described. The new technique is applied to (diamagnetic) benzoic acid-d(1) (C(6)H(5)CO(2)D) and (paramagnetic) copper(II) chloride dihydrate-d(4) (CuCl(2).2D(2)O).

Journal Article↗

PubMed2005

Rotor-synchronized acquisition of quadrupolar satellite-transition NMR spectra: practical aspects and double-quantum filtration.

The very broad resonances of quadrupolar (spin I > 1/2) nuclei are resolved by magic angle spinning (MAS) into a large number of spinning sidebands, each of which often remains anisotropically broadened. The quadrupolar interaction can be removed to a first-order approximation if the MAS NMR spectrum is acquired in a rotor-synchronized fashion, aliasing the spinning sidebands onto a centreband and thereby increasing the signal-to-noise ratio in the resulting, possibly second-order broadened, spectrum. We discuss the practical aspects of this rotor-synchronization in the direct (t(2)) time domain, demonstrating that the audiofrequency filters in the receiver section of the spectrometer have a significant impact on the precise timings needed in the experiment. We also introduce a novel double-quantum filtered rotor-synchronized experiment for half-integer spin quadrupolar (spin I = 3/2, 5/2, etc.) nuclei that makes use of central-transition-selective inversion pulses to both excite and reconvert double-quantum coherences and yields a simplified spectrum containing only the ST(1) (m(I) = +/-1/2 <--> +/-3/2) satellite-transition lineshapes. For spin I = 5/2 nuclei, such as (17)O and (27)Al, this spectrum may exhibit a significant resolution increase over the conventional central-transition spectrum.

Journal Article↗

PubMed2004

Spin-locking of half-integer quadrupolar nuclei in nuclear magnetic resonance of solids: creation and evolution of coherences.

Spin-locking of half-integer quadrupolar nuclei, such as 23Na (I=3/2) and 27Al (I=5/2), is of renewed interest owing to the development of variants of the multiple-quantum and satellite-transition magic angle spinning (MAS) nuclear magnetic resonance experiments that either utilize spin-locking directly or offer the possibility that spin-locked states may arise. However, the large magnitude and, under MAS, the time dependence of the quadrupolar interaction often result in complex spin-locking phenomena that are not widely understood. Here we show that, following the application of a spin-locking pulse, a variety of coherence transfer processes occur on a time scale of approximately 1/omegaQ before the spin system settles down into a spin-locked state which may itself be time dependent if MAS is performed. We show theoretically for both spin I=3/2 and 5/2 nuclei that the spin-locked state created by this initial rapid dephasing typically consists of a variety of single- and multiple-quantum coherences and nonequilibrium population states and we discuss the subsequent evolution of these under MAS. In contrast to previous work, we consider spin-locking using a wide range of radio frequency field strengths, i.e., a range that covers both the "strong-field" (omega1 >> omegaQPAS and "weak-field" (omega1 << omegaQPAS limits. Single- and multiple-quantum filtered spin-locking experiments on NaNO2, NaNO3, and Al(acac)3, under both static and MAS conditions, are used to illustrate and confirm the results of the theoretical discussion.

Journal Article↗

PubMed2003

High-resolution 17O NMR spectroscopy of wadsleyite (beta-Mg2SiO4).

The sensitivity of high-resolution 17O (I = 5/2) NMR spectroscopy of solids has advanced significantly in recent years. Here, we show that excellent results are now obtainable from milligram quantities of 17O-enriched materials, thereby allowing the technique to be applied to silicate phases synthesized under very high pressures in a multiple-anvil apparatus. We report the first 17O NMR study of beta-Mg2SiO4 (9.6 mg of 35% 17O-enriched material, synthesized at p = 16 GPa and T = 1873 K), a dense phase believed to have a significant role in the Earth's mantle. Using STMAS at magnetic fields of B0 = 9.4 and 11.7 T and MQMAS at B0 = 18.8 T, we have resolved and assigned all four crystallographically distinct O sites and determined their chemical shift and quadrupolar parameters.

Journal Article↗

PubMed2003

SCAM-STMAS: satellite-transition MAS NMR of quadrupolar nuclei with self-compensation for magic-angle misset.

Several methods are available for the acquisition of high-resolution solid-state NMR spectra of quadrupolar nuclei with half-integer spin quantum number. Satellite-transition MAS (STMAS) offers an approach that employs only conventional MAS hardware and can yield substantial signal enhancements over the widely used multiple-quantum MAS (MQMAS) experiment. However, the presence of the first-order quadrupolar interaction in the satellite transitions imposes the requirement of a high degree of accuracy in the setting of the magic angle on the NMR probehead. The first-order quadrupolar interaction is only fully removed if the sample spinning angle, chi, equals cos(-1)(1/3) exactly and rotor synchronization is performed. The required level of accuracy is difficult to achieve experimentally, particularly when the quadrupolar interaction is large. If the magic angle is not set correctly, the first-order splitting is reintroduced and the spectral resolution is severely compromised. Recently, we have demonstrated a novel STMAS method (SCAM-STMAS) that is self-compensated for angle missets of up to +/-1 degrees via coherence transfer between the two different satellite transitions ST(+)(m(I)=+3/2<-->+1/2) and ST(-)(m(I)=-1/2<-->-3/2) midway through the t(1) period. In this work we describe in more detail the implementation of SCAM-STMAS and demonstrate its wider utility through 23Na (I=3/2), 87 Rb (I=3/2), 27 Al (I=5/2), and 59 Co (I=7/2) NMR. We discuss linewidths in SCAM-STMAS and the limits over which angle-misset compensation is achieved and we demonstrate that SCAM-STMAS is more tolerant of temporary spinning rate fluctuations than STMAS, resulting in less "t(1) noise" in the two-dimensional spectrum. In addition, alternative correlation experiments, for example involving the use of double-quantum coherences, that similarly display self-compensation for angle misset are investigated. The use of SCAM-STMAS is also considered in systems where other high-order interactions, such as third-order quadrupolar effects or second-order quadrupole-CSA cross-terms, are present. Finally, we show that the sensitivity of the experiment can be improved through the use of amplitude-modulated pulses.

Journal Article↗

PubMed2003

Refocussing of chemical and paramagnetic shift anisotropies in 2H NMR using the quadrupolar-echo experiment.

A simple two-pulse spin-echo experiment is shown to refocus inhomogeneous broadening arising from both chemical and/or paramagnetic shift anisotropy and a first-order I=1 quadrupolar interaction. The method is shown to yield 2H NMR spectra of a paramagnetic solid (CuCl2.2D2O) and of a non-paramagnetic solid (D2C2O4.2D2O) that are significantly less distorted than those provided by the conventional quadrupolar-echo method. The technique will thus prove useful in studies of motion and dynamics where detailed analysis of the 2H lineshape is performed.

Journal Article↗

PubMed2002

High-resolution NMR spectroscopy of quadrupolar nuclei in solids: satellite-transition MAS with self-compensation for magic-angle misset.

Several methods are available for obtaining high-resolution NMR spectra of half-integer spin quadrupolar nuclei, such as (11)B, (23)Na (I = (3)/(2)) and (17)O, (27)Al (I = (5)/(2)), in powdered solids. Satellite-transition magic-angle spinning (STMAS) uses only conventional magic-angle spinning (MAS) hardware and, it has been claimed, improves significantly upon the signal-to-noise ratio obtained with the widely adopted multiple-quantum MAS (MQMAS) experiment. The STMAS technique, however, requires that the sample rotation axis be set to the magic angle (cos(-1)(1/ radical 3) = 54.736 degrees ) with respect to the magnetic field B(0) with an accuracy of better than +/-0.004 degrees, and this stringent requirement has severely limited the use of the method. Here, we propose a novel version of STMAS that self-compensates for magic-angle missets of up to +/-1.0 degrees and yet retains a sensitivity comparable with MQMAS. This SCAM-STMAS experiment is demonstrated on RbNO(3) using (87)Rb (I = (3)/(2)) NMR and on kyanite (Al(2)SiO(5)) using (27)Al (I = (5)/(2)) NMR.

Journal Article↗

PubMed2002

Satellite-transition MAS NMR of spin I = 3/2, 5/2, 7/2, and 9/2 nuclei: sensitivity, resolution, and practical implementation.

The satellite-transition MAS (STMAS) experiment offers an alternative approach to established methods such as dynamic angle spinning (DAS), double rotation (DOR), and multiple-quantum MAS (MQMAS) for obtaining high-resolution NMR spectra of half-integer quadrupolar nuclei. Unlike the multiple-quantum experiment, STMAS involves two-dimensional correlation of purely single-quantum coherences; satellite transitions in t(1) (or F(1)) and the central transition in t(2) (or F(2)). To date, STMAS has primarily been demonstrated for nuclei with spin quantum numbers I = 3/2 and, to a lesser extent, I > 5/2. However, many chemically relevant nuclei possess I > 3/2, such as (17)O and (27)Al (both I = 5/2), (59)Co (I = 7/2), and (93)Nb (I = 9/2). Here, we discuss the application of STMAS to nuclei with spin quantum numbers from I = 3/2 to 9/2. First, we consider the practical implementation of the STMAS experiment using (87)Rb (I = 3/2) NMR as an example. We then extend the discussion to include nuclei with higher spin quantum numbers, demonstrating (27)Al, (45)Sc (I = 7/2), (59)Co, and (93)Nb STMAS experiments on both crystalline and amorphous samples. We also consider the possibility of experiments involving satellite transitions other than m(I) = +/- 1/2 <--> +/- 3/2 and, using (93)Nb NMR, demonstrate the correlation of all single-quantum satellite transitions up to and including m(I) = +/- 7/2 <--> +/- 9/2. The absolute chemical shift scaling factors in these experiments are discussed, as are the implications for isotropic resolution.

Journal Article↗