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H W Spiess

Publications and source records attributed to H W Spiess.

At least 19 recordsLinked to original sources
PubMed2006

Bulk chemical shifts in hydrogen-bonded systems from first-principles calculations and solid-state-NMR.

We present an analysis of bulk (1)H NMR chemical shifts for a series of biochemically relevant molecular crystals in analogy to the well-known solvent NMR chemical shifts. The term bulk shifts denotes the change in NMR frequency of a gas-phase molecule when it undergoes crystallization. We compute NMR parameters from first-principles electronic structure calculations under full periodic boundary conditions and for isolated molecules and compare them to the corresponding experimental fast magic-angle spinning solid-state NMR spectra. The agreement between computed and experimental lines is generally very good. The main phenomena responsible for bulk shifts are packing effects (hydrogen bonding and pi-stacking) in the condensed phase. By using these NMR bulk shifts in well-ordered crystalline model systems composed of biologically relevant molecules, we can understand the individual spectroscopic signatures of packing effects. These local structural driving forces, hydrogen bonding, pi-stacking, and related phenomena, stand as a model for the forces that govern the assembly of much more complex supramolecular aggregates. We show to which accuracy condensed-phase ab initio calculations can predict structure and structure-property relationships for noncovalent interactions in complex supramolecular systems.

Journal Article↗
PubMed2006

Diffusion in binary gas mixtures studied by NMR of hyperpolarized gases and molecular dynamics simulations.

The dependence of the individual mean square displacement of rare gases in binary mixtures is studied by a combined experimental and theoretical approach. We show that the diffusion constant can be varied in a considerable range by changing the molar fractions of the mixtures. On the experimental side, NMR diffusion measurements are done on hyperpolarized 3He and 129Xe, mixed with several inert buffer gases, in the presence of a magnetic field gradient. The results are compared to diffusion coefficients obtained from atomistic molecular dynamics simulations based on Lennard-Jones type potentials of the corresponding gas mixtures, and to appropriate analytical expressions, yielding very good mutual agreement. This study is the first quantitative validation of the effects of the mutual interactions between gas particles on the individual diffusion properties. It is shown that the dependency of gas phase diffusion properties on the local chemical environment may not be neglected, e.g. in diffusion-controlled chemical reactions.

Complex Mixtures↗
PubMed2006

Orientation dynamics in isotropic phases of model oligofluorenes: glass or liquid crystal.

Orientation molecular dynamics were investigated in a series of "defect-free" oligofluorenes by depolarized dynamic light scattering and dynamic NMR spectroscopy. Typical liquid crystalline pretransitional dynamics were observed upon cooling the isotropic phase to the liquid crystalline phase with strong increase of the scattered intensity and slowing down of the characteristic time of the probed collective relaxation. This is well accounted for by the Landau-de Gennes theory, however, with a strong temperature dependence of the viscosity coefficient, reflecting the proximity of the glass transition. For the trimer the two transitions almost overlap and the molecular orientation coincide with the alpha-relaxation associated with the glass transition. The NMR measurements confirm that the time scale of the dynamics is completely governed by the glass process, yet the geometry of the motion is anisotropic, yielding order parameters ranging from 0.15 to 0.25 for the long axis in the liquid crystalline phase. The glass transition is therefore geometrically restricted with poorly ordered mesophase which is consistent with the weak transverse phonons in the light scattering experiment down to Tg+20 K.

Journal Article↗
PubMed2005

Structure of amorphous poly-(ethylmethacrylate): a wide-angle x-ray scattering study.

The local, temperature dependent structure of poly-(ethylmethacrylate) was studied with wide-angle x-ray scattering. The results were set into context to recent wide-angle neutron scattering results as well as to the results from a multitude of studies of the dynamics of this polymer. The temperature dependence of the wide angle x-ray results point to the development of local order which is backed by the neutron scattering results and which is connected to characteristic temperatures of the relaxation dynamic T(g) and T(c). The poly-(ethylmethacrylate) was studied in its predominantly syndiotactic as well as predominantly isotactic state displaying vastly different local structures as manifested in the x-ray results.

Journal Article↗
PubMed2004

Imaging of a mixture of hyperpolarized 3He and 129Xe.

With the use of hyperpolarized gases, a great number of experiments have been carried out in order to improve the diagnostics of the lung, both from a structural and a functional point of view. 3He is best suited for structural studies, whereas 129Xe gives more detailed information about the functionality of the lung because it enters the bloodstream. In this work, we propose the use of a gas mixture to perform consecutive analysis of lung structure and functionality upon the delivery of a single bolus of gas. We show images of a helium-xenon gas mixture in the presence of a small amount of liquid toluene in order to demonstrate how both nuclei can be detected independently, extracting the spectroscopic information provided by the 129Xe spectra and obtaining an image with high sensitivity for 3He. A second experiment performed on a dissected mouse lung was used to demonstrate how the mixture of gases can enhance sensitivity in the larger airways of the lung.

Animals↗
PubMed2001

An investigation of the hydrogen-bonding structure in bilirubin by 1H double-quantum magic-angle spinning solid-state NMR spectroscopy.

The complex hydrogen-bonding arrangement in the biologically important molecule bilirubin IXalpha is probed by using 1H double-quantum (DQ) magic-angle spinning (MAS) NMR spectroscopy. Employing fast MAS (30 kHz) and a high magnetic field (16.4 T), three low-field resonances corresponding to the different hydrogen-bonding protons are resolved in a 1H MAS NMR spectrum of bilirubin. These resonances are assigned on the basis of the proton-proton proximities identified from a two-dimensional rotor-synchronized 1H DQ MAS NMR spectrum. An analysis of 1H DQ MAS spinning-sideband patterns for the NH protons in bilirubin allows the quantitative determination of proton-proton distances and the geometry. The validity of this procedure is proven by simulated spectra for a model three-spin system, which show that the shortest distance can be determined to a very high degree of accuracy. The distance between the lactam and pyrrole NH protons in bilirubin is determined to be 0.186 +/- 0.002 nm (corresponding to a dominant dipolar coupling constant of 18.5 +/- 0.5 kHz). The analysis also yields a distance between the lactam NH and carboxylic acid OH protons of 0.230 +/- 0.008 nm (corresponding to a perturbing dipolar coupling constant of 9.9 +/- 1.0 kHz) and an H-H-H angle of 122 +/- 4 degrees. Finally, a comparison of 1H DQ MAS spinning-sideband patterns for bilirubin and its dimethyl ester reveals a significantly longer distance between the two NH protons in the latter case.

Bilirubin↗
PubMed2001

Structure assignment in the solid state by the coupling of quantum chemical calculations with NMR experiments: a columnar hexabenzocoronene derivative.

We present a quantum chemical ab initio study which demonstrates a new combined experimental and theoretical approach, whereby a comparison of calculated and experimental (1)H NMR chemical shifts allows the elucidation of structural arrangements in solid-state molecular ensembles, taking advantage of the marked sensitivity of the (1)H chemical shift to intermolecular interactions. Recently, Brown et al. have shown that, under fast magic-angle spinning (MAS) at 35 kHz, the resolution in a (1)H NMR spectrum of the solid phase of an alkyl-substituted hexabenzocoronene (HBC) derivative is sufficient to observe the hitherto unexpected resolution of three distinct aromatic resonances ( J. Am. Chem. Soc. 1999, 121, 6712). Exploiting the additional information about proton proximities provided by (1)H double-quantum (DQ) MAS NMR spectroscopy, it was shown that the results are qualitatively consistent with the aromatic cores packing in a manner similar to that in unsubstituted HBC. Using the HBC-C(12) molecule as an example, we show here that the new combined experimental and theoretical approach allows the observed (1)H chemical shifts to be related in a quantitative manner to the intermolecular structure. In the quantum chemical calculations, a series of model systems of stacked HBC oligomers are used. On account of the marked dependence of the (1)H chemical shift to ring currents arising from nearby aromatic rings, the calculated (1)H chemical shifts are found to be very sensitive to the stacking arrangement of the HBC molecules. Moreover, the ring current effect is found to be particularly long range, with a considerable influence of the second neighbor, at a distance of 700 pm, being observed.

Dimerization↗
PubMed2001

Recoupled polarization-transfer methods for solid-state (1)H--(13)C heteronuclear correlation in the limit of fast MAS.

An in-depth account of the effects of homonuclear couplings and multiple heteronuclear couplings is given for a recently published technique for (1)H--(13)C dipolar correlation in solids under very fast MAS, where the heteronuclear dipolar coupling is recoupled by means of REDOR pi-pulse trains. The method bears similarities to well-known solution-state NMR techniques, which form the framework of a heteronuclear multiple-quantum experiment. The so-called recoupled polarization-transfer (REPT) technique is versatile in that rotor-synchronized (1)H--(13)C shift correlation spectra can be recorded. In addition, weak heteronuclear dipolar coupling constants can be extracted by means of spinning sideband analysis in the indirect dimension of the experiment. These sidebands are generated by rotor encoding of the reconversion Hamiltonian. We present generalized variants of the initially described heteronuclear multiple-quantum correlation (HMQC) experiment, which are better suited for certain applications. Using these techniques, measurements on model compounds with (13)C in natural abundance, as well as simulations, confirm the very weak effect of (1)H--(1)H homonuclear couplings on the spectra recorded with spinning frequencies of 25--30 kHz. The effect of remote heteronuclear couplings on the spinning-sideband patterns of CH(n) groups is discussed, and (13)C spectral editing of rigid organic solids is shown to be practicable with these techniques.

Journal Article↗
PubMed2001

Inverse detection and heteronuclear editing in 1H-15N correlation and 1H-1H double-quantum NMR spectroscopy in the solid state under fast MAS.

Signal enhancement in heteronuclear correlation spectra as well as signal selection in 1H experiments can be achieved through inverse, i.e., 1H, detection in the solid state under fast MAS conditions. Using recoupled polarization transfer (REPT), a heteronuclear 1H-15N single-quantum correlation (HSQC) experiment is presented whose symmetrical design allows the frequency dimensions to be easily interchanged. By observing the 15N dimension indirectly and detecting on 1H, the sensitivity is experimentally found to be increased by factors between 5 and 10 relative to conventional 15N detection. In addition, the inverse 1H-15N REPT-HSQC scheme can be readily used as a filter for the 1H signal. As an example, we present the combination of such a heteronuclear filter with a subsequent 1H-1H DQ experiment, yielding two-dimensional 15N-edited 1H-1H DQ MAS spectra. In this way, specific selection or suppression of 1H resonances is possible in solid-state MAS experiments, by use of which the resolution can be improved and information can be unravelled in 1H spectra.

Journal Article↗
PubMed2001

High-resolution 1H NMR spectroscopy in the solid state: very fast sample rotation and multiple-quantum coherences.

In the past few years, solid-state 1H NMR spectroscopy under fast magic-angle spinning (MAS) has developed into a versatile tool for elucidating structure and dynamics. Dipolar multiple-quantum (MQ), in particular double-quantum (DQ), MAS spectroscopy has been applied to a variety of materials and provided unique insight, e.g., into the structure of hydrogen-bonded systems. This review intends to present solid-state 1H DQ and MQ MAS spectroscopy in a systematic fashion with a particular emphasis on methodological aspects, followed by an overview of applications.

Magnetic Resonance Spectroscopy↗
PubMed2000

Dead-time free measurement of dipole-dipole interactions between electron spins.

A four-pulse version of the pulse double electron-electron resonance (DEER) experiment is presented, which is designed for the determination of interradical distances on a nanoscopic length-scale. With the new pulse sequence electron-electron couplings can be studied without dead-time artifacts, so that even broad distributions of electron-electron distances can be characterized. A version of the experiment that uses a pulse train in the detection period exhibits improved signal-to-noise ratio. Tests on two nitroxide biradicals with known length indicate that the accessible range of distances extends from about 1.5 to 8 nm. The four-pulse DEER spectra of an ionic spin probe in an ionomer exhibit features due to probe molecules situated both on the same and on different ion clusters. The former feature provides information on the cluster size and is inaccessible with previous methods.

Electron Spin Resonance Spectroscopy↗
PubMed2000

Sideband patterns from rotor-encoded longitudinal magnetization in MAS recoupling experiments.

Recent multiple-quantum MAS NMR experiments have shown that a change in the rotor phase (and, hence, in the Hamiltonian) between the excitation and reconversion periods can lead to informative spinning-sideband patterns. However, such "rotor encoding" is not limited to multiple-quantum experiments. Here it is shown that longitudinal magnetization can also be rotor-encoded. Both homonuclear and heteronuclear rotor encoding of longitudinal magnetization (RELM) experiments are performed on dipolar-coupled spin-1/2 systems, and the corresponding sideband patterns in the indirect dimension are analyzed. In both cases, only even-order sidebands are produced, and their intensity distribution depends on the durations of the recoupling periods. In heteronuclear experiments using REDOR-type recoupling, purely dipolar sideband patterns that are entirely free of effects due to the chemical-shielding anisotropy can be generated. Advantages and disadvantages of the heteronuclear RELM experiment are discussed in the context of other methods used to measure heteronuclear dipolar couplings.

Alanine↗
PubMed2000

Magic-angle sample spinning electron paramagnetic resonance--instrumentation, performance, and limitations.

An electron paramagnetic resonance (EPR) setup for line narrowing experiments with fast sample spinning at variable angles between the rotation axis and the static magnetic field is described and applied in the magic-angle sample spinning (MAS) EPR experiment at X-band frequencies (9.5 GHz). Sample spinning speeds up to 17 kHz at temperatures down to 200 K can be achieved with rotors of 4-mm outer and 2.5-mm inner diameter without severe losses in microwave amplitude compared to standard pulse EPR probeheads. A phase cycle is introduced that provides pure absorption MAS EPR spectra and allows one to distinguish between positive and negative frequency offsets (pseudo-quadrature detection). Possible broadening mechanisms in MAS EPR spectra are discussed. It is demonstrated both by theory and by experiment that the MAS EPR experiment requires excitation bandwidths that are comparable to the total spectral width, since otherwise destructive interference between contributions of spins with similar resonance offsets suppresses the signal. Experimental observations on the E(1) center in gamma-irradiated silica glass and on the SO(-)(3) radical in gamma-irradiated sulfamic acid are reported.

Electron Spin Resonance Spectroscopy↗
PubMed2000

[Possible mutagenic effects of magnetic fields].

AIM: To assess the potential mutagenic effect of static magnetic fields of 1.5 T and 7 T, gradient fields and high frequency magnetic fields. METHOD: We used the Salmonella mutagenicity test (Ames test), which detects mutations in a gene of a histidine-requiring (his-) strain to produce a histidine-independent (his+) strain, Exposure to a static magnetic field of 1.5 T and 7.2 T, in a bipolar magnetic gradient and additionally in a high frequency field took place with and without known genotoxic chemicals. RESULTS: No differences in the number of revertants between the bacterial strains of exposed and control cells could be detected and the exposure with known genotoxic chemicals showed no significant difference in mutagenicity. CONCLUSION: In conclusion our data do not provide evidence that exposure to a static magnetic field exerts effects on the mutagenicity in our standard tester strains and whether the exposure took place in a diagnostic 1.5 T MR scanner which is used in the clinical routine or at 7.2 T which is a much stronger field made no difference. Also an exposure in a gradient field or in a high frequency field did not show any alteration in the number of revertants.

Electromagnetic Fields↗
PubMed1999

Heteronuclear double-quantum MAS NMR spectroscopy in dipolar solids.

A new pulse sequence for high-resolution solid-state heteronuclear double-quantum MAS NMR spectroscopy of dipolar-coupled spin-12 nuclei is introduced. It is based on the five-pulse sequence known from solution-state NMR, which is here applied synchronously to both spin species. The heteronuclear double-quantum (HeDQ) spinning-sideband patterns produced by this experiment are shown to be sensitive to the heteronuclear distance, as well as the relative orientations of the chemical-shift and dipolar tensors. In particular, it is shown that the HeDQ patterns exhibit an enhanced sensitivity to the chemical shielding tensors as compared with the single-quantum spinning-sideband patterns. The detection of HeDQ patterns via the I and S spins is discussed. The isolated (13)C-(1)H spin pair in deuterated ammonium formate with (13)C in natural abundance was chosen as a model system, and the perturbing influence of dipolar couplings to surrounding protons on the (13)C-(1)H DQ coherence is discussed. The pulse sequence can also be used as a heteronuclear double-quantum filter, hence providing information about heteronuclear couplings, and thus allowing the differentiation of quaternary and CH(n) bonded carbons. The elucidation of (13)C-(1)H dipolar proximities is presented for a sample of bisphenol A polycarbonate with (13)C in natural abundance, recorded with a broadband version of the synchronized five-pulse sequence.

Magnetic Resonance Spectroscopy↗
PubMed1999

Combined reduced 4D 13C exchange and 1H spin diffusion experiment for determining the length scale of dynamic heterogeneities.

A multidimensional static solid-state NMR experiment is described that combines 13C exchange sequences with 1H spin diffusion. It realizes a spatial correlation of different reorientation rates. By means of this experiment the length scale of dynamic heterogeneities can be measured directly. The pulse sequence and phase cycle as well as the experimental setup procedure and data analysis are described in detail. It complements the previous letter on this subject where a brief report of the main results were presented (U. Tracht et al., 1998, Phys. Rev. Lett. 81, 2727). Application of this experiment to an amorphous polymer in the supercooled state yields a length scale of immobile regions of about 3 nm.

Magnetic Resonance Spectroscopy↗
PubMed1999

Recoupled polarization transfer heteronuclear 1H-13C multiple-quantum correlation in solids under ultra-fast MAS.

A new approach for high-resolution solid-state heteronuclear multiple-quantum MAS NMR spectroscopy of dipolar-coupled spin-12 nuclei is introduced. The method is a heteronuclear chemical shift correlation technique of abundant spins, like 1H with rare spins, like 13C in natural abundance. High resolution is provided by ultra-fast MAS and high magnetic fields, high sensitivity being ensured by a direct polarization transfer from the abundant protons to 13C. In a rotor-synchronized variant, the method can be used to probe heteronuclear through-space proximities, while the heteronuclear dipolar coupling constant can quantitatively be determined by measuring multiple-quantum spinning-sideband patterns. By means of recoupling, even weak heteronuclear dipolar interactions are accessible. The capabilities of the technique are demonstrated by measurements on crystalline L-tyrosine hydrochloride salt.

Magnetic Resonance Spectroscopy↗