Multiple-pulse Methods Research Articles

Anatomising proton NMR spectra with pure shift 2D J-spectroscopy: A cautionary tale

Analysis of proton NMR spectra has been a key tool in structure determination for over 60years. A classic tool is 2D J-spectroscopy, but common problems are the difficulty of obtaining the absorption mode lineshapes needed for accurate results, and the need for a 45° shear of the final 2D spectrum. A novel 2D NMR method is reported here that allows straightforward determination of homonuclear couplings, using a modified version of the PSYCHE method to suppress couplings in the direct dimension. The method illustrates the need for care when combining pure shift data acquisition with multiple pulse methods.

Attenuation Characteristics of Ultrasonic Velocity and Acoustic-Based TC4 Alloy Uniformity Nondestructive Evaluation

ultrasound characteristics of the material microstructure nondestructive evaluation of its experimental sample TC4 titanium alloy microstructure of two-phase microstructure:. Α-phase and α + β phase. Phase material microstructure and grain area ratio directly affects the material uniformity, uniformity of material differences caused by different grain characteristics which ultrasound parameters. Flooding experiments using ultrasonic method for measuring the material longitudinal wave velocity; measuring the sound attenuation coefficient, combined with the area ratio of the sample α using multiple pulse method. Associated with the sample area ratio between the speed of sound and sound attenuation analysis of α-phase titanium alloy TC4 through the material properties of ultrasound to characterize the degree of uniformity of the material microstructure, nondestructive evaluation of materials in order to achieve the purpose of the microstructure.

Structural Studies of OH in Nominally Anhydrous Minerals Using NMR

Nuclear magnetic resonance (NMR) is a technique that is used very widely throughout science and medicine. There are approaching 20 journals devoted exclusively to NMR and magnetic resonance imaging, and well in excess of 10,000 papers published annually which involve this family of techniques. It has found extraordinarily diverse applications, and its applications to inorganic solids such as minerals represent a very small part of NMR as a whole. There had been occasional NMR studies of minerals since the discovery of the NMR effect in 1946, but until the early 1980s, the large width of NMR resonances in the solid state precluded widespread application of the technique. The most important factor in the application of NMR to minerals was the development of magic angle spinning (MAS), a technique for narrowing lines in solid state NMR. More recently the development of more complex sample spinning arrangements and multiple pulse methods together with the availability of ever higher magnetic fields opens up more and more possibilities for NMR in mineralogy and geochemistry. NMR is far too diverse and complex for the whole subject to be covered here, so this review will be tightly focused on aspects which relate to understanding the structural role of water in nominally anhydrous minerals. Two review papers (Kirkpatrick 1988; Stebbins 1988) in the “Reviews in Mineralogy” volume on Spectroscopic Methods in Mineralogy and Geology provide an excellent starting point for Earth scientists wishing to learn more about NMR in general. More recent reviews aimed at Earth scientists include those by Fechtelkord (2004) and Kohn (2004), more general reviews of NMR of inorganic solids include Engelhardt and Michel (1987) and MacKenzie and Smith (2002). The web page maintained by J.P. Hornak ( http://www.cis.rit.edu/htbooks/nmr/ ) is also an extremely good resource. NMR is a multinuclear technique, which means …

Closed solution to the Baker-Campbell-Hausdorff problem: exact effective Hamiltonian theory for analysis of nuclear-magnetic-resonance experiments.

A closed solution to the Baker-Campbell-Hausdorff problem is described. The solution, which is based on the Cayley-Hamilton theorem, allows the entanglement between exponential operators to be described by an exact finite series expansion. Addressing specifically the special unitary Lie groups SU(2), SU(3), and SU(4), we derive expansion formulas for the entangled exponential operator as well as for the effective Hamiltonian describing the net evolution of the quantum system. The capability of our so-called exact effective Hamiltonian theory for analytical and numerical analysis is demonstrated by evaluation of multiple-pulse methods within liquid- and solid-state nuclear-magnetic-resonance spectroscopy. The examples include composite pulses for inversion, decoupling, and dipolar recoupling, as well as coherence-order- and spin-state-selective double- to single-quantum conversion, homonuclear dipolar decoupling, finite rf excitation for quadrupolar nuclei, heteronuclear coherence transfer, and gates for quantum computation.

Solid-State NMR Investigation of Paramagnetic Nylon-6 Clay Nanocomposites. 1. Crystallinity, Morphology, and the Direct Influence of Fe3+ on Nuclear Spins

Several exfoliated nylon-6/clay nanocomposites (NnC's) were investigated and compared with pure nylon-6 using solid-state NMR, both proton and 13C. NnC's had nominally 5 mass % clay and were generated both by blending and by in situ polymerization (IsP). Most of the studied NnC's contained layered, naturally occurring montmorillonite clays having nonstoichiometric amounts of nonexchangeable Mg2+ and Fe3+ ions that substitute into octahedral Al3+ sites along the midplane of the 1-nm-thick clay layers. The Fe3+ ions impart a useful paramagnetism to the clay. Each Mg2+ ion leaves an embedded negative charge that must be neutralized with some cation at the surface of the clay. All clays were initially treated with a cationic so-called organic modifier (OM), often a substituted ammonium ion, which increases the clay layer spacing, attaching ionically to the surface of the clay layers. Clay is found to promote growth of the γ-crystalline phase of nylon-6 for both blended and IsP NnC's; α-crystallites are characteristic of the pure nylon-6. Stability of the γ-phase to annealing at 214 °C was investigated. Conversion of γ- to α-crystallinity during annealing was minimal, except for an injection-molded IsP NnC, which had been exposed to a temperature of 295 °C during molding. This high processing temperature produced an irreversible change. An attempt was made to understand, at least qualitatively, the nature of the spectral density of magnetic fluctuations associated with the paramagnetic Fe3+ sites in the clay. For this purpose, we looked directly at the influence of Fe3+ on the 13C and proton observables in organically modified clays (OMC). We agree with other investigators that the spectral density of paramagnetic fluctuations at the surface of the clay is determined mainly by spin-exchange interactions between Fe3+ sites; thus, the spectral density can be altered by changing the Fe3+ concentration. Moreover, we find that the spectral density is very wide, having strong contributions all the way from mid-kHz fluctuations to MHz fluctuations near the proton Larmor frequencies. Significant variations in the α/γ ratio were also observed in the injection-molded disk, which reflect either a processing-induced heterogeneity in clay dispersion or a significant variation in cooling history from region to region. Proton spin diffusion and multiple-pulse methods were utilized to compare morphologies for a diamagnetic NnC and nylon-6 with the same thermal histories. Long spacing, crystallinity, and the mobility of the noncrystalline nylon-6 segments are very similar for NnC's and nylon-6.

The role of pulse sequences in controlling ultrafast intramolecular dynamics with four-wave mixing

This article seeks to provide a fundamental understanding of time-resolved four-wave mixing (FWM) processes based on a large body of experimental measurements on a model system consisting of isolated iodine molecules. The theoretical understanding is based primarily on a diagrammatic approach. Doublesided Feynman diagrams are used to classify and describe the coherent FWM processes involved in the signal obtained with each pulse sequence. Different pulse sequences of degenerate femtosecond pulses are shown to control the optical phenomena observed, that is transient grating, reverse-transient grating, photon echo and virtual photon echo. The experimental data reveal clear differences between the nonlinear optical phenomena. We find that the virtual photon echo sequence k1 - k2 + k3 is the most efficient for controlling the observation of ground - or excited-state dynamics. The strategy followed to make this assessment was to compare transients when the time delay between two of the three pulses set in or out of phase with the excited-state vibrational dynamics. We have obtained a signal from pulse sequences k1 + k2 - k3 for which FWM signal generation for this two-electronic-level system is forbidden. This signal can be explained by the cascading of a first-order polarization and a second-order process to generate the FWM signal. The implications of our findings are discussed in the context of multiple-pulse methods for the control of intramolecular dynamics.

High-resolution N14 NMR in polycrystalline solids

The N14 NMR spectra of solids are usually very broad due to the presence of large quadrupole coupling constants. However, even partial excitation of the whole spectrum can give valuable information. With magic angle spinning (MAS), the spectrum consists of a number of peaks, but normally the centerband cannot be readily distinguished from the spinning sidebands. Multiple-pulse methods of sideband elimination, such as TOSS and PASS, cannot be used for N14 because of its short spin–spin relaxation time. On the other hand, the sidebands can be eliminated by systematic data treatment. First, the signal-to-noise ratio (S/N) is enhanced by co-adding all the peaks in a MAS spectrum in a periodic way. Then, several spectra obtained at different spinning rates are added or multiplied together to identify the centerband. In the centerband region of the spectrum obtained from the addition method, the residual sidebands can be distinguished from the weak signals by the use of logical or digital filtering. Results obtained by using these methods to treat the spectra of two mixtures of KNO3, Pb(NO3)2, and NH4Cl are shown. The experimental requirements are not very stringent, the S/N ratio is good, and peaks covering a large range of chemical shifts can be readily observed with high-resolution.

Radio-frequency μSR experiments at the ISIS pulsed muon facility

This paper explores the use of pulsed radio-frequency (RF) techniques to remove the frequency limitations imposed on conventional transverse muon spin rotation (μSR) experiments at a pulsed muon source by the finite muon pulse width. The implementation of the 90° pulse technique is demonstrated by observing the free precession signal of diamagnetic muons implanted in polythene, the change in signal amplitude as a function of RF pulse length is plotted and the precise condition for a 90° pulse determined. The technique is evaluated by comparing measurements made using conventional spin rotation experiments to those employing pulsed RF methods. The potential for applying standard NMR multiple-pulse methods to the μSR experiment is considered and the use of two-pulse RF sequences (90° x −τ−90° x and 90° x −τ−180° x ) to form a muon spin echo demonstrated.

Some perspectives on the interpretation of proton NMR spin diffusion data in terms of polymer morphologies

Proton spin diffusion data yield morphological information over dimensions covering approximately the 2–50 nm range. In this article, the interpretation of such data for polymers is emphasized, recognizing that the mathematical framework for much of this interpretation already exists in the literature. Practical issues are considered, for example, a useful scaling of plotted data is suggested, key attributes of the data are identified and ambiguities in the mapping of data into morphological models are spelled out. Discussion is limited to two-phase systems, where it is assumed that, by employing multiple-pulse methods, polarization gradients can be generated, whose spatial sharpness is limited solely by the morphological definition of the interfaces. Interpretation of data in terms of morphology and stoichiometry is emphasized, where stoichiometric issues pertain only to chemically heterogeneous systems. Extraction of stoichiometric information from spin diffusion data is not commonly attempted; the discussion included herein allows for the possibility that the composition of phases may be chemically mixed. Methods for generating gradients are discussed only briefly. A standardized spin diffusion plot is proposed and the initial slope of this plot is focussed on for providing information about morphology and stoichiometry. Ambiguities of interpretation considered include the dimensionality of the deduced morphology and, for systems with chemical heterogeneity, the uniqueness of the compositional characterization of each phase. In addition, finite difference methods are used to simulate entire spin diffusion curves for idealized lamellar and hexagonal rod/matrix morphologies. Comparisons of these curves show that distinguishing 1-D and 2-D morphologies on the basis of experimental data is unlikely to be successful over the range of stoichiometries where such morphologies are expected. Several examples of spin diffusion data are presented. Brief treatments of the following topics are included: finite interface width, estimation of spin diffusion constants, and incorporation of longitudinal relaxation effects. Finally, a short experimental discussion on the preparation of polarization gradients is given, including those preparations which make use of differences in the multiple-pulse relaxation time, T lxz . It is noted that T lxz decays may be strongly perturbed in the presence of magic angle spinning; therefore, strategies are also outlined for minimizing these effects.

Proton Spin Diffusion Studies of Polymer Blends Having Modest Monomer Size: 1. Polystyrene/poly(xylylene ether), a Miscible Blend

Proton spin diffusion experiments, utilizing multiple-pulse methods, have been conducted on three samples of a 50/50 blend of polystyrene (PS) and poly(xylylene ether) (PXE) as well as on the PS and PXE homopolymers. The PS/PXE blend is considered miscible on a molecular scale; this was verified by demonstrating the strength of the early-time spin diffusion between the PS and PXE protons. An attempt was made to estimate the average number of dissimilar monomers surrounding a repeat unit of a given type, and for this discussion a model of a simple cubic lattice was adopted. This exercise involved several other assumptions and was semiquantitative

Properties and derivatives of the solid-state imaging sequence zig-zag

The zig-zag six-pulse nuclear magnetic resonance line-narrowing sequence, used in conjunction with sinusoidally driven magnetic field gradients, reduces the demands on radio-frequency power common to multiple-pulse methods, and allows images to be obtained which are largely free from distortions due to bulk susceptibility, chemical shift, and magnet inhomogeneity broadening. In this paper fundamental properties of the sequence are calculated to first order in the Magnus expansion. It is shown theoretically that the sequence may be improved substantially by means of compensated derivatives of 12 and 24 pulses.

Multiple-pulse methods of1H N.M.R. imaging of solids: second-averaging

Multiple-pulse cycles composed of short (approximately 2–4 μs) gradient pulses intercalated between solid echoes are examined to find useful approaches to solid-state imaging. The usefulness of a cycle is judged on how well it fulfils requirements that the dipolar terms in the Magnus expansion of the average Hamiltonian of the cycle vanish to zeroth order for finite radio-frequency pulse lengths, and to first order in a delta-function pulse approximation; that all important first-order dipolar cross-terms also vanish; and that chemical-shift evolution is suppressed. For imaging in solids, we demonstrate a phase-shifted version of the MREV-8 sequence with gradient pulses of alternating polarity applied in the long windows. The addition of phase shifts to certain pulses eliminates resonance offset and chemical-shift interactions via ‘second-averaging’, while the gradient is unaffected by this averaging since it is applied along the axis of second averaging. An image is shown of a poly(methyl methacrylate) p...

Proton spin diffusion as a tool for characterizing polymer blends

AbstractProton spin diffusion studies for characterizing minimum domain dimensions have been carried out on three blends: a 40/60 blend of nylon 6,6 with the aromatic rigid rod polymer poly(benzo[a,d]dithiazol‐2,6‐diyl‐1,4‐phenylene), PBZT, a 47/53 blend of an amorphous nylon with PBZT, and a 50/50 blend of a polyetherimide (PEI) and a polybenzimidazole (PBI). Polarization gradients necessary for these experiments were generated using both chemical shift differences (via multiple pulse techniques) and linewidth differences. Polarization readout techniques included proton lineshape deconvolution, multiple pulse proton lineshapes and 13C CPMAS spectra utilizing short CP times. The two nylon/PBZT blends are expected to phase separate from thermodynamic arguments; however, kinetic considerations, more than thermodynamics, determine domain size. In the 40/60 blend, the minimum domain dimensions of each of the nylon and the PBZT phases were about 4 nm with some scattered larger crystals of nylon. In the 47/53 blend, mixing in some regions indicated domain dimensions similar to the 40/60 blend. In contrast to the 40/60 blend, however, the 47/53 blend was still far from internal spin polarization equilibrium after spin diffusion times of 140 ms. The implication is that the sample‐average composition is not found over dimensions like 40 nm; the problem is that the possible morphological explanations are manifold. By investigating the proton rotating frame relaxation, T1p, the possibility that some of the PBZT domains are isolated from the nylon, in a spin diffusion sense, was eliminated. It is more likely that about half of the nylon protons are isolated by spin diffusion from the PBZT protons on a 140 ms timescale. The PEI/PBI blend is a compatible blend of two aromatic polymers where mixing on a molecular scale is expected. We were interested in a measurement of the lower limit of domain size using proton spin diffusion. This lower limit turned out to be about 2.5 nm based on low temperature T1p measurements as opposed to room temperature multiple pulse methods. The latter measurements monitored the disappearance of a polarization gradient between the PEI methyl protons and all the remaining protons. The superiority of the T1p measurements over the multiple pulse method for establishing the smaller minimum domain dimension is not a general result and reasons are discussed. Finally, some general remarks about characterizing polymer blends by solid state NMR, particularly blends which have undergone spinodal phase separation, are included.

The initiation mechanism of isoprene polymerization on a heterogeneous ziegler system, as studied by a multiple pulse NMR method

High-frequency NMR spectroscopy together with multiple-pulse methods was used to study the transformations occurring during the polymerization of isoprene with a heterogeneous catalyti Ziegler complex at elevated temperatures. From the structure of terminal fragments in macromolecules, the occurrence of spontaneous chain termination processes in the system was indicated, with the formation of a metal-hydride bond capable of subsequent exchange reactions. The initiation mechanism could be explained by means of the model of Cosse, modified by a consideration of the π-allylic nature of the active centre of polymerization.

Characterization of some coals and coal intercalations by high-resolution 1H (BR-24/MAS at 270 MHz) and 13C (CP/MAS) n.m.r. in solids

High resolution 1H n.m.r. studies combining the BR-24 multiple-pulse method after BURUM and RHIM with fast magic-angle sample spinning (MAS), and high resolution solid-state 13C n.m.r. measurements with cross polarization (CP) and MAS have been used for investigating: a number of coals of different ranks; GDR soft brown coals characterized by textural differences; and typical intercalations to be found in such soft brown coals. The resolution level of 1H n.m.r. spectra achieved so far at a resonance frequency of 270 MHz permits determination of essential functional groups. The coalification series ranging from soft brown coal to anthracite shows an increase of the aromaticity values ƒ a C and also an increase of the ƒ a H values, which is analogous to the increases in reflectance and carbon content. There are marked differences between the functional groups to be found in detrital and xylite-containing soft brown coals. Tertiary (Miocene) brown coals from the 2nd Lower Lusatian seam (GDR) (i.e. unbedded, poorly bedded, bedded, xylite-containing unbedded, and gelified bedded coals) differ significantly due to variations in micropetrographical composition. Varying amounts of cellulose and lignin are contained in the xylites of the 2nd Lower Lusatian seam. 13C and 1H n.m.r. measurements are employed to determine the main constituents of substances such as pyropissite, retinite, fusite and ‘monkey hairs’ (fossilized rubber).

ChemInform Abstract: Modern NMR Techniques for Structure Elucidation

The rapid growth during the last decade of multiple pulse methods in one (1D) and two-dimensional (2D) NMR has produced over 100 new techniques of potential use in high-resolution NMR. This review sets out to identify and to illustrate those new methods which are most useful in the determination of molecular structure. Ten basic techniques are considered, including 2D homo- and hetero-nuclear chemical shift correlation by direct and by relayed coherence transfer, and 1D polarization transfer. Numerous variations on the basic techniques, for example multiple quantum filtration, are also discussed. Particular attention is paid to the problems of sensitivity and of the optimum choice of experimental techniques for different classes of structural problem.

Modern NMR techniques for structure elucidation

AbstractThe rapid growth during the last decade of multiple pulse methods in one (1D) and two‐dimensional (2D) NMR has produced over 100 new techniques of potential use in high‐resolution NMR. This review sets out to identify and to illustrate those new methods which are most useful in the determination of molecular structure. Ten basic techniques are considered, including 2D homo‐ and hetero‐nuclear chemical shift correlation by direct and by relayed coherence transfer, and 1D polarization transfer. Numerous variations on the basic techniques, for example multiple quantum filtration, are also discussed. Particular attention is paid to the problems of sensitivity and of the optimum choice of experimental techniques for different classes of structural problem.

The application of double-quantum and spin echo nuclear magnetic resonance spectroscopy to the assignment of 1H chemical shifts of RNA oligomers

Specific assignment of the non-exchangeable base and ribose anomeric protons is difficult for oligoribonucleotides with a high pyrimidine content. Problems arise from the narrow range of pyrimidine H-6 chemical shifts and the overlap of pyrimidine H-5 resonances with the ribose anomeric proton signals. To solve these problems two multiple pulse methods were used which depend on the scalar coupling between H-6 and H-5. The first method was double-quantum nmr which established the connectivity between specific pyrimidine H-6 and H-5 resonances. A refocussing pulse was added to the end of the double-quantum pulse sequence to improve the identification of several H-6 and H-5 connectivities in a single experiment. The second procedure involved homonuclear decoupled spin echo nmr to remove H-5 signals selectively from the ribose anomeric proton region. These techniques are illustrated by the assignment of the spectra of the oligoribonucleotides UpUpC, UpUpU, GpCpUpC, and UpApGpCpUpU.

Resolution of proton-phosphorus dipolar splitting in a KH2PO4 single crystal with the NMR multiple pulse method WHH4

Resolution of proton-phosphorus dipolar splitting in a KH2PO4 single crystal with the NMR multiple pulse method WHH4

Measurement of the high-resolution proton magnetic resonance spectrum of solid polystyrene by various NMR multiple pulse methods

Measurement of the high-resolution proton magnetic resonance spectrum of solid polystyrene by various NMR multiple pulse methods