Nutation Experiments Research Articles

Time dependent measurements of nitrous oxide and carbon dioxide collisional relaxation processes by a frequency down-chirped quantum cascade laser: Rapid passage signals and the time dependence of collisional processes

Intrapulse quantum cascade laser spectrometers are able to produce both saturation and molecular alignment of the gas sample. This is due to the rapid sweep of the radiation through the absorption features. The intrapulse time domain spectra closely resemble those recorded in coherent optical nutation experiments. In the present paper, the frequency down-chirped technique is employed to investigate the nitrous oxide-foreign gas collisions. We have demonstrated that the measurements may be characterized by the induced polarization dominated and collision dominated measurement limits. The first of these is directly related to the time dependence of the long range collision cross sections. Among the collisional partners considered, carbon dioxide shows a very unusual behavior of rapid polarization damping, resulting in the production of symmetrical line shapes at very low gas buffer pressures. In the collision dominated regime, the pressure broadening parameters, which we have derived, are comparable at slow chirp rates, with those derived from other experimental methods. By comparing the pressure broadening coefficients of Ar, N(2), and CO(2) with those of He, making use of the chirp rate independence of the pressure broadening by helium, we have shown that at higher chirp rates there is clear evidence of the chirp-rate dependence of the pressure broadening parameters of N(2) and CO(2).

The utility of phase alternated pulses for the measurement of dipolar couplings in 2D-SLF experiments

The measurement of hetero-nuclear dipolar coupling using two-dimensional separated local field (SLF-2D) NMR experiments is a powerful technique for the determination of the structure and dynamics of molecules in the solid state and in liquid crystals. However, the experiment is sensitive to a number of factors such as the Hartmann–Hahn match condition, proton frequency off-set and rf heating. It is shown here that by the use of phase alternated pulses during spin-exchange the effect of rf mismatch on the dipolar coupling measurement can be compensated over a wide range of off-sets. Phase alternation together with time and amplitude modulation has also been considered and incorporated into a pulse scheme that combines spin exchange with homonuclear spin decoupling based on magic sandwich sequence and named as SAMPI4. Such time and amplitude averaged nutation experiments use relatively low rf power and generate less sample heating. One of these schemes has been applied on liquid crystal samples and is observed to perform well and yield spectra with high resolution.

Decomposition of a NMR signal comprising a single resonance in exchange with a Pake doublet: Application to the analysis of 1H NMR spectra of water molecules embedded in synthetic clays

Conventional 1H NMR spectra of water in clays generally consist of several broad overlapping resonances. However, when a type of water is partially oriented, it leads to a Pake doublet superposed to the resonances corresponding to different types of non-oriented water. With the help of a recent method based on nutation experiments, the proportions of the different kinds of water could be determined. Thus, we are able to determine interesting NMR parameters like dipolar couplings, exchange rate, and line broadening. These determinations are complemented by spin relaxation times and self-diffusion measurements which bring additional information about the dynamical behavior of water.

Electron spin–lattice and spin–spin relaxation study of a trinuclear iron(iii) complex and its relevance in quantum computing

Electron spins of molecular magnets are promising candidates for large scale quantum information processing because they exhibit a large number of low-lying excited states. In this paper X-band pulse electron paramagnetic resonance spectroscopy is used to determine the intrinsic relaxation times T1 and T2 of a molecular magnet with an S = 1/2 ground state, namely the neutral trinuclear oxo-centered iron (III) complex, [Fe3(micro3-O)(O2CPh)5(salox)(EtOH)(EtOH)(H2O)]. The temperature dependence of the spin-lattice relaxation time T1 between 4.5 and 11 K shows that the Orbach relaxation process is dominant with the first excited state lying 57 cm(-1) above the ground state, whereas the phase memory time T(M) is of the order of 2.6 micros and exhibits a modest temperature dependence. These results together with previous magnetic measurements give further insight into the magnetic properties of the complex. The coherent manipulation of the electron spins is also examined by means of transient nutation experiments.

A Morphological Study of Poly[bis(trifluoroethoxy)phosphazene] Using High Resolution Solid-State 1H, 19F, 31P and 13C NMR Spectroscopy

High-resolution 19F, 1H, 31P and 13C solid-state NMR methods were assessed to ascertain their suitability for studying the morphological behavior in the crystalline domain of phosphazene polymers with partially fluorinated side-chains. Poly[bis(trifluoroethoxy)phosphazene] (PBFP) was used as a sample system. Fast magic angle spinning (MAS), along with simultaneous 19F and 1H decoupling using the xy-16 sequence, were employed, as this has proven to greatly improve resolution in 13C spectra of perfluorinated materials. Information obtained from Discrimination Induced by Variable Amplitude Minipulses (DIVAM) nutation experiments and cross-polarization (CP) methods aided the deconvolution analysis used to identify all components in the 1H, 19F and 13C signals. DIVAM nutation experiments were also used to discriminate between signals from the amorphous and crystalline domain. The crystallinity in the solvent-cast PBFP was determined to be approximately 70%, which was seen to increase to approximately 80% in the heat-treated material, for all nuclei studied. A preliminary assignment was made for the crystalline signals in the 1H and 13C spectra to the α-, β- and γ-phases. Therefore, high-resolution 13C and 1H methods are valuable tools for morphological investigations into this class of polymer.

Electron paramagnetic resonance and non‐resonant microwave absorption of single wall carbon nanotubes

AbstractSingle wall carbon nanotubes (SWNT) prepared by the “super growth” method exhibit electron paramagnetic resonance (EPR) signals, which originate from temperature activated detrapping of spins. Non‐resonant microwave absorption in the temperature range below 20 K is indicative for the opening of a small gap at the Fermi level for tubes of metallic character, which might indicate a transition into a superconducting (SC) state. The expected concurrent microwave field inhomogeneity has been detected by a transient nutation experiment. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

1H nutation experiments under low-amplitude radiofrequency fields

It is shown theoretically and experimentally that, under weak radiofrequency (RF) field conditions, nutation frequency depends on relaxation times. The Fourier transform of a nutation curve, which is obtained by plotting signal amplitude as a function of RF field application time, yields separate bands corresponding to the components of an unresolved composite signal. This can be effectively achieved provided that these components possess different relaxation characteristics. Using proton nuclear magnetic resonance, this new method is applied to the study of water in clays and leads to the proportion of different types of water. The case where one water species appears in the form of a Pake doublet is considered.

Separation of components of a 1H NMR composite signal by nutation experiments under low amplitude radio-frequency fields. Application to the water signal in clays

NMR nutation experiment consists in probing the evolution of nuclear magnetization perpendicularly to a radio-frequency field applied for a variable time. It is shown theoretically and experimentally that, under weak radio-frequency field conditions, the nutation frequency depends on relaxation rates. The Fourier transform of the nutation curve, which is obtained by plotting the signal amplitude as a function of the rf field application time, yields separate bands corresponding to the components of the unresolved composite signal. This can be only achieved if these components possess different relaxation characteristics. This new method is applied to the study of water in clays.

14N Pulsed nuclear quadrupole resonance. 2. Effect of a single radio-frequency pulse in the general case

A novel theory, based on density operator calculations, is provided for assessing the nuclear quadrupole resonance behaviour of a spin 1 (14N) subjected to a single radio-frequency pulse. It is for a powder sample in zero magnetic field for an electric field gradient tensor without symmetry. A complete set of equations is obtained for the quantities of interest. It is derived from the general Liouville–von Neumann equation and from a proper basis on which the density operator is expanded. Theoretical results, in terms of signal evolution as a function of the pulse length (nutation experiments), show that the same nutation curve is expected for the three different transitions which exist when the electric field gradient tensor is without symmetry. This latter nutation curve is, however, different from that which prevails in the case of an axially symmetric tensor, this apparent discrepancy being easily resolved on theoretical grounds. Experimental data (for NaNO2, electric field gradient tensor without symmetry) are checked against values of the radio-frequency field amplitude provided by NMR measurements performed with the same equipment. Good agreement between theory and experiment is obtained.

14N pulsed nuclear quadrupole resonance. 1. Nutation experiments in the case of an axially symmetric electric field gradient tensor

An exact theory based on density matrix calculations is provided for assessing the nuclear quadrupole resonance (NQR) behaviour of a spin 1 (14N) subjected to a single radio-frequency pulse. It concerns a powder sample in zero magnetic field with the assumption of an axially symmetric electric field gradient tensor. Theoretical results, in terms of signal evolution as a function of the pulse length (nutation experiments), are checked against values of the radio-frequency field amplitude provided by NMR measurements performed with the same equipment. Good agreement between theory and experiment is obtained.

1H, 13C, and 15N NMR Pulse Width Determination for Labeled Proteins in H2O Solution: Nutation Pulse Sequences Including WATERGATE and Analysis by MUSASHI

Abstract We developed MUSASHI, a method to determine NMR 90 degree pulse width by nonlinear least square curve fitting. A nutation experiment is carried out and then the data set is sent to MUSASHI program. We report new nutation pulse sequences for labeled proteins in water including WATERGATE scheme aimed at obtaining data for MUSASHI. The new sequences and MUSASHI make it possible to obtain 1H, 13C, and 15N pulse width required to double- and triple-resonance experiments in a short time.

NMR spectroscopy in inhomogeneous B0 and B1 fields with non-linear correlation

Resolved NMR spectra from samples in inhomogeneous B 0 and B 1 fields can be obtained with the so-called “ex situ” methodology, employing a train of composite or adiabatic z-rotation RF pulses to periodically refocus the inhomogeneous broadening during the detection of the time-domain signal. Earlier schemes relied on a linear correlation between the inhomogeneous B 0 and B 1 fields. Here the pulse length, bandwidth, and amplitude of the adiabatic pulses of the hyperbolic secant type are adjusted to improve the refocusing for a setup with non-linear correlation. The field correlation is measured using a two-dimensional nutation experiment augmented with a third dimension with varying RF carrier frequency accounting for off-resonance effects. The pulse optimization is performed with a computer algorithm using the experimentally determined field correlation and a standard adiabatic z-rotation pulse as a starting point for the iterative optimization procedure. The shape of the z-rotation RF pulse is manipulated to provide refocusing for the conditions given by the sample-, magnet-, and RF-coil geometry.

Characterization of heteronuclear decoupling through proton spin dynamics in solid-state nuclear magnetic resonance spectroscopy.

The work presented here aims at understanding the performance of phase modulated heteronuclear decoupling sequences such as Cosine Modulation or Two Pulse Phase Modulation. To that end we provide an analytical description of the intrinsic behavior of Cosine Modulation decoupling with respect to radio-frequency-inhomogeneity and the proton-proton dipolar coupling network. We discover through a Modulation Frame average Hamiltonian analysis that best decoupling is obtained under conditions where the heteronuclear interactions are removed but notably where homonuclear couplings are recoupled at a homonuclear Rotary Resonance (HORROR) condition in the Modulation Frame. These conclusions are supported by extensive experimental investigations, and notably through the introduction of proton nutation experiments to characterize spin dynamics in solids under decoupling conditions. The theoretical framework presented in this paper allows the prediction of the optimum parameters for a given set of experimental conditions.

High-resolution NMR spectroscopy in inhomogeneous B0 and B1 fields by two-dimensional correlation

Recently, there has been much interest in methods for obtaining high-resolution NMR spectra in inhomogeneous B 0 and B 1 fields and in their application to so-called ‘ex situ’ spectroscopy, where the sample and magnet/probe assembly are spatially separated. Here we discuss the implementation of the well-known two-dimensional nutation experiment as a method for correlating B 0 and B 1 inhomogeneities and, hence, achieving a high-resolution NMR spectrum. The advantages of this approach lie in its simplicity, its spatial (‘depth’) resolution, and in its not requiring a linear correlation of fields, i.e., B 1= αB 0+ k, across the sample.

Spin States of C603-and C120On-(n= 2, 3, 4) Anions Using Electron Spin Transient Nutation Spectroscopy

Electron spin transient nutation (ESTN) experiments show that the spin multiplicity of the ground state of C-60(3-) in frozen solution is a doublet with S = 1/2. In purified samples, there is no evidence for excited states or other species with higher multiplicity. In the anions Of C120On- (n = 2, 3, 4), where the CW EPR experiments have shown that a mixture of species is present, ESTN experiments confirm that a doublet with S = 1/2 is associated with the 3- anion and triplets with S = 1 are associated with the 2- and 4- anions. A weak nutation peak attributable to m(s) = -1/2 1/2 transitions within a quartet state may arise from association of anions with spins of 1/2 and 1 in solute aggregates.

Application of two-dimensional pulsed EPR nutation spectroscopy to a disordered system with largeg-anisotropy

Two-dimensional (2-D) pulsed electron paramagnetic resonance (EPR) nutation spectroscopy is applied to a disordered system with strongly anisotropicg-factors. The analysis and interpretation of a 2-D nutation spectrum is performed by theoretical calculations appropriate for ferric myoglobin cyanide in frozen solution. The observed and calculated 2-D spectra show broad signals which correspond to the principalg-values. The advantage of nutation spectroscopy is demonstrated by measuring theg x=0.93 signal, which cannot be observed in a conventional field-swept spectrum due to largeg strain broadening. It is shown that the 2-D nutation experiment can be considered as a sort of an angle-selected method but a nutation spectrum gives a powder pattern even for magnetic field settings at extreme field values of an EPR spectrum. This is in contrast to angle-selected electron nuclear double resonance, which gives a single-crystal type spectrum. The advantage of sine Fourier transform (FT) over complex FT is demonstrated for data processing in the nutation domain. Problems of experimental and data-processing procedures are also discussed.

The exclusion of excited triplets as dominating paramagnetic species in doped PPy

The deviations from the Curie susceptibility were observed for doped polypyrrole (PPy). Here, we examined the possibility that exited triplet species resulting from the combination between polarons yield such deviations. For the examination, the electron spin transient nutation (ESTN) method was applied to directly determine the spin quantum number of observed spins in the ESR spectra. The nutation experiments revealed the absence of a peak corresponding to S=1 and, moreover, the FID spectra denied the definite coexistence of two species. Thus, we conclude that the excited triplets should be excluded as main species giving the non-Curie susceptibility. In contrast, the doping-level dependences of the susceptibility were well explained in terms of the superposition of the Curie susceptibility and the Pauli one. Therefore, we conclude that the non-Curie components observed for doped PPy are attributed to the Pauli susceptibility as reported for several conducting polymers.

Pure-phase two-dimensional niobium-93 nutation spectroscopic study of lead metaniobate and the piezoelectric lead magnesium niobate

An NMR technique to measure pure-phase two-dimensional nutation NMR spectra, that yields higher resolution than traditional nutation experiments is reported. Using this technique 93 Nb nutation NMR spectra of PbNb 2O 6 and the technologically important Pb(Mg 1/3Nb 2/3)O 3 (PMN) have been measured and the quadrupolar coupling constant of the niobium site in PbNb 2O 6 ( C Q=19±2 MHz) determined. Estimates of the quadrupolar coupling constants for three different resonances associated with different niobium(V) sites in PMN ( C Q<1.2 MHz, ≈17 MHz, and >62 MHz) are also reported.

Selective Injection of Magnetization by Slow Chemical Exchange in NMR

In a system in slow dynamic equilibrium two NMR methods are shown to be suitable for injecting magnetization from one resonance to another by means of slow chemical exchange. The combined outputs of the methods may be employed to measure the value of the off-rate constant κoff in the complex. The methods are implemented experimentally using the complex of molecules composed of the enzyme Esherichia coli dihydrofolate reductase (DHFR) and the ligand folate. In an equilibrium solution with DHFR, folate is known to undergo chemical exchange between a free state and a bound state. The modified synchronous nutation method is applied to a spin of the folate molecule in the free and bound states; magnetization transfer occurs between the two sites due to the underlying exchange process. As a preliminary step for the application of the synchronous nutation method, a new one-dimensional 1H NMR technique is proposed which facilitates the assignment of the resonance of a spin in the bound state, provided the resonance of its exchange partner in the free state is known. This experiment is also used to obtain quantitative estimates of the transverse relaxation rate constant of the bound resonance. The numerical procedure necessary to analyze the experimental results of the synchronous nutation experiment is presented.

90� pulse width calibrations: How to read a pulse width array

One of the most important parts of setting up a Fourier transform–nuclear magnetic resonance experiment is the calibration of the 90° pulse width. There are many ways to calibrate this parameter, but running the experiment as a single array, with the pulse widths ranging from 0° to 450° or more (a nutation experiment), offers a distinct advantage. From this one dataset, a trained eye can evaluate—by simple inspection—as many as nine different aspects of the sample and the spectrometer. These not only include the pw90, but also probe radiofrequency homogeneity, probe arcing, T1 “abuse” (insufficient relaxation delays), radiation damping, off-resonance effects, relaxation during the pulse, nonlinear pulse behavior, and lead pickup. ©1999 John Wiley & Sons, Inc. Concepts Magn Reson 11: 165–180, 1999