Nutation NQR Research Articles

Determination of mutual orientation of tensor interactions from two-dimensional nutation NQR-spectra of solids

Expressions for singularities of 2D correlation nutation NQR-spectra are obtained. A method is proposed for measuring orientation of two tensor interactions in powder specimens, which relies on specimen re-orientation in an rf-coil within the pulse period between the second and third pulses of a sequence of stimulated echo signal and detection of a 2D nutation NQR spectrum. Feasibilities of the method are discussed and simulated spectra for the case of nuclei with the spin I = 3/2 are reported.

Two-Dimensional Nutation NQR Broad-Line Spectra in Oriented Samples

The NQR nutation method to determine the electric field gradient asymmetry parameter η in systems, where the resonance line is so broad that the radio frequency field can excite only a portion of the nuclear spins, is presented. In this situation, the recently developed spectroscopic methods are not applicable. Two-dimensional nutation NQR spectra of oriented powders are calculated and used to determine η at particular frequencies along a broad NQR line. The proposed method is useful for single crystals, oriented powders, glasses, and disordered systems even for small values of the asymmetry parameter. Therefore it can be used to evaluate fluctuations in η and the quadrupole coupling constant e2qQ due to inhomogeneities. We demonstrate the application of this method to oriented chalcogenide semiconducting glasses.

Off-Resonance Nutation NQR Spectra

Various methods for measuring multivariate nutation NQR spectra under off-resonance conditions are considered. A method is proposed which is based on optimum filtration. An experimental Cl-35 3D nutation NQR spectrum of chloral hydrate is reported for the first time.

Nutation Spectra of Nuclear Quadrupole Resonance in Off-Resonance Conditions

The modes of recording the multidimensional NQR nutation spectra have been analyzed using different off-resonance methods. A method of recording the nutation spectra in off-resonance conditions, based on optimal filtration, has been proposed. For the first time, an experimental spectrum of 3D-35Cl NQR nutation of chloral hydrate is presented.

Orientation Of Tensorial Interactions Determined From Two-Dimensional Nutation Exchange Nqr And Nmr Powder Spectra

The method for determining the mutual orientation of molecular interaction tensors in powders is described. The technique is based on 2D nutation exchange NQR and NMR spectroscopy. It is shown that the 2D nutation exchange spectra exhibit characteristic ridges, which reflect the motional mechanism in a model-independent fashion. The angles through which the molecule rotates can be read from elliptical ridges in the 2D spectra. The 2D nutation exchange NMR and NQR powder patterns are calculated for spins I = 1 and I = 3/2 for different symmetry of reorienting molecular groups.

Two-dimensional Exchange and Nutation Exchange Nuclear Quadrupole Resonance Spectroscopy

A theoretical treatment of the 2D exchange NQR pulse sequence is presented and applied to a quantitative study of exchange processes in molecular crystals. It takes into account the off-resonance irradiation, which critically influences the spin dynamics. The response to the threepulse sequence of a system of spins I = 3/2 in zero applied field, experiencing electric quadrupole couplings, is analysed. The mixing dynamics by exchange and the expected cross-peak intensities as a function of the frequency offset have been derived. The theory is illustrated by a study of the optimization procedure, which is of crucial importance for the detection of the cross- and diagonal-peaks in a 2D-exchange spectrum. The systems investigated are hexachloroethane and tetrachloroethylene. They show threefold and twofold reorientational jumps about the carbon-carbon axis, respectively. A new method of direct determination of rotational angles based on two-dimensional nutation exchange NQRspectroscopy is proposed. Themethod involves the detection of exchange processes through NQR nutation spectra recorded after the mixing interval. The response of a system of spins I = 3/2 to the three-pulse sequence with increasing pulse widths is analyzed. It is shown that the 2D-nutation exchange NQR spectrum exhibits characteristic ridges, which manifest the motional mechanism in a model-independent fashion. The angles through which the molecule rotates can be read directly from elliptical ridges in the 2D spectrum, which are also sensitive to the asymmetry parameter of the electric field gradient tensor.

Two-dimensional nutation exchange NQR spectroscopy: direct determination of rotational angles

A new method for the direct determination of rotational angles based on 2-dimensional nutation exchange NQR spectroscopy is proposed. The method involves the detection of exchange processes through NQR nutation spectra recorded after the mixing interval. The response is analysed of a system of spins I = 3/2 in zero applied field, experiencing electric quadrupole couplings, to the three-pulse sequence with incrementing pulse widths. The systems investigated here were hexachloroethane and tetrachloroethylene, which show threefold and twofold reorientational jumps about the carbon-carbon axis, respectively. It is shown that the 2D nutation exchange NQR spectrum exhibits characteristic ridges, which reveal the motional mechanism in a model-independent fashion. The angles through which the molecule rotates can be read directly from elliptical ridges in the 2D spectrum, which are also sensitive to the value of asymmetry parameter of the electric field gradient tensor.

Application of Nutations to Studying the Exchange Processes by the Method of Nuclear Quadrupole Resonance Spectroscopy

A method of direct determining the discontinuous rotational reorientation angles from NQR exchange spectra based on recording of two-dimensional nutations of magnetization after the mixing period is suggested. The possible modification of the Jeener pulse sequence with independently varying duration of pulses for obtaining a 2D exchange nutation spectrum is described. Shapes of singularities in the 2D nutation spectrum of CCl2- and CCl3-group reorientations, which depend on the angle θS between C–Cl bonds and on the asymmetry parameter for nonequivalent positions of nuclei participating in the exchange are examined. The 2D exchange NQR nutation spectra of 35Cl in powders prepared from C2Cl4 and C2Cl6 molecular crystals are modeled. The advantages and capabilities of the method are discussed.

Lineshape Analysis of 2D NMR and NQR Nutation Spectra of Integer and Half-Integer Quadrupolar Nuclei

Abstract A lineshape analysis of the NMR and NQR powder nutation spectra of integer and half-integer quadrupolar nuclei is presented. Simulated NMR nutation spectra of spin I = 1 and 3 nuclei are reported. For I = 1 the formulas for the singularities of the NMR nutation powder patterns as a functions of η, e2Qq and 7B x are given. The NQR nutation powder patterns for spin I = 3/2, 5/2, 7/2, and 9/2 for different induced transitions are calculated, and some experimental aspects of the method are discussed. A universal empirical formula to facilitate the determination of the asymmetry parameter from the NQR nutation frequency singularities for any arbitrary spin or transition is found. The NQR nutation spectra for half-integer and integer spins are compared. For integer spins the two-frequency excitation of the nutation spectrum is analysed. Application of the 2D-nutation lineshape analysis for the determination of the quadrupole interaction parameters is emphasised.

Influence of the rf Field Inhomogeneity on Nutation NQR Spectra of Spin 3/2 Nuclei in Powders

Abstract The influence of the rf field inhomogeneity on the NQR nutation spectra of spin / = 3/2 nuclei in powder samples is investigated. To eliminate the rf field inhomogeneity effects, a method of reconstruction of the NQR nutation spectra, based on finding the deconvolution of the Fourier nutation spectrum with a function of the rf field distribution, is used. The method is successfully demonstrated for simulated and experimental NQR nutation spectra of 35C1 in TiCl4 . The lineshape analysis of reconstructed nutation spectra allowing the determination of the EFG asymmetry parameter is given. The real advantage of the proposed method is that the high-resolution nutation spectrum may be obtained for a sample filling up the entire coil.

Determination of the asymmetry parameter of EFG tensor from moments of NQR nutation spectra in powders

A novel approach to determination of the asymmetry parameter of the EFG tensor from zero-field nutation NQR spectra of the spinI = 3/2 nuclei in powder samples is reported. The proposed theoretical treatment uses lineshape analysis of the nutation NQR spectra by the method of line moments. The analytical formulas for the lineshape of the powder nutation spectrum are given. It is shown that the asymmetry parameter can be determined from the second moment 〈ω2〉 and the frequency of only one singularity ω2 of the nutation spectrum. It is also shown that the asymmetry parameter can be determined from the second and fourth spectrum moments alone. The method is successfully demonstrated for the simulated nutation NQR spectra of the spinI = 3/2 nuclei in powder samples.

2D Off-Resonance Nutation NQR Spectroscopy of Spin 3/2 Nuclei

Abstract The effects of off-resonance irradiation in nutation NQR experiments are demonstrated both experimentally and theoretically. The theoretical description of the off-resonance effects in 2D nutation NQR spectroscopy is given, and general exact formulas for the asymmetry parameter are obtained. It is shown that the outcome of the off-resonance nutation experiments depends on the data acquisition procedure, leading to one- or three-line nutation spectra. To explain this fact and to describe the off-resonance nutation experiment properly, the transient response theory of a quadrupolar spin system to an RF pulse was modified using the wave-function approach. The 2D separation of interactions technique has been applied to separate a static and a randomly time-fluctuating dynamic part of the quadrupole interaction in the antiferroelectric phase of poly-crystalline ammonium dihydrogen arsenate. Two-dimensional off-resonance nutation NQR spectroscopy has been used to determine the full quadrupolar tensor of spin-3/2 nuclei in several molecular crystals containing the 35Cl and 75As nuclei. The off-resonance phenomena in 2D NQR are very important from the practical point of view (determination of ƞ in multiple or broad-line spectrum) and also provide interesting information on the dynamic properties of a quadrupolar spin system.

Enhanced Information Recovery in 2D On-and Off-Resonance Nutation NQR using the Maximum Entropy Method

Abstract Two-dimensional zero-field nutation NQR spectroscopy has been used to determine the full quadrupolar tensor of spin - 3/2 nuclei in serveral molecular crystals containing the 3 5 Cl and 7 5 As nuclei. The problems of reconstructing 2D-nutation NQR spectra using conventional methods and the advantages of using implementation of the maximum entropy method (MEM) are analyzed. It is shown that the replacement of conventional Fourier transform by an alternative data processing by MEM in 2D NQR spectroscopy leads to sensitivity improvement, reduction of instrumental artefacts and truncation errors, shortened data acquisition times and suppression of noise, while at the same time increasing the resolution. The effects of off-resonance irradiation in nutation experiments are demonstrated both experimentally and theoretically. It is shown that off-resonance nutation spectroscopy is a useful extension of the conventional on-resonance experiments, thus facilitating the determination of asymmetry parameters in multiple spectrum. The theoretical description of the off-resonance effects in 2D nutation NQR spectroscopy is given, and general exact formulas for the asymmetry parameter are obtained. In off-resonance conditions, the resolution of the nutation NQR spectrum decreases with the spectrometer offset. However, an enhanced resolution can be achieved by using the maximum entropy method in 2D-data reconstruction.

Rapid Measurement of Nutation NQR Spectra in Powders Using an RF Pulse Train

Abstract The method of two-dimensional nutation NQR spectroscopy, introduced by Harbison in 1989, has been successfully used for determining of the asymmetry parameter of the EFG tensor for spin-3/2 nuclei in both powdered and monocrystal samples in the absence of an external magnetic field. The inconvenience inherent in the method, however, is the long time required, because data acquisition must be repeated for various RF pulse lengths. We discuss a method to reduce the time of the nutation experiment by using a sequence of identical short RF pulses of length r w and distance τ. It is shown that for an NQR frequency ω0 , frequency offset Δω, and pulse parameters satisfying the relation ω0τ + Δωtw = 2πk (k = 1, 2, 3,..., n) a synchronism of pulse action takes place and the nutation interferogram can be measured "stroboscopically" between the pulses. The maximum time saving factor that can be obtained as compared to the conventional nutation experiment is of the order of the number of pulses used in a pulse train. The method has been successfully applied for determination of the asymmetry parameter for one of the two 3 5 Cl sites in polycrystalline 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride).

Off-Resonance Nutation NQR Spectroscopy of Powders

Abstract The peculiarities of nutation NQR spectra of nuclei with spin I=3/2 in powders are considered in the case of a resonance offset. An equation, which takes into account the offset is given for the determination of the asymmetry η. It is shown that structural of monocrystals by means of nutation spectroscopy are possible.

Off-Resonance Effects in Two-Dimensional Zero-Field Nutation NQR Spectroscopy

Two-dimensional zero-field nutation NQR spectroscopy enables one to determine the quadrupole coupling constant and the asymmetry parameter in a single experiment on powder samples. The method has been originally developed for the conditions which apply to narrow-line solids so that the nutation frequency is much larger than the NQR linewidth. The irradiation must also be on-resonance. In broad-line solids, these conditions are not fulfilled, and consequently the nutation frequency changes over the NQR lineshape with the irradiation offset. The nutation lineshape varies with offset over the NQR spectrum in a complicated way. The rather simple nutation lineshapes, which can be used for the asymmetry parameter η determination, are obtained only in the on-resonance irradiated parts of the NQR spectrum. Even there, the presence of a finite homogeneous linewidth smears and obscures the characteristic nutation lineshapes, which for narrow homogeneous linewidths exhibit singularities and shoulders. In this paper the theoretical description of the off-resonance effects in the 2D nutation NQR spectroscopy is given and the results are compared to the experiment for a broad-line proton glass Rb 0.98(NH 4) 0.02 H 2AsO 4 at T = 77 K.

Singularties of the shape of the nonresonance nutation spectra of NQR powders

The singularities of nutation NQR spectra for nuclei with spin J=1, 3/2 in the absence of resonance are considered for powders. It is shown that when determining the asymmetry parameter η from the frequency characteristics of the nutation spectrum one must take into account the frequency shift of the spectrometer relative to exact resonance. Structural studies of single crystals by nutation spectroscopy are demonstrated to be feasible.

On the Reconstruction of NQR Nutation Spectra in Solids with Powder Geometry

Abstract In single crystals the NQR nutation frequency depends on the relative orientation of the coil and the quadrupole axes. In powders the nutation lineshape is a superposition of spectra from the randomly oriented single crystals, so that powder patterns appear in such experiments if the recon­struction is performed by the Fourier transform method. In this paper an alternative reconstruction method of nutation spectra is suggested making use of the Hankel Transform. In this way the nutation spectra are simplified. Singularities arising with experiments for the determination of the asymmetry parameter η can easily be resolved. In the particular case of an axially symmetric quadrupolar tensor and a homogeneous radiofrequency field one can reduce the powder pattern to a single line without heterogeneous broadening with respect to orientation. Further improvement o f the nutation spectra can be achieved by taking advantage of the maximum entropy method, which strongly reduces apodisation and noise problems. Applications of the new data manipulation techniques to N Q R imaging methods published elsewhere and 2D zero-field N Q R spectroscopy are reported.