Mariusz Maćkowiak

Determination of the symmetry of the electric field gradient tensor for spin—32 unclei in polycrystalline 2,4,6-trichloro-1,3,5-triazine by two-dimensional nutation FT/maximum entropy method NQR spectroscopy

Abstract An approach for the evaluation of the asymmetry parameter, η, of the electric field gradient tensor for spin - 3 2 systems from two-dimensional nutation spectroscopy using the maximum entropy method (MEM) is presented. The superiority of the MEM over the traditional fast Fourier transform (FFT) method is demonstrated in NQR spectral analysis of two-dimensional (2D) data. A comparison of the theoretically simulated 2D spectrum (FFT and MEM) with that recorded experimentally is shown to permit the evaluation of η with improved sensitivity. Use of the MEM in 2D-NQR spectroscopy can lead to sensitivity improvements, reduction of instrumental artefacts and truncation errors, shortened data acquisition times and automatic suppression of noise, while at the same time increasing the resolution. The method has been successfully demonstrated for the two 35 Cl sites in polycrystalline 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride).

NQR study of high-pressure deformation of the hydrogen bond potential

Abstract A variational correlated ground state wavefunction theory is employed for predicting and interpreting the pressure dependence of NQR frequency and isotope shift for several deuterated and undeuterated hydrogen bonded crystals. The evolution of the hydrogen bond potential with increasing pressure and the effects of deuterium isotope substitution on the quantum-fluctuation-driven phenomena are discussed. NQR-spectra parameters are strongly dependent on the bond length involved as well as the mass of tunneling particle. The correlation between the magnitude of geometric isotope effect and the NQR-frequency isotope shift as well as the value of pressure coefficients δν NQR /δ p have been found and explained on the basis of ground-state variational theory in terms of microscopic parameters of a compressed single hydrogen bond.

Size effects in the conduction electron spin resonance of anthracite and higher anthraxolite.

Electron paramagnetic resonance spectroscopy of conduction electrons, i.e. Conduction Electron Spin Resonance (CESR), is a powerful tool for studies of carbon samples. Conductive samples cause additional effects in CESR spectra that influence the shape and intensity of the signals. In cases where conduction electrons play a dominant role, whilst the influence of localized paramagnetic centres is small or negligible, the effects because of the spins on conduction electrons will dominate the spectra. It has been shown that for some ratios of the bulk sample sizes (d) to the skin depth (δ), which depend on the electrical conductivity, additional size effects become visible in the line asymmetry parameter A/|B|, which is the ratio of the maximum to the absolute, minimum value of the resonance signal. To study these effects the electrical direct current–conductivity and CESR measurements are carried out for two amorphous bulk coal samples of anthracite and a higher anthraxolite. The observed effects are described and discussed in terms of the Dyson theory. Copyright

High pressure NQR studies of hydrogen bonds for complexes of pentachlorophenol with nitrogen bases

Abstract The NQR spectrum of 35 Cl nuclei for complexes of pentachlorophenol with nitrogen bases has been studied as a function of pressure and temperature. It is shown that the value of the pressure coefficient of the NQR frequency is related to the degree of proton transfer. A distinct anomaly of the pressure coefficient of the NQR frequency in the vicinity of 50% proton transfer hydrogen bonds has been observed. The phenomena have been interpreted on the basis of the Matsushita and Matsubara model and by assuming the pressure dependence of proton transfer equilibrium. The influence of the crystal effect and torsional vibrations on the temperature-pressure characteristics of the NQR spectrum is discussed.

PEANUT experiment in NQR spectroscopy for I=3/2

The experiment with phase inversion and phase-inverted echo-amplitude detected nutation (PEANUT) was introduced in the nuclear quadrupole resonance (NQR). Formulas were obtained describing the NQR (I=3/2) experiment. Exemplary experiments are provided confirming the predicted particularities of the PEANUT spectra in NQR Cl-35. It is proposed to apply the method for the purpose of determination the asymmetry parameter of the electric field gradient (EFG) tensor in powders with the help of the analysis of PEANUT interferograms. Application of two-dimensional PEANUT experiments, in which the nutation frequencies correlate with the resonance NQR frequencies, can substantially simplify the interpretation of complex spectra.

Determination of rotational angles from two-dimensional exchange NMR and NQR spectra of powder samples

Abstract The reconstruction of angular distributions from two-dimensional (2D) magnetic resonance spectra of powders is described. The analysis involves no model assumptions and is applicable to 2D exchange NMR as well as 2D nutation exchange NQR. The proposed method is based on the constrained analysis of selected dominant ridges of a 2D powder spectrum. Practical applications of the proposed technique are demonstrated with experimental 13 C 2D exchange NMR spectrum of dimethylsulfone and calculated 35 Cl 2D nutation exchange NQR spectra of chloral hydrate. Rotational angle distributions for methyl groups in dimethylsulfone and CCl 3 group in chloral hydrate have been found. The chemical shift anisotropy of 13 C in dimethylsulfone was determined with high accuracy from 2D exchange NMR powder spectra.

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.

NQR Study of molecular dynamics of kda uetder high hydrostatic pressure

Abstract The Influence of hydrostatic pressure up to 2.5 kbar on the NQR spectrum of the 75As nuclei in KH2AsO4 is studied from 77 K to Tc. Using the disappearance of the NQR signal at T to monitor the ferroelectric phase transition, the pressure dependence of Tc Was studied leading to dTc/dp=-2.1 deg/kbar. The energies of the H2AsO4 group tanneling and dipole-dipole interaction as functions of pressure, as well as the change in volume at the transitions, are determined, The pressure dependences of the H2AsO4 group tunneling energy and dipole-dipole interaotion derived from the pressure shift of Curie temperature permitted the desoription of the temperature and pressure dependences of the NQR frequencies throughout the entire range of temperature and pressure applied. The particular utility of NQR under high hydrostatic pressure for studies of the lattice dynamics of ferroelectric substances has been stressed.

p-Chlorobenzotrichloride revisited: novel analytical treatments of 2D exchange NQR in molecular crystals

A theoretical treatment of the two‐dimensional exchange nuclear quadrupole resonance pulse sequence is presented and applied to the quantitative study of exchange processes in molecular crystals. The theory is successfully tested for the exchange spectra of hindered trichloromethyl groups of p‐chlorobenzotrichloride, where the conventional approach without taking into account the off‐resonance phenomena has failed. The mixing dynamics by exchange and the expected diagonal‐ and cross‐peak intensities as a function of the mixing time have been derived. It is shown that the off‐resonance effects are of crucial importance for the quantitative description of the exchange spectra. Copyright

Off-resonance effects in two-dimensional exchange NMR at zero-field

The effects of off-resonance irradiation in 2D exchange NMR at zero-field are analysed. A theoretical treatment of the 2D exchange NMR pulse sequence is presented and applied to the quantitative study of exchange processes in molecular crystals. It takes into account the off-resonance irradiation, which critically influences the spin dynamics. The response of a system of spins I=3/2 in zero applied field to the three-pulse sequence is analysed. The mixing dynamics by exchange and the expected cross-peak intensities as a function of the frequency offset has been derived. It is shown that the off-resonance effects are of crucial importance for the quantitative description of the exchange spectra. The theory is successfully tested for the exchange spectra of hindered trichloromethyl groups of p-chloroanilinium trichloroacetate, where the conventional approach without taking into account the off-resonance phenomena has failed.