D. Ya. Osokin

Two-frequency multiple-pulse sequences in nitrogen-14 NQR

The theoretical investigation of the multiple-pulse sequences in which the radio-frequency pulses are applied simultaneously at two transitions are presented. The time-independent solution of the quantum Liouville equation for the average Hamiltonian of inhomogeneous broadening in the sequence is obtained by the projection operator technique. This solution predicts the multiple echo signals on the third transition that is not irradiated by radio-frequency pulses.

Two-frequency composite pulses in NQR

The application of multiple-pulse sequences consisting of two-transition composite pulses for remote nuclear quadrupole resonance detection of explosives and narcotics is discussed. The pulse sequence for obtaining these pulses is described and it is shown that these pulses are equivalent to the pulse applied at the third (nonirradiated) transition. The loss of the signal intensity in this case is absent.

Toggling-frame NQR imaging

NQR imaging based on spin precession in toggling reference frame and radio frequency field encoding is proposed. The profile of a NaNO2 monocrystalline phantom obtained using the method proposed is demonstrated. The application of the method in remote NQR detection and NMR surface coil techniques is suggested.

Transient NQR signals in the effective field of multipulse trains

This paper is a theoretical analysis of time-dependent nutation and echo signals in the effective field of multipulse trains in NQR. The results of experimental investigations of the same aspects were reported in earlier papers. The various features of dipole-dipole interactions in three-level quadrupole spin systems are discussed. It is shown that, in contrast to NMR, the dipole Hamiltonian in the interaction representation determined by the quadrupole Hamiltonian contains only three diagonal components of the dipole-dipole interaction tensor. On the other hand, the strong inhomogeneous broadening characteristic of NQR hinders exact measurement of these components by ordinary methods. The theoretical analysis suggests that the decay of the echo-signal envelope in the effective field of multipulse trains is determined solely by the dipole relaxation time, which serves as justification of a new experimental method used in measuring the characteristics of the dipole-dipole interaction tensor in spin systems with an inhomogeneously broadened spectrum.

Ab initio calculation of 14N and 35Cl nuclear quadrupole coupling constants in aziridine and Cl-aziridine

Abstract Ab initio calculations of electric field gradients (EFG) at 14 N and 35 Cl nuclei and conversion to the corresponding nuclear quadrupole coupling constants are reported for aziridine and Cl-aziridine molecules. The structural parameters for Cl-aziridine used for these calculations are obtained as a result of gradient optimization at the HF/4–31G ∗ level. The mutual orientation of the molecular axes and principal axes of the EFG tensor is presented. To evaluate the main contribution from various bonds and lone pairs to the EFG tensor components, a localized molecular orbital (LMO) analysis is made. The comparison of the results of the calculation with experiment is discussed.

Theory of two-frequency excitation in14N NQR

A simple and effective technique for calculating the transient signals in three-level spin systems irradiated at one and two frequencies is described. All possible signals are presented and discussed. The application of the two-frequency excitation technique for the investigation of the inhomogeneity of the crystalline lattice is suggested.

Nonempirical calculations of NQR parameters in some three-membered rings

Abstract Ab initio calculations (RHF/6–31G∗) of the electric field gradients (EFGs) at 14N,17O, 2H and 33S and the corresponding nuclear quadrupole coupling constants (NQCCs) are reported for a number of three-membered rings containing atoms of the first and the second rows. The results of the calculations are in good agreement with the available experimental data. The orientations of the principal axes of the EFG tensor at the quadrupole nuclei are determined. The origin of the EFG is analyzed within the framework of the LMO approach. To compare the charge density distributions in aziridine and Cl-aziridine, the charge density maps are obtained using the canonical molecular orbitals.

Determination of the local inhomogeneity of a crystal lattice using a two-frequency nuclear quadrupole resonance method

Nonuniform line broadening in quadrupole spin systems is analyzed. It is shown theoretically that this broadening is of the tensor type. This forms the basis of the method for analyzing the distribution of local inhomogeneities in the crystal lattice, which is verified experimentally on a sodium nitride sample.

Optimal filtering in multipulse sequences for nuclear quadrupole resonance detection

The application of the multipulse sequences in nuclear quadrupole resonance (NQR) detection of explosive and narcotic substances has been studied. Various approaches to increase the signal to noise ratio (SNR) of signal detection are considered. We discussed two modifications of the phase-alternated multiple-pulse sequence (PAMS): the 180° pulse sequence with a preparatory pulse and the 90° pulse sequence. The advantages of optimal filtering to detect NQR in the case of the coherent steady-state precession have been analyzed. It has been shown that this technique is effective in filtering high-frequency and low-frequency noise and increasing the reliability of NQR detection. Our analysis also shows the PAMS with 180° pulses is more effective than PSL sequence from point of view of the application of optimal filtering procedure to the steady-state NQR signal.

A two-frequency coherent pulse NQR spectrometer

A two-frequency coherent pulse spectrometer has been designed for seeking and recording nuclear quadrupole resonance spectra and studying coherence transfer processes, quasi-stationary states, and transient processes in multipulse sequences. The functional diagram of this spectrometer can be used in production of equipment for remote detection of explosives and drugs, which will require only an increase in the output power of the transmitter.