Actual Problems of Computer Parametric Identification of the NMR and NQR spectra: a Review

Determining a computer model of a spin echo to simulate its amplitude and obtain information about the magnitudes and orientations of the local magnetic field, the direction of the gradient of the crystalline electric field in a sample, is an important task. The paper considers a number of topical issues that enable solving the problem of computer parametric identification of the NMR and NQR spectra. Analysis of the data presented in literature revealed several software programs for simulation of the shapes of NQR lines associated with quadrupole interactions of nuclei in powder solids. It has been established that many computer programs for modeling NMR/NQR experiments developed over the past four decades can be categorized as follows: NMR analytical tools (usually implemented in Mathematica), specialized applications (for example, NOESY spectrum simulator), application programming interfaces, API; the generic NMR modeling program. The available means of computer simulation of the spin echo are described in regard with obtaining information about its amplitude, the magnitude and orientation of the local magnetic field, and the direction of the gradient of the crystalline electric field in the sample. Considerable attention is paid to the imitation of accurate NMR/NQR spectra using the fast diagonalization procedure, as well as an effective averaging scheme for Olderman, Straw, Grant powders. The conducted analysis revealed that Quadrupolar Exact SofTware (QUEST) is one of the most accurate programs for modeling quadrupole nuclei of powder samples under static conditions. The QUEST spectral simulation is based on summation in the full quadrupole Hamiltonian. This theory is valid and holds for all transitions and at an arbitrary direction of the magnetic field. When modeling the NQR spectra, it is necessary to use low Larmor frequency (∼ 0.0001 MHz), but not equal to 0. This is due to the fact that the pure NQR peaks are infinitely sharp and, therefore, it is difficult to summarize them if they appear between the bins in the histogram. Since the calculations in QUEST are performed in units of the absolute frequency, when using a very narrow spectral window, the required accuracy in the frequency values of each bin in the histogram may be missing.