A. M. Tokmachev

Cluster models in the quantum-chemical analysis of coordination of nitroxide probes to Lewis acid sites on the surface of Al2O3

The results of ESR-spectroscopic and quantum-chemical investigations of the coordination of 2,2,6,6-tetramethylpiperidineN-oxyl, 2,2,3,4,5,5-hexamethyl-3-imidazolidineN-oxyl, 2,2,4,5,5-pentamethyl-3-imidazolineN-oxyl, 2,4,5,5-tetramethyl-2-phenyl-3-imidazolineN-oxyl, and 2,4,5,5-tetramethyl-2-octyl-3-imidazolineN-oxyl to Lewis acid sites (LAS) on alumina surface are described systematically and analyzed. The cluster models of LAS accepted in radiospectroscopy and based on experimental data on g-factors and hyperfine coupling constants with N and Al nuclei in the corresponding donor-acceptor complexes are discussed. Within the framework of the unrestricted Hartree—Fock (UHF) method using the STO-3G, STO-6G, 3–21G, and 6–31G basis sets and also in terms of semiempirical MNDO, AM1, and PM3 procedures, comparative quantum-chemical analysis of the structural, spin, electrostatic, energy, and radiospectroscopic characteristics of the coordination of the model cluster LAS to the simplest representative of nitroxides is performed. Three illustrative types of structures of the resulting surface complex are considered. A semiquantitative interpretation of the whole set of features found experimentally for the coordination of nitroxide probes to the surface LAS on alumina is given.

Quantum chemical procedure of automatic correction of optimized geometric parameters of free radicals

Experimental geometric parameters of the HCO σ-electron radical are compared with those optimized for total energy within the scope of widely distributed DFT, ab initio and semiempirical quantum chemical schemes. The comparative analysis of the whole set of geometric and magnetic resonance characteristics of the theoretical and experimental radical structures points to the possibility of systematical correction of the optimized geometric parameters of free radicals on the basis of ESR data. A specialized automatic computing procedure allowing to put into practice such a correction is developed and evaluated.

Coordination of nitroxyl radical probes to the Lewis acid sites on the surface of gallium oxide: A quantum-chemical analysis

The results of ESR investigations and quantum-chemical studies on the coordination of 2,2,6,6-tetramethylpiperidine-N-oxyl to the Lewis acid sites (LAS) on the surface of gallium oxide are reported and analyzed. The structural, spin, electrostatic, and energy characteristics of the coordination of the simplest nitroxyl radical, H2NO., to the Ga(OH)3 model cluster-type LAS were calculated by the unrestricted Hartree—Fock method in the 6–31G basis set and using the semiempirical PM3 approximation. The properties of gallium-containing nitroxyl surface complexes and their aluminium-containing analogs are compared. According to the results of cluster quantum-chemical calculations, the LAS on the surface of gallium oxide are stronger than those on the alumina surface.

Quantum-chemical analysis of the cluster models of coordination of imidazoline nitroxides on the silica surface

The results of ESR-spectroscopic and quantum-chemical studies of the coordination of 2,2,4,5,5-pentamethyl-3-imidazoline-N-oxyl and 2-phenyl-2,4,5,5-tetramethyl-3-imidazoline-N-oxyl with the surface acid sites (AS) of silica are analyzed. Cluster models of AS, corresponding to one- and two-point coordination and accepted in radiospectroscopy on the basis ofg-factors and constants of hyperfine splitting at the N nuclei in the resulting donor-acceptor complexes, are discussed. Within the framework of the unrestricted Hartree-Fock method using the MNDO approach, a comparative quantum-chemical analysis of the structural, spin, electrostatic, energy, and radiospectroscopic characteristics of the coordination of the model cluster AS to the imidazoline nitroxide probes is performed. For the two-center adsorption, three types of AS structures are considered, together with the paramagnetic surface complexes they form. A consistent semi-quantitative interpretation of the whole set of features found experimentally for the coordination of imidazoline nitroxides to the surface AS on silica is given.

Evaluation of MNDO approximation in quantum-chemical calculations ofg-tensors of free radicals

The MNDO approximation was tested for applicability in calculations of theg-tensors of free radicals. The representative test set included 15 free radicals for which the isotropic and anisotropic hyperfine coupling constants were calculated previously in the framework of the MNDO approach.

Evaluation of MNDO approximation in quantum-chemical calculations of anisotropic hyperfine coupling tensors in free radicals

The MNDO approximation was tested for applicability in quantum-chemical calculations of anisotropic hyperfine coupling tensors using the same set of 17 free radicals as that used previously in evaluating a new procedure for quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in the framework of the same approach.

On quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in free radicals

A scheme of consitent quantum-chemical calculations of constants of isotropic hyperfine coupling (IHFC) with protons (aHiso) in free radicals is considered for the case where the spin populations ρsH are determined in the basic set of symmetrically orthogonalized atomic orbitals taking model σ-and π-electron fragments as an example. The competence of using two coefficients of proportionalityK(H) when estimating the proton IHFC constants by semiempirical quantum-chemical methods was demonstrated. Theoretical substantiation of empirical values of the above coefficients previously used is revealed.

Evaluation of a new procedure for quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in free radicals

A new procedure for quantum-chemical estimates of the constants of isotropic hyperfine coupling (IHFC) with protons using the orbital spin populations calculated in the basis set of symmetrically orthogonalized (according to Löwdin) atomic orbitals is tested taking 16 well-studied simplest π-electron and σ-electron radicals as examples. The most probable reasons for and possible ways of correcting large deviations of calculated IHFC constants from experimental values are considered. The efficiency of the semiempirical MNDORU scheme, which makes it possible to consistently estimate the delocalization and spin-polarization contributions to the constants of IHFC with protons in free radicals, is demonstrated.

CORRELATIONS BETWEEN MAGNETIC RESONANCE PARAMETERS OF ESR AND NMR SPECTRA

Empirically established correlations between magnetic resonance parameters of free radicals (g-factors, isotropic hyperfine coupling constans) and isostructural molecules (chemical shifts, isotropic spin-spin coupling constants) or structurally similar ligands in paramagnetic transition metal complexes (isotropic chemical shifts) are systematized and critically discussed. Quantum-chemical analysis of the suggested spin distribution damping coefficients in model systems and structurally similar chemical compounds is performed. Based on the results obtained, physicochemical interpretation of the observed correlations between the parameters of ESR and NMR spectra is given.