Density Functional Theory Predictions of Isotropic Hyperfine Coupling Constants

Abstract

The reliability of density functional theory (DFT) in the determination of the isotropic hyperfine coupling constants (hfccs) of the ground electronic states of organic and inorganic radicals is examined. Predictions using several DFT methods and 6-31G, TZVP, EPR-III and cc-pVQZ basis sets are made and compared to experimental values. The set of 75 radicals here studied was selected using a wide range of criteria. The systems studied are neutral, cationic, anionic; doublet, triplet, quartet; localized, and conjugated radicals, containing 1H, 9Be, 11B, 13C, 14N, 17O, 19F, 23Na, 25Mg, 27Al, 29Si, 31P, 33S, and 35Cl nuclei. The considered radicals provide 241 theoretical hfcc values, which are compared with 174 available experimental ones. The geometries of the studied systems are obtained by theoretical optimization using the same functional and basis set with which the hfccs were calculated. Regression analysis is used as a basic and appropriate methodology for this kind of comparative study. From this analysis, we conclude that DFT predictions of the hfccs are reliable for B3LYP/TZVP and B3LYP/EPR-III combinations. Both functional/basis set scheme are the more useful theoretical tools for predicting hfccs if compared to other much more expensive methods.

Role of zero-point vibrational corrections to carbon hyperfine coupling constants in organic π radicals

“…Thus, following our previous work, 30 the equilibrium and effective geometries as well as force fields at these geometries have been determined using the B3LYP exchange-correlation functional [38][39][40][41] and the TZV2P basis set, 42 and computations of the isotropic HFCCs and their second derivatives with respect to normal modes have been done using the DFT-RU approach [35][36][37] (numerical differentiation step size has been set to 0.0075 a.u.). In the carbon HFCCs calculations, we also employed the B3LYP exchange-correlation functional, [38][39][40][41] which is typically recommended for computation of HFCCs of radicals, 15,17 and the Huz-IIIsu3 basis set, [43][44][45][46] which is tailored for evaluation of HFCCs. To validate our choice of exchange-correlation functional and basis set for carbon HFCCs, we tested the performance of our methodology on a single carbon atom in its 3 P ground state.…”

“…[7][8][9][10][11][12][13][14][15][16][17][18] The lack of such analysis tools forced early investigations of EPR data to rely on rules-of-thumb or simple principles that relate the spin density to the measured hyperfine coupling constants (HFCCs). One such relation is the McConnell relation that states that the spin density ρ C on the carbon of a C-H fragment in an organic π radical is linearly dependent on the isotropic hyperfine constant of hydrogen.…”

“…One such relation is the McConnell relation that states that the spin density ρ C on the carbon of a C-H fragment in an organic π radical is linearly dependent on the isotropic hyperfine constant of hydrogen. [1][2][3] Although such relations or structure-property tools have frequently been used in the field of EPR, [4][5][6] recent advances in the quantum chemical modeling of HFCCs in organic radicals [7][8][9][10][11][12][13][14][15][16][17][18] offer an alternative way for exploring the physical origin of these constants.…”

EPR Spectroscopic and Computational Characterization of the Hydroxyethylidene-Thiamine Pyrophosphate Radical Intermediate of Pyruvate:Ferredoxin Oxidoreductase

“…In the case of models 1′-4′, the positions of heavy atoms assigned in the electron density maps from the crystal structure, were frozen during the geometry optimization and only the positions of the H-atoms were allowed to vary. RMSD's between the heavy atoms of the geometry-optimized structures (models 1 -4) and the corresponding X-ray coordinates (models 1′ -4′) were obtained by overlaying the structures and employing a simulated annealing algorithm (26) to minimize the differences in atomic coordinates.Single-point calculations were then performed on the optimized structures using the B3LYP hybrid functional in combination with the DFT-optimized valence triple-ζ basis, TZVP, which has been shown to give accurate estimates for hyperfine parameters of nuclei from the first three rows of the periodic table (27,28). Estimates of the g-tensors of each model were obtained from single-point calculations with the ORCA 2.4-41 software package using the same B3LYP/ TZVP scheme (18).…”

Distribution of r·p in Atomic Systems

“…21 It can be measured experimentally or determined by standard ab initio methods. 22, 23 The second, Π(0), appears in the Maclaurin expansion of the spherically averaged momentum density 24 and can be obtained from high-energy electron impact experiments 25 or ab initio calculations. 26,27 The origin posmom density S(0) is also important, for it directly measures the probability of circular electronic trajectories.…”