Isotropic Hyperfine Coupling Constants Research Articles

ChemInform Abstract: Interactions Between Ionic Liquids and Radicals

AbstractReview: 102 refs.

Linear oligoarylamines: electrochemical, EPR, and computational studies of their oxidative states.

A series of straight-chain oligoarylamines were synthesized and examined by electrochemical, spectroelectrochemical, electron paramagnetic resonance techniques, and density functional theory (DFT) calculation. Depending on their electrochemical characteristics, these oligoarylamines were classified into two groups: one containing an odd number and the other an even number of redox centers. In the systems with odd redox centers (N1, N3, and N5), each oxidation was associated with the loss of one electron. As for the systems with even redox centers (N2, N4, and N6), oxidation occurred by taking N2 as a unit. Absorption spectra of linear oligoarylamines at various oxidative states were obtained to investigate their charge transfer behaviors. Moreover, DFT-computed isotropic hyperfine coupling constants and spin density were in accordance with the EPR experiment, and gave a close examination of oligoarylamines at charged states.

Synthesis and Properties of Fullerene C60 and C70 Spin Probes Containing Isoindoline Nitroxides

Three fullerene isoindoline nitroxides N-methyl-3,4-fulleropyrrolidine-2-spiro-5′- (1′,1′,3′,3′-tetramethylisoindolin-2′-yloxyl), (C60-(TMIO)m, and C70-(TMIO)n) were synthesized by the covalent bonding of 5-formyl-1,1,3,3-tetramethyl isoindolin-2-yloxyl to the fullerenes C60 and C70. Significantly, the X-ray photoelectron spectra indicated the characteristic N 1s signals of NO. at 402 eV. The atomic force microscope morphologies showed that the average particle sizes of C60-(TMIO)m and C70-(TMIO)n were 38 and 15 nm. The electrochemical experiments indicated that fullerene bound isoindoline nitroxides retained similar electrochemical properties and redox reaction mechanisms as the parent nitroxides. The electron paramagnetic resonance spectra of the fullerene isoindoline nitroxides all exhibited the hyperfine splittings and characteristic spectra of tetramethyl isoindoline nitroxides, with typical nitroxide g-values and nitrogen isotropic hyperfine coupling constants. Therefore, these fullerene isoindoline nitroxides may be considered as potential candidates for novel biological spin probes using electron paramagnetic resonance spectroscopy.

Modeling EPR parameters of nitrogen containing conjugated radical cations

DFT investigation on conjugated radical cations containing14N nucleus to obtain accurate isotropic hyperfine coupling constants.

Structural differences between the active sites of the Ni-A and Ni-B states of the [NiFe] hydrogenase: an approach by quantum chemistry and single crystal ENDOR spectroscopy.

The two resting forms of the active site of [NiFe] hydrogenase, Ni-A and Ni-B, have significantly different activation kinetics, but reveal nearly identical spectroscopic features which suggest the two states exhibit subtle structural differences. Previous studies have indicated that the states differ by the identity of the bridging ligand between Ni and Fe; proposals include OH(-), OOH(-), H2O, O(2-), accompanied by modified cysteine residues. In this study, we use single crystal ENDOR spectroscopy and quantum chemical calculations within the framework of density functional theory (DFT) to calculate vibrational frequencies, (1)H and (17)O hyperfine coupling constants and g values with the aim to compare these data to experimental results obtained by crystallography, FTIR and EPR/ENDOR spectroscopy. We find that the Ni-A and Ni-B states are constitutional isomers that differ in their fine structural details. Calculated vibrational frequencies for the cyano and carbonyl ligands and (1)H and (17)O hyperfine coupling constants indicate that the bridging ligand in both Ni-A and Ni-B is indeed an OH(-) ligand. The difference in the isotropic hyperfine coupling constants of the β-CH2 protons of Cys-549 is sensitive to the orientation of Cys-549; a difference of 0.5 MHz is observed experimentally for Ni-A and 1.9 MHz for Ni-B, which results from a rotation of 7 degrees about the Cα-Cβ-Sγ-Ni dihedral angle. Likewise, the difference of the intermediate g value is correlated with a rotation of Cys-546 of about 10 degrees.

Scalar Relativistic Calculations of Hyperfine Coupling Constants Using Ab Initio Density Matrix Renormalization Group Method in Combination with Third-Order Douglas–Kroll–Hess Transformation: Case Studies on 4d Transition Metals

We have developed a new computational scheme for high-accuracy prediction of the isotropic hyperfine coupling constant (HFCC) of heavy molecules, accounting for the high-level electron correlation effects, as well as the scalar-relativistic effects. For electron correlation, we employed the ab initio density matrix renormalization group (DMRG) method in conjunction with a complete active space model. The orbital-optimization procedure was employed to obtain the optimized orbitals required for accurately determining the isotropic HFCC. For the scalar-relativistic effects, we initially derived and implemented the Douglas-Kroll-Hess (DKH) hyperfine coupling operators up to the third order (DKH3) by using the direct transformation scheme. A set of 4d transition-metal radicals consisting of Ag atom, PdH, and RhH2 were chosen as test cases. Good agreement between the isotropic HFCC values obtained from DMRG/DKH3 and experiment was archived. Because there are no available gas-phase values for PdH and RhH2 radicals in the literature, the results from the present high-level theory may serve as benchmark data.

Interaction of the Mu-cyclohexadienyl radical with metallic (Au, Pt) nanoparticles in mesoporous silica

μSR and ALCR techniques have been used to investigate the structure and dynamics of the Mu-cyclohexadienyl radical interacting with Au and Pt metal nanoparticles (MNPs) supported in mesoporous silica (SBA-15). Surprisingly, coherent precession signals are observed and the isotropic hyperfine coupling constants are almost the same in loaded and unloaded samples, implying that the electronic structure of MuC6H6 is only weakly perturbed by the presence of the MNPs. We propose the observed radicals are shielded from the metallic surfaces by a benzene coating on the MNPs. The Δ1 resonance is observable in MNP-loaded samples at higher temperatures than in the unloaded SBA-15. This is attributed to stronger binding of MuC6H6 to the benzene coated MNPs.

Quantum simulation for muoniated and deuterated methyl radicals in implicit water solvent: combined ab initio path integral molecular dynamics and the polarizable continuum model simulation study

We have combined ab initio path integral molecular dynamics (PIMD) simulation and the polarizable continuum model (PCM) method to efficiently incorporate solvent effects into nuclear quantum fluctuation of molecular systems. Our combined ab initio PIMD–PCM simulation was applied to muoniated and deuterated methyl radical immersed in implicit water solvent to gain information on solvent and isotope effects from one simulation run. We found that solvent effects lead to the bond elongation and a decrease in the magnitude of isotropic hyperfine coupling constants. These are consistent with the trends in conventional static calculations and experiments. In addition, the performance of cavity models (universal force field, united atom specified for Kohn–Sham and these hybrid models) and the conservation of the PIMD–PCM Hamiltonian were accessed. We confirmed that solvent effects on nuclear quantum fluctuation are efficiently computed using our combined simulation of quantum solute in implicit solvent.

Calculated optimized structures and hyperfine coupling constants of some radical adducts of α-phenyl-N-tert-buthyl nitrone in water and benzene solutions

The ground state optimized structures of some radical adducts of α-phenyl-N-tert-buthyl nitrone (PBN) in water and benzene solutions were calculated by using DFT (B3LYP, B3PW91 and PBEPBE) and HF methods with 6-311++G(d, p), 6-31G(d, p) and LanL2DZ levels. As trapped radicals, F, Cl, Br, H, OH, CN, NCO, and N3 were used. The calculated isotropic hyperfine coupling constants of all the trapped radicals were seen to be in good agreement with the corresponding experimental data. The hyperfine coupling constant due to the β proton of nitroxide radical was seen to be effected with the opposite spin density of oxygen nucleus bonded to the nitrogen. From all the calculated data it was concluded that for hyperfine calculations the DFT method is superior relative to the HF method. Also the geometrical parameters for the ground state optimized structures of all the radical adducts were listed, and the binding energies of all the trapped radicals were obtained.

EPR study of methyl radical in van-der-Waals solids

High-resolution EPR spectra of CH3 radicals trapped in solid N2 matrix were obtained in a temperature range 7–28K. The analysis of the spectrum suggests that, in the whole temperature range, the radical performs both fast rotational motion around the C3-symmetry axis and libration motion about the C2-symmetry axes. A comparison study of the methyl libration motion in gas matrices of linear molecules, N2, CO, N2O and CO2, was carried out using a model of the infinite potential well with flat bottom. As a result, averaged angular deviations of the radical orientation and dimensions of the potential wells were estimated. We assessed contributions of both the classical motion and the quantum-mechanical correction to the radical reorientation. A possibility of utilizing the trapped CH3 radical as a spin probe to study order–disorder structural phase transition at the microscopic level was discussed. An empirical formula obtained earlier for the matrix shift of the isotropic hyperfine coupling (hfc) constant of CH3 in matrices of spherical particles was shown to hold true for matrices of linear molecules as well. The experimental results and their treatment made it possible to report an estimate of the hfc tensor anisotropy for the free methyl radical. This information is of special interest to the theory of atoms and molecules.

Radical sites in humic acids: A theoretical study on protocatechuic and gallic acids

Protocatechuic and gallic acids (PCA/GA) were used as models for studying the radical sites in humic acids. Neutral, anionic, and anion radical conformers were investigated by means of density functional theory in gas phase and solvent, respectively. For the most stable radical structures electron spin density and EPR parameters such as g-tensors and isotropic hyperfine coupling constants (HFCCs) were calculated using the B3LYP functional. Additionally, the performance of several other popular functionals of the DFT theory including BP86, B97D, M06, TPSS, TPSSh and B2PLYP was evaluated. Obtained g-tensors are strongly anisotropic having two principal components (gxx, gyy) lying in the plane of the aromatic ring. The difference between the g-tensors of PCA and GA anion radicals are minimal. Inclusion of environmental effects (explicit consideration of hydrogen bonds plus continuum solvation) has a significant impact on g-tensor components and HFC constants shifting their values closer to available experimental data. In summary, good agreement of the B3LYP results with experiment is obtained and from the set of the additional DFT functionals used only the hybrid variant of the TPSSh functional provides good results. The calculated values of g-tensors and HFCCs support the hypothesis that polyphenolic fragments are main sources of transient and native radical sites in structurally complex humic acids.

Methoxy Dihedral Angles of Ubiquinone Contribute More than 160 mV to the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of the Photosynthetic Reaction Center

Ubiquinone is an almost universal, membrane-associated redox mediator. Its redox properties are substantially determined by its environment in the binding sites of proteins and by the dihedral angles of the methoxy groups relative to the ring plane. In this work, we use the photosynthetic reaction center as a model system for understanding the role of methoxy conformations in determining the redox potential of the ubiquinone/semiquinone couple. Despite the abundance of X-ray crystal structures for the reaction center, the quinone site resolution has thus far been too low to provide a reliable measure of the methoxy dihedral angles of the primary and secondary quinones, QA and QB. We have performed HYSCORE on isolated reaction centers with ubiquinones 13C-labeled at the headgroup methyl and methoxy groups, and the isotropic and anisotropic hyperfine coupling constants were estimated. Comparison of our data to quantum mechanically derived models and available crystal structures gives a best fit for the 2-methoxy dihedral angle of QB as 50° more out of plane than in QA. This assignment corresponds to a redox potential gap (ΔEm) between QA and QB of ≈180 mV. This is consistent with the failure of a ubiquinone analog lacking the 2-methoxy group to function as QB in mutant reaction centers with ΔEm ≈160-195 mV. We conclude the 2-methoxy group of ubiquinone provides an essential mechanism by which the reaction center tunes the redox potential difference necessary for electron transfer. The influence of the methoxy groups on the second electron transfer is under investigation through similar EPR studies of the biradical, QA-QB-, generated in mutant reaction centers and at high pH in wild type reaction centers.

Interactions between ionic liquids and radicals

Ionic liquids were investigated with both stable radicals on the basis of 2,2,6,6-tetramethylpiperidine-1-yloxyl (TEMPO) and photogenerated lophyl radicals. The ionic liquids are composed either of bis(trifluoromethylsulfonyl)imide (NTf2) as anion and various cations or they contain an imidazolium ion in combination with various anions. The cations include imidazolium, pyrrolidinium, piperidinium, polymethine or ammonium ions. Furthermore, BF4−, PF6−, triflate, camphorsulfonate, lactate, tosylate or tris(pentafluoroethyl)trifluorophosphate (FAP) are the counter ions in the imidazolium salts. The structural variation of the ionic liquids results in differences in glass formation, semicrystallinity, or crystallinity, as well as in viscosity differences. Furthermore, a vinyl substituent at the imidazolium ion and a methacryloyloxyethyl substituent at the ammonium ion result in polymerizable ionic liquids that were converted via a radical mechanism in amorphous polymerized ionic liquids with a glass transition temperature, which is significantly higher compared to the ionic liquids. An additional substituent at TEMPO causes additional hydrogen bond formation or additional Coulomb interactions with the individual ions of the ionic liquids compared to TEMPO. This influences the mobility of these radicals in the ionic liquid expressed by differences in the average rotational correlation time (τrot). The mobility of the radicals in the ionic liquids as function of the temperature describes ionic liquids either as continuum in analogy to molecular solvents using the Stokes–Einstein model, that is the case for 1-butyl-3-methylimidazolium NTf2, or as medium where free volume effects are important for the mobility of a solute in the ionic liquid using the model of Spernol, Gierer, and Wirtz. The 1-butyl-3-methylimidazolium BF4− fits well into the latter. Furthermore, the isotropic hyperfine coupling constant (Aiso(14N)) of the stable radicals gives information about micropolarity of the ionic liquids only if the mobility of the radical is high enough in the ionic liquid. In addition to the rotational mobility of the stable radicals, the photogenerated lophyl radicals give information about translational diffusion of radicals and solvent cage effects in the ionic liquids. The application of the Eyring equation results mostly in the expected negative values of the activation entropy for the transition state that is typical for bimolecular reactions. Only few examples show a less negative or positive activation entropy for the bimolecular reaction, which may be attributed to radical recombination within the solvent cage to a high extent. The results obtained during investigation of radicals in ionic liquids are important to understand the radical processes in ionic liquids that may occur for example in dye sensitized solar cells, photo or thermally induced reactions or radical polymerizations in ionic liquids.

Metaphenylene-Based Nitroxide Diradicals: A Protocol To Calculate Intermolecular Coupling Constant in a One-Dimensional Chain

Intramolecular magnetic exchange coupling constants are determined for seven isolated metaphenylene-based dinitroxide diradicals by unrestricted density functional methodology (UDFT) using a number of hybrid functionals such as B3LYP, B3LYP-D3, M06-2X, HSE, and LC-ωPBE. Geometry optimizations for both triplet and broken symmetry solutions are performed with the 6-311G(d,p) basis set for all the molecules. In all cases, B3LYP somewhat overestimates the coupling constant, and M06-2X produces a more realistic value. The range-separated HSE and LC-ωPBE functional yield large deviations from experiment. The nature of spin coupling agrees with the spin alternation rule and the calculated spin densities, in conjunction with the McConnell rule. It can also be explained in terms of the nondisjoint Single Occupied Molecular Orbital effect. Furthermore, it correlates with the calculated NICS(1) isotropic and zz and hyperfine coupling constants. We also put forward a method for the determination of the intramolecular (J) and intermolecular (J') coupling constants from quantum chemical calculations on a one-dimensional chain of weakly bound diradicals. Two expressions are derived for the energies of different spin states in terms of J and J'. Exemplary UDFT computations are done on the N-mers (N = 2-6) of two diradicals for which the crystal coordinates are available. The intramolecular and intermolecular coupling constants are determined from the calculated UDFT energies. These are indeed in general agreement with the measured coupling constants.

Massively parallel implementations of coupled-cluster methods for electron spin resonance spectra. I. Isotropic hyperfine coupling tensors in large radicals

Coupled cluster (CC) methods provide highly accurate predictions of molecular properties, but their high computational cost has precluded their routine application to large systems. Fortunately, recent computational developments in the ACES III program by the Bartlett group [the OED∕ERD atomic integral package, the super instruction processor, and the super instruction architecture language] permit overcoming that limitation by providing a framework for massively parallel CC implementations. In that scheme, we are further extending those parallel CC efforts to systematically predict the three main electron spin resonance (ESR) tensors (A-, g-, and D-tensors) to be reported in a series of papers. In this paper inaugurating that series, we report our new ACES III parallel capabilities that calculate isotropic hyperfine coupling constants in 38 neutral, cationic, and anionic radicals that include the (11)B, (17)O, (9)Be, (19)F, (1)H, (13)C, (35)Cl, (33)S,(14)N, (31)P, and (67)Zn nuclei. Present parallel calculations are conducted at the Hartree-Fock (HF), second-order many-body perturbation theory [MBPT(2)], CC singles and doubles (CCSD), and CCSD with perturbative triples [CCSD(T)] levels using Roos augmented double- and triple-zeta atomic natural orbitals basis sets. HF results consistently overestimate isotropic hyperfine coupling constants. However, inclusion of electron correlation effects in the simplest way via MBPT(2) provides significant improvements in the predictions, but not without occasional failures. In contrast, CCSD results are consistently in very good agreement with experimental results. Inclusion of perturbative triples to CCSD via CCSD(T) leads to small improvements in the predictions, which might not compensate for the extra computational effort at a non-iterative N(7)-scaling in CCSD(T). The importance of these accurate computations of isotropic hyperfine coupling constants to elucidate experimental ESR spectra, to interpret spin-density distributions, and to characterize and identify radical species is illustrated with our results from large organic radicals. Those include species relevant for organic chemistry, petroleum industry, and biochemistry, such as the cyclo-hexyl, 1-adamatyl, and Zn-porphycene anion radicals, inter alia.

Calculating Hyperfine Couplings in Large Ionic Crystals Containing Hundreds of QM Atoms: Subsystem DFT Is the Key

We present an application of the linear scaling frozen density embedding (FDE) formulation of subsystem DFT to the calculation of isotropic hyperfine coupling constants (hfcc's) of atoms belonging to a guanine radical cation embedded in a guanine hydrochloride monohydrate crystal. The model systems range from an isolated guanine to a 15,000 atom QM/MM cluster where the QM region is comprised of 36 protonated guanine cations, 36 chlorine anions, and 42 water molecules. Our calculations show that the embedding effects of the surrounding crystal cannot be reproduced by small model systems nor by a pure QM/MM procedure. Instead, a large QM region is needed to fully capture the complicated nature of the embedding effects in this system. The unprecedented system size for a relativistic all-electron isotropic hfcc calculation can be approached in this work because the local nature of the electronic structure of the organic crystals considered is fully captured by the FDE approach.

Pyramidal distortion of the NO2 group in the radical anion of 2,4,6-trimethoxynitrobenzene

Electrochemical reduction in DMF is used to produce the radical anion of 2,4,6-trimethoxynitrobenzene with an EPR spectrum that shows hyperfine splitting for 15N nuclei of the nitro group and 13C nuclei of the benzene ring given the natural abundance of the isotopes. According to UB3LYP/6-31+G* quantum chemical calculations considering solvation in the PCM model, the nitro group in the equilibrium conformation of the RA of 2,4,6-trimethoxynitrobenzene is rotated relative to the benzene ring plane by an angle close to 90° and has a pyramidal structure. The calculated isotropic hyperfine coupling constants for this conformation are closest to the experimental results.

Extension of the AMBER Force Field for Nitroxide Radicals and Combined QM/MM/PCM Approach to the Accurate Determination of EPR Parameters of DMPO-H in Solution.

A computational strategy that combines both time-dependent and time-independent approaches is exploited to accurately model molecular dynamics and solvent effects on the isotropic hyperfine coupling constants of the DMPO-H nitroxide. Our recent general force field for nitroxides derived from AMBER ff99SB is further extended to systems involving hydrogen atoms in β-positions with respect to NO-moiety. The resulting force-field has been employed in a series of classical molecular dynamics simulations, comparing the computed EPR parameters from selected molecular configurations to the corresponding experimental data in different solvents. The effect of vibrational averaging on the spectroscopic parameters is also taken into account, by second-order vibrational perturbation theory involving semidiagonal third energy derivatives together first and second property derivatives.

CIDNP as a tool for rapid rearrangements: New insights into cyclobutanoid 4‐center/3‐electron radical cations

CIDNP has been often utilized as a mechanistic tool for radical reactions. Here, we show that detailed insights into the structure of carbon‐centered radical cations with unusual structures, which, otherwise, are hardly detectable, can be obtained. Hydrocarbon cage compounds containing syn‐periplanar cyclobutane moieties were oxidized by photoinduced one‐electron transfer to chloranil at room temperature in CD2Cl2. The 1H CIDNP polarization intensities of geminate radical pairs can be translated into isotropic hyperfine coupling constants and reveal radical cations with remarkable electronic structures. Density functional theory calculations are in line with the experimental results. Copyright © 2013 John Wiley & Sons, Ltd.

An electron paramagnetic resonance and density-functional theory study on the methyl isotropic hyperfine coupling constants in gamma-irradiated 2,6-di-tert-butyl-4-methylphenol

Single crystal of gammairradiated 2,6-di-tert-butyl-4-methylphenol (BHT) was investigated using an electron paramagnetic resonance (EPR) technique at different orientations in the magnetic field at room temperatures. Taking into consideration the chemical structure and the experimental spectra of the irradiated single crystal of BHT, we assumed that one phenoxyltype paramagnetic species was produced having an unpaired electron localized at the methyl fragment side of the phenyl ring. Depending on this assumption, one possible radical was modeled using the B3LYP/6-311+G(d) level of density-functional theory. EPR parameters were calculated for these modeled radical using the B3LYP/TZVP and B3LYP/EPR-III level. The averaged value of isotropic hydrogen hyperfine coupling constants of rotating methyl functional group of phenoxyl radical is calculated for the first time. Theoretically calculated values of the modeled radical are in reasonably good agreement with the experimental data determined from the spectra (differences in averaged coupling constant values smaller than 5%, and differences in isotropic g values fall into 1 ppt).