Values Of Hyperfine Coupling Constants Research Articles

Hyperfine Decoupling of ESR Spectra Using Wavelet Transform.

The objective of spectral analysis is to resolve and extract relevant features from experimental data in an optimal fashion. In continuous-wave (cw) electron spin resonance (ESR) spectroscopy, both values of a paramagnetic center and hyperfine splitting caused by its interaction with neighboring magnetic nuclei in a molecule provide important structural and electronic information. However, in the presence of - and/or -anisotropy and/or large number of resonance lines, spectral analysis becomes highly challenging. Either high-resolution experimental techniques are employed to resolve the spectra in those cases or a range of suitable ESR frequencies are used in combination with simulations to identify the corresponding and values. In this work, we present a wavelet transform technique in resolving both simulated and experimental cW-ESR spectra by separating the hyperfine and super-hyperfine components. We exploit the multiresolution property of wavelet transforms that allow the separation of distinct features of a spectrum based on simultaneous analysis of spectrum and its varying frequency. We retain the wavelet components that stored the hyperfine and/or super-hyperfine features, while eliminating the wavelet components representing the remaining spectrum. We tested the method on simulated cases of metal-ligand adducts at L-, S-, and X-band frequencies, and showed that extracted values, hyperfine and super-hyperfine coupling constants from simulated spectra, were in excellent agreement with the values of those parameters used in the simulations. For the experimental case of a copper(II) complex with distorted octahedral geometry, the method was able to extract and hyperfine coupling constant values, and revealed features that were buried in the overlapped spectra.

Structure, Stability, and Properties of Boron Encapsulated Complexes of C60, C59B, and C59N.

Density functional studies are performed for the boron atom encapsulated complexes of C60, C59B, and C59N using B3LYP and B3LYP-GD2 functionals with 6-311G* basis set. The study shows that the complexes B@C59B and B@C59N can exist in different forms which differ in their structure as well as electronic and magnetic properties. The nature of interaction between the guest and the host is analyzed based on the stabilization energies of complexes for their different spin states. The electronic properties such as vertical electron affinity (VEA), vertical ionization energy (VIE), and energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the complexes are calculated. The transfer of electron spin density and spin-spin coupling of electrons between the guest and the host are examined. The study reveals that ferromagnetic core@shell spin coupling occurs between the host and the guest species of the complexes B@C59B and B@C59N in their triplet state without any transfer of spin density. The values of hyperfine coupling constant (hfcc) for the heteroatom of the cage and for the encapsulated boron atom of the complexes are analyzed, and it indicates that the value of hfcc for the encapsulated boron decreases significantly due to confinement.

A path integral molecular dynamics study of the hyperfine coupling constants of the muoniated and hydrogenated acetone radicals

The on-the-fly ab initio density functional path integral molecular dynamics (PIMD) simulations, which can account for both the nuclear quantum effect and thermal effect, were carried out to evaluate the structures and “reduced” isotropic hyperfine coupling constants (HFCCs) for muoniated and hydrogenated acetone radicals (2-muoxy-2-propyl and 2-hydoxy-2-propyl) in vacuo. The reduced HFCC value from a simple geometry optimization calculation without both the nuclear quantum effect and thermal effect is −8.18 MHz, and that by standard ab initio molecular dynamics simulation with only the thermal effect and without the nuclear quantum effect is 0.33 MHz at 300 K, where these two methods cannot distinguish the difference between muoniated and hydrogenated acetone radicals. In contrast, the reduced HFCC value of the muoniated acetone radical by our PIMD simulation is 32.1 MHz, which is about 8 times larger than that for the hydrogenated radical of 3.97 MHz with the same level of calculation. We have found that the HFCC values are highly correlated with the local molecular structures; especially, the Mu—O bond length in the muoniated acetone radical is elongated due to the large nuclear quantum effect of the muon, which makes the expectation value of the HFCC larger. Although our PIMD result calculated in vacuo is about 4 times larger than the measured experimental value in aqueous solvent, the ratio of these HFCC values between muoniated and hydrogenated acetone radicals in vacuo is in reasonable agreement with the ratio of the experimental values in aqueous solvent (8.56 MHz and 0.9 MHz); the explicit presence of solvent molecules has a major effect on decreasing the reduced muon HFCC of in vacuo calculations for the quantitative reproduction.

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.

Accurate Prediction of Hyperfine Coupling Constants in Muoniated and Hydrogenated Ethyl Radicals: Ab Initio Path Integral Simulation Study with Density Functional Theory Method.

We performed ab initio path integral molecular dynamics (PIMD) simulations with a density functional theory (DFT) method to accurately predict hyperfine coupling constants (HFCCs) in the ethyl radical (CβH3-CαH2) and its Mu-substituted (muoniated) compound (CβH2Mu-CαH2). The substitution of a Mu atom, an ultralight isotope of the H atom, with larger nuclear quantum effect is expected to strongly affect the nature of the ethyl radical. The static conventional DFT calculations of CβH3-CαH2 find that the elongation of one Cβ-H bond causes a change in the shape of potential energy curve along the rotational angle via the imbalance of attractive and repulsive interactions between the methyl and methylene groups. Investigation of the methyl-group behavior including the nuclear quantum and thermal effects shows that an unbalanced CβH2Mu group with the elongated Cβ-Mu bond rotates around the Cβ-Cα bond in a muoniated ethyl radical, quite differently from the CβH3 group with the three equivalent Cβ-H bonds in the ethyl radical. These rotations couple with other molecular motions such as the methylene-group rocking motion (inversion), leading to difficulties in reproducing the corresponding barrier heights. Our PIMD simulations successfully predict the barrier heights to be close to the experimental values and provide a significant improvement in muon and proton HFCCs given by the static conventional DFT method. Further investigation reveals that the Cβ-Mu/H stretching motion, methyl-group rotation, methylene-group rocking motion, and HFCC values deeply intertwine with each other. Because these motions are different between the radicals, a proper description of the structural fluctuations reflecting the nuclear quantum and thermal effects is vital to evaluate HFCC values in theory to be comparable to the experimental ones. Accordingly, a fundamental difference in HFCC between the radicals arises from their intrinsic molecular motions at a finite temperature, in particular the methyl-group behavior.

Synthesis and electrochemical oxidation of hybrid compounds: dihydropyridine-fused coumarins

In this paper, a series of six dihydropyridine-fused coumarins were synthesized and electrochemically characterized in dimethylformamide (DMF).Dihydropyridine ring oxidation on glassy carbon electrode (GCE) for condensed heterocyclic compounds revealed a single anodic peak. Oxidation potential values correlated fairly well with substituent effects at 9-position. The overall oxidation mechanism involved 2-electrons and 2-protons as determined by chronoamperometry.Controlled-potential electrolysis followed by UV-Visible spectroscopy proves that dihydropyridine-fused coumarins are electrochemically oxidized in DMF giving rise to the aromatic pyridine derivative. ESR experimental spectra show a triplet, due to the C-centered dihydropyridyl radical trapped with N-tert-butylamine-α-phenylnitrone (PBN). Hyperfine coupling constant values (aN) of dihydropyridine-fused coumarins were higher than corresponding values for non-fused ones. These results could be due to the effect of the coupling of the dihydropyridine moiety with the coumarin ring over the splitting constant.

Toward Reliable Prediction of Hyperfine Coupling Constants Using Ab Initio Density Matrix Renormalization Group Method: Diatomic (2)Σ and Vinyl Radicals as Test Cases.

The density matrix renormalization group (DMRG) method is used in conjunction with the complete active space (CAS) procedure, the CAS configuration interaction (CASCI), and the CAS self-consistent field (CASSCF) to evaluate hyperfine coupling constants (HFCCs) for a series of diatomic (2)Σ radicals (BO, CO(+), CN, and AlO) and vinyl (C2H3) radical. The electron correlation effects on the computed HFCC values were systematically investigated using various levels of active space, which were increasingly extended from single valence space to large-size model space entailing double valence and at least single polarization shells. In addition, the core correlation was treated by including the core orbitals in active space. Reasonably accurate results were obtained by the DMRG-CASSCF method involving orbital optimization, while DMRG-CASCI calculations with Hartree-Fock orbitals provided poor agreement of the HFCCs with the experimental values. To achieve further insights into the accuracy of HFCC calculations, the orbital contributions to the total spin density were analyzed at a given nucleus, which is directly related to the FC term and is numerically sensitive to the level of correlation treatment and basis sets. The convergence of calculated HFCCs with an increasing number of renormalized states was also assessed. This work serves as the first study on the performance of the ab initio DMRG method for HFCC prediction.

Zero-Point Corrections for Isotropic Coupling Constants for Cyclohexadienyl Radical, C6H7 and C6H6Mu: Beyond the Bond Length Change Approximation

Zero-point vibrational level averaging for electron spin resonance (ESR) and muon spin resonance (µSR) hyperfine coupling constants (HFCCs) are computed for H and Mu isotopomers of the cyclohexadienyl radical. A local mode approximation previously developed for computation of the effect of replacement of H by D on 13C-NMR chemical shifts is used. DFT methods are used to compute the change in energy and HFCCs when the geometry is changed from the equilibrium values for the stretch and both bend degrees of freedom. This variation is then averaged over the probability distribution for each degree of freedom. The method is tested using data for the methylene group of C6H7, cyclohexadienyl radical and its Mu analog. Good agreement is found for the difference between the HFCCs for Mu and H of CHMu and that for H of CHMu and CH2 of the parent radical methylene group. All three of these HFCCs are the same in the absence of the zero point average, a one-parameter fit of the static HFCC, a(0), can be computed. That value, 45.2 Gauss, is compared to the results of several fixed geometry electronic structure computations. The HFCC values for the ortho, meta and para H atoms are then discussed.

Calculations of hyperfine coupling constant of the TMPD molecule

The calculations of the hyperfine coupling constants (HFCC) of tetramethyl-p-Phenylenediamine (TMPD) are presented. Several DFT functionals are employed and the results are compared with the UHF and MP2 level of theory. Sensitivity of HFCC to the choice of the basis set is investigated. Impact of the different conformers of TMPD and the equivalence of methyl group hydrogens on the HFCC values is considered. The solvent effects are introduced via polarizable continuum model and compared with in vacuo calculations. Last but not least, molecular dynamics is employed to include the solvent molecules as well as the dynamics explicitly, accounting similar conditions comparing to the liquid phase EPR experiment.

EPR study of gamma-irradiated amphi-phenylglyoxime single crystals

Gamma-irradiated single crystals of Amphi-phenylglyoxime (APGO) were investigated using electron paramagnetic resonance (EPR) at different orientations in a magnetic field at room temperature (298 K). Considering the chemical structure and the experimental spectra of the irradiated single-crystals of APGO, we assumed that two different paramagnetic species, labeled as R ⁎ and R ⁎⁎, are either two iminoxy radicals formed by the abstraction of a H atom from different oxime branches or are different conformations of an iminoxy radical. Pursuant to this assumption, RA- and RB-type iminoxy radicals were modeled by the abstraction of H atoms from different oxime branches, and conformational analysis of these modeled radicals was performed using the semi-empirical AM1 and B3LYP/6-31+ G(d,p) methods. EPR parameters were calculated for the modeled radicals using the B3LYP method and EPR-III basis set. Theoretically calculated values of the most stable conformers (RA-1 and RB-1) of the modeled radicals are in good agreement with the experimental EPR parameters determined from the spectra (differences in isotropic hyperfine coupling constant values <5%, and differences in isotropic g values fall into 1 ppt). Thus, from the findings of the present study, we strongly suggest that the experimentally observed R ⁎ and R ⁎⁎ radicals in the single crystal of amphi-phenylglyoxime are the most stable conformers of RA- and RB-type modeled iminoxy radicals, respectively. The experimental g factors and hyperfine coupling constants were found to be anisotropic, with average values of g=2.0052, A( 14N)=29.50 G, A( 1H)=25.30 G for R ⁎, and g=2.0057, A( 14N)=34.50 G for R ⁎⁎.

Further Insights into the Environmental Effects on the Computed Hyperfine Coupling Constants of Nitroxides in Aqueous Solution

We investigated the main two factors influencing the mean hyperfine coupling constants of small nitroxide radicals in aqueous solution, i.e., the out-of-plane displacement of their nitrogen atom and the environmental effects (solvent effects), by means of the approach we previously developed and fine-tuned to study the solvation of the dimethyl nitroxide radical. Our methodology efficiently combines classical molecular dynamics based on a polarizable force field at the nanosecond scale and quantum mechanics/molecular mechanics (QM/MM) computations to account for the bulk instantaneous electrostatic environmental effect. Our method has been applied to five small nitroxides, namely methyl nitroxide, ethyl nitroxide, dimethyl nitroxide, di-tert-butyl nitroxide, and PROXYL. The theoretical nitrogen hyperfine coupling constant values for the five nitroxides in solution are in good agreement with experiment (difference of 0.3 G on average). Our approach showed that the solvent shift in nitroxide hyperfine coupling constants is almost constant whatever the nitroxide, and, particularly, whatever the nitroxide NO moiety's accessibility to the solvent. This result contrasts with earlier results derived from 10 ps scale trajectories based on Car-Parrinello molecular dynamics approach. However, we show that if we consider on average these latter results, they are in agreement with our conclusion. We also present an attempt to identify the origin of this result by analyzing the solvent contributions in terms of effects of the nitroxide first hydration shell and of the bulk, and by investigating the relation between these two contributions and the solvent structure at the vicinity of the NO moiety.

Density functional theory – electron paramagnetic resonance study of gamma-irradiated single crystal of amphi-chloroglyoxime

The density functional theory was studied to examine the suggested type of model radicals RI, RII and RIII that have been formed upon gamma-irradiation of single crystals of amphi-chloroglyoxime (ACG). RI, RII and RIII type model radicals were obtained by abstraction of Cl and H atoms from ACG. The possible conformations of these model radicals were obtained using the semi-empirical Austin Model 1 method. Subsequently, hyperfine coupling constants and atomic spin density calculations of these conformations were performed using the Becke-3-Lee-Yang-Parr functional in combination with a split-valence Gaussian basis set. Theoretically calculated values were compared with the experimental values. Calculated isotropic and anisotropic hyperfine coupling constant values of conformations of model radical RII were in good agreement with experimental values. However, the agreement was rather poor in the case of the model radicals RI and RIII. Thus, the findings from the present study clearly suggest that the experimentally observed radical should be the model radical RII.

Joint Theoretical Experimental Investigation of the Electron Spin Resonance Spectra of Nitroxyl Radicals: Application to Intermediates in in Situ Nitroxide Mediated Polymerization (in Situ NMP) of Vinyl Monomers

Density functional theory (DFT) calculations have been performed to address the structure of nitroxide intermediates in controlled radical polymerization. In a preliminary step, the reliability of different theoretical methods has been substantiated by comparing calculated hyperfine coupling constants (HFCCs) to experimental data for a set of linear and cyclic alkylnitroxyl radicals. Considering this tested approach, the nature of different nitroxides has been predicted or confirmed for (a) the reaction of C-phenyl- N- tert-butylnitrone and AIBN, (b) N- tert-butyl-alpha-isopropylnitrone and benzoyl peroxide, (c) tert-butyl methacrylate polymerization in the presence of sodium nitrite as mediator, and (d) for the reaction of a nitroso compound with AIBN. Values of HFCC experimentally determined have been confirmed by DFT calculations.

Magnetic properties of undoped and doped poly[(3-methoxy)4,6-toluene] (P3mt)

The magnetic properties of two types of poly[(3-methoxy)4,6-toluene] (P3mt 1 and P3mt 2) are studied before and after doping with iodine or organic acids (CF 3COOH or CH 3SO 3H) in a range of temperature varying from 110 to 460 K. Under 250 K and independent of the state, the magnetic susceptibility χ obeys Curie law dependence. Up to this temperature, χ is governed by a Pauli law contribution. This fact can be related to localized or delocalized spin. Furthermore, iodine doping induces distortion in the polymers that affects the ESR line distribution. In the case of organic acids doping process, a hyperfine structure with hyperfine coupling constant values between 2.2 and 2.8 G is observed. As the spin concentration decreases, we deduce that in the case of P3mt 2, there is a de-doping process.

DFT calculations of magnetic parameters for molybdenum complexes and hydroxymethyl intermediates trapped on silica surface

Density functional theory (DFT) calculations of EPR parameters and their structure sensitivity for selected surface paramagnetic species involved in oxidative dehydrogenation of methanol over silica grafted molybdenum catalyst were investigated. Two surface complexes, Mo 4 c /SiO 2 and {O − Mo 4 c }/SiO 2, as well as CH 2OH radical trapped on the SiO 2 matrix were taken as the examples. The spin-restricted zeroth order regular approximation (ZORA) implemented in the Amsterdam Density Functional suite was used to calculate the electronic g tensor for those species. The predicted values were in satisfactory agreement with experimental EPR results. Five different coordination modes of the CH 2OH radical on the silica surface were considered and the isotropic 13C, 17O, and 1H hyperfine coupling constants (HFCC) of the resultant surface complexes were calculated. Structure sensitivity of the HFCC values was discussed in terms of the angular deformations caused by hydrogen bonding with the silica surface.

Effect of tetrapyrrole-based photosensitizers on spin trapping by 5,5-dymethyl-1-pyrroline N-oxide

Kinetics of the effect of three different tetrapyrrole-based photosenzitizers, pheophorbide a, meso-tetrahydroxyphenyl chlorin, and 3,7-bis(2-carboxyethyl)-2,8,12,17-tetramethyl-13,18-bis[(5-propoxycarbonyl-3-4-dihidroxy-phenyloxy) ethyl] porphyrin, on spin trapping with of 5,5-dymethyl-1-pyrroline N -oxide (DMPO) – a method widely applied in photodynamic therapy research – was investigated in the chemical model system of initiated oxidation of ethylbenzene. Spin adducts were measured by ESR spectroscopy, and kinetics of the consumption of the spin trap and accumulation of main products of oxidation (acetophenon, 1-phenyl-ethanol and 1-phenyl-ethyl-hydroperoxide) were followed using high performance liquid chromatography. The ESR spectrum of the non-illuminated system corresponded to an adduct formed between the spin trap and α–phenylethyl peroxyl radical, while under illumination, the spectrum indicated competitive formation of another adduct. Based on analysis of the corresponding g factor and hyperfine coupling constant values of the latter adduct ( g = 2.00587, αN = 13.57 G , αH = 2.27 G ), the nitrone ring has presumably been split up and an adduct was formed between α-phenylethyl radical and 4-methyl-4-nitrosopentanoic acid – a product of the oxidation of the parent spin trap by singlet oxygen. Computer modeling based on detailed reaction mechanism and fitted to the measured data confirmed this assumption. The rate constants gave values of 2.3 × 105 M −1. s −1 for the formation of the adduct and 1.0 × 104 M −1. s −1 for its decay by interaction with free radicals offering a more detailed quantitative understanding of spin trapping in the presence of tetrapyrrole photosensitizers under illumination.

Fast-Flow EPR Spectroscopic Observation of the Isoniazid, Iproniazid, and Phenylhydrazine Hydrazyl Radicals

Hydrazyl radical intermediates have been suggested as important intermediates in the biochemistry of hydrazides and hydrazines. Although spin-trapping studies have intercepted those species previously, there has been no report of the direct observation of the unstable hydrazyl radicals of isoniazid and iproniazid. We have employed the fast-flow technique in electron paramagnetic resonance (EPR) spectroscopy to measure spectra for the short-lived hydrazyl radicals of a family of hydrazides, including the pharmacologically important compounds isoniazid and iproniazid, as well as for a series of phenylhydrazines. Our investigations of the phenylhydrazine radical and the related chloro-substituted analogues have allowed definitive assignments of the hyperfine coupling constants of that toxicologically important free radical. Theoretical values of hyperfine coupling constants, calculated by density functional formalism, provided a guide to assignments for the hydrazyl species and confirmed the experimentally based assignments for phenylhydrazyl radical.

P-2-231 - Local neurosarcoidosis radiographically misdiagnosed as a meningioma

The hyperfine coupling constant to the proton in the 4 position of the nitroxide spin label 4-protio-3-carbamoyl-2,2,5,5-tetraperdeuteromethyl-3-pyrrohne-1-yloxy (mHCTPO) has been measured in seven solvents of varying polarities at 259 and 323 K. All of the proton hyperfine coupling constants in 3-carbamoyl-2,2,5,5,-tetramethyl-3-pyrroline-1-yloxyl (CTPO) have been measured by NMR at 500 MHz in CD3OD at 307 and 323 K. These results together with corrected results in the literature have been fitted to an empirical equation which yields accurate values of the hyperfine coupling constants from measured values of the nitrogen hyperfine coupling constant and the temperature. Gaussian linewidths, necessary to correct for inhomogeneous broadening in unresolved spin-label spectra, may be calculated from an empirical equation involving the nitrogen hyperfine coupling constant and the temperature.

ESR characterization and kinetics of the enzymatically obtained M(II)-dobutamine-o-semiquinone system

Dobutamine is a synthetic catecholamine used in the treatment of different cardiac pathologies associated with organic heart diseases, myocardial infarction and cardiac surgery. The ESR spectra of M2+-o-semiquinone radicals, obtained by the oxidation of dobutamine HCl with horseradish peroxidase and hydrogea peroxide in the presence of several bivalent metal ions of IIA (Mg, Ca, Sr, Ba) and IIB (Zn, Cd) groups, have been reported. ESR experimental spectra have been simulated and completely characterized. The dependence of the proton hyperfine coupling constant values on the complexing ion radius has been discussed together with the differences in the radicals concentrations. Furthermore, the formation and decay kinetics of Zn2+-dobutamine-o-semiquinone have been analyzed at varying H2O2 concentrations, and the corresponding kinetic constants have been calculated.

Ab Initio Investigation of the Vibronic and Magnetic Hyperfine Effects in theX2ΠuState of[formula

The ab initio variational method for handling the Renner–Teller effect in tetraatomic molecules, developed in a previous work, is extended to enable the calculation of the vibronically averaged values for the hyperfine coupling constants. The approach is applied to compute the vibronic mean values for the isotropic hyperfine coupling constants (hfcc's) of hydrogen and boron centers in theX2Πuelectronic state of[formula]. The present study clearly shows that for the isotropic hfcc of the boron center the incorporation of the vibronic effects is essential already for the lowest vibronic state. The isotropic hfcc's calculated for the lowest vibronic state (Aiso(11B) = 12.6 MHz,Aiso(H) = 47.2 MHz) differ considerably from the isotropic hfcc's calculated at the equilibrium geometry (Aiso(11B) = 1.3 MHz,Aiso(H) = 47.9 MHz). The strong influence of the vibronic effects can also be seen inAiso(11B) calculated for[formula]which should be identical to that of[formula]within the Born–Oppenheimer approximation. Including vibronic effects, the values differ by about 20%.