Role of the geometry, restricted rotations and solvents on the computed 2,2'-diphenyl-1-picrylhydrazyl hyperfine tensors

The nuclear hyperfine tensor (A) components of the 2,2'-diphenyl-1-picrylhydrazyl neutral radical are computed using the UB1LYP hybrid density functional method. Solvent interactions via hydrogen bonding are found to play a crucial role in the position of the two phenyl rings relative to the picryl moiety. Under these conditions, the calculated isotropic hyperfine tensor components of the N 1 and N 2 hydrazyl backbone are within approximately 1.3 Gauss (G) of the experimental values determined by EPR and ENDOR spectroscopy. Just as important are the effects of restricted rotations of the phenyl rings on these tensors. Rotational averaging using a Maxwell-Boltzmann type distribution improves the agreement between theory and experiment to less than 1.0 G. In addition, rotational averaging of the twelve isotropic proton coupling constants has also been performed. They come within 0.3 G of the experimental values. Thus, for the first time, all the nuclear hyperfine tensor components of this large class of molecules are accurately calculated without resorting to post Hartree-Fock techniques.     

Probing the Electronic Structure of Bacteriochlorophyll Radical Ions—A Theoretical Study of the Effect of Substituents on Hyperfine Parameters

In reaction centers (RCs) of photosynthesis, a light‐induced charge separation takes place creating radical cations and anions of the participating cofactors. In photosynthetic bacteria, different bacteriochlorophylls (BChl) are involved in this process. Information about the electronic structure of the BChl radical cations and anions can be obtained by measuring the electron spin density distribution via the electron–nuclear hyperfine interaction using EPR and ENDOR techniques. In this communication, we report isotropic hyperfine coupling constants (hfcs) of the BChl b and g radical cations and anions, calculated by density functional theory, and compare them with the more common radical ions of BChl a and with available experimental data. The observed differences in the computed hyperfine data are discussed in view of a possible distinction between these species by EPR/ENDOR methods. In addition, ¹⁴N nuclear quadrupole coupling constants (nqcs) computed for BChl a, b, g, and also for Chl a in their charge neutral, radical cation and radical anion states are presented. These nqcs are compared with experimental values obtained by ESEEM spectroscopy on several different radical ions.     

Zero-point vibrational corrections to isotropic hyperfine coupling constants in polyatomic molecules

The present work addresses isotropic hyperfine coupling constants in polyatomic systems with a particular emphasis on a largely neglected, but a posteriori significant, effect, namely zero-point vibrational corrections. Using the density functional restricted-unrestricted approach, the zero-point vibrational corrections are evaluated for the allyl radical and four of its derivatives. In addition for establishing the numerical size of the zero-point vibrational corrections to the isotropic hyperfine coupling constants, we present simple guidelines useful for identifying hydrogens for which such corrections are significant. Based on our findings, we critically re-examine the computational procedures used for the determination of hyperfine coupling constants in general as well as the practice of using experimental hyperfine coupling constants as reference data when benchmarking and optimizing exchange-correlation functionals and basis sets for such calculations.     

A theoretical study of nuclear spin coupling constants and hyperfine coupling constants of se-heterocyclics

MO calculations based on the finite perturbation theory in the INDO approximation have been carried out on selenophene, eighteen of its monosubstituted derivatives and benzo (b)selenophene. The calculated nuclear spin coupling constants satisfactorily reproduce signs, magnitudes, internal orders and some trends of the experimental values. Comparison of different ZDO calculations provides information on the relative importance of σ and π pathways for the various coupling constants in selenophene and benzo(b)selenophene. Unrestricted Hartree-Fock calculations at the INDO level have been performed on the radical anions of dibenzoselenophene and 2,1,3-benzoselenadiazole, the phenoselenazine radical cation, the phenoselenazine neutral radical and the phenoselenazine nitroxide. The isotropic hyperfine coupling constants have been found to be generally in satisfactory agreement with experiment.     

Ab initio MO LCAO UHF calculations of magnetic hyperfine interactions in σ radicals. Isotropic and anisotropic couplings of NO2 and CO2−

The magnetic hyperfine coupling constants in NO₂ and CO₂^? have been computed by an initio methods. Spin annihilation is found to be essential in order to obtain useful results for the dipolar couplings, but has much less influence on the isotropic couplings. The electric quadrupole coupling constants have also been evaluated, and are in good agreement with available experimental data.     

1H and 13C hyperfine coupling constants of the tryptophanyl cation radical in aqueous solution from microsecond time-resolved CIDNP

Relative values of the 1H and 13C isotropic hyperfine couplings in the cationic oxidized tryptophan radical TrpH*+ in aqueous solution are determined. The data are obtained from the photo-CIDNP (chemically induced dynamic nuclear polarization) enhancements observed in the microsecond time-resolved NMR spectra of the diamagnetic products of photochemical reactions in which TrpH*+ is a transient intermediate. The method is validated using the tyrosyl neutral radical Tyr*, whose 1H and 13C hyperfine couplings have previously been determined by electron paramagnetic resonance spectroscopy. Good agreement is found with hyperfine coupling constants for TrpH*+ calculated using density functional theory methods but only if water molecules are explicitly included in the calculation.     

An investigation of capto-dative interactions in muonated cyclohexadienyl radicals.

On the basis of the muon hyperfine coupling constants, it is concluded that the pairs of substituents in cyclohexadienyl radicals operate on the spin-density distribution in a “capto-dative” manner, as they do in benzyl radicals, when direct conjugation with the radical centre is possible; however, for other arrangements of the substituents, opposing effects are found.     

Hydrogen bond geometries from electron paramagnetic resonance and electron-nuclear double resonance parameters: density functional study of quinone radical anion-solvent interactions

Density functional theory was used to study the impact of hydrogen bonding on the p-benzosemiquinone radical anion BQ(*-) in coordination with water or alcohol molecules. After complete geometry optimizations, (1)H, (13)C, and (17)O hyperfine as well as (2)H nuclear quadrupole coupling constants and the g-tensor were computed. The suitability of different model systems with one, two, four, and 20 water molecules was tested; best agreement between theory and experiment could be obtained for the largest model system. Q-band pulse (2)H electron-nuclear double resonance (ENDOR) experiments were performed on BQ(*-) in D(2)O. They compare very well with the spectra simulated by use of the theoretical values from density functional theory. For BQ(*-) in coordination with four water or alcohol molecules, rather similar hydrogen-bond lengths between 1.75 and 1.78 A were calculated. Thus, the computed electron paramagnetic resonance (EPR) parameters are hardly distinguishable for the different solvents, in agreement with experimental findings. Furthermore, the distance dependence of the EPR parameters on the hydrogen-bond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected dependencies on the H-bond length R(O.H). A correlation was obtained for the g-tensor. It is shown that the point-dipole model is suitable for the estimation of hydrogen-bond lengths from anisotropic hyperfine coupling constants of the bridging (1)H nuclei for H-bond lengths larger than approximately 1.7 A. Furthermore, the estimation of H-bond lengths from (2)H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.     

Density functional theory study of the beta-carotene radical cation and deprotonated radicals

The beta-carotene radical cation and deprotonated neutral radicals were studied at the density functional theory (DFT) level using different density functionals and basis sets: B3LYP/3-21G, SVWN5/6-31G*, BPW91/DGDZVP2, and B3LYP/6-31G**. The geometries, total energies, spin distributions, and isotropic and anisotropic hyperfine coupling constants of these species were calculated. Deprotonation of the methyl group at the double bond of the cyclohexene ring of the carotenoid radical cation at 5 or 5' produces the most stable neutral radical because of retention of the pi-conjugated system while less stable deprotonation at 9 or 9' and 13 or 13' of the chain methyl groups causes significant distortion of the conjugation. The predicted methyl hyperfine coupling constants of 13-16 MHz of the neutral radicals are in good agreement with the previous electron nuclear double resonance (ENDOR) spectrum of photolyzed beta-carotene on a solid support. DFT calculations on the beta-carotene radical cation in a polar water environment showed that the polar environment does not cause significant changes in the proton hyperfine constants from those in the isolated gas-phase molecule. DFT calculated methyl proton hyperfine coupling constants of less than 7.2 MHz are in agreement with those reported for the radical cation in photosystem II (PS II) and those found in the absence of UV light for the radical cation on a silica alumina matrix.     

Magnetic interaction of pyridyl-substituted thioaminyl stable free radicals

N-(2-Pyridylthio)-2,6-diaryl-4-R-phenylaminyls (R = Ph, 4-ClC(6)H(4), MeCO, CN, EtOCO) and N-(4-pyridylthio)-2,6-diaryl-4-R-phenylaminyls (R = Ph, 4-ClC(6)H(4), EtOCO) were prepared and isolated as radical crystals. Their ESR spectra were measured, and the NS and pyridyl nitrogen and anilino meta and pyridyl ortho and para proton hyperfine coupling constants were determined. The spin-density calculations based on the density functional theory were performed by the UBecke 3LYP hybrid method using the STO 6-31G basis set. X-ray crystallographic analyses were performed for three radicals, and their structures were discussed in detail. The magnetic susceptibility measurements were carried out for the nine kinds of isolated radicals with a SQUID magnetometer. One radical showed ferromagnetic coupling (2J/k(B) = 44 K), and the others showed antiferromagnetic behavior. The magnetic interactions observed are explained on the basis of the crystal structures.     

Effects of static and dynamic perturbations on isotropic hyperfine coupling constants in some quinone radicals

The effects of solvent dielectric response on the isotropic hyperfine coupling constants of the 1,4-benzoquinone, 1,4-naphthoquinone and 9,10-anthraquinone anions and 1,4-naphthalenediol cation radicals were studied by electron spin resonance (ESR) spectroscopy and by the theoretical density functional method within the polarizable continuum model. Experimental results demonstrate that the isotropic hyperfine coupling constants can be obtained with high accuracy and that the effects of solvent impurities can be minimized by careful sample preparation. The results obtained correlate well with theoretical predictions from density functional theory calculations. For 1,4-naphthalenediol both the solvent dielectric response as well as rotational averaging of the hydroxy groups were calculated. The overall results highlight the importance of static and dynamic perturbations to the couplings and aid in the assignation process of the couplings to specific magnetic nuclei.     

ESR and calculations on electronic structure of phenalenyl and phenalenyl derivative radicals

Calculations on electronic structure of the perinaphthenyl radical and phenalenyl derivative radicals responsible for the composition of the ESR spectrum of marine diesel under heating were performed to obtain support for the experimental ESR results. The parameters calculated were the hyperfine coupling constants (A), which were then used for comparison with the experimental data. The energy‐minimized structures were obtained using the density functional theory method. In all cases, the symmetry system was taken into account in theoretical calculations. The differences between experimental and theoretical values were below 7% for nearest hydrogens in molecules, named hyperfine coupling constant A (first neighbors) and 18% for farthest hydrogens atoms named hyperfine coupling constants A′ (second neighbors), for all structures analyzed. Copyright © 2014 John Wiley & Sons, Ltd.     

Calculating the electron paramagnetic resonance parameters of exchange coupled transition metal complexes using broken symmetry density functional theory: application to a MnIII/MnIV model compound

The capability of the density functional broken symmetry approach for the calculation of various EPR parameters of exchange coupled metal clusters is demonstrated by studying the experimentally well-investigated [Mn(III)Mn(IV)(mu-O)(2)(mu-OAc)DTNE](2+) complex. Geometry optimizations of the complex in its broken symmetry and high spin states yielded structures with two distinct manganese sites and geometrical parameters in good agreement with the X-ray structure. Exchange coupling constants were calculated from the energy differences between the high spin and broken symmetry states using the Heisenberg spin Hamiltonian. Very good agreement between theory and experiment was achieved with the B3LYP hybrid functional. The g-tensor calculations were performed employing the coupled perturbed Kohn-Sham equations. A strategy for the computation of g-tensor site values is presented and provides single-site g-tensors that are in good agreement with the expectations for Mn(III) and Mn(IV), respectively. Spin projection gave the g-tensor of the coupled manganese complex in very good agreement with the experimental results. Complete (55)Mn hyperfine tensors, including spin-orbit contributions, were calculated and spin-projected. The source of anisotropy in this system could be traced back to the Mn(III) ion in line with the experimental results. The isotropic manganese hyperfine coupling constants were underestimated by factors between 1.4 and 2.5. It is shown that this deficiency is systematic in character and not anchored in the broken symmetry approach. Nuclear quadrupole splitting of the (55)Mn nuclei is shown to be small in this system. In addition, (14)N and (1)H ligand hyperfine data were calculated and compared well with the experimental results. The quality of the extended point-dipole model was demonstrated in application to (1)H anisotropic hyperfine coupling constants.     

2D Ligand ENDOR and TRIPLE resonance studies of Cu(II)-doped DTGS single crystals

The deuteron hyperfine and quadrupole coupling tensors in the bis(glycinato)-Cu(II) ((ND₂CH₂COO)₂Cu) complex in ferroelectric triglycine sulfate (DTGS) single crystals were measured by ENDOR spectroscopy. These data indicate that the molecular symmetry of the Cu complex is C₁. From TRIPLE resonance experiments the signs of the isotropic coupling constants were determined. Structural information was obtained from the principal values of the quadrupole coupling tensors of the deuterium atoms and their direction cosines. Using the relationship between the deuterium quadrupole constant and ND bond lengths, the positions of the deuterons in bis(glycinato)-Cu(II) were estimated. The negative signs of the isotropic hyperfine coupling constants were explained by spin polarization of the deuteron 1s orbitals. The values of the anisotropic hyperfine coupling constants are discussed in the framework of conventional molecular-orbital theory and can be understood quite satisfactorily.     

An ab initio study of the hyperfine structure in the X2 Pi electronic state of CCCH

The results of an ab initio study of the magnetic hyperfine structure in the X (2)Pi electronic state of CCCH are reported. The potential surfaces for two components of the X (2)Pi electronic state were computed by means of an extensive configuration interaction approach. The electronically averaged hyperfine coupling constants of H and (13)C for (12)C (12)C (12)CH, (13)C (12)C (12)CH, (12)C (13)C (12)CH, and (12)C (12)C (13)CH are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The model Hamiltonian is expressed in terms of the normal bending coordinates. It is found that, due to the generally strong geometry dependence of the hyperfine coupling constants, it is necessary to carry out the vibronic averaging of the corresponding functions in order to obtain the values which can be compared to the results of the measurements. The results of the present study help to reliably interpret the experimental data previously published. They also predict the yet unobserved hyperfine structure in excited vibronic states.     

Electron traps on oxide surfaces: (H+)(e-) pairs stabilized on the surface of 17O enriched CaO.

(H+)(e-) pairs generated at the surface of polycrystalline CaO are analyzed for the first time in terms of the interaction of the unpaired electron spin with the nuclear spin of the 17O anions of the surface. CaO crystals enriched in the 17O isotope are prepared and the corresponding hyperfine coupling constants are measured in electron paramagentic resonance (EPR) spectra. The results are analyzed on the basis of cluster model density functional theory calculations. The computed hyperfine coupling constants for (H+)(e-) pairs formed on the edge, corner, and reverse corner sites of the CaO surface allow a tentative assignment of two observed spectral features to specific morphological surface sites.     

Hyperfine coupling constants of the azafullerenes C19N, C59N, C69N, and C75N

The isomers of the nitrogen-substituted fullerenes (azafullerenes) C19N, C59N, C69N, and C75N are examined using all-electron Gaussian atomic orbital basis density functional theory, to determine the doublet radical geometries and hyperfine coupling constants. We find that the inaccuracy of previously calculated hyperfine coupling constants of C59N resulted from a poor treatment of the geometry optimization. We find that UB3LYP minimization of the radical geometry in the 6-31G basis, followed by single-point evaluation of the hyperfine constants in which an expanded basis is used on the atomic sites of interest, forms an efficient compromise between computational cost and accuracy with respect to experimental hyperfine constants. Using this approach, we assign the hyperfine signals observed in experiments on the C69N radical by calculating the hyperfine coupling constants for all five of the isomers and examine the electron spin density distribution. Finally, we present predicted hyperfine coupling constants for the isomers of C19N and C75N for use in the interpretation of future experiments.     

The electronic structure of the photoexcited triplet state of free-base (tetraphenyl)porphyrin by time-resolved electron-nuclear double resonance and density functional theory

The photoexcited triplet states of free-base porphyrin (H(2)P) and free-base tetraphenylporphyrin (H(2)TPP) have been investigated by time-resolved electron paramagnetic resonance and electron-nuclear double resonance in a toluene glass at 80 K. Both the zero-field splitting parameters, D and E, and the proton A(zz) hyperfine coupling tensor components could be determined. D is about 13% larger in H(2)P than in H(2)TPP. In contrast, however, the A(zz) hyperfine coupling tensor components showed differences of less than 2%. To aid the understanding of these results, the electronic structures of H(2)P and H(2)TPP have been modeled using density functional theory. The geometrical structures of both molecules in their lowest triplet states were calculated using the Becke3 Lee-Yang-Parr composite exchange correlation functional and the 6-31G* basis set. Hyperfine couplings for these structures were calculated using the same functional but with the extended EPR-II basis set. These allow unambiguous assignment of the experimentally determined couplings. The theoretical values for H(2)P and H(2)TPP agree with the experimental values in that the presence of the phenyl groups has only a small effect on the unpaired electron spin-density distribution. The difference in sensitivity of the zero-field splitting parameters and the hyperfine couplings to mesophenyl substitution is discussed in terms of the wave functions of the four frontier orbitals of porphyrins introduced by Gouterman.     

Characterization of cationic diarylethene by electron spin resonance and absorption spectra-ratio of open/closed-ring isomers

Electrochemical cyclization/cycloreversion reactions of a diarylethene, 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene, are examined experimentally by electron spin resonance (ESR) and absorption spectra. To understand the ESR spectrum, the hyperfine coupling constants are calculated by the density functional theory (DFT) with the B3LYP exchange-correlation functional. The averaged values of the hyperfine coupling constants are approximated by imposing the C(2) symmetry on the structure of the diarylethene. We found that the spectral width of the ESR is significantly different between the open- and closed-ring isomers. This is due to the difference in the pi-conjugation between two isomers. The ESR spectral width analysis could, thus, be used to identify the isomerization of the radical species, which involve the change of the pi-conjugation. The experimentally observed spectrum is found to be the mixture of the open- and closed-ring isomers of the diarylethene. The excitation energies of the cationic diarylethenes are further identified by the SAC-CI calculations.     

自由基CH2CO+的平衡构型和超精细耦合常数的从头计算

本文用UHF从头计算法处理π自由基CH_2CO~+, 确定基态并研究其平衡构型, 电子结构和自旋性质。在自旋性质的计算中, 本文使用了由Amos和Snyder~(1)提出的方法。通过从UHF波函数中投影去掉最主要的污染自旋成分, 得到近似的自旋纯态波函数。由这种波函数得到的关于自旋性质的计算结果比UHF结果大为改善, 计算的hfc常数与ESR实验结果符合得相当好。