Unpaired Spin Density Distribution Research Articles

Guanidinate Yttrium Complexes Containing Bipyridyl and Bis(benzimidazolyl) Radicals.

Ancillary ligand scaffolds that sufficiently stabilize a metal ion to allow its coordination to an open-shell ligand are scarce, yet their development is essential for next-generation spin-based materials with topical applications in quantum information science. To this end, a synthetic challenge must be met: devising molecules that enable the binding of a redox-active ligand through facile displacement and clean removal of a weakly coordinating anion. Here, we probe the accessibility of unprecedented radical-containing rare-earth guanidinate complexes by combining our recently discovered yttrium tetraphenylborate complex [{(Me3Si)2NC(NiPr)2}2Y][(μ-η6-Ph)(BPh3)] with the redox-active ligands 2,2'-bipyridine (bpy) and 2,2'-bis(benzimidazole) (Bbim), respectively, under reductive conditions. Our endeavor resulted in the first evidence of guanidinate complexes that contain radicals, namely, a mononuclear bipyridyl radical complex, {(Me3Si)2NC(NiPr)2}2Y(bpy•) (1), and a dinuclear bis(benzimidazolyl) radical-bridged complex, [K(crypt-222)][{(Me3Si)2NC(NiPr)2}2Y]2(μ-Bbim•) (2'). The latter was achieved by an in situ reduction of [{(Me3Si)2NC(NiPr)2}2Y]2(μ-Bbim) (2), which was isolated from a salt metathesis reaction. 1 and 2 were characterized by X-ray crystallography and IR and UV-vis spectroscopy. Variable-temperature electron paramagnetic resonance spectroscopy was applied to gain insight into the distribution of unpaired spin density on 1 and 2'. Density functional theory calculations were conducted on 1 and 2' to elucidate further their electronic structures. The redox activity of 1 and 2' was also probed by electrochemical methods.

The Ubiquinol Binding Site of Cytochrome bo3 from Escherichia coli Accommodates Menaquinone and Stabilizes a Functional Menasemiquinone.

Cytochrome bo3, one of three terminal oxygen reductases in the aerobic respiratory chain of Escherichia coli, has been well characterized as a ubiquinol oxidase. The ability of cytochrome bo3 to catalyze the two-electron oxidation of ubiquinol-8 requires the enzyme to stabilize the one-electron oxidized ubisemiquinone species that is a transient intermediate in the reaction. Cytochrome bo3 has been shown recently to also utilize demethylmenaquinol-8 as a substrate that, along with menaquinol-8, replaces ubiquinol-8 when E. coli is grown under microaerobic or anaerobic conditions. In this work, we show that its steady-state turnover with 2,3-dimethyl-1,4-naphthoquinol, a water-soluble menaquinol analogue, is just as efficient as with ubiquinol-1. Using pulsed electron paramagnetic resonance spectroscopy, we demonstrate that the same residues in cytochrome bo3 that stabilize the semiquinone state of ubiquinone also stabilize the semiquinone state of menaquinone, with the hydrogen bond strengths and the distribution of unpaired spin density accommodated for the different substrate. Catalytic function with menaquinol is more tolerant of mutations at the active site than with ubiquinol. A mutation of one of the stabilizing residues (R71H in subunit I) that eliminates the ubiquinol oxidase activity of cytochrome bo3 does not abolish activity with soluble menaquinol analogues.

Radicals formed from proton loss of carotenoid radical cations: A special form of carotenoid neutral radical occurring in photoprotection

In an organized assembly in Arabidopsis thaliana plant, proton loss from the radical cation of zeaxanthin (Zea+) was found to occur under intense illumination, a possible component in photoprotection. A stable neutral radical is formed because of the favorable proton loss at C4(4′) position(s) of the terminal ends of Zea+ that extends the unpaired spin density distribution (notation Zea(4) or Zea(4′) by symmetry). Proton loss from the radical cation of β-carotene (β-car+) to available proton acceptors was also detected in a PSII sample upon irradiation. A controversial optical absorption peak at 750nm predicted by DFT was attributed to β-carotene neutral radical formed by proton loss at C4(4′) position(s) situated on the terminal end(s) (notation β-car(4) or β-car(4′) by symmetry), also detected in solid copper-containing MCM-41 molecular sieves (Cu-MCM-41) and by hydrogen atom transfer from β-carotene to hydroxyl radical. Unlike the PSII organized assembly where proton loss occurs from the terminal ends of the radical cation, in the Cu-MCM-41 unorganized assembly proton loss occurs from all positions including the methyl groups on the polyene chain forming neutral radicals with different EPR couplings, different unpaired spin density distribution and different optical absorption peaks. We emphasize here the properties of these neutral radicals for various carotenoids formed under high-light conditions and in different media (solution, solid siliceous materials, and photosynthetic samples).

The semiquinone at the Qi site of the bc1 complex explored using HYSCORE spectroscopy and specific isotopic labeling of ubiquinone in Rhodobacter sphaeroides via (13)C methionine and construction of a methionine auxotroph.

Specific isotopic labeling at the residue or substituent level extends the scope of different spectroscopic approaches to the atomistic level. Here we describe 13C isotopic labeling of the methyl and methoxy ring substituents of ubiquinone, achieved through construction of a methionine auxotroph in Rhodobacter sphaeroides strain BC17 supplemented with l-methionine with the side chain methyl group 13C-labeled. Two-dimensional electron spin echo envelope modulation (HYSCORE) was applied to study the 13C methyl and methoxy hyperfine couplings in the semiquinone generated in situ at the Qi site of the bc1 complex in its membrane environment. The data were used to characterize the distribution of unpaired spin density and the conformations of the methoxy substituents based on density functional theory calculations of 13C hyperfine tensors in the semiquinone of the geometry-optimized X-ray structure of the bc1 complex (Protein Data Bank entry 1PP9) with the highest available resolution. Comparison with other proteins indicates individual orientations of the methoxy groups in each particular case are always different from the methoxy conformations in the anion radical prepared in a frozen alcohol solution. The protocol used in the generation of the methionine auxotroph is more generally applicable and, because it introduces a gene deletion using a suicide plasmid, can be applied repeatedly.

Density-functional global optimization of (La2O3)n clusters

Structures of stoichiometric (La(2)O(3))(n) (n = 1-6) clusters have been systematically studied by theoretical calculations. Global minimum structures for these clusters are determined by genetic algorithm based global optimizations at density functional level. The ground state structure for La(6)O(9) was found to be highly symmetric with point group O(h) and the centered oxygen atom has the coordination number as large as six, which is the same as the highest coordination number of oxygen atoms in bulk La(2)O(3). Analysis of the binding energies shows that La(6)O(9) has a high stability among the studied clusters. The energies of the highest occupied∕lowest unoccupied molecular orbitals, vertical ionization energy, and vertical electron affinity of each cluster are provided. Electronic structure of La(6)O(9) is discussed by analysis of the frontier molecular orbitals and unpaired spin density distributions of charged clusters.

C–H bond activation by nanosized scandium oxide clusters in gas-phase

Scandium oxide cluster cations are prepared by laser ablation and reacted with n-butane in a fast flow reactor. A reflectron time-of-flight mass spectrometer is built to detect the cluster distribution before and after the reactions. Hydrogen atom abstraction (HAA) products (Sc 2O 3) N H + ( N = 1–22), (Sc 2O 3) N O 4H + ( N = 4–22) and their deuterated compounds are observed upon the cluster interactions with n-C 4H 10 and n-C 4D 10, respectively. This indicates that C–H bond activation of n-butane over atomic clusters as large as Sc 44O 66 + and Sc 44O 70 + can take place in gas phase. The experimentally determined rate constants and values of kinetic isotopic effect for HAA vary significantly with the cluster sizes. Density functional theory (DFT) calculations are performed to study the structures and reactivity of small clusters (Sc 2O 3) 1–3 +. The DFT results suggest that the experimentally observed C–H bond activation by (Sc 2O 3) N + is facilitated by oxygen-centred radicals bridgingly bonded in the clusters. The nature of unpaired spin density distributions within the clusters may be responsible for the experimentally observed size-dependent reactivity.

C–H Bond Activation by Oxygen-Centered Radicals over Atomic Clusters

Saturated hydrocarbons, or alkanes, are major constituents of natural gas and oil. Directly transforming alkanes into more complex organic compounds is a value-adding process, but the task is very difficult to achieve, especially at low temperature. Alkanes can react at high temperature, but these reactions (with oxygen, for example) are difficult to control and usually proceed to carbon dioxide and water, the thermodynamically stable byproducts. Consequently, a great deal of research effort has been focused on generating and studying chemical entities that are able to react with alkanes or efficiently activate C-H bonds at lower temperatures, preferably room temperature. To identify low-temperature methods of C-H bond activation, researchers have investigated free radicals, that is, species with open-shell electronic structures. Oxygen-centered radicals are typical of the open-shell species that naturally occur in atmospheric, chemical, and biological systems. In this Account, we survey atomic clusters that contain oxygen-centered radicals (O(-•)), with an emphasis on radical generation and reaction with alkanes near room temperature. Atomic clusters are an intermediate state of matter, situated between isolated atoms and condensed-phase materials. Atomic clusters containing the O(-•) moiety have generated promising results for low-temperature C-H bond activation. After a brief introduction to the experimental methods and the compositions of atomic clusters that contain O(-•) radicals, we focus on two important factors that can dramatically influence C-H bond activation. The first factor is spin. The O(-•)-containing clusters have unpaired spin density distributions over the oxygen atoms. We show that the nature of the unpaired spin density distribution, such as localization and delocalization within the clusters, heavily influences the reactivity of O(-•) radicals in C-H bond activation. The second factor is charge. The O(-•)-containing clusters can be negatively charged, positively charged, or neutral overall. We discuss how the charge state may influence C-H bond activation. Moreover, for a given charge state, such as the cationic state, it can be demonstrated that local charge distribution around the O(-•) centers can also significantly change the reactivity in C-H bond activation. Through judicious synthetic choices, spin and charge can be readily controllable physical quantities in atomic clusters. The adjustment of these two properties can impact C-H bond activation, thus constituting an important consideration in the rational design of catalysts for practical alkane transformations.

Hydrogen Bonding and Spin Density Distribution in the QBSemiquinone of Bacterial Reaction Centers and Comparison with the QASite

In the photosynthetic reaction center from Rhodobacter sphaeroides, the primary (Q(A)) and secondary (Q(B)) electron acceptors are both ubiquinone-10, but with very different properties and functions. To investigate the protein environment that imparts these functional differences, we have applied X-band HYSCORE, a 2D pulsed EPR technique, to characterize the exchangeable protons around the semiquinone (SQ) in the Q(A) and Q(B) sites, using samples of (15)N-labeled reaction centers, with the native high spin Fe(2+) exchanged for diamagnetic Zn(2+), prepared in (1)H(2)O and (2)H(2)O solvent. The powder HYSCORE method is first validated against the orientation-selected Q-band ENDOR study of the Q(A) SQ by Flores et al. (Biophys. J.2007, 92, 671-682), with good agreement for two exchangeable protons with anisotropic hyperfine tensor components, T, both in the range 4.6-5.4 MHz. HYSCORE was then applied to the Q(B) SQ where we found proton lines corresponding to T ≈ 5.2, 3.7 MHz and T ≈ 1.9 MHz. Density functional-based quantum mechanics/molecular mechanics (QM/MM) calculations, employing a model of the Q(B) site, were used to assign the observed couplings to specific hydrogen bonding interactions with the Q(B) SQ. These calculations allow us to assign the T = 5.2 MHz proton to the His-L190 N(δ)H···O(4) (carbonyl) hydrogen bonding interaction. The T = 3.7 MHz spectral feature most likely results from hydrogen bonding interactions of O1 (carbonyl) with both Gly-L225 peptide NH and Ser-L223 hydroxyl OH, which possess calculated couplings very close to this value. The smaller 1.9 MHz coupling is assigned to a weakly bound peptide NH proton of Ile-L224. The calculations performed with this structural model of the Q(B) site show less asymmetric distribution of unpaired spin density over the SQ than seen for the Q(A) site, consistent with available experimental data for (13)C and (17)O carbonyl hyperfine couplings. The implications of these interactions for Q(B) function and comparisons with the Q(A) site are discussed.

Interaction and Reaction of Ethylene and Oxygen on Six-Atom Silver Clusters Supported on LTA Zeolite

Mixed silver−sodium containing zeolite LTA was prepared by aqueous ion exchange. Reduction of oxidized Ag/NaA (12 wt %) using hydrogen leads to the formation of a paramagnetic cluster possessing a well-defined structure of six equivalent Ag nuclei. The hyperfine structure of the silver clusters appeared only in Ag+-exchanged zeolite. Continuous wave and pulse EPR techniques revealed that the unpaired spin density distribution was strongly affected by adsorption of C2H4 at room temperature, and the initially symmetric structure of the reduced Ag6+ cluster turned less symmetric or less compact, lifting the equivalence of the silver atoms. The extremely weak isotropic hyperfine coupling of the protons was resolved by Mims 1H-ENDOR and HYSCORE. The temperature dependence of the electron spin−lattice relaxation was studied by pulse EPR applying a two-pulse spin−echo method. The inversion recovery of the C2H4/Ag6+ system was on the order of milliseconds, exhibiting a biexponential character. There was no effect on the hyperfine coupling of the Ag6+ cluster when it was exposed to 16O2 at room temperature. The interaction of 17O2 with the surface of the Ag6+ cluster was monitored using CW EPR. This revealed some anisotropy of the hyperfine structure of the cluster at 3.0 K. The sudden disappearance of the anisotropy in the range 7−10.5 K is ascribed to motional effects by 17O2. The hyperfine structure of the Ag6+ cluster disappeared after coadsorbing C2H4 and 16O2, and a new spectrum appeared, suggesting that an intermediate or product of a catalytic reaction is observed. In-situ EXAFS measurements were performed at the Ag K-edge of the hydrogen reduced silver clusters and after adsorbing C2H4 and coadsorbing C2H4 and O2 on the reduced clusters at room temperature. The local structural parameters were derived from a three-shell fitting model including a split Ag−Ag shell and a single Ag−O shell. Formation of a quite monodisperse structure was favored by in-situ reduction, and the structure of the reduced cluster was compatible with six silver atoms, based on the Ag−Ag coordination of four. The interatomic distance was contracted by <10% as compared to the bulk distance. The multiple-shell fitting proved the increase of the carbon plus oxygen coordination number after coadsorbing C2H4 and O2 which accounts for about one added oxygen molecule per cluster. EPR methods prove the electronic structure alterations of the clusters, seen as shifts of the confined unpaired electron spin density.

Differential Polarization of Spin and Charge Density in Substituted Phenoxy Radicals

We have carried out a theoretical study of a series of para-substituted phenoxy radicals in an effort to understand the factors influencing spin and charge density distribution in open-shell systems. The calculations reveal that the distribution of spin and charge are not correlated: cases were found for which spin and charge move together, whereas for other substituents the two quantities exhibit spatially distinct intramolecular polarizations. Charge density variations across the series were found to correlate well with both the Hammett (sigma(p)) and Hammett-Brown (sigma(p)+) constants for each substituent, indicating that inductive and/or resonance effects are primarily responsible for the polarization of charge within the molecule. In contrast, the distribution of unpaired spin density could not be adequately accounted for using any of the typical Hammett-type spin delocalization constants cited in the literature. We uncovered an empirical correlation between the polarization of spin density and the alpha-HOMO-alpha-LUMO gap of the substituted phenoxy radicals: this led to the development of a simple model based on a three-electron, two-orbital bonding scheme in which mixing between the HOMO of the substituent and the SOMO of the phenoxy moiety serves to define the nature and extent of unpaired spin polarization throughout the molecule. This analysis yielded a correlation coefficient of r > 0.97 for the 15 substituents examined in the study; spin polarization effects in compounds that exhibited the greatest deviation from this correlation could also be readily explained within the context of the model. The underlying reason for the ability to differentially polarize spin and charge likely stems from the fact that net unpaired spin density is completely carried by the unpaired electron (and thus tracks the spatial characteristics of the SOMO), whereas charge density reflects the behavior of all of the electrons of the system. These results could have implications in the field of molecular magnetism, suggesting a means for synthetically tuning the magnitude of intramolecular exchange interactions, as well as providing guidance for the design of catalysts for radical-radical coupling reactions.

Measuring Ti(III)−Carotenoid Radical Interspin Distances in TiMCM-41 by Pulsed EPR Relaxation Enhancement Method

Interspin distances between the Ti(3+) ions and the carotenoid radicals produced inside TiMCM-41 pores by photoinduced electron transfer from 7'-apo-7'-(4-carboxyphenyl)-beta-carotene (coordinated to Ti(3+)), canthaxanthin (formed as a random distribution of isomers), and beta-ionone (model for a short-chain polyene) to Ti(3+) framework sites were determined using the pulsed EPR relaxation enhancement method. To estimate the electron transfer distances, the temperature dependence of relaxation rates was analyzed in both siliceous and metal-substituted siliceous materials. The phase memory times, T(M), of the carotenoid radicals were determined from the best fits of two-pulse ESEEM curves. The spin-lattice relaxation times, T(1), of the Ti(3+) ion were obtained from the inversion recovery experiment with echo detection on a logarithmic time scale in the temperature range of 10-150 K. The relaxation enhancement for the carotenoid radicals in TiMCM-41 as compared to that in MCM-41 is consistent with an interaction between the radical and the fast relaxing Ti(3+) ion. For canthaxanthin and beta-ionone, a dramatic effect on the carotenoid relaxation rate, 1/T(M), occurs at 125 and 40 K, respectively, whereas for carboxy-beta-carotene 1/T(M) increases monotonically with increasing temperature. The interspin distances for canthaxanthin and beta-ionone were estimated from the 1/T(M) - 1/T(M0) difference, which corresponds to the Ti(3+) contribution at the temperature where the maximum enhancement in the relaxation rate occurs. Determination of the interspin distances is based on calculations of the dipolar interaction, taking into consideration the unpaired spin density distribution along the 20-carbon polyene chain, which makes it possible to obtain a fit over a wider temperature interval. A distribution of the interspin distances between the carotenoid radical and the Ti(3+) ion was obtained with the best fit at approximately 10 A for canthaxanthin and beta-ionone and approximately 9 A for 7'-apo-7'-(4-carboxyphenyl)-beta-carotene with an estimated error of +/-3 A. The interspin distances do not depend on 1/T(M) - 1/T(M0) for carboxy-beta-carotene which shows no prominent peak in the relaxation rate over the temperature range measured.

Electronic structure of [Ga 2(tren) 2(CA sq,cat)](BPh 4) 2(BF 4): An EPR, ENDOR, and density functional study

The bimetallic [M 1M 2(tren) 2(CA n− )] m+ series, where M = Ga III or Cr III and CA is the chloranilate ligand which can take on diamagnetic (CA cat,cat) 4− or paramagnetic (CA sq,cat) 3− forms, comprises an electronically diverse series of compounds ranging from the closed-shell [Ga 2(tren) 2(CA cat,cat)] 2+ to the S = 5/2 ground state of [Cr 2(tren) 2(CA sq,cat)] 3+. This report deals with the interpretation of the EPR and ENDOR spectra of [Ga 2(tren) 2(CA sq,cat)](BPh 4) 2(BF 4) ( 2) and the related derivative [Ga 2(tren) 2(DHBQ)](BPh 4) 2(BF 4) ( 2a) (where DHBQ is the fully deprotonated trianionic form of 2,5-dihydroxy-1,4-benzoquinone) in an effort to further characterize the electronic structure of this radical species. The X-band (∼9.5 GHz) EPR spectrum of complex 2 acquired in a butyronitrile/propionitrile glass at 4 K reveals a rhombic g-tensor with g xx = 2.0100, g yy = 2.0097, and g zz = 2.0060 with hyperfine interactions due to spin delocalization onto the two Ga nuclei ( a xx = 4.902 G, a yy = 4.124 G, a zz = 3.167 G); the origin of the hyperfine coupling was confirmed by analysis of the room temperature spectra of complexes 2 and 2a. The low-temperature spectrum of complex 2 also indicates the presence of a triplet electronic state characterized by a g-value of 2.009 and axial zero-field splitting of D = 150 G (0.012 cm −1) as determined from measurements carried out at both X- and W-band (∼95 GHz) frequencies. This triplet state is believed to arise due to a weak intermolecular Heisenberg exchange interaction between two aggregating complexes. ENDOR measurements on complex 2a at 20 K allowed for a determination of the magnitude of hyperfine coupling to the protons associated with the radical bridge as well as providing a rare example of an ENDOR signal arising from coupling to a gallium nucleus. Finally, these results were combined with literature data on the free semiquinone form of the bridging ligand in order to assess the extent to which density functional theory can predict unpaired spin density distribution in a complex molecule of this type. Although differences between theory and experiment were noted, DFT was able to provide a reasonably accurate picture of the electronic structure of this system as well as provide insight into the spin polarization mechanism(s) responsible for the observed hyperfine interactions.

The Effect of Oxidation and Reduction of Chlorophyll a on Its Geometry, Vibrational and Spin Density Properties as Revealed by Hybrid Density Functional Methods

The oxidation and reduction of a chlorophyll a model molecule is investigated using hybrid density functional, B3LYP, calculations. The geometry, vibrational modes, and spin density distributions of the free radical forms are investigated. Small bond length changes on oxidation or reduction are shown to lead to significant variations in certain vibrational mode frequency values. Bond length changes can be readily accounted for by the electron density distributions of the frontier HOMO and LUMO orbitals. Harmonic frequency shifts calculated for oxidation and reduction on the 131CO stretching frequency show very good agreement with experimental determinations. The unpaired spin density distribution for the cation and anion free radicals is also computed, and calculated 14N and 25Mg isotropic hyperfine coupling constants are in good agreement with experiment.

Hybrid density functional studies of a bacteriopheophytina model and its anion radical form: Geometry, spin densities, and hyperfine couplings

Hybrid density functional calculations are performed on models of the bacteriopheophytin a molecule, which acts as an electron acceptor in the initial electron transfer events of bacterial photosynthesis. The geometry of the neutral and one-electron reduced free radical form are calculated. Little variation in the main π framework geometry is predicted to occur on one-electron reduction. The electron density of the SOMO for the anion free radical form is also investigated, and the unpaired spin density distribution is also analyzed. Anisotropic and isotropic hyperfine couplings are calculated and discussed on the basis of the unpaired spin density distributions. The electron density of the SOMO is shown to extend on to the carbonyl group attached to ring A; the extent of this delocalization being dependent on the twist angle, this groupmakes with the main ring plane with the magnitude of the 1H hyperfine couplings of the 2methyl group strongly dependent on the value of this twist angle. Good agreement is observed between calculated and experimentally measured isotropic hyperfine couplings. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1292–1298, 1999

The Electronic Structure of the Bacteriopheophytin a Anion Radical, in Vivo

Density functional calculations are used to calculate the electronic structure of the bacteriopheophytin a radical anion, φA, formed in the initial electron-transfer reactions of bacterial photosynthesis. Using the hybrid B3LYP functional together with the double ζ basis set EPR−II, 13C, 1H, 17O, and 14N isotropic and anisotropic hyperfine couplings are calculated and explained by reference to the electron density of the highest occupied molecular orbital and of the unpaired spin distribution around the radical. Good agreement is observed between calculated and experimental hyperfine couplings. Hydrogen bonding to the carbonyl group of ring E leads to minor changes in the unpaired spin density distribution and the resultant hyperfine couplings. The electronic structure of the anion radical form of the other bacteriopheophytin a molecule, φB, found along the inactive-electron-transfer (B) branch, is also studied, and the calculated electronic properties are compared with φA. It is shown that, whereas the e...

Energy Component Analysis of the Pseudo-Jahn−Teller Effect in the Ground State of the Triafulvalene Anion, Pentafulvalene Cation, and Heptafulvalene Anion Radicals

To understand the nature of the pseudo-Jahn−Teller (JT) effect, an energy component analysis of the total energy was carried out in the ground state of the titled ion radicals by using the MCSCF method with 6-31G(d) basis set. Examination of the energy components comprising in the total energy reveals that in the radicals the stability of a less symmetrical nuclear configuration (C2v) is attributable commonly to the energy lowering of the internuclear repulsion term and the kinetic and interelectronic repulsion terms due to σ electrons. These observations are consistent with an expansion of the molecular skeleton brought about by the pseudo-JT distortion. In the triafulvalene anion radical, it is further found that the nuclear−electron attractive and interelectronic repulsive terms due to π electrons also contribute to the stability of the C2v structure. In the pentafulvalene cation and heptafulvalene anion radicals, on the other hand, the interelectronic repulsive and nuclear−electron attractive terms due to π electrons contribute to the stability of the C2v structure, respectively. These differences are accounted for in terms of a charge polarization attributed to the migration of π electrons. Moreover, characteristic electronic properties inherent in the radicals are discussed with much attention to the charge and unpaired spin-density distributions in the distorted C2v structure.

Proton NMR study of the influence of hydrophobic contacts on protein prosthetic group recognition in bovine and rat ferricytochrome b5

The proton nuclear magnetic resonance spectra of the soluble fragment of native bovine and genetically engineered wild-type rat ferricytochrome b5 reconstituted with a wide variety of hemes chemically modified at 2- and/or 4-positions have been recorded and analyzed. While all but one nonsymmetric heme yielded comparable amounts of the two heme orientations immediately after reconstitution, the relative proportion of the two orientations at equilibrium varied widely. The unpaired spin density distribution in the heme pi system leads to substituent hyperfine shift patterns in these paramagnetic complexes that are completely diagnostic of the heme orientation in the protein matrix. An empirical assignment strategy is outlined and applied which allows unequivocal assignment of the absolute orientation of a derivatized heme within the protein matrix. Using a series of hemes lacking 2-fold symmetry solely due to a single substitution, the preferences for localized site occupation of vinyls, methyls, and hydrogens are developed. The large differences in relative stability of the two orientations of native protohemin in the two cytochromes b5 is shown to result from the additivity of localized effects for the bovine protein and the near cancellation of competing effects in the rat protein. The major determinant of the heme orientation is judged to be a repulsive interaction between a vinyl and a hydrophobic cluster of amino acids including positions 23 and 25. The differences in this heme orientational preference among bovine, rat, and chicken ferricytochromes b5 could be correlated with the relative steric bulk of the residues at positions 23 and 25.(ABSTRACT TRUNCATED AT 250 WORDS)

Single-crystal electron paramagnetic resonance study of triplet excitons in [Fe(mesitylene)2+2][C3(C(CN)2)−3

Single crystals of [Fe(mesitylene)2+2][C3(C(CN)2)−3]2 exhibit an intense EPR spectrum with anisotropy and fine structure typical of an electronic triplet species (S=1). From measurements of the fine-structure splitting as a function of angle in three orthogonal crystal planes, it has been established that the spin–spin interaction tensor is nearly axial, D=(−218, 116, 105 MHz), and that the g tensor is almost isotropic with giso =2.0027. The zero-field splitting parameters were determined as ‖D‖=327.8 MHz (117 G) and ‖E‖=5.7 MHz (2 G). Comparison of the principal directions of D with vectors within the known crystal structure shows that the triplet comprises a radical pair formed between nearest neighboring anions. The principal values and directions of D for the biradical compare well with those calculated for the unpaired spin density distribution deduced for the monoradical from its isotropic spectrum and from INDO calculations.

EPR-derived electronic characteristics of zerovalent manganese carbonyl di(tertiary phosphine) radicals

radical~-'Mn(CO)~(P(n-Bu)~)~, 'Mn(CO)3(P(i-Bu)3)2, 'Mn(CO)3(P(i-Pr)3)2, and 'MII(CO),(P(O-~-P~)~)~. Anisotropic g shifts and metal hyperfine and phosphorus superhyperfine spin Hamiltonian parameters are reported and compared to values reported in the literature for 'Mn(CO)5. It is thus possible to examine some of the effects of varying ligand, geometry, and electronic parameters upon the characteristics of the EPR spectra obtained and upon the unpaired spin density localized in the manganese 3d9 and 4s metal orbitals and delocalized onto the phosphorus ligands. An LCAO-MO perturbation treatment of g and A values for these square-pyramidal d7 systems yields reasonable estimates of the distribution of unpaired spin density, upper and lower limits of core polarization, and the energy difference, AE, between the ground state and d?r excited state. of metal-centered organometallic species in hydride abstraction,8-lo electron transfers,11-13 ligand substituti~ns,~~~-~~~~l~-'~ atom transfer^?^^^'^ and oxidative additions/reductive elimination~.~J~ Because of rapid radical recombination it is difficult to prepare persistent zerovalent manganese-centered radicals: consequently, few examples are found in the literature?,l0 Persistent Mn(C0)3L2 metal-centered radicals have been generated, where the ligand, L, is either a tertiary phosphine or pho~phite.~J~J~ These species are isoelectronic with the pentacyanocobaltate radical anion, CO(CN)~~-, a well-characterized radical,6 and with the unsub- stituted manganese(0) pentacarbonyl radical.21v22 This paper presents a characterization via multifrequency EPR spectroscopy of a series of 'Mn(CO),L, radicals. We chose to study 'Mn- (CO)~(P(~-BU)~I~ , 'Mn(CO)dP(i-Bu)31,, 'Mn(CO)3(p(i-pr)312, and 'Mn(C0)3(P(O-i-Pr)3), in order to examine the effects of varying steric and electronic parameter~~~,~~ upon the charac- teristics of the EPR spectra obtained and upon the unpaired spin density localized in the manganese 3d,z and 4s metal orbitals and delocalized onto the phosphorus ligands. Anisotropic g shifts and metal hyperfine and phosphorus superhyperfine spin Hamiltonian parameters are reported and compared to values reported for 'Mn(C0)5.21 An LCAO-MO perturbation treatment of g and A values for these square-pyramidal d7 systems yields reasonable estimates of the distribution of unpaired spin density, upper and lower limits of core polarization, and the energy difference, AE, between the ground state and da excited state. A less detailed study of 'Mn(C0)3L2 radicals has previously appeared.25

Endor investigation of tellurium donors in silicon

Paramagnetic Te + donors in silicon form deep level impurities. They were investigated with electron nuclear double resonance (ENDOR). The superhyperfine interaction with 12 shells of 29Si nuclei could be determined and it was confirmed that S = 1 2 . 10 % of the unpaired spin density are on the central nucleus, 40 % in the 4 nearest Si neighbours along (111), altogether 75 % are within the 12 shells (108 nuclei). The shf structure is very similar to that of S + in Si investigated earlier by Ludwig. From the ENDOR analysis alone it cannot be decided whether Se + is interstitial or on a lattice site. The distribution of unpaired spin density over the neighbour shells is discussed for both sites. It seems, however, easier to explain it assuming the interstitial site.