EPR, ENDOR and HYSCORE study of X-ray induced centres in K2YF5 thermoluminescent phosphors

X-Ray irradiation at room temperature produces several paramagnetic centres in rare-earth activated K2YF5 crystals, whose thermal annealing behaviour can be linked with the occurrence of thermoluminescence (TL) glow peaks. In this paper, continuous wave (CW) and pulsed paramagnetic resonance techniques are used to study the structure of a very stable radiation-induced centre, which may be involved in the TL peak at approximately 390 degrees C reported for Ce- and Tb-activated crystals. From the spectra the centre's g tensor and hyperfine (nuclear quadrupole) tensors for several 19F and 39K neighbouring nuclei are extracted, but no self-hyperfine interaction could be detected. Based on the analysis of the interaction tensors, a model is constructed consisting of an oxygen-related radical (e.g. O(-) or O2(-)) on a substitutional F(-) position in the mirror plane of the YF7 polyhedra. Such a centre most probably corresponds to a trapped-hole state.     

An ENDOR and HYSCORE investigation of a reaction intermediate in IspG (GcpE) catalysis

IspG is a 4Fe-4S protein that carries out an essential reduction step in isoprenoid biosynthesis. Using electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies on labeled samples, we have specifically assigned the hyperfine interactions in a reaction intermediate. These results help clarify the nature of the reaction intermediate, supporting a direct interaction between the unique fourth Fe in the cluster and C2 and O3 of the ligand.     

An EPR and ENDOR investigation of a series of diazabutadiene-group 13 complexes

Paramagnetic diazabutadienegallium(II or III) complexes, [(Ar-DAB)2Ga] and [{(Ar-DAB*)GaX}2] (X = Br or I; Ar-DAB = {N(Ar)C(H)}2, Ar = 2,6-diisopropylphenyl), have been prepared by reactions of an anionic gallium N-heterocyclic carbene analogue, [K(tmeda)][:Ga(Ar-DAB)], with either "GaI" or [MoBr2(CO)2(PPh3)2]. A related InIII complex, [(Ar-DAB*)InCl2(thf)], has also been prepared. These compounds were characterised by X-ray crystallography and EPR/ENDOR spectroscopy. The EPR spectra of all metal(III) complexes incorporating the Ar-DAB ligand, [(Ar-DAB(.))MX(2)(thf)(n)] (M = Al, Ga or In; X = Cl or I; n = 0 or 1) and [(Ar-DAB)2Ga], confirmed that the unpaired spin density is primarily ligand centred, with weak hyperfine couplings to Al (a = 2.85 G), Ga (a = 17-25 G) or In (a = 26.1 G) nuclei. Changing the N substituents of the diazabutadiene ligand to tert-butyl groups in the gallium complex, [(tBu-DAB*)GaI2] (tBu-DAB={N(tBu)C(H)}2), changes the unpaired electron spin distribution producing 1H and 14N couplings of 1.4 G and 8.62 G, while the aryl-substituted complex, [(Ar-DAB*)GaI2], produces couplings of about 5.0 G. These variations were also manifested in the gallium couplings, namely aGa approximately 1.4 G for [(tBu-DAB*)GaI2] and aGa approximately 25 G for [(Ar-DAB*)GaI2]. The EPR spectra of the gallium(II) and indium(II) diradical complexes, [{(Ar-DAB*)GaBr}2], [{(Ar-DAB*)GaI}2], [{(tBu-DAB*)GaI}2] and [{(Ar-DAB*)InCl}2], revealed doublet ground states, indicating that the Ga-Ga and In-In bonds prevent dipole-dipole coupling of the two unpaired electrons. The EPR spectrum of the previously reported complex, [(Ar-BIAN*)GaI2] (Ar-BIAN = bis(2,6-diisopropylphenylimino)acenaphthene) is also described. The hyperfine tensors for the imine protons, and the aryl and tert-butyl protons were obtained by ENDOR spectroscopy. In [(Ar-DAB*)GaI2], gallium hyperfine and quadrupolar couplings were detected for the first time.     

Direct catalytic asymmetric Michael reaction of hydroxyketones: asymmetric Zn catalysis with a Et2Zn/linked-BINOL complex

Full details of our direct Michael addition of unmodified ketones using new asymmetric zinc catalysis are described. Et(2)Zn/(S,S)-linked-BINOL complexes were successfully applied to direct 1,4-addition reactions of hydroxyketones. The first generation Et(2)Zn/(S,S)-linked-BINOL 1 = 2/1 system was effective for 1,4-addition of 2-hydroxy-2'-methoxyacetophenone (3). Using 1 mol % of (S,S)-linked-BINOL 1 and 2 mol % of Et(2)Zn, we found that a 1,4-addition reaction of beta-unsubstituted enone proceeded smoothly at 4 degrees C to afford products in high yield (up to 90%) and enantiomeric excess (up to 95%). In the case of beta-substituted enones, however, the first generation Et(2)Zn/(S,S)-linked-BINOL 1 = 2/1 system was not at all effective. The second generation Et(2)Zn/(S,S)-linked-BINOL 1 = 4/1 with MS 3A system was developed and was effective for various beta-substituted enones to afford products in good dr, yield (up to 99%), and high enantiomeric excess (up to 99% ee). With the Et(2)Zn/1 = 4/1 systems, catalyst loading for beta-unsubstituted enone was reduced to as little as 0.01 mol % (substrate/chiral ligand = 10 000). The new system was also effective for 1,4-addition reactions of 2-hydroxy-2'-methoxypropiophenone (9) to afford chiral tert-alcohol in high enantiomeric excess (up to 96% ee). Mechanistic investigations as well as transformations of the Michael adducts into synthetically versatile intermediates are also described.     

An EPR/ENDOR study of the asymmetric hydrogen bond between the quinone electron acceptor and the protein backbone in Photosystem I

Hydrogen bonding to the photoaccumulated secondary acceptor radical anion A₁^√− in photosystem (PS) I has been studied using pulsed Q-band ENDOR spectroscopy. With deuterated quinone in protonated PS I particles it is demonstrated that the observed radical anion has only one hydrogen-bond hyperfine coupling (hfc) tensor with tensor components above the 2 MHz range. Below 2 MHz the protein matrix protons dominate and a second weak H-bond could not be detected. The spectral resolution of pulsed Q-band ENDOR is critically required to separate the signals of the H-bond proton from those of the primary chlorophyll acceptor, A₀^√−, which cannot be avoided to be formed to some extent in the photoaccumulation procedure. The determined H-bond hfc tensor of A₁^√− is found to be close to axial symmetry with a small isotropic component, as expected from a predominantly dipolar electron–proton spin interaction in a hydrogen-bond. The principal tensor components are A=(+)7.7, MHz A=(−)4.9 MHz, A_iso=(−)0.7 MHz. The magnitude of the dipolar tensor corresponds to an unusually short H-bond which can be estimated from the point-dipole approximation (1.5±0.1 Å). Based on previous studies with A- and B-branch specific site-directed mutants of the A₁ site of PS I and the chosen photoaccumulation protocol, the observed A₁^√− radical anion can be assigned to the Q_K–A site of the A-branch. The observed H-bond hfc tensor is compared to those determined for related quinone radical anions observed in frozen protic solution as well as in the Q_A site of type II bacterial reaction centers.     

Photochemically generated carotenoid radicals on Nafion film and silica gel: an EPR and ENDOR study

Carotenoid cation radicals have been observed at 120 K, by EPR and proton ENDOR measurements, to be formed upon 77 K photolysis of thin films of Nafion or silica gel coated with the carotenoids, β-carotene and canthaxanthin. The powder ENDOR spectra consist of resolvable lines due to couplings larger than 2.8 MHz but smaller than 17 MHz assigned to the methyl protons of the carotenoid cation radical and to an -proton of the planar polyene chain.     

Direct catalytic asymmetric aldol reaction of hydroxyketones: asymmetric Zn catalysis with a Et(2)Zn/linked-BINOL complex

Full details of our newly developed catalyses with asymmetric zinc complexes as mimics of class II zinc-containing aldolase are described. A Et(2)Zn/(S,S)-linked-BINOL complex was developed and successfully applied to direct catalytic asymmetric aldol reactions of hydroxyketones. A Et(2)Zn/(S,S)-linked-BINOL 1 = 2/1 system was initially developed, which efficiently promoted the direct aldol reaction of 2-hydroxy-2'-methoxyacetophenone (7d). Using 1 mol % of (S,S)-linked-BINOL 1 and 2 mol % of Et(2)Zn, we obtained 1,2-dihydroxyketones syn-selectively in high yield (up to 95%), good diastereomeric ratio (up to 97/3), and excellent enantiomeric excess (up to 99%). Mechanistic investigation of Et(2)Zn/(S,S)-linked-BINOL 1, including X-ray analysis, NMR analysis, cold spray ionization mass spectrometry (CSI-MS) analysis, and kinetic studies, provided new insight into the active oligomeric Zn/(S,S)-linked-BINOL 1/ketone 7d active species. On the basis of mechanistic investigations, a modified second generation Et(2)Zn/(S,S)-linked-BINOL 1 = 4/1 with molecular sieves 3A (MS 3A) system was developed as a much more effective catalyst system for the direct aldol reaction. As little as 0.1 mol % of (S,S)-linked-BINOL 1 and 0.4 mol % of Et(2)Zn promoted the direct aldol reaction smoothly, using only 1.1 equiv of 7d as a donor (substrate/ligand = 1000). This is the most efficient, in terms of catalyst loading, asymmetric catalyst for the direct catalytic asymmetric aldol reaction. Moreover, the Et(2)Zn/(S,S)-linked-BINOL 1 = 4/1 system was effective in the direct catalytic asymmetric aldol reaction of 2-hydroxy-2'-methoxypropiophenone (12), which afforded a chiral tetrasubstituted carbon center (tert-alcohol) in good yield (up to 97%) and ee (up to 97%), albeit in modest syn-selectivity. Newly developed (S,S)-sulfur-linked-BINOL 2 was also effective in the direct aldol reaction of 12. The Et(2)Zn/(S,S)-sulfur-linked-BINOL 2 = 4/1 system gave aldol adducts anti-selectively in good ee (up to 93%). Transformations of the aldol adducts into synthetically versatile intermediates were also described.     

Influence of counterions on the structure of bis(oxazoline)copper(ii) complexes; an EPR and ENDOR investigation

X- and Q-band EPR and ENDOR spectroscopy was used to study the structure of a series of heteroleptic and homoleptic copper bis(oxazoline) complexes, based on the (-)-2,2'-isopropylidenebis[(4S)-4-phenyl-2-oxazoline] ligand and bearing different counterions (chloride versus triflate); labelled [Cu(II)()]. The geometry of the two heteroleptic complexes, [Cu(II)()] and [Cu(II)()], depended on the choice of counterion. Formation of the homoleptic complex was only evident when the Cu(II)(OTf)(2) salt was used (Cu(II)(Cl)(2) inhibited the transformation from heteroleptic to homoleptic complexes). The hyperfine and quadrupole parameters for the surrounding ligand nuclei were determined by ENDOR. Well resolved (19)F and (1)H couplings confirmed the presence of both coordinated water and TfO(-) counterions in [Cu()].     

Determination of the structural environment of the tyrosyl radical in prostaglandin H2 synthase-1: a high frequency ENDOR/EPR study

The catalytically active tyrosyl radical which gives rise to the "wide doublet" (WD1) signal in ovine Prostaglandin H2 Synthase-1 has been studied using high frequency (HF) pulsed ENDOR and EPR. A hydrogen-bonded deuteron was directly detected in HFENDOR (130 GHz) spectra of 1H2O/2H2O-exchanged samples. The HFENDOR spectral simulations required a distribution in hydrogen bond distances to achieve proper fits. This range of distances was consistent with that used to model the distribution in gX values detected in pulsed HFEPR spectra. Possible hydrogen-bonding partners, as well as implications regarding the mechanism of self-inactivation for PGHS, are discussed.     

EPR investigation of TiO2 nanoparticles with temperature-dependent properties

An in situ electron paramagnetic resonance (EPR) study has been carried out for anatase (Hombikat UV100) and rutile TiO(2) nanoparticles at liquid helium (He) temperature (4.2 K) under UV irradiation. Rutile titania was synthesized by ultrasonic irradiation with titanium tetrachloride (TiCl(4)) as the precursor. XRD and Raman results evidence the crystallinity of titania phases. The nature of trapped electrons and holes has been investigated by EPR spectroscopy under air and vacuum conditions. Illumination of TiO(2) powder (anatase and rutile) at 4.2 K resulted in the detection of electrons being trapped at Ti(4+) sites within the bulk and holes trapped at lattice oxide ions at the surface. The stability of electron traps was very sensitive to temperature in both phases of TiO(2). The annealing kinetics of the EPR detected radicals has been studied from 4.2 K to ambient temperature and also for calcined titania particles from 523 to 1273 K.     

EPR/ENDOR study of an X-irradiated single crystal of: 1-triphenylphosphoranylidene-2-propanone: The role of hydrogen bonds in the trapping of radiogenic radicals

As shown from the crystal structure, the oxygen atom of Ph₃P=CH---C(O)CH₃ forms both intra and intermolecular hydrogen bonds. X-irradiation of this compounds produces a room-temperature-stable radical which was studied by single crystal EPR/ENDOR spectroscopy. Comparison of the experimental hyperfine couplings with those obtained from ab initio calculations shows that the radical cation Ph₃P⁺---CH=C(OH)CH₂ is formed under radiolysis. The principal directions of the hyperfine tensors indicate that, in this process, some of the hydrogen bonds are broken and that the radical undergoes a drastic reorientation around the Ph₃P---C bond.     

ENDOR/HYSCORE studies of the common intermediate trapped during nitrogenase reduction of N2H2, CH3N2H, and N2H4 support an alternating reaction pathway for N2 reduction

Enzymatic N(2) reduction proceeds along a reaction pathway composed of a sequence of intermediate states generated as a dinitrogen bound to the active-site iron-molybdenum cofactor (FeMo-co) of the nitrogenase MoFe protein undergoes six steps of hydrogenation (e(-)/H(+) delivery). There are two competing proposals for the reaction pathway, and they invoke different intermediates. In the 'Distal' (D) pathway, a single N of N(2) is hydrogenated in three steps until the first NH(3) is liberated, and then the remaining nitrido-N is hydrogenated three more times to yield the second NH(3). In the 'Alternating' (A) pathway, the two N's instead are hydrogenated alternately, with a hydrazine-bound intermediate formed after four steps of hydrogenation and the first NH(3) liberated only during the fifth step. A recent combination of X/Q-band EPR and (15)N, (1,2)H ENDOR measurements suggested that states trapped during turnover of the α-70(Ala)/α-195(Gln) MoFe protein with diazene or hydrazine as substrate correspond to a common intermediate (here denoted I) in which FeMo-co binds a substrate-derived [N(x)H(y)] moiety, and measurements reported here show that turnover with methyldiazene generates the same intermediate. In the present report we describe X/Q-band EPR and (14/15)N, (1,2)H ENDOR/HYSCORE/ESEEM measurements that characterize the N-atom(s) and proton(s) associated with this moiety. The experiments establish that turnover with N(2)H(2), CH(3)N(2)H, and N(2)H(4) in fact generates a common intermediate, I, and show that the N-N bond of substrate has been cleaved in I. Analysis of this finding leads us to conclude that nitrogenase reduces N(2)H(2), CH(3)N(2)H, and N(2)H(4) via a common A reaction pathway, and that the same is true for N(2) itself, with Fe ion(s) providing the site of reaction.     

Analysis of an asymmetric addition with a 2:1 mixed lithium amide/n-butyllithium aggregate

A 2:1 lithium amide/ n-butyllithium aggregate 1 is investigated as an asymmetric addition template in hydrocarbon solvents. Several different chiral lithium amides were synthesized from l-valine and tested in the asymmetric addition of n-BuLi to various aldehydes. Enantiomeric excesses up to 83% were obtained in the case of the addition of n-BuLi to pivaldehyde at -116 degrees C in pentane. (1)H and (13)C INEPT DOSY were utilized to characterize a new trimeric complex 12 between 2 equiv of lithium amide and 1 equiv of lithium alkoxide. This mixed aggregate strongly indicates the possibility of product-induced chirality inhibition that is detrimental to the enantioselectivity of asymmetric addition reaction.     

Measurement of Angstrom to Nanometer Molecular Distances with 19F Nuclear Spins by EPR/ENDOR Spectroscopy

Spectroscopic and biophysical methods for structural determination at atomic resolution are fundamental in studies of biological function. Here we introduce an approach to measure molecular distances in bio‐macromolecules using ¹⁹F nuclear spins and nitroxide radicals in combination with high‐frequency (94 GHz/3.4 T) electron–nuclear double resonance (ENDOR). The small size and large gyromagnetic ratio of the ¹⁹F label enables to access distances up to about 1.5 nm with an accuracy of 0.1–1 Å. The experiment is not limited by the size of the bio‐macromolecule. Performance is illustrated on synthesized fluorinated model compounds as well as spin‐labelled RNA duplexes. The results demonstrate that our simple but strategic spin‐labelling procedure combined with state‐of‐the‐art spectroscopy accesses a distance range crucial to elucidate active sites of nucleic acids or proteins in the solution state.     

Theoretical investigation on mechanism of asymmetric Michael addition of trans-1-nitro-2-phenylethylene to 2-methylpropionaldehyde catalyzed by a Cinchona alkaloid-derived primary amine

Using cinchona alkaloid-derived primary amine as catalyst and benzoic acid as co-catalyst, Michael-type addition reactions between enolizable carbonyl compounds and nitroalkenes have been extensively studied; however, our understanding of the mechanism is far from complete. In this paper, a theoretical study is presented for the Michael addition reaction between trans-1-nitro-2-phenylethylene and 2-methylpropionaldehyde catalyzed by 9-epi-QDA and benzoic acid. By performing DFT and ab initio calculations, we have identified a detailed mechanism. The calculations indicated that four continuous steps are involved in the overall reaction: (1) the formation of an iminium intermediate, (2) an addition reaction between the iminium and trans-1-nitro-2-phenylethylene, (3) the proton transfer process, and (4) hydrolysis and regeneration of the catalyst. The rate-determining step is the second proton transfer from the amine group to β-carbon of trans-1-nitro-2-phenylethylene, and the enantioselectivity is also controlled by this step. The calculated results provide a general model that explains the mechanism and enantioselectivity of the title reaction.     

The mechanism of asymmetric reduction catalyzed by a C2 symmetric bis-amino alcohol catalyst

By means of¹H,¹³C,¹¹B NMR, polar transfer DEPT135, DEPT90 and 2D NMR experiments, IR, etc, the structure of the diaminodihydroxyl ligand and its derivatives used in the asym-metric reduction of prochiral ketones were detected. The catalysts derived from the ligand and borane formedin situ were also studied and the structure transformations in solution were monitored. The structure of the catalyst was proved to be a new type of dual-centered catalyst — bisoxazaborolidine.     

Observation of an Organic Acid Mediated Spin State Transition in a Co(II)-Schiff Base Complex: An EPR, HYSCORE, and DFT Study

The interactions of a weak organic acid (acetic acid, HOAc) with a toluene solution of the Co(II)-Schiff base type complex, (R,R')-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino Co(II) (labeled [Co(1)]), was investigated using EPR, HYSCORE, and DFT computations. This activated [Co(II)(1)] system is extremely important within the context of asymmetric catalysts (notably the hydrolytic kinetic resolution of epoxides) despite the lack of detailed structural information about the nature of the paramagnetic species present. Under anaerobic conditions, the LS [Co(II)(1)] complex with a |yz, (2)A(2)? ground state is converted into a low-spin (LS) and a high-spin (HS) complex in the presence of the acid. The newly formed LS state is assigned to the coordinated [Co(II)(1)]-(HOAc) complex, possessing a |z(2), (2)A(1)? ground state (species A; g(x) = 2.42, g(y) = 2.28, g(z) = 2.02, A(x) = 100, A(y) = 120, A(z) = 310 MHz). The newly formed HS state is assigned to an acetate coordinated [Co(II)(1)]-(OAc(-)) complex, possessing an S = (3)/(2) spin ground state (species B, responsible for a broad EPR signal with g ≈ 4.6). These spin ground states were confirmed with DFT calculations using the hybrid BP86 and B3LYP functionals. Under aerobic conditions, the LS and HS complexes (species A and B) are not observed; instead, a new HS complex (species C) is formed. This complex is tentatively assigned to a paramagnetic superoxo bridged dimer (AcO(-))[Co(II)(1)···O(2)(-)Co(III)(1)](HOAc), as distinct from the more common diamagnetic peroxo bridged dimers. Species C is characterized by a very broad HS EPR signal (g(x) = 5.1, g(y) = 3.9, g(z) = 2.1) and is reversibly formed by oxygenation of the LS [Co(II)(1)]-(HOAc) complex to the superoxo complex [Co(III)(1)O(2)(-)](HOAc), which subsequently forms the association complex C by interaction with the HS [Co(II)(1)](OAc(-)) species. The LS and HS complexes were also identified using other organic acids (benzoic and propanoic acid). Thermal annealing-quenching experiments revealed the additional presence of [Co(III)(1)O(2)(-)](HOAc) adducts, corroborating the presence of species C and the presence of diamagnetic dimer complexes in the solution, such as the EPR silent (HOAc)[Co(III)(1)(O(2)(2-))Co(III)(1)](HOAc). Overall, it appears that a facile interconversion of the [Co(1)] complex, possessing a LS ground state, occurs in the presence of acetic acid, producing both HS and LS Co(II) states, prior to formation of the oxidized active form of the catalyst, [Co(III)(1)](OAc(-)).     

EPR investigation of plasma-chemical resist etching in O2 and O2/CF4 discharges

During the etching of AZ 1350 photoresist in O₂ and O₂/CF₄ discharges, ground-state concentrations of atoms (O, F, and H), and small radicals (OH, HO₂, RO₂) were measured in the discharge afterglow by EPR spectroscopy. In the case of CF₄/O₂ discharges, the dependence of O and F atom concentrations on the etch time reflects both surfäce oxidation and fluorination reactions in accordance with existing etch models. In the case of high-rate resist etching in pure O₂ discharges, high concentrations of product radicals (H, OH and HO₂) were detected and compared with resist free O₂/H₂O discharges. Kinetic modeling of the afterglow reactions reveals that the mean lifetime and, accordingly, the diffusion length of the etchant species O(³P) is drastically reduced in rapid reactions with OH and HO₂. The results are used to simulate both etch homogeneity and the loading effect in a simple etch model.