27Al Hyperfine Research Articles

The Stark Effect, Zeeman Effect, and Transition Dipole Moments for the B2Σ+−X2Σ+ Band of Aluminum Monoxide, AlO

Abstract The experimentally measured radiative lifetimes and branching ratios were combined to determine the transition dipole moments for the B 2Σ+(v = 0–3) → X 2Σ+(v = 0–6) bands of aluminum monoxide, AlO, and compared with theoretical predictions. The B 2Σ+–X 2Σ+ (0, 1) band of a molecular beam sample of AlO was recorded at high spectral resolution both field-free and in the presence of static electric and magnetic fields. The 27Al(I = 5/2) hyperfine interaction in the B 2Σ+(v = 0) state was analyzed. The observed Stark shifts were analyzed to produce permanent electric dipole moments of 1.94(8) D and 4.45(3) D for the B 2Σ+(v = 0) and X 2Σ+(v = 1) states, respectively. It is demonstrated that the observed Zeeman spectra can be simulated using an effective Hamiltonian with the associated expected g-factors for both the X 2Σ+(v = 1) and B 2Σ+(v = 0) states.

Relationship investigation of structure and properties of Nd3+: Ga2O3-Al2O3-PbO-CaO via Raman, infrared, NMR and EPR spectroscopy

A detailed study structure and luminescence properties of Nd3+: Ga2O3-Al2O3-PbO-CaO laser glasses was conducted. The network structure was mainly dominated by Al (IV) or Ga (IV). With the increase of the Al2O3 content, the Al (IV) content increases, whereas the Ga (IV) content decreases. Hyperfine sublevel correlation (HYSCORE) spectra reveal 27Al and 207Pb weak hyperfine coupling, which indicates that these nuclei are in the second- or higher-order Nd3+ coordination sphere. No 69Ga or 71Ga signals can be detected in the three-pulse electron spin echo envelope modulation (3P-ESEEM) and HYSCORE spectra. For germanate glass, the local environment of the Nd3+ ions is characterized by the Pb ligand, for aluminum gallium glasses, the Nd3+ ions are surrounded by Al3+ and Pb2+ ions, and for aluminum glass, Nd3+ ions are mainly surrounded by 27Al cations. The change in the structure and in the local environment of Nd3+ ions results in a change in the luminescence properties of glasses.

Aluminum Atom Activation of C-S Bonds: An EPR Study of the Intermediates Formed in the Reaction Between Aluminum Atoms and Dialkyl Sulfides.

The major mononuclear aluminum species obtained in the reaction of aluminum atoms with dimethyl sulfide (DMS) and diethyl sulfide (DES), under cryogenic conditions, are the C-S insertion products, CH3AlSCH3 and CH3CH2AlSCH2CH3, respectively, showing that aluminum atoms can activate C-S bonds. The magnetic parameters were extracted from the EPR spectrum. Support for the spectral assignment was obtained by repeating the experiments with 13C-labeled dimethyl sulfide yielding 13CH3AlS13CH3. DFT calculations using the M06 functional have proven to be effective in predicting the isotropic and anisotropic Al hyperfine interaction values of the aluminum atom C-S insertion products. In addition, the thioalkyl radicals, CH3SCH2• and CH3CH2SCH•CH3 were formed in the aluminum atom reaction.

Low-temperature binding of NO adsorbed on MIL-100(Al)-A case study for the application of high resolution pulsed EPR methods and DFT calculations.

The low-temperature binding of nitric oxide (NO) in the metal-organic framework MIL-100(Al) has been investigated by pulsed electron nuclear double resonance and hyperfine sublevel correlation spectroscopy. Three NO adsorption species have been identified. Among them, one species has been verified experimentally to bind directly to an 27Al atom and all its relevant 14N and 27Al hyperfine interaction parameters have been determined spectroscopically. Those parameters fit well to the calculated ones of a theoretical cluster model, which was derived by density functional theory (DFT) in the present work and describes the low temperature binding of NO to the regular coordinatively unsaturated Al3+ site of the MIL-100(Al) structure. As a result, the Lewis acidity of that site has been characterized using the NO molecule as an electron paramagnetic resonance active probe. The DFT derived wave function analysis revealed a bent end-on coordination of the NO molecule adsorbed at that site which is almost purely ionic and has a weak binding energy. The calculated flat potential energy surface of this species indicates the ability of the NO molecule to freely rotate at intermediate temperatures while it is still binding to the Al3+ site. For the other two NO adsorption species, no structural models could be derived, but one of them is indicated to be adsorbed at the organic part of the metal-organic framework. Hyperfine interactions with protons, weakly coupled to the observed NO adsorption species, have also been measured by pulsed electron paramagnetic resonance and found to be consistent with their attribution to protons of the MIL-100(Al) benzenetricarboxylate ligand molecules.

Influence of particle size on the paramagnetic components of kaolins from different origins

AbstractElectron paramagnetic resonance (EPR) spectra of different particle size fractions of four kaolins from diverse sources in North America, Europe and Asia have been investigated in order to characterize their paramagnetic properties and heterogeneity. There were major differences in the sources of the EPR signals from transition metals; V and Mn were structural, Fe was both structural and as associated oxides, and Cu was in the form of an adsorbed ion. The radiation-induced free radical signals commonly known as the A- and B-centres were observed in three of the deposits; however, in addition to the previously reported 27Al hyperfine structure associated with the B-centre, we also observed much smaller 27Al hyperfine structure on the g┴ feature of the A-centre. The other kaolin sample produced four free radical signals that have not previously been reported in kaolins. Each had substantial 1H hyperfine splitting; three are interpreted as corresponding to defect centres associated with Si-OH groups, and the other to a Si hole surrounded by protonated O atoms. The EPR spectra changed progressively with particle size, and measurements on the Asian specimens after grinding showed major differences in the Fe3+ signals from the same particle size fractions separated from the natural samples, thus supporting previous reports that grinding results in major structural changes in the minerals.

Single-crystal EPR and DFT study of a VIAl–O−–VIAl center in jeremejevite: electronic structure and 27Al hyperfine constants

Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al6B5O15(F, OH)3, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an 27Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g 1 = 2.02899(1), g 2 = 2.02011(2), g 3 = 2.00595(1); A 1/g e β e = −0.881(1) mT, A 2/g e β e = −0.951(1) mT, and A 3/g e β e = −0.972(2) mT, with the orientations of the g 3- and A 3-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g 1- and A 1-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a VIAl–O−–VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental 27Al hyperfine coupling constants and directions, supporting the proposed structural model. The VIAl–O−–VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the VIAl–O−–VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.

Activation of C–O and C–C Bonds and Formation of Novel HAlOH-Ether Complexes: An EPR Study of the Reaction of Ground-State Al Atoms with Methylethyl Ether and Diethyl Ether

Reaction mixtures, containing Al atoms and methylethyl ether (MEE) or diethyl ether (DEE) in an adamantane matrix, were prepared with the aid of a metal-atom reactor known as a rotating cryostat. The EPR spectra of the resulting products were recorded from 77-260 K, at 10 K intervals. Al atoms were found to insert into methyl-O, ethyl-O, and C-C bonds to form CH(3)AlOCH(2)CH(3), CH(3)OAlCH(2)CH(3), and CH(3)OCH(2)AlCH(3), respectively, in the case of MEE while DEE produced CH(3)CH(2)AlOCH(2)CH(3) and CH(3)AlCH(2)OCH(2)CH(3), respectively. From the intensity of the transition lines attributed to the Al atom C-O insertion products of MEE, insertion into the methyl-O bond is preferred. The Al hyperfine interaction (hfi) extracted from the EPR spectra of the C-O insertion products was greater than that of the C-C insertion products, that is, 5.4% greater for the DEE system and 7% greater for the MEE system. The increase in Al hfi is thought to arise from the increased electron-withdrawing ability of the substituents bonded to Al. Besides HAlOH, resulting from the reaction of Al atoms with adventitious water, novel mixed HAlOH:MEE and HAlOH:DEE complexes were identified with the aid of isotopic studies involving H(2)(17)O and D(2)O. The Al and H hfi of HAlOH were found to decrease upon complex formation. These findings are consistent with the nuclear hfi calculated using a density functional theory (DFT) method with close agreement between theory and experiment occurring at the B3LYP level using a 6-311+G(2df,p) basis set.

Single-crystal EPR and ENDOR study of an AlO center in prehnite: implications for aluminum-associated oxyradicals in layer silicates

Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated prehnite (Jeffrey mine, Quebec, Canada) measured at 298 and 160 K reveal an aluminum-associated oxygen hole center (Al—O − ). Spin Hamiltonian parameters g and A ( 27 Al) fitted from the 298 K spectra suggest that this Al—O − center represents hole trapping on an apical hydroxyl oxygen atom (after removal of the proton) coordinated to an octahedral Al 3+ ion ( i.e. , an [ • OAlO 4 (OH)] center from the [(OH)AlO 4 (OH)] precursor, where • denotes the unpaired spin). Pulsed electron nuclear double resonance (ENDOR) spectra measured at 25 K allow the identification and quantitative analysis of two sets of 27 Al hyperfine structures and five proton hyperfine structures, which are all consistent with the proposed structural model. Isothermal and isochronal annealing experiments show that this center is bleached out completely at 375 °C, but can be readily restored by gamma-ray irradiation, and exhibits second-order decay kinetics. These results from the Al—O − center in prehnite provide support for and new insights into Clozel et al . (1995)’s VI Al—O − — VI Al model for B-centers in kaolinite.

Radiation-induced defects in quartz: a multifrequency EPR study and DFT modelling of new peroxy radicals

Abstract A natural quartz, annealed first at 800°C and then irradiated with a 3.5 MeV electron beam, has been investigated by single-crystal electron paramagnetic resonance (EPR) spectroscopy at X- and W-band frequencies from 110 K to 298 K. The W-band EPR spectra allow better separation of two previously reported radiation-induced defects (D and E) and improved determinations of their spin Hamiltonian parameters. These defects have similar g tensors with the gmax axes approximately along an O—O pair and gmin axes perpendicular to the short Si—O bonds, but different 27Al hyperfine structures. Centre E is also characterized by a 29Si hyperfine structure (A/geβe = ~0.4 mT). These spin Hamiltonian parameters, together with results from density functional theory (DFT) calculations, suggest centre E to be a new variant of peroxy radicals in quartz, whereas a peroxy radical model for centre D remains tentative. Thermal stabilities and decay kinetics of centres D and E have been investigated by use of isochronal and isothermal annealing experiments on a neutron-irradiated quartz and six smoky quartz crystals in druses from a U deposit.

Theoretical modeling of the Al paramagnetic center and its precursors in stishovite

Previous electron paramagnetic resonance (EPR) spectroscopic study of gamma-ray-irradiated stishovite at 77 K detected an Al hole center, which was proposed to be an [O2 3−–Al3+] defect. First-principles quantum-mechanical calculations show that the unpaired spin is 85% localized on one of the six oxygen atoms at an AlO6 octahedron, while the calculated 27Al hyperfine constants are similar to those determined by EPR experiments. Theoretical results allow us to propose the Al center to represent an [AlO6]0 defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that the diamagnetic precursors [AlO6/H+]0, [AlO6/Li+]0, and [AlO6/Na+]0 are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO6/Li+]0 and [AlO6/Na+]0 defects suggest that the monovalent cations such as Li+ and Na+ are potentially important in accommodating Al in stishovite in the lower mantle.

Radiation-induced defects in quartz. III. Single-crystal EPR, ENDOR and ESEEM study of a peroxy radical

The X- and W-band single-crystal electron paramagnetic resonance spectra of an electron-irradiated natural quartz permit quantitative analysis of a 29Si hyperfine structure (A ~12.6 MHz) and an 27Al hyperfine structure (A ≤ 0.8 MHz) for a previously reported hole-like center. The 29Si hyperfine structure arises from interaction with two equivalent Si atoms and is characterized by the direction of the unique A axis close to a Si–O bond direction. The 27Al hyperfine structure, confirmed by pulsed electron nuclear double resonance and electron spin echo envelope modulation spectra, is characterized by the unique A axis approximately along a twofold symmetry axis. These 29Si and 27Al hyperfine data, together with published theoretical results on peroxy radicals in SiO2 as well as our own density functional theory (DFT) calculations on model peroxy centers, suggest this hole-like center to have the unpaired spin on a pair of oxygen atoms linked to two symmetrically equivalent Si atoms and a substitutional Al3+ ion across the c-axis channel, a first peroxy radical in quartz. The nuclear quadrupole matrix P also suggests that the Al3+ ion corresponds closely to the diamagnetic precursor to the [AlO4]0 center.

Radiation-induced defects in quartz. II. Single-crystal W-band EPR study of a natural citrine quartz

Single-crystal electron paramagnetic resonance (EPR) spectra of a natural citrine quartz without any artificial irradiation, measured at W-band frequencies (∼94 GHz) and temperatures of 77, 110 and 298 K, allow better characterization of three previously-reported Centers (#6, #7 and B) and discovery of three new defects (B′, C′ and G′). The W-band EPR spectra reveal that Centers #6 and #7 do not reside on twofold symmetry axes, contrary to results from a previous X-band EPR study. The W-band spectra also show that the previously reported Center B is a mixture of two defects (B and B′) with similar g matrices but different-sized 27Al hyperfine structures. Center C′ has similar principal g values to the previously reported Center C but is distinct from the latter by a larger 27Al hyperfine structure with splittings from 0.10 to 0.22 mT. Also, Center G′ has a similar g matrix to the previously reported Center G but a different 27Al hyperfine structure with splittings from 0.41 to 0.53 mT. These spin-Hamiltonian parameters, together with observed thermal properties and microwave-power dependence, suggest that Centers #6 and #7 probably represent O23− type defects. Centers B and B′ are probably superoxide radicals (O2−) with the unpaired spin localized on the same pair of oxygen atoms around a missing Si atom but linked to a substitutional Al3+ ion each at different neighboring tetrahedral sites. Similarly, Centers G and G′ are most likely superoxide radicals with the unpaired spin localized on another pair of oxygen atoms around a missing Si atom and linked to a substitutional Al3+ ion each at different neighboring tetrahedral sites. Center C′ is probably an ozonide radical associated with a missing Si atom and linked to a substitutional Al3+ ion at the neighboring tetrahedral site. This study exemplifies the value of high-frequency EPR for discrimination of similar defect centers and determination of small local structural distortions that are often difficult to resolve in conventional X- and Q-band EPR studies.

NATURAL RADIATION-INDUCED DAMAGE IN QUARTZ. III. A NEW OZONIDE RADICAL IN DRUSY QUARTZ FROM THE ATHABASCA BASIN, SASKATCHEWAN

Quartz crystals collected in druses, including one annealed at 400°C, from a fracture in the hanging-wall sandstone at the McArthur River uranium deposit in the Paleoproterozoic Athabasca basin, northern Saskatchewan, have been investigated by single-crystal X-band electron paramagnetic resonance (EPR) spectroscopy at room temperature and 110 K. The single-crystal EPR spectra allow us to distinguish eight paramagnetic defects, including one new center and seven previously reported centers. These centers, most of which were discovered in artificially irradiated crystals and only a few had been suggested to occur in natural quartz by powder EPR, are herewith positively established for the first time as natural radiation-induced defects by single-crystal EPR. The new center is similar in principal g-factor values to the ozonide radical (O 3 − ) in various minerals and synthetic compounds, and is characterized by its g-maximum and g-minimum axes approximately parallel to two O–O edges of the SiO 4 quasi-tetrahedron in the quartz structure. This geometry is also compatible with an ozonide radical formed from a silicon vacancy. The observed linewidths of the new ozonide radical vary from 0.087 to 0.257 mT, which are attributed to unresolved site-splittings or unresolved hyperfine splittings. This new ozonide radical is distinct from a previously reported ozonide radical in citrine quartz, which is characterized by the presence of a small 27 Al hyperfine structure. The new ozonide radical is probably linked to a Si atom in the neighboring tetrahedron and hence represents a general case in quartz, whereas the previously reported ozonide radical linked to a neighboring Al atom is a special variant in Al-bearing quartz. The presence of natural radiation-induced defects in drusy quartz is attributable to the presence of uranium in mineralized assemblages nearby or late hydrothermal fluids.

Radiation-damage-induced defects in quartz. I. Single-crystal W-band EPR study of hole centers in an electron-irradiated quartz

Single-crystal W-band electron paramagnetic resonance (EPR) spectra of an electron-irradiated quartz, measured at room temperature, 110 and 77 K, disclose three previously reported hole centers (#1, G and an ozonide radical). The W-band EPR spectra of these three centers clearly resolve six magnetically nonequivalent sites each, whereas previous X- and Q-band EPR studies reported Centers #1 and the ozonide radical to consist of only three symmetry-related components and interpreted them to reside on twofold symmetry axes in the quartz structure. The calculated g matrices of Center #1 and the ozonide radical show that deviations from twofold symmetry axes are <10°, which are probably attributable to distortion related to neighboring charge compensating ions. The W-band EPR spectra of Center G not only result in improved g matrices but also allow quantitative determination of the nuclear hyperfine (A) and quadrupole (P) matrices of its 27Al hyperfine structure that was incompletely resolved before. In particular, the g-maximum and g-minimum principal axes of Center G are approximately along two pairs of O–O edges of the SiO4 tetrahedron, while the unique A principal axis is approximately along a Si–Si direction. These new spin-Hamiltonian parameters suggest that Center G most likely involves trapping of a hole between two oxygen atoms related to a silicon vacancy and stabilized by an Al3+ ion in the neighboring tetrahedron (hence an O2n−–Al3+ defect, where n is either 1 or 3).

High Field 27Al ENDOR Reveals the Coordination Mode of Cu2+ in Low Si/Al Zeolites

The siting of Cu2+ in zeolites with low exchange levels has been a subject for debate due to the lack of experimental evidence that provide directly the interaction between the Cu2+ ion and the zeolite framework. High field 27Al ENDOR provided highly resolved orientation selective ENDOR spectra from which both the 27Al hyperfine and quadrupole principal components and orientations relative to the g tensor principal axis system were determined for a dehydrated Cu2+ exchanged zeolite X with Si/Al = 1. The results show that all three Cu-Al distances in the six-member ring are equivalent, in contrast to DFT predictions using cluster models.

Electron Paramagnetic Resonance Study of the Mononuclear Al Species Formed in the Reaction of Al Atoms and Cyclopropylamine at 77 K in an Adamantane Matrix

Ground-state Al atoms have been reacted with cyclopropylamine (CpNH2) in an adamantane matrix at 77 K. The four paramagnetic species generated in the reaction have been studied by electron paramagnetic resonance (EPR) spectroscopy and their magnetic parameters extracted from the EPR spectra. The isotropic Al hyperfine interactions (hfis) for these radicals vary from 801 to 962 MHz and are consistent with values previously reported for structurally similar Al-centered radicals. The major species formed in the reaction has a highly anisotropic a tensor indicative of an Al-centered radical with a substantial amount of the unpaired spin density residing in the Al 3p orbital. In addition, there is EPR evidence that the insertion products, CpNH(AlH) and CpAlNH2, are formed in the reaction, suggesting that Al atoms can activate the N-H and C-N bonds of amines. It has been shown that a molecule of CpNH2 coordinates to CpNH(AlH), causing the Al, N and H hfi of the N-H insertion product to decrease. The values of the nuclear hfis calculated for the insertion products, using a density functional theory (DFT) method, are comparable to the experimental values.

Pulsed ENDOR Study of Cu(I)−NO Adsorption Complexes in Cu−L Zeolite

The local environments of Cu(I)-NO adsorption complexes formed in zeolites Cu-L and Cu-ZSM-5 were studied by electron spin resonance (ESR), pulsed electron nuclear double resonance (ENDOR), and hyperfine sublevel correlation spectroscopy (HYSCORE). Cu(I)-NO complexes have attracted special interest because they are important intermediates in the catalytic decomposition of nitric oxide over copper exchanged zeolites. Recently, detailed structures of the complexes in Cu-ZSM-5 zeolites, O2-Al-O2-Cu(I)-NO, have been proposed on the basis of quantum chemical calculations (Pietrzyk, et al. J. Phys. Chem. B 2003, 107, 6105. Dedecek, et al. Phys. Chem. Chem. Phys. 2002, 4, 5406). 27Al pulsed ENDOR and HYSCORE experiments allowed the hyperfine coupling parameters of an aluminum nuclei found in the vicinity of the Cu(I)-NO complex formed in zeolite Cu-L to be estimated. The data indicate that the aluminum atom is located in the third coordination sphere of the adsorbed NO molecule in agreement with the suggested geometry of the adsorption sites. Broad distributions of aluminum nuclear quadrupole and hyperfine coupling parameters and short electron spin relaxation times of the Cu(I)-NO species prevented the determination of the 27Al hyperfine couplings for zeolite Cu-ZSM-5.

EPR studies on defects in sol–gel derived alumina films

We have studied the defect structures and mechanism for the luminescence of non-doped, Tb 3+-doped and Tb 3+–Zn 2+ codoped alumina films treated at various temperatures from 300 to 800 °C. The electron paramagnetic resonance (EPR) spectra observed before UV-irradiation are attributable to the non-bridging oxygen radicals; Al–O–O at treatment temperatures of 300–500 °C and Al–O at 800 °C. The UV-light irradiation of the non-doped alumina films gives the EPR spectra with the 27Al hyperfine splitting. There exists a good correlation between the treatment temperature dependence on the light-induced EPR signal intensity of the trapped electron and the NMR peak intensity due to the five-coordinated aluminum ion. We concluded that the electron is captured at the oxygen vacancy connected with the five-coordinated aluminum. The broad EPR spectra due to the ground state of the Tb 3+ ion were observed in the Tb 3+-doped and Tb 3+–Zn 2+ codoped alumina films. The Tb 3+ concentration dependence of the EPR signal intensity due to the non-bridging oxygens suggests that Tb 3+ prefers to form Al–O–Tb bonds and prevent to form a cluster of rare earth ions itself. The light-induced EPR signal of the electron trapped at oxygen vacancy next the five-coordinated aluminum was also observed for the Tb 3+–Zn 2+ codoped alumina film, which produced a long-lasting luminescence. The photogenerated trapped electron remained for several minutes under the room temperature. These experimental facts indicate that the five-coordinated aluminum ions act as the trapping sites to produce the long-lasting luminescence.

Interactions of Cu(II) Ions with Framework Al in High Si:Al Zeolite Y as Determined from X- and W-Band Pulsed EPR/ENDOR Spectroscopies

A combination of continuous wave (CW) electron paramagnetic resonance (EPR), pulsed EPR, and pulsed electron−nuclear double resonance (ENDOR) techniques were used to obtain structural information about the Cu2+ ions in hydrated, room-temperature evacuated, and dehydrated Cu−Y (Si:Al = 12 and 5) with a particular emphasis on framework Al interactions. W-band 1H ENDOR was used to probe the water ligands, whereas X-band hyperfine sublevel correlation (HYSCORE) spectroscopic measurements were employed to detect 27Al hyperfine couplings. The X-band CW EPR spectra show that a total of three Cu2+ species (A, B, and C) are present in the samples. ENDOR measurements of hydrated and evacuated Cu−Y-12 indicate that species A and B have a complete coordination sphere of water and, on the basis of the absence of 27Al signals in the HYSCORE spectrum, the Cu2+ is not directly bonded to the zeolite framework. Evacuation converted species B to species A. 1H ENDOR spectra combined with simulations show that upon freezing, the equatorial and axial water ligands of species A and B have a distribution of orientations with respect to the Cu−O bond. The CW EPR spectrum of dehydrated Cu−Y-12 shows a single species (species C) and the HYSCORE spectrum exhibits cross-peaks from 27Al with an isotropic coupling, aiso, of 1.5 MHz. Unlike Cu−Y-12, evacuated Cu−Y-5 consists of species A and C, and the HYSCORE spectrum clearly shows a doublet of 27Al cross-peaks with aiso = 3.0 MHz assigned to species C. Upon dehydration, the 27Al coupling decreases to 2.6 MHz. This indicates that in species C the Cu2+ is bound to framework oxygens which are bonded to an Al nucleus. For Si:Al = 5, the zeolite framework becomes negative enough that it can replace water ligands even after mild evacuation. Simulations of 27Al HYSCORE spectra indicate that species C is Cu2+ bound to the framework oxygens primarily near a single Al nucleus with a large quadrupole coupling constant.

Evidence for changes in the nucleotide conformation in the active site of H(+)-ATPase as determined by pulsed EPR spectroscopy.

The conformation of di- and triphosphate nucleosides in the active site of ATPsynthase (H(+)-ATPase) from thermophilic Bacillus PS3 (TF1) and their interaction with Mg(2+)/Mn(2+) cations have been investigated using EPR, ESEEM, and HYSCORE spectroscopies. For a ternary complex formed by a stoichiometric mixture of TF1, Mn(2+), and ADP, the ESEEM and HYSCORE data reveal a (31)P hyperfine interaction with Mn(2+) (|A((31)P)| approximately 5.20 MHz), significantly larger than that measured for the complex formed by Mn(2+) and ADP in solution (|A((31)P)| approximately 4.50 MHz). The Q-band EPR spectrum of the Mn.TF1.ADP complex indicates that the Mn(2+) binds in a slightly distorted environment with |D| approximately 180 x 10(-4) cm(-1) and |E| approximately 50 x 10(-4) cm(-1). The increased hyperfine coupling with (31)P in the presence of TF1 reflects the specific interaction between the central Mn(2+) and the ADP beta-phosphate, illustrating the role of the enzyme active site in positioning the phosphate chain of the substrate for efficient catalysis. Results with the ternary Mn.TF1.ATP and Mn.TF1.AMP-PNP complexes are interpreted in a similar way with two hyperfine couplings being resolved for each complex (|A((31)P(beta))| approximately 4.60 MHz and |A((31)P(gamma))| approximately 5.90 MHz with ATP, and |A((31)P(beta))| approximately 4.20 MHz and |A((31)P(gamma))| approximately 5.40 MHz with AMP-PNP). In these complexes, the increased hyperfine coupling with (31)P(gamma) compared with (31)P(beta) reflects the smaller Mn.P distance with the gamma-phosphate compared with the beta-phosphate as found in the crystal structure of the analogous enzyme from mitochondria [3.53 vs 3.70 A (Abrahams, J. P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628)] and the different binding modes of the two phosphate groups. The ESEEM and HYSCORE data of a complex formed with Mn(2+), ATP, and the isolated beta subunit show that the (31)P hyperfine coupling is close to that measured in the absence of the protein, indicating a poorly structured nucleotide site in the isolated beta subunit in the presence of ATP. The inhibition data obtained for TF1 incubated in the presence of Mg(2+), ADP, Al(NO(3))(3), and NaF indicate the formation of the inhibited complex with the transition state analogue namely Mg.TF1.ADP.AlF(x) with the equilibrium dissociation constant K(D) = 350 microM and rate constant k = 0.02 min(-1). The ESEEM and HYSCORE data obtained for an inhibited TF1 sample, Mn.TF1.ADP.AlF(x), confirm the formation of the transition state analogue with distinct spectroscopic footprints that can be assigned to Mn.(19)F and Mn.(27)Al hyperfine interactions. The (31)P(beta) hyperfine coupling that is measured in the inhibited complex with the transition state analogue (|A((31)P(beta))| approximately 5.10 MHz) is intermediate between those measured in the presence of ADP and ATP and suggests an increase in the bond between Mn and the P(beta) from ADP upon formation of the transition state.