Unpaired Electron Spin Research Articles

Fourier Transform Infrared Studies of Ammonia Photochemistry in Solid Parahydrogen

We present 193 nm in situ photochemical studies of NH3 isolated in solid parahydrogen (pH2) at 1.8 K using Fourier Transform Infrared (FTIR) spectroscopy. By recording FTIR spectra during and after irradiation we are able to identify and assign a number of rovibrational transitions to ortho-NH2(X(2)B1) and NH(X(3)Σ(-)). Spectroscopic analysis shows that these two radical species rotate freely in solid pH2 and that effects of the unpaired electron spin remain essentially unchanged from the gas phase. We provide detailed mechanistic studies that show the nascent ortho-NH2 photoproduct is rapidly cooled within the pH2 matrix to the ground vibrational and rotational state before (1) subsequent photodissociation or (2) tunneling-driven reaction (k(tun) = 1.88(17) min(-1)) with the pH2 host to produce ortho-NH3 in a defect site. Once the ortho-NH3 is produced in this defect site it slowly converts (k(conv) = 7.72(51) × 10(-3) min(-1)) back to a single substitution site even at 1.8 K. We demonstrate the in situ photolysis of NH3 can be utilized to generate NH doped pH2 solids that are relatively stable at low temperature. However, the ortho-NH2 + pH2 → ortho-NH3 + H back reaction substantially limits the sequential two-photon conversion of NH3 to NH. These studies also reveal that extended photolysis of the NH3/pH2 system results in the generation of high concentrations of orthohydrogen that must result from repeated cycles of photodissociation and NH2 back reaction within the pH2 host.

Effects of annealing on the structure of ion beam sputtered amorphous tantalum oxide: Oxygen-17 NMR spectra and relaxation times

Unlike well-studied network-structured oxide glasses such as silicates and borates, the structures of amorphous films of heavy transition metal oxides such as tantalum oxide (Ta2O5) are poorly known, in spite of their importance in applications such as the multilayer mirror coatings in optical interferometers, such as the Laser Interferometer Gravitational Wave Observatory (LIGO). 17O MAS NMR can provide new and unique structural information about these materials, but requires isotopic enrichment. For optical coatings formed by ion-beam sputtering (IBS), annealing at temperature up to 600°C is known to improve properties such as optical absorption and thermal noise. In this paper we report NMR results on tantala films deposited by IBS in an 17O-enriched environment, which allows the structure to be examined by NMR for the first time without an added step of isotope exchange at elevated temperature. Subsequent annealing at temperatures from 300 to 600°C shows only subtle changes in the proportions of two- and three-coordinated oxygen anions and in bond distances, which may be related to minor structural relaxation and property changes. In contrast, the nuclear spin-relaxation rates increase dramatically on annealing in oxygen gas, suggesting the progressive removal of defects in the as-grown films that contain paramagnetic, unpaired electron spins. EPR spectra qualitatively confirm this observation. The most likely candidate for this effect is the oxidation of lower-valent tantalum cations (e.g. Ta4+) and thus the removal of a slight oxygen deficit present in the as-deposited films. This may lead to reduction in optical absorption and possibly to improvements in other physical properties during annealing.

A Radical Anion of Structurally Constrained Triphenylborane

Chemical reduction of the structurally constrained triphenylborane 1 with K produced a radical anion. EPR analysis demonstrated the delocalization of the unpaired electron spin density over the entire π skeleton to a greater extent than is the case for a nonconstrained Ph3B radical anion. DFT calculations indicated that the coplanarization of the benzene rings with the boron plane and the shortening of the B–C bond lengths by the structural constraint are responsible for the spin delocalization. The geometry optimization also suggested a bowl-shaped conformation as a viable local minimum structure in addition to the planar conformation. X-ray crystal structure analysis indeed revealed the nonplanar structure of the radical anion 1•–.

Metastable Solids—Terra Incognita Awaiting Discovery

Metastable Solids—Terra Incognita Awaiting Discovery

Pulsed electron spin nutation spectroscopy for weakly exchange-coupled multi-spin molecular systems with nuclear hyperfine couplings: a general approach to bi- and triradicals and determination of their spin dipolar and exchange interactions

Weakly exchange-coupled biradicals have attracted much attention in terms of their dynamic nuclear polarisation application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits in quantum information processing/quantum-computing technology. Analogues multi-partite molecular systems are important in entering a new phase of the relevant fields. Many stable organic biradicals known so far have nitrogen nuclei at their electron spin sites, where singly occupied molecular orbitals are dominating and large hyperfine couplings occur. A salient feature of such weakly exchange-coupled molecular systems in terms of electronic spin structures is underlain by small zero-field splitting (ZFS) parameters comparable with nuclear hyperfine and/or exchange interactions. Pulse-based electron spin nutation (ESN) spectroscopy of weakly exchange-coupled biradicals, applicable to oriented or non-oriented media, has proven to be a useful and facile approach to the determination of ZFS parameters, which reflect relatively short distances between unpaired electron spins. In the present study, we first treat two-dimensional single-crystal ESN spectroscopy (Q-band) of a 15N-labelled weakly exchange-coupled biradical, showing the nuclear hyperfine effects on the ESN phenomena from both the experimental and theoretical side. ESN spectroscopy is transition moment spectroscopy, in which the nutation frequency as a function of the microwave irradiation strength ω1 (angular frequency) for any cases of weakly exchange-coupled systems can be treated. The results provide a testing ground for the simplified but general approach to the ESN analysis. In this study, we have invoked single-crystal electron-electron double resonance measurements on a typical biradical well incorporated in a diamagnetic host lattice and checked the accuracy of our ESN analysis for the spin dipolar tensor and exchange interaction. Next, we extend the general approach to analogues multi-partite molecular systems such as stable organic triradical, in which the exchange interaction can be governed by a significant amount of the delocalisation of three unpaired spins over the molecular frame of a triangular structure. The triangular structure maintains π-conjugation in which each spin-bearing nitroxide at the vertex participates and the exchange interaction is greatly controlled by the dihedral angle between the π-conjugation plane and nitroxide moiety at the vertex. In this context, the ZFS parameters do not correspond to spin distances (1.0 nm) in a straightforward manner, but reflect a salient electronic structure associated with both the π-electron network and the symmetry property of the triradical under study. Thus, both the D-value and exchange interaction J have been controlled in this study. In order to interpret the experimental ZFS parameters and exchange interaction, which is three-order of magnitude reduced in the present poly(methyl methacrylate) polymer matrix compared with that in the crystal, sophisticated quantum chemical calculations of the ZFS tensor and exchange interaction were carried out and reproduced the experimental values, concluding that the present triradical of the triangular structure undergoes significant twisting at the nitroxide sites in the polymer matrix. In this study, we observed electron spin resonance forbidden transitions between the Ms-manifolds belonging to the spin-doublet ground state and spin-quartet excited state. The observation enables us to derive the magnitude of the exchange coupling.

Luminomagnetic K2 Gd1− x Zr(PO4 )3 :Tb x 3+ phosphor with intense green fluorescence and paramagnetism

Multimetallic complex phosphate K2Gd1−xZr(PO4)3:Tbx3+ (0 ≤ x ≤ 50) synthesized by solid state diffusion exhibit intense green fluorescence and paramagnetic behaviour. Monophasic particles of dimensions ranging from few hundred nanometers to few micrometers could be obtained. Photo excitation at 274 nm corresponding to Gd3+ transition8S7/2–6IJ and Gd3+–Tb3+ charge transfer followed by emission from 5DJ(J = 3,4) to 7FJ(J = 3,4,5,6) levels of Tb3+ ions is observed to be the most effective photoluminescence mechanism. Paramagnetic nature was proved by VSM study. Variation of paramagnetism in undoped K2GdZr(PO4)3 and K2Gd1−xZr(PO4)3:Tbx3+ particles and quantitative analysis by EPR indicate the role of unpaired 4f electron spin of trivalent rare earth ions in the lattice for making the particles paramagnetic.

Peculiarities of molecular structure of 1-vinylimidazole: High resolution NMR study

The peculiarities of molecular structure of 1-vinylimidazoles were studied using 1Н and 13С NMR spectra of its complexes. Mechanisms of delocalization of unpaired electron spin density in the complex CoCl2⋅4(1-vinylimidazole) were revealed. Characteristic features of 1-vinylimidazole coordination with (C2H5)3SnCl were established. Unlike paramagnetic cobalt complex, the structure of VIM diamagnetic complex with triethylhalogenstananne R represents a trigonal bipyramid. The Sn atom in this complex has sp3d-hybridization and three alkyl moieties trend towards coplanarity interacting with sp2-orbitals of tin. The long-range spin–spin interaction between protons of 1-vinylimidazole and the vinyl group coupled with the pyrrole nitrogen N1, typical for coplanar arrangement the imidazole cycle and vinyl moiety was found.

Hyperfine interactions of narrow-line trityl radical with solvent molecules

The electron nuclear dipolar interactions responsible for some dynamic nuclear polarization (DNP) mechanisms also are responsible for the presence formally in CW EPR spectra of forbidden satellite lines in which both the electron spin and a nuclear spin flip. Such lines arising from 1H nuclei are easily resolved in CW EPR measurements of trityl radicals, a popular family of DNP reagents. The satellite lines overlap some of the hyperfine features from 13C in natural abundance in the trityl radical, but their intensity can be easily determined by simple simulations of the EPR spectra using the hyperfine parameters of the trityl radical. Isotopic substitution of 2H for 1H among the hydrogens of the trityl radical and/or the solvent allows the dipolar interactions from the 1H on the trityl radical and from the solvent to be determined. The intensity of the dipolar interactions, integrated over all the 1H in the system, is characterized by the traditional parameter called reff. For the so-called Finland trityl in methanol, the reff values indicate that collectively the 1H in the unlabeled solvent have a stronger integrated dipolar interaction with the unpaired electron spin of the Finland trityl than do the 1H in the radical and consequently will be a more important DNP route. Although reff has the dimensions of distance, it does not correspond to any simple physical dimension in the trityl radical because the details of the unpaired electron spin distribution and the hydrogen distribution are important in the case of trityls.

Monotrimethylene-Bridged Bis-p-phenylenediamine Radical Cations and Dications: Spin States, Conformations, and Dynamics

The properties of p-phenylenediamine- (PD-) based systems substantially depend on the molecular topology. The singly bridged PD analogues HMPD and OMPD in which the PD rings are connected by a flexible linker reveal particular electronic properties in their radical cations and dications. The EPR and UV-vis spectra of HMPD(2+••) were found to be exceptionally temperature-sensitive, following a change from the extended conformation (doublet-doublet state) predominant at room temperature to the π-stacked conformation (singlet state) prevailing at dry-ice temperature. Changing the single bridge from (CH(2))(3) to dimethylated CH(2)CMe(2)CH(2) in OMPD(2+••) causes considerably less of the π-stacked conformation to be present at low temperature as a result of the steric interactions with the methyl groups of the bridge. In contrast to HMPD(2+••) and OMPD(2+••), in which the positive charges are localized separately in each PD(+•) ring, in the extended conformation, exchange of the electron ("hole hopping") between the two PD units (fast at the time scale of EPR experiments) was observed for HMPD(+•) and OMPD(+•). This process slows in a reversible manner with decreasing temperature, thus forming the radical cation with the unpaired electron spin density predominantly on one PD core, at low temperatures. Accordingly, a subtle balance between conformational changes, electron delocalization, and spin states could be established.

Electron paramagnetic resonance in positively charged Au25 molecular nanoclusters

In this study, we investigated the unpaired electrons and singly occupied molecular orbitals (SOMO) of positively charged Au(25) molecular clusters using solid-state electron paramagnetic resonance (EPR). The EPR powder spectra of the positively charged (Au(25) (+)) and neutral (Au(25) (0)) species of Au(25) are discussed and compared. Our study demonstrates that Au(25) (+) is paramagnetic with a SOMO that is mostly localized about the central gold atom in the core of the molecule and possesses a strong p-type atomic character. The unpaired electron spin is demonstrated to strongly interact with the nuclear spins from other (197)Au nuclei in the core of Au(25) (+) molecules and the hyperfine tensor describing such interaction was extracted from the comparison of the EPR spectra with quantum mechanical simulations assuming an anisotropic structure of the core. Our simulations suggest that the core of Au(25) (+) molecular clusters is more distorted than in the corresponding neutral counterpart. They also confirm previous hypotheses suggesting that the icosahedral core of Au(25) (+) experiences contraction with decreasing temperature.

Co-precipitation synthesis and photoluminescence properties of K2GdZr (PO4)3:Eu3+—a deep red luminomagnetic nanophosphor

Nanoparticles of Eu3+ activated K2GdZr(PO4)3 has been successfully synthesized by controlled inclusive co-precipitation method in high alkaline environment to enable complex crystalline phase formation. Much enhanced deep red luminescence, broadened emission bands with unusually prominent 5D0–7F4 transition at 699nm are defining characteristics of the nanoparticles compared to bulk counterpart synthesized by solid state reaction route. Among various excitation pathways such as charge transfer from O2−–Eu3+, Gd3+–Eu3+, the direct excitation of Eu3+ at 394nm is the most effective as revealed by photoluminescence and time resolved studies. Occurrence and variation of superparamagnetism in undoped and Eu3+ doped nanoparticles indicate the role of unpaired 4f electron spin of Gd3+ in making the nanoparticles superparamagnetic. A room temperature cost effective synthesis process of Eu3+ doped multimetallic complex phosphate supermagnetic nanophosphor can pave way for applications requiring such functionality.

Dissection of Hydrogen Bond Interaction Network around an Iron–Sulfur Cluster by Site-Specific Isotope Labeling of Hyperthermophilic Archaeal Rieske-Type Ferredoxin

The electronic structure and geometry of redox-active metal cofactors in proteins are tuned by the pattern of hydrogen bonding with the backbone peptide matrix. In this study we developed a method for selective amino acid labeling of a hyperthermophilic archaeal metalloprotein with engineered Escherichia coli auxotroph strains, and we applied this to resolve the hydrogen bond interactions with the reduced Rieske-type [2Fe-2S] cluster by two-dimensional pulsed electron spin resonance technique. Because deep electron spin-echo envelope modulation of two histidine (14)N(δ) ligands of the cluster decreased non-coordinating (15)N signal intensities via the cross-suppression effect, an inverse labeling strategy was employed in which (14)N amino acid-labeled archaeal Rieske-type ferredoxin samples were examined in an (15)N-protein background. This has directly identified Lys45 N(α) as providing the major pathway for the transfer of unpaired electron spin density from the reduced cluster by a "through-bond" mechanism. All other backbone peptide nitrogens interact more weakly with the reduced cluster. The extension of this approach will allow visualizing the three-dimensional landscape of preferred pathways for the transfer of unpaired spin density from a paramagnetic metal center onto the protein frame, and will discriminate specific interactions by a "through-bond" mechanism from interactions which are "through-space" in various metalloproteins.

Quantum Coherence and Sensitivity of Avian Magnetoreception

Migratory birds and other species have the ability to navigate by sensing the geomagnetic field. Recent experiments indicate that the essential process in the navigation takes place in the bird's eye and uses chemical reaction involving molecular ions with unpaired electron spins (radical pair). Sensing is achieved via geomagnetic-dependent dynamics of the spins of the unpaired electrons. Here we utilize the results of two behavioral experiments conducted on European robins to argue that the average lifetime of the radical pair is of the order of a microsecond and therefore agrees with experimental estimations of this parameter for cryptochrome--a pigment believed to form the radical pairs. We also find a reasonable parameter regime where the sensitivity of the avian compass is enhanced by environmental noise, showing that long coherence time is not required for navigation and may even spoil it.

Pulsed electron paramagnetic resonance spectroscopy powered by a free-electron laser

Electron paramagnetic resonance (EPR) spectroscopy interrogates unpaired electron spins in solids and liquids to reveal local structure and dynamics; for example, EPR has elucidated parts of the structure of protein complexes that other techniques in structural biology have not been able to reveal. EPR can also probe the interplay of light and electricity in organic solar cells and light-emitting diodes, and the origin of decoherence in condensed matter, which is of fundamental importance to the development of quantum information processors. Like nuclear magnetic resonance, EPR spectroscopy becomes more powerful at high magnetic fields and frequencies, and with excitation by coherent pulses rather than continuous waves. However, the difficulty of generating sequences of powerful pulses at frequencies above 100 gigahertz has, until now, confined high-power pulsed EPR to magnetic fields of 3.5 teslas and below. Here we demonstrate that one-kilowatt pulses from a free-electron laser can power a pulsed EPR spectrometer at 240 gigahertz (8.5 teslas), providing transformative enhancements over the alternative, a state-of-the-art ∼30-milliwatt solid-state source. Our spectrometer can rotate spin-1/2 electrons through π/2 in only 6 nanoseconds (compared to 300 nanoseconds with the solid-state source). Fourier-transform EPR on nitrogen impurities in diamond demonstrates excitation and detection of EPR lines separated by about 200 megahertz. We measured decoherence times as short as 63 nanoseconds, in a frozen solution of nitroxide free-radicals at temperatures as high as 190 kelvin. Both free-electron lasers and the quasi-optical technology developed for the spectrometer are scalable to frequencies well in excess of one terahertz, opening the way to high-power pulsed EPR spectroscopy up to the highest static magnetic fields currently available.

Combination of Chemical and Physical Methods to Produce High-Quality Epitaxial Zn1−x Mn x O Thin Films

We have carefully analyzed the epitaxial growth and magnetic properties of Zn1−xMnxO (x=0–0.1) thin films fabricated by pulsed laser deposition on c-Al2O3(0001) substrates. The ablation targets are prepared by pressing nano-sized Zn1−xMnxO (x=0–0.1) powders which were previously synthesized by the polymeric precursor method. The films show excellent crystalline quality with full width at half-maximums varying from ∼0.07–1∘. Some films show well-defined magnetic hysteresis loops both at 300 and 5 K, which suggests the presence of ferromagnetic order. Nevertheless, we find that the ferromagnetism in the samples depends strongly on the oxygen partial pressure in the deposition chamber. This finding suggests that the observed ferromagnetism in Mn-doped ZnO is not an extrinsic property of the system. It is probable that unpaired electron spins, which have their origin in the oxygen vacancies, are responsible for the observed ferromagnetic behavior.

Accurate Structure and Dynamics of the Metal-Site of Paramagnetic Metalloproteins from NMR Parameters Using Natural Bond Orbitals

A natural bond orbital (NBO) analysis of unpaired electron spin density in metalloproteins is presented, which allows a fast and robust calculation of paramagnetic NMR parameters. Approximately 90% of the unpaired electron spin density occupies metal–ligand NBOs, allowing the majority of the density to be modeled by only a few NBOs that reflect the chemical bonding environment. We show that the paramagnetic relaxation rate of protons can be calculated accurately using only the metal–ligand NBOs and that these rates are in good agreement with corresponding rates measured experimentally. This holds, in particular, for protons of ligand residues where the point-dipole approximation breaks down. To describe the paramagnetic relaxation of heavy nuclei, also the electron spin density in the local orbitals must be taken into account. Geometric distance restraints for 15N can be derived from the paramagnetic relaxation enhancement and the Fermi contact shift when local NBOs are included in the analysis. Thus, the NBO approach allows us to include experimental paramagnetic NMR parameters of 15N nuclei as restraints in a structure optimization protocol. We performed a molecular dynamics simulation and structure determination of oxidized rubredoxin using the experimentally obtained paramagnetic NMR parameters of 15N. The corresponding structures obtained are in good agreement with the crystal structure of rubredoxin. Thus, the NBO approach allows an accurate description of the geometric structure and the dynamics of metalloproteins, when NMR parameters are available of nuclei in the immediate vicinity of the metal-site.

The electronic structure of the neutral isoalloxazine semiquinone within Anabaena flavodoxin: New insights from HYSCORE experiments

A complete study of Anabaena flavodoxin in the neutral semiquinone state by means of the EPR pulse technique HYSCORE is here presented. The results provide new information about the hyperfine interactions of the unpaired electronic spin and the nuclei in the isoalloxazine ring. This allows a better knowledge of the electronic structure of the neutral flavin radical within the protein. Combination of these results with other previously obtained by using other EPR related techniques allowed producing a very precise mapping of the flavin spin distribution in the neutral semiquinone state. This information can be very useful for determining the relationship between the electronic structure and mechanisms in flavoproteins. An experimental protocol for measuring the electronic structure details available to date is suggested.

Charge transfer effects on the paramagnetic properties of the [M(C8H8)2] and M(C5H5)(C8H8); M = Ti, Zr, Hf and Th, complexes

A systematic study of a series of (19e) [M(COT)2]n, (n=−1 for Th and n=+1 for Ti, Zr, Hf) and (17e) MCpCOT, where Cp and COT represent cyclopentadienyl [C5H5]− and cyclooctatetraene [C8H8]−2, respectively, has been performed using relativistic density functional theory with the two components ZORA Hamiltonian for the inclusion of scalar and spin–orbit effects. The EPR parameters were obtained as second order properties employing the EPR/NMR algorithms with an inclusion of spin–orbit coupling. The obtained g tensor values are in good agreement with the experimental results and the calculations reveals that each ground state in all systems are nonbonding and of mostly of dz2 character. There is a low spin density at protons site resulting from a direct delocalization of the unpaired electron spin on the 1s orbitals.

Crystal structures of (Mg 1- x ,Fe x )SiO 3 postperovskite at high pressures

X-ray diffraction experiments on postperovskite (ppv) with compositions (Mg(0.9)Fe(0.1))SiO(3) and (Mg(0.6)Fe(0.4))SiO(3) at Earth core-mantle boundary pressures reveal different crystal structures. The former adopts the CaIrO(3)-type structure with space group Cmcm, whereas the latter crystallizes in a structure with the Pmcm (Pmma) space group. The latter has a significantly higher density (ρ = 6.119(1) g/cm(3)) than the former (ρ = 5.694(8) g/cm(3)) due to both the larger amount of iron and the smaller ionic radius of Fe(2+) as a result of an electronic spin transition observed by X-ray emission spectroscopy (XES). The smaller ionic radius for low-spin compared to high-spin Fe(2+) also leads to an ordered cation distribution in the M1 and M2 crystallographic sites of the higher density ppv structure. Rietveld structure refinement indicates that approximately 70% of the total Fe(2+) in that phase occupies the M2 site. XES results indicate a loss of 70% of the unpaired electronic spins consistent with a low spin M2 site and high spin M1 site. First-principles calculations of the magnetic ordering confirm that Pmcm with a two-site model is energetically more favorable at high pressure, and predict that the ordered structure is anisotropic in its electrical and elastic properties. These results suggest that interpretations of seismic structure in the deep mantle need to treat a broader range of mineral structures than previously considered.

Nuclear spin–lattice relaxation and paramagnetic defects in carbon nanomaterials

We review recent nuclear spin–lattice relaxation studies of carbon nanomaterials, such as nanodiamonds, carbon onions, activated carbon fibers, graphene and carbon nanoscrolls. We show that a significant reduction in the relaxation time in nanoparticles compared with that in the bulk compounds and a stretched exponential magnetization recovery are caused by the interaction of nuclear spins with unpaired electron spins of paramagnetic defects, which creates an effective channel for the nuclear relaxation. We present a theoretical approach for such kind of relaxation in static and magic angle spinning regimes and explain a difference in the data received in these regimes. We also extend our approach for the case of additional contributions to the relaxation resulting from interaction of nuclear spin with conduction electrons and with adsorbed paramagnetic oxygen molecules. The developed approach allows correct interpreting of the NMR relaxation data and receiving useful information on properties of nanomaterials from the NMR measurements.