Longitudinally detected electron spin resonance: Recent developments

Multiple-quantum spectroscopies are reviewed in the frame of electron paramagnetic resonance. Some properties of different nonlinear techniques are discussed for both transverse and longitudinal detection. The connections of effects recently presented with the procedure of longitudinal detection of electron paramagnetic resonance (LODESR) in presence of double transverse irradiation are stressed. Peculiarities of LODESR spectroscopy and its capabilities in facing problems related to relaxations in presence of very slow dynamic processes are evidentiated. Recent results show the vitality of the LODESR technique, that in the future could be applied to new fields, owing to experimental updating.     

Signatures of the fast dynamics in glassy polystyrene: first evidence by high-field electron paramagnetic resonance of molecular guests

The reorientation of one small paramagnetic molecule (spin probe) in glassy polystyrene (PS) is studied by high-field electron paramagnetic resonance spectroscopy at two different Larmor frequencies (190 and 285 GHz). Two different regimes separated by a crossover region are evidenced. Below 180 K the rotational times are nearly temperature independent with no apparent distribution. In the temperature range of 180-220 K a large increase of the rotational mobility is observed with the widening of the distribution of correlation times which exhibits two components: (i) a deltalike, temperature-independent component representing the fraction of spin probes w which persist in the low-temperature dynamics; (ii) a strongly temperature-dependent component, to be described by a power distribution, representing the fraction of spin probes 1-w undergoing activated motion over an exponential distribution of barrier heights g(E). Above 180 K a steep decrease of w is evidenced. The shape and the width of g(E) do not differ from the reported ones for PS within the errors. For the first time the large increase of the rotational mobility of the spin probe at 180 K is ascribed to the onset of the fast dynamics detected by neutron scattering at T(f)=175+/-25 K.     

Diels‐alder reactions of 2‐(2‐nitroethenyl)‐1H‐pyrroles and their oxygen and sulfur analogs with quinones

Diels‐Alder reaction of 2‐(E‐2‐nitroethenyl)‐1H‐pyrrole ( 2a ) with 1,4‐benzoquinone gave the desired benzo[e]indole‐6, 9(3H)‐dione ( 4a ) in 10% yield versus a 26% yield (lit. 86% [5]) of the known N‐methyl compound ( 4b ) from the N‐(or 1)‐methyl compound ( 2b ). Protection of the nitrogen of 2a with a phenylsul‐fonyl group ( 2c ) gave a 9% yield of the corresponding N‐(or 3)‐phenylsulfonyl compound ( 4c ). The reaction of 2b with 1,4‐naphthoquinone gave in 6% yield (lit. 64% [5]) the known 3‐methylnaphtho[2,3‐e]‐indole‐6, 9(3H)‐dione ( 6 ). The reaction of 2‐(E‐2‐nitroethenyl)furan ( 8a ) gave a small yield of the desired naphtho[2,1‐b]furan‐6, 9‐dione ( 9a ), recognized by comparing its NMR spectrum with that of 4b. The corresponding reaction of 2‐(E‐2‐nitroethenyl)thiophene ( 8b ) gave a 4% yield of naphtho[2,1‐ b ]thiophene‐6,9‐dione ( 9b ), previously prepared in 24% yield [12] in a three‐step procedure involving 2‐ethenylthiophene. Introducing an electron‐releasing 2‐methyl substituent into 8a and 8b gave 12a and 12b , which, upon reaction with 1,4‐benzoquinone, gave 2‐methylnaphtho[2,1‐b]furan‐6, 9‐dione ( 13a ) and its sulfur analog ( 13b ) in yields of 4 and 8%, respectively.     

High-frequency and -field electron paramagnetic resonance of high-spin manganese(III) in tetrapyrrole complexes

High-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopy has been used to study three complexes of high spin Manganese(III), 3d4, S = 2. The complexes studied were tetraphenylporphyrinatomanganese(III) chloride (MnTPPCI), phthalocyanatomanganese(III) chloride (MnPcCl), and (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III) (MnCor). We demonstrate the ability to obtain both field-oriented (single-crystal like) spectra and true powder pattern HFEPR spectra of solid samples. The latter are obtained by immobilizing the powder, either in an n-eicosane mull or KBr pellet. We can also obtain frozen solution HFEPR spectra with good signal-to-noise, and yielding the expected true powder pattern. Frozen solution spectra are described for MnTPPCl in 2:3 (v/v) toluene/CH2Cl2 solution and for MnCor in neat pyridine (py) solution. All of the HFEPR spectra have been fully analyzed using spectral simulation software and a complete set of spin Hamiltonian parameters has been determined for each complex in each medium. Both porphyrinic complexes (MnTPPCl and MnPcCl) are rigorously axial systems, with similar axial zero-field splitting (zfs): D approximately -2.3 cm(-1), and g values quite close to 2.00. In contrast, the corrole complex, MnCor, exhibits slightly larger magnitude, rhombic zfs: D approximtely -2.6 cm(-1), absolute value(E) approximately 0.015 cm(-1), also with g values quite close to 2.00. These results are discussed in terms of the molecular structures of these complexes and their electronic structure. We propose that there is a significant mixing of the triplet (S = 1) excited state with the quintet (S= 2) ground state in Mn(III) complexes with porphyrinic ligands, which is even more pronounced for corroles.     

High-Field Electron Paramagnetic Resonance Reveals a Stable Glassy Fraction up to Melting in Semicrystalline Poly(dimethylsiloxane)

The reorientation of the guest 4-methoxy-TEMPO (spin probe) in the disordered fraction of semicrystalline poly(dimethylsiloxane) (PDMS) is investigated by high-field electron paramagnetic resonance (HF-EPR) at 190 and 285 GHz. Accurate numerical simulations of the HF-EPR lineshapes evidence that the reorientation times of the spin probes are distributed between the melting temperature \(T_{\rm m}\) and \(T_{\rm m}\)—30 K. The distribution exhibits, in addition to a broad component, a narrow component with low mobility up to the PDMS melting point. It is shown that the temperature dependence of the reorientation time of the spin probes with low mobility is the same of the spin probes in glassy PDMS. The result suggests that the low-mobility fraction is localized in the so-called rigid amorphous fraction.     

One-step synthesis of gold and silver hydrosols using poly(N-vinyl-2-pyrrolidone) as a reducing agent

Synthesis of gold and silver hydrosols was carried out in a one-step process by reduction of aqueous solutions of metal salts using poly(N-vinyl-2-pyrrolidone) (PVP). Both kinds of metal nanoparticles were obtained without the addition of any other reducing agent, at low temperatures and using water as the synthesis solvent. Shape, size, and optical properties of the particles could be tuned by changing the employed PVP/metal salt ratio. It is proposed that PVP acts as the reducing agent suffering a partial degradation during the nanoparticles synthesis. Two possible mechanisms are proposed to explain the reduction step: direct hydrogen abstraction induced by the metal ion and/or reducing action of macroradicals formed during degradation of the polymer. Initial formation of the macroradicals might be associated with the metal-accelerated decomposition of low amounts of peroxides present in the commercial polymer.     

Direct determination of the single-ion anisotropy in a one-dimensional magnetic system by high-field EPR spectroscopy; synthesis, EPR, and X-ray structure of NixZn1-x(C2O4)(dmiz)2

The synthesis, X-ray structure, and EPR measurements of the integer-spin linear-chain antiferromagnet [Ni(ox)(dmiz)2] (where ox = C2O4(2-) and dmiz = 1,2-dimethylimidazole) are presented. The sign and size of the single-ion zero field splitting (Zfs) of the divalent Ni have been determined by high field/high-frequency EPR spectroscopy. The spectra of powder samples of the derivatives [NixZn1-x(C2O4)(dmiz)2] for x = 0.09 and 0.07, at frequencies ranging from 110 to 440 GHz allowed the accurate determination of the zfs parameters D and E, with D = 1.875(4) cm(-1) and E = 0.38 cm(-1). The X-ray structure has been determined from measurements on a single crystal with x = 0.07. Structural parameters are as follows: a = 14.5252(7) A, b = 12.1916(8) A, c = 8.6850(8) A,beta = 97.460(6)degrees in space group C2/c. The zigzag chain contains octahedrally coordinated metal ions with two cis-oriented N-coordinated dmiz ligands and two cis-oriented, tetradentate bridging oxalato(2-) ligands, together resulting in a MN2O4 donor set. The structure was refined to a conventional R value of 0.073 for 1,051 observed reflections. Zn-O distances are 2.167(5) A and Zn-N = 2.098 A. Coordination angles vary for cis angles from 78.4 to 100.7 degrees, with trans angles varying from 163.9 degrees to 165.5 degrees.     

Multifrequency EPR spectra of molecular oxygen in solid Air

Multifrequency EPR spectra in the 94 to 550 GHz range were performed on solid air samples condensed at 5 K in the waveguide of a single pass probe. The spectra of molecular oxygen were observed and interpreted in the frame of the spin Hamiltonian model as axial S = 1 spectra with a zero field splitting parameter D = 3.572(3) cm⁻¹. The result of this study is relevant in the field of high field–high frequency EPR application in which solid air O₂ is a common paramagnetic impurity.     

Refinement of local and long-range structural order in theophylline-binding RNA using (13)C-(1)H residual dipolar couplings and restrained molecular dynamics.

13C-(1)H residual dipolar couplings (RDC) have been measured for the bases and sugars in the theophylline-binding RNA aptamer, dissolved in filamentous phage medium, and used to investigate the long-range structural and dynamic behavior of the molecule in the solution state. The orientation dependent RDC provide additional restraints to further refine the overall structure of the RNA-theophylline complex, whose long-range order was poorly defined in the NOE-based structural ensemble. Structure refinement using RDC normally assumes that molecular alignment can be characterized by a single tensor and that the molecule is essentially rigid. To address the validity of this assumption for the complex of interest, we have analyzed distinct domains of the RNA molecule separately, so that local structure and alignment tensors experienced by each region are independently determined. Alignment tensors for the stem regions of the molecule were allowed to float freely during a restrained molecular dynamics structure refinement protocol and found to converge to similar magnitudes. During the second stage of the calculation, a single alignment tensor was thus applied for the whole molecule and an average molecular conformation satisfying all experimental data was determined. Semirigid-body molecular dynamics calculations were used to reorient the refined helical regions to a relative orientation consistent with this alignment tensor, allowing determination of the global conformation of the molecule. Simultaneously, the local structure of the theophylline-binding core of the molecule was refined under the influence of this common tensor. The final ensemble has an average pairwise root mean square deviation of 1.50 +/- 0.19 A taken over all heavy atoms, compared to 3.5 +/- 1.1 A for the ensemble determined without residual dipolar coupling. This study illustrates the importance of considering both the local and long-range nature of RDC when applying these restraints to structure refinements of nucleic acids.     

A High-Field EPR Study of the Accelerated Dynamics of the Amorphous Fraction of Semicrystalline Poly(dimethylsiloxane) at the Melting Point

The reorientation of a small paramagnetic tracer in poly(dimethylsiloxane) (PDMS) has been investigated by high-field electron paramagnetic resonance spectroscopy at a Larmor frequency of 285 GHz. The tracer is confined in the disordered phase of the semicrystalline PDMS. A sudden change of the rotational dynamics is observed close to the melting point (213 K) of the crystallites. This points to strong coupling between the crystalline and the disordered fractions of PDMS. Below the glass transition ( \(T_\mathrm{g} \sim 150 \mathrm{K}\) ), the tracer reorientation occurs via small angle jumps, with no apparent distribution of the correlation times. Above \(T_\mathrm{g}\) , a power-law distribution of correlation times is evidenced.