Electronic properties of the linear ladder polymer,poly[4-(3-pyridyl),8-methyl,2,3–6,7-quinolino] (PPMQ)

Electronic paramagnetic resonance (EPR) and conductivity of pristine and iodine-doped PPMQ were studied. The pristine polymer EPR signal exhibited a Lorentzian line shape. Unpaired spin density measurements indicated that the spin concentrations of the undoped polymer lie in the range of one spin per 150–190 repeat units at room temperature. The peak-to-peak width doubled, the line shape became asymmetric and the spin concentration in the polymer increased slightly after doping with iodine. EPR saturation experiments show that the spin lattice relaxation time T₁ is sensitive to trace impurity. Considerable reduction of T₁ after doping with iodine shows strong coupling between the spin system and N-iodonium nucleus. Conductivity increases up to 5 orders of magnitude by iodine doping; at room temperature, the best value found was 0.017 S/cm. The activation energy for conductance after doping is about half that of pristine polymer.     

Electron Spin Resonanance Studies of Acid-Doped Polyaniline

The electron spin resonance spectra of various acid-doped polyanilines were studied. There are three significant changes in the ESR spectra upon acid-doping as the acid concentration of the doping solution is varied from pH<3 to pH>1: (1) The linewidth decreases by a factor of three. (2) The intensity increases. (3) The spin density increases by a factor of twenty. By comparing the variation of the electric conductivity with the variation of the spin density upon acid-doping, it is proposed that the polyaniline prepared by chemical oxidation consists of various oxidized and reduced units, and that the pi-electron rearrangement upon acid-doping results in highly conductive radical cations.     

Dependence of Distance Distributions Derived from Double Electron–Electron Resonance Pulsed EPR Spectroscopy on Pulse‐Sequence Time

Pulsed double electron–electron resonance (DEER) provides pairwise P(r) distance distributions in doubly spin labeled proteins. We report that in protonated proteins, P(r) is dependent on the length of the second echo period T owing to local environmental effects on the spin‐label phase memory relaxation time T_m. For the protein ABD, this effect results in a 1.4 Å increase in the P(r) maximum from T=6 to 20 μs. Protein A has a bimodal P(r) distribution, and the relative height of the shorter distance peak at T=10 μs, the shortest value required to obtain a reliable P(r), is reduced by 40 % relative to that found by extrapolation to T=0. Our results indicate that data at a series of T values are essential for quantitative interpretation of DEER to determine the extent of the T dependence and to extrapolate the results to T=0. Complete deuteration (99 %) of the protein was accompanied by a significant increase in T_m and effectively abolished the P(r) dependence on T.     

Semiconducting poly(dicyanoacetylenes)

Poly(dicyanoacetylene) (PDCA) has been synthesized and characterized. The pristine polymer has EPR g-value, linewidth, unpaired spin concentration, spin—spin relaxation time (T₂), and room temperature dc conductivity (σ_RT) very similar to those of pristine cis-polyacetylene (PA), but shorter spin—lattice relaxation time (T₁). Saturation doping with iodine has little effect on most EPR characteristics of the polymer except for a slight increase in T₁. The doped PDCA has σ_RT value of only 5 X 10^-9 (Ω cm)^-1, indicating either low carrier concentration and/or carrier mobility. Partial cyclization of the nitrile groups by heating at 400°C of PDCA produces l-PDCA with significant increases in unpaired spin concentration and σ_RT but marginal effects on other properties. Saturation doping of l-PDCA with iodine increases σ_RT to 7 × 10^-3 (Ω cm)^-1 without appreciable changes in EPR characteristics. The dopants in both polymers can be removed by evacuation indicating only weak charge transfer interactions. The possible stereoelectronic contribution toward the property differences between the PDCA polymers and PA are discussed.     

Magnetic resonance imaging of spatially resolved acrylamide photopolymerization

Magnetic resonance imaging was employed to examine spatially and temporally resolved photopolymerization of acrylamide gels. Fast exchange between free and bound water results in single exponential T₂ decay, where 1/T₂ scales linearly with polymer concentration. Measured T₂s are sensitive to the experimental conditions; however, the 1/T₂ relationship to polymer concentration allows a straightforward interpretation of image contrast changes during photopolymerization. The polymer appears to form at a nearly constant rate until the monomer concentration is significantly depleted. Conventional spin‐echo images and quantitative CPMG‐weighted spin‐echo images were acquired. Photopolymerization of a partially masked sample produced a sharp transition (1 mm width) between polymer and monomer regions of the sample. The image intensity is uniform throughout the illuminated region of the sample, indicating uniform polymer formation. Interrupting the illumination quenches polymer formation. Copyright © 2003 John Wiley & Sons, Ltd.     

Behavior of Two Almost Identical Spins during the CPMG Pulse Sequence

Multiple‐spin‐echo experiments have found wide use in nuclear magnetic resonance spectroscopy. In particular, the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence is used to determine transverse relaxation times T₂. Herein it is demonstrated, both theoretically and experimentally, that for a pair of almost identical spins‐1/2 the experimental setup can have a profound effect on the observed spin dynamics. It is shown that, in the case of dipolar relaxation, the measured T₂ values can roughly vary between the limits of identical and unlike spins, just depending on the repetition rate of π pulses with respect to chemical shift separation. Such an effect can, in the extreme narrowing regime, amount to a 50 % difference.     

Slow Magnetic Relaxation of Ni(III) Complexes toward Molecular Spin Qubits

Molecule-based magnetic materials are promising candidates for molecular spin qubits, which utilize spin relaxation behavior. Various kinds of transition metal complexes with S=1/2 have been reported to act as spin qubits with long spin-spin relaxation times (T₂). However, the spin qubit properties of low-spin Ni(III) complexes are not as well known since Ni(III) compounds are often unstable. We report here the slow magnetic relaxation behavior and T₂ values for three kinds of low-spin Ni(III) based complexes with S=1/2 under magnetically diluted conditions. [Ni(cyclam)X₂]Y (cyclam=1,4,8,11-tetraazacyclotetradecane) with octahedral structures and [Ni(mnt)₂]⁻ (mnt=maleonitriledithiolate) with a square-planar structure underwent slow magnetic relaxations in the presence of a dc magnetic bias field. From electron spin resonance (ESR) spectroscopy, the Ni(III) complexes exhibited observable T₂, indicating that Ni(III) complexes are promising candidates for use as molecule-based spin qubits.     

A comparison of magnetic resonance imaging methods for fluid content imaging in porous media

Quantitative measurements are important for imaging fluid content in porous media. Conventional MRI methods suffer from contrast because of relaxation times in porous media, resulting in measurements of apparent fluid content, not the true fluid content. We compare four magnetic resonance imaging methods for fluid content imaging in several water‐saturated reservoir core plugs: frequency‐encoded spin echo, single point ramped imaging with T₁ enhancement, hybrid spin echo single point imaging (SE‐SPI), and T₂ mapping SE‐SPI. 1‐D profiles obtained with each method were compared in terms of image quality, image sensitivity, and quantification of water content. The image quality of short T₂ lifetime samples suffered from blurring in hybrid SE‐SPI images. Image sensitivity was the highest in the profiles obtained with frequency‐encoded spin echo. The quantification of frequency‐encoded spin echo, T₂ mapping SE‐SPI, and hybrid SE‐SPI suffered in core plugs with a significant population of short T₂ components because of T₂ attenuation. Overall, single point ramped imaging with T₁ enhancement was found to be the most general method for fluid content imaging. Copyright © 2013 John Wiley & Sons, Ltd.     

Lamellar thickness of crystallizable ethene runs in ethene-propene copolymers by solid state NMR

Crystallizable runs of ethene in ethene-propene copolymers can be identified in ¹³C CPMAS NMR spectra as a resonance at 33 ppm. In the absence of spin diffusion, the variation in intensity of this resonance with a ¹H spin lock will reflect the intrinsic T^H_1ρ. Spin diffusion leads to a more complex relaxation decay, which reflects the local polymer morphology. Simulations of the spin diffusion process have been carried out for a simplified two-phase model for the morphology with the aim of determining whether the lamellar thickness of the crystalline and amorphous regions can be found from the T^H_1ρ observed via the ¹³C NMR spectrum. Calculations covering the expected range of the input parameters, namely the spin diffusion coefficients, domain lengths, and intrinsic relaxation times, show that, providing the intrinsic relaxation time in the amorphous phase is known, an accurate estimate of the crystalline and amorphous lamellar thicknesses can be made. Analysis of simulated T^H_1ρ decays indicate that, in general, the time constant of the fastest decaying component can be identified with the intrinsic relaxation time of the amorphous phase. © 1994 John Wiley & Sons, Inc.     

A Nuclear Singlet Lifetime of More than One Hour in Room‐Temperature Solution

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are supremely important techniques with numerous applications in almost all branches of science. However, until recently, NMR methodology was limited by the time constant T₁ for the decay of nuclear spin magnetization through contact with the thermal molecular environment. Long‐lived states, which are correlated quantum states of multiple nuclei, have decay time constants that may exceed T₁ by large factors. Here we demonstrate a nuclear long‐lived state comprising two ¹³C nuclei with a lifetime exceeding one hour in room‐temperature solution, which is around 50 times longer than T₁. This behavior is well‐predicted by a combination of quantum theory, molecular dynamics, and quantum chemistry. Such ultra‐long‐lived states are expected to be useful for the transport and application of nuclear hyperpolarization, which leads to NMR and MRI signals enhanced by up to five orders of magnitude.     

Semiconducting poly(monocyanoacetylenes)

Poly(monocyanoacetylenes) (PMCA) were synthesized by anionic, Ziegler–Natta, metathesis, and photo initiations. The Ziegler–Natta-catalyzed polymers probably have highly stereoregular cis-transoid structure that contains very few defects and the nitrile groups are difficultly cyclized. It has M?_n = 1100. PMCA obtained by anionic polymerization at ?78°C has M?_n ～ 4800; it is rich in trans-transoid structures but probably contains other isomeric units as well. The unpaired spin concentrations in these polymers are very high, comparable to that in trans-polyacetylene (PA) isomerized above 150°C. UV irradiation initiated rapid polymerization of cyanoacetylene in solid state at low temperature but the products were bleached in color after long irradiation. The unpaired spins in PMCA are immobile; nitrile cyclization causes some decrease in EPR linewidth and increase in room-temperature conductivity (σ_RT). There was also a large increase in unpaired spin concentrations to about 200 monomer units/spin. Iodine doping increases σ_RT to about 10^?3 (ω cm)^?1 but the dopant is readily removed by evacuation and the polymer returns to its original insulating state. The properties of pristine and doped PMCA, such as EPR g-value, ΔH_pp, T₁, T₂, and σ_RT are very similar. The similarities persist after cyclization and doping for this pair of polymers. These properties are also compared with those of poly(methylacetylene), poly(phenylacetylene), poly(dicyanoacetylene) and PA, and the significance is discussed.     

Electron spin relaxation time of Ni(II) ion in hexapyrazole zinc(II) dinitrate at 300 K

Understanding the electron spin relaxation properties of paramagnetic species is a fundamental requirement to use them as a probe to measure distances between sites in biomolecules by electron paramagnetic resonance (EPR) spectroscopy. Even though Ni(II) ion is an essential trace element for many species, relaxation properties are not well understood. Herein, the polycrystalline sample of Ni(II) ion magnetically diluted in Zn(Pyrazole)₆(NO₃)₂ (Ni/ZPN) has been studied in detail by EPR spectroscopy to explore the electron spin relaxation time. Progressive continuous-wave (CW) EPR power saturation study on Ni/ZPN at 300 K yielded 907 mW as the P_1/2 value. The cavity constant (K_Q) has been calculated using tempol in PVA-BA glass matrix and the product of electron spin-lattice relaxation time (T₁) and spin–spin relaxation time (T₂) for Ni/ZPN at 300 K has been reported for the first time.     

Electron spin relaxation times and rapid scan EPR imaging of pH‐sensitive amino‐substituted trityl radicals

Carboxy‐substituted trityl (triarylmethyl) radicals are valuable in vivo probes because of their stability, narrow lines, and sensitivity of their spectroscopic properties to oxygen. Amino‐substituted trityl radicals have the potential to monitor pH in vivo, and the suitability for this application depends on spectral properties. Electron spin relaxation times T₁ and T₂ were measured at X‐band for the protonated and deprotonated forms of two amino‐substituted triarylmethyl radicals. Comparison with relaxation times for carboxy‐substituted triarylmethyl radicals shows that T₁ exhibits little dependence on protonation or the nature of the substituent, which makes it useful for measuring O₂ concentration, independent of pH. Insensitivity of T₁ to changes in substituents is consistent with the assignment of the dominant contribution to spin lattice relaxation as a local mode that involves primarily atoms in the carbon and sulfur core. Values of T₂ vary substantially with pH and the nature of the aryl group substituent, reflecting a range of dynamic processes. The narrow spectral widths for the amino‐substituted triarylmethyl radicals facilitate spectral‐spatial rapid scan electron paramagnetic resonance imaging, which was demonstrated with a phantom. The dependence of hyperfine splittings patterns on pH is revealed in spectral slices through the image. Copyright © 2014 John Wiley & Sons, Ltd.     

Spin‐Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization

A potentially biocompatible class of spin‐labeled macromolecules, spin‐labeled (SL) heparins, and their use as nuclear magnetic resonance (NMR) signal enhancers are introduced. The signal enhancement is achieved through Overhauser‐type dynamic nuclear polarization (DNP). All presented SL‐heparins show high ¹H DNP enhancement factors up to E=?110, which validates that effectively more than one hyperfine line can be saturated even for spin‐labeled polarizing agents. The parameters for the Overhauser‐type DNP are determined and discussed. A striking result is that for spin‐labeled heparins, the off‐resonant electron paramagnetic resonance (EPR) hyperfine lines contribute a non‐negligible part to the total saturation, even in the absence of Heisenberg spin exchange (HSE) and electron spin‐nuclear spin relaxation (T_1ne). As a result, we conclude that one can optimize the use of, for example, biomacromolecules for DNP, for which only small sample amounts are available, by using heterogeneously distributed radicals attached to the molecule.     

Mobility of anionic spin probes in water-containing nylon 6 film

The rotational mobility of anionic spin probes in water-containing nylon 6 film was investigated by means of electron spin resonance (ESR) measurements for comparison with the results for nonionic spin probes reported previously. The extrema separation of the ESR spectra, 2Az′ increased with time owing to the evaporation of water. In the higher temperature region, 2Az′ increased steeply with time at first, and then more slowly, whereas for the nonionic spin probes, 2Az′ increased gradually and monotonically with time. This fact suggests that the anionic probe molecules are more strongly affected by water than the nonionic ones, i.e., the former probes are located in hydrophilic regions and the latter in hydrophobic regions. T_50G, which can be empirically correlated with the glass transition temperature of the polymer T_g decreased with increasing water content. The decreasing tendency for the anionic spin probes was stronger than that for the nonionic ones. This fact also indicates that the local environment around the probe molecules varies from probe to probe. The rotational correlation time τ_R decreased markedly with an increase in water content. The Arrhenius plots of τ_R showed two crossover points. The crossover points in the higher temperature region T_n decreased greatly with increasing water content. The difference in T_n between dried and water-containing films was larger than that for T_50G. The activation energy for rotation, E, also decreased with increasing water content. It is suggested that water concentrates around the anionic spin probes and makes their rotation much easier.     

pH Dependence of T1 for 13C-Labelled Small Molecules Commonly Used for Hyperpolarized Magnetic Resonance Imaging

Hyperpolarization is a method to enhance the nuclear magnetic resonance signal by up to five orders of magnitude. However, the hyperpolarized (HP) state is transient and decays with the spin-lattice relaxation time (T₁), which is on the order of a few tens of seconds. Here, we analyzed the pH-dependence of T₁ for commonly used HP ¹³C-labelled small molecules such as acetate, alanine, fumarate, lactate, pyruvate, urea and zymonic acid. For instance, the T₁ of HP pyruvate is about 2.5 fold smaller at acidic pH (25 s, pH 1.7, B₀=1 T) compared to pH close to physiological conditions (66 s, pH 7.3, B₀=1 T). Our data shows that increasing hydronium ion concentrations shorten the T₁ of protonated carboxylic acids of most of the analyzed molecules except lactate. Furthermore it suggests that intermolecular hydrogen bonding at low pH can contribute to this T₁ shortening. In addition, enhanced proton exchange and chemical reactions at the pK_a appear to be detrimental for the HP-state.     

Ultrafast Two‐Dimensional NMR Relaxometry for Investigating Molecular Processes in Real Time

Nuclear spin–lattice (T₁) and spin–spin (T₂) relaxation times provide versatile information about the dynamics and structure of substances, such as proteins, polymers, porous media, and so forth. Multidimensional experiments increase the information content and resolution of NMR relaxometry, but they also multiply the measurement time. To overcome this issue, we present an efficient strategy for a single‐scan measurement of a 2D T₁–T₂ correlation map. The method shortens the experimental time by one to three orders of magnitude as compared to the conventional method, offering an unprecedented opportunity to study molecular processes in real‐time. We demonstrate that, despite the tremendous speed‐up, the T₁–T₂ correlation maps determined by the single‐scan method are in good agreement with the maps measured by the conventional method. The concept of the single‐scan T₁–T₂ correlation experiment is applicable to a broad range of other multidimensional relaxation and diffusion experiments.     

An Investigation of Photo‐ and Pressure‐Induced Effects in a Pair of Isostructural Two‐Dimensional Spin‐Crossover Framework Materials

Two new isostructural iron(II) spin‐crossover (SCO) framework (SCOF) materials of the type [Fe(dpms)₂(NCX)₂] (dpms=4,4′‐dipyridylmethyl sulfide; X=S ( SCOF‐6(S) ), X=Se ( SCOF‐6(Se) )) have been synthesized. The 2D framework materials consist of undulating and interpenetrated rhomboid (4,4) nets. SCOF‐6(S) displays an incomplete SCO transition with only approximately 30 % conversion of high‐spin (HS) to low‐spin iron(II) sites over the temperature range 300–4 K (T_1/2=75 K). In contrast, the NCSe^? analogue, SCOF‐6(Se) , displays a complete SCO transition (T_1/2=135 K). Photomagnetic characterizations reveal quantitative light‐ induced excited spin‐state trapping (LIESST) of metastable HS iron(II) sites at 10 K. The temperature at which the photoinduced stored information is erased is 58 and 50 K for SCOF‐6(S) and SCOF‐6(Se) , respectively. Variable‐pressure magnetic measurements were performed on SCOF‐6(S) , revealing that with increasing pressure both the T_1/2 value and the extent of spin conversion are increased; with pressures exceeding 5.2 kbar a complete thermal transition is achieved. This study confirms that kinetic trapping effects are responsible for hindering a complete thermally induced spin transition in SCOF‐6(S) at ambient pressure due to an interplay between close T_1/2 and T(LIESST) values.     

Variable temperature 19F solid‐state NMR study of the effect of electrostatic interactions on thermally‐stimulated molecular motions in perfluorosulfonate ionomers

This study uses variable temperature ¹⁹F solid‐state nuclear magnetic resonance (SSNMR) spectroscopy to determine the influence of electrostatic interactions on the T₁, T_1ρ, and T₂ values of Nafion®. Because of a “homogenizing” of the T₁'s as a result of spin diffusion, it was not possible to resolve from the T₁ experiments the relative motions of the side‐ and main‐chain. The initial increase in T_1ρ as a function of increasing temperature has been attributed to backbone rotations that increase with increasing temperature. The maxima observed in the T_1ρ plots suggest a change in the dominant relaxation mechanism at that temperature. The similarity in relaxation behavior of the side‐ and main‐chains suggests that the motions are dynamically coupled, because of the fact that the side‐chain is directly attached to the main‐chain. Two T_1ρ values were observed for the main‐chain at high temperatures, which has been attributed to a thermally activated ion‐hopping process. The results of T₂ studies show that correlated motions of the side‐ and main‐chain exist at low temperatures. However, at elevated temperatures the T₂ values for the side‐chain increase rapidly while remaining relatively constant for the main‐chain, indicating an onset of mobility of the side‐chains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2177–2186, 2007     

35Cl NQR studies of 1‐chloro‐2,4‐dinitrobenzene and 1,2‐dichloro‐3‐nitrobenzene as a function of pressure and temperature

The temperature and pressure dependences of ³⁵Cl nuclear quadrupole resonance (NQR) frequency and spin–lattice relaxation time (T₁) were investigated for 1‐chloro‐2,4‐dinitrobenzene and 1,2‐dichloro‐3‐nitrobenzene. T₁ was measured in the temperature range 77–300 K. Furthermore, the NQR frequency (ν) and T₁ for these compounds were measured as a function of pressure up to 5.1 kbar at 300 K. Relaxation was found to be due to the torsional motion of the molecule and the reorientation motion of the nitro group. By analysing the temperature dependence of T₁, the activation energy for the reorientation motion of the nitro group was obtained. The temperature dependence of the average torsional lifetimes of the molecules and the transition probabilities W₁ and W₂ for the Δm = ±1 and Δm = ±2 transitions, were also obtained. Both compounds showed a non‐linear variation of NQR frequency with pressure. The pressure coefficients were observed to be positive. A thermodynamic analysis of the data was carried out to determine the constant‐volume temperature coefficients of the NQR frequency. The spin–lattice relaxation time T₁ for both the compounds was found to be weakly dependent on pressure, showing that the relaxation is mainly due to the torsional motions. Copyright © 2002 John Wiley & Sons, Ltd.