Paramagnetic Molecules Research Articles

Carbon-encapsulated iron nanoparticles used to generate magnetic field and to enhance substrate transport at electrode surface

A thin layer of iron nanoparticles coated tightly with carbon was used to modify glassy carbon electrode. As a result an increase in voltammetric peak current of ferroceneacetate anion by circa 30% was seen; this was due to an increase in real electrode area. Further increase, to 62%, took place after the application of a relatively low (3.4mT or 34Gs) magnetic field, and finally, in all following scans, starting from the second one, the voltammetric peaks were higher by 165% compared to those for the bare electrode. The latter effect was caused by the interactions between the paramagnetic molecules and the non-uniform magnetic field. The magnetic effect was found to depend on the concentration of electroactive species.

Analytic Calculation of Isotropic Hyperfine Structure Constants Using the Normalized Elimination of the Small Component Formalism

Based on the normalized elimination of the small component relativistic formalism, a new approach to the calculation of hyperfine structure parameters of paramagnetic molecules is developed and implemented. The new method is tested in the calculation of the isotropic hyperfine structure constant for a series of open-shell molecules containing mercury. The results of calculations carried out in connection with ab initio methods of increasing complexity demonstrate the high accuracy of the formalism developed. In view of its computational simplicity, the new approach provides the basis for an efficient and accurate calculation of the HFS parameters of large molecules.

1H NMR relaxation in glycerol solutions of nitroxide radicals: Effects of translational and rotational dynamics

(1)H spin-lattice relaxation rates in glycerol solutions of selected nitroxide radicals at temperatures between 200 K and 400 K were measured at 15 MHz and 25 MHz. The frequency and temperature conditions were chosen in such a way that the relaxation rates go through their maximum values and are affected by neither the electron spin relaxation nor the electron-nitrogen nucleus hyperfine coupling, so that the focus could be put on the mechanisms of motion. By comparison with (1)H spin-lattice relaxation results for pure glycerol, it has been demonstrated that the inter-molecular electron spin-proton spin dipole-dipole interactions are affected not only by relative translational motion of the solvent and solute molecules, but also by their rotational dynamics as the interacting spins are displaced from the molecular centers; the eccentricity effects are usually not taken into account. The (1)H relaxation data have been decomposed into translational and rotational contributions and their relative importance as a function of frequency and temperature discussed in detail. It has been demonstrated that neglecting the rotational effects on the inter-molecular interactions leads to non-realistic conclusions regarding the translational dynamics of the paramagnetic molecules.

A Stable Hybrid Bisphosphonate Polyoxometalate Single‐Molecule Magnet

A new functionalizable polyoxometalate platform: The heptanuclear CoII polyoxometalate complex [{(B-α- PW9O34)Co3(OH)(H2O)2(O3PC(O)- (C3H6NH3)PO3)}2Co]14− represents a unique example of a magnetic, stable, functionalizable polyoxotungstate platform, and behaves as a single-molecule magnet (see figure).

Magnetism of HNO3-adsorbed nanoporous network of nanographene sheets

Nonbonding π-electron states (edge-state) localized at the graphene edges are the origin of electronic/chemical activities of nanographene, and the localized spin of this edge state gives unconventional magnetic features to nanographene. The magnetic properties of nanoporous carbon consisting of a 3D disordered network of nanographite domains (each consists of a stack of 3–4 nanographene sheets) are investigated in relation to the host-guest interaction with nitric acid molecules possessing significant oxidizing power. The adsorption of HNO3 molecules induces a two-step reduction in the edge-state spin concentration due to the charge transfer from nanographene to HNO3 molecules; charge transfer with the exterior nanographene sheets of the nanographite domain in the first step, and subsequent intercalation into the interior nanographene sheets in the second step. Besides the charge transfer reaction, it is found that the paramagnetic NO molecules are produced owing to the decomposition of HNO3 and they are dimerized to form nonmagnetic (NO)2 at low temperatures. This finding suggests that the chemically active nanographene having edge states plays an important role in enhancing charge transfer and the concomitant decomposition of the guest HNO3 molecules.

NMR Relaxation and ESR Lineshape of Anisotropically Rotating Paramagnetic Molecules

A. Kubica, A. Milewska, M. Noinska, K. Earle and D. Krukd,∗ Institute of Physics, Jagiellonian University, W.S. Reymonta 4, 30-059 Krakow, Poland The University of Economy in Bydgoszcz, Garbary 2, 82-299 Bydgoszcz, Poland Physics Department, University at Albany (SUNY), 1400 Washington Ave., Albany, NY 12222, USA University of Warmia and Mazury in Olsztyn, Faculty of Mathematics and Computer Science

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.

Microstructured objects produced by the supramolecular hierarchical assembly of an organic free radical gathering hydrophobic-amphiphilic characteristics

The different weak non-covalent interactions established between “hydrophobic–amphiphilic” fluorescent and paramagnetic organic free-radical molecules, and solvents result in the formation of hierarchically self-assembled multifunctional superhydrophobic materials with very unusual shapes.

Rotationally resolved magnetic vibrational circular dichroism of the paramagnetic molecule NO

Magnetic vibrational circular dichroism (MVCD) enables the measurement of molecular magnetic moments with modest spectral resolution. Due to its paramagnetism, NO gives a much stronger spectral response, about 3 orders of magnitude more intense, than do typical diamagnetic molecules. The molecule thus provides a convenient test for the experiment and theory of paramagnetic rotors. We have measured and analyzed the MVCD, equivalent to the molecular Zeeman spectra, of NO in co-linear magnetic fields of 0.1, 0.2, 2, 4 and 8 Tesla. Similar MVCD intensities were observed for both the (2)Π(1/2) and (2)Π(3/2) components of NO, particularly for high J values, which demonstrates a considerable deviation from pure Hund's case (a) for NO. The g(J)-values for the (2)Π(1/2) components of NO, which can be determined from our experimental spectra by moment analysis, agree well with the predicted values from Radford's theory. For the (2)Π(3/2) components, we tested this theory by simulating the MVCD and absorption spectra, and comparing them with our experimental spectra by use of moment analysis to show that they match well in terms of magnetic properties. While 0.2 T experiments easily develop sufficient MVCD for analysis of NO spectra and these low field intensities have a linear field dependence, spectra in the strong fields accessible in our study showed non-linear response due to onset of saturation effects. We also observed a strong field dependence for the absorption intensities for some (2)Π(3/2) components that was not encompassed in the theoretical model. Finally, a full coupling scheme provided analytical MVCD and absorption intensities that were in good agreement with the experimental values.

Design of magnetic coordination complexes for quantum computing

A very exciting prospect in coordination chemistry is to manipulate spins within magnetic complexes for the realization of quantum logic operations. An introduction to the requirements for a paramagnetic molecule to act as a 2-qubit quantum gate is provided in this tutorial review. We propose synthetic methods aimed at accessing such type of functional molecules, based on ligand design and inorganic synthesis. Two strategies are presented: (i) the first consists in targeting molecules containing a pair of well-defined and weakly coupled paramagnetic metal aggregates, each acting as a carrier of one potential qubit, (ii) the second is the design of dinuclear complexes of anisotropic metal ions, exhibiting dissimilar environments and feeble magnetic coupling. The first systems obtained from this synthetic program are presented here and their properties are discussed.

Stern-Gerlach deflection of field-free aligned paramagnetic molecules

The effects of laser-induced pre-alignment on the deflection of paramagnetic molecules by inhomogeneous static magnetic field are studied. Depending on the relevant Hund's coupling case of the molecule, two different effects were identified: either suppression of the deflection by laser pulses (Hund's coupling case (a) molecules, such as ClO), or a dramatic reconstruction of the broad distribution of the scattering angles into several narrow peaks (for Hund's coupling case (b) molecules, such as O(2) or NH). These findings are important for various applications using molecular guiding, focusing and trapping with the help of magnetic fields.

Possible structural role of the disaccharide unit in Fe-bleomycin before and after oxygen activation

Our previous investigation of the solution structure of Fe(II)-bleomycin pointed toward the carbamoyl group in the mannose moiety or a water molecule as possible alternative axial ligands to the metal center in this metallo-bleomycin. The possibility of a solvent molecule occupying the apical position trans to the primary amine has not been ruled out yet. In order to explore this possibility even further, the coordination chemistry of azide-bound Fe(II)-bleomycin was investigated with the use of NMR applied to paramagnetic molecules. Fe(II)- and apo-bleomycin were also re-visited. Comparison of the NMR results for both Fe(II)-bound molecules obtained in the present study strongly suggests that the carbamoyl oxygen is ligated to Fe(II), and it is released from coordination upon azide binding. This event is suggested based on the diminished paramagnetic character exhibited by the carbohydrate moiety in Fe(II)-azide-bleomycin when compared with its parent metal complex. A possible structural role for the glucopyranose fragment, which changes throughout the process that starts with metallo-bleomycin formation and ends with DNA binding, is discussed. The study of the coordination of azide by Fe(II)-bleomycin through NMR has not been reported previously. Unlike magnetic CD data, NMR offers a residue-by-residue account of the possible structural changes that take place in Fe(II)-bleomycin after azide binding.

Coupling Single Molecule Magnets to Ferromagnetic Substrates

We investigate the interaction of TbPc(2) single molecule magnets (SMMs) with ferromagnetic Ni substrates. Using element-resolved x-ray magnetic circular dichroism, we show that TbPc(2) couples antiferromagnetically to Ni films through ligand-mediated superexchange. This coupling is strongly anisotropic and can be manipulated by doping the interface with electron acceptor or donor atoms. We observe that the relative orientation of the substrate and molecule anisotropy axes critically affects the SMM magnetic behavior. TbPc(2) complexes deposited on perpendicularly magnetized Ni films exhibit enhanced magnetic remanence compared to SMMs in the bulk. Contrary to paramagnetic molecules pinned to a ferromagnetic support layer, we find that TbPc(2) can be magnetized parallel or antiparallel to the substrate, opening the possibility to exploit SMMs in spin valve devices.

A moving magnetic trap decelerator: a new source of cold atoms and molecules

We present an experimental realization of a moving magnetic trap decelerator, where paramagnetic particles entrained in a cold supersonic beam are decelerated in a co-moving magnetic trap. Our method allows for an efficient slowing down of both paramagnetic atoms and molecules to near stopping velocities. We show that under realistic conditions we will be able to trap and decelerate a large fraction of the initial supersonic beam. We present our first results on deceleration in a moving magnetic trap by bringing metastable neon atoms to near rest. Our estimated phase space volume occupied by decelerated particles at a final velocity of 50 m s−1 shows an improvement of two orders of magnitude as compared to currently available deceleration techniques.

Spectroscopic investigations of nanoporous SiO 2 impregnated with Ag β-diketonates from supercritical solution of carbon dioxide

Porous materials based on SiO 2 containing nanoparticles of silver (NP Ag) are perspective objects for different medico-biological and optoelectronic applications. In the present work nanoporous glasses Vycor (pore size ∼4 nm) and synthetic opal matrices (OM, void size ∼40 nm) impregnated with β-diketonates of silver (Ag(hfac)COD) via solution of supercritical carbon dioxide were studied. Paramagnetic molecules Cu(hfac) 2 were used as spectroscopic probes permitting to obtain the information about the incorporation of similar organometallic molecules into these matrices. Spectra of absorption and electronic paramagnetic resonance (EPR) in the samples of porous SiO 2 containing Ag(hfac)COD were studied before and after heat treatment in air atmosphere. In both materials impregnated at temperature above 50 °C the absorption band at 420–430 nm (the plasmon resonance (PR) band)) typical for nanoparticles of metallic Ag is observed. This band increases in the intensity at heat treatment or at an increase in temperature of impregnation processing. Features of the formation of the PR band in each case depends on sizes and shape of voids in the samples as well as on conditions of heat treatment. The states of paramagnetic centers formed after Ag(hfac)COD introduction are analyzed.

Paramagnetic Relaxation Enhancement (PRE) as a Tool for Probing Diffusion in Environmentally Relevant Porous Media

The transport diffusivity of the paramagnetic molecule 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was measured by monitoring its influence on the NMR transverse relaxation time (T₂) on surrounding water protons - also known as paramagnetic relaxation enhancement (PRE). Due to the nature of the PRE effect, few paramagnetic molecules are able to simultaneously reduce the T₂ of many NMR active nuclei, which represents a significant gain in sensitivity. In an aqueous solution, the minimal detectable TEMPO concentration was around 70 ppm. The value of the diffusivity was estimated by fitting the relaxation data, collected as a function of time, with the appropriate solutions of the second Fick's law in respect to the corresponding sample geometry and dimensions. Considering the experimentally determined TEMPO relaxivity in water ("TEMPO-water relaxivity"; R(TEMPO) = (1.05 ± 0.12) × 10⁻³ ppm⁻¹ s⁻¹), the obtained diffusion coefficients (D) of TEMPO in homogeneous solution and in a water saturated sand column (D(bulk) = (6.7 ± 0.4) × 10⁻¹⁰ m² s⁻¹ and D(sand) = (1.4 ± 0.5) × 10⁻¹⁰ m² s⁻¹, respectively) are in good agreement with the expected values (literature values: D(bulk) = 6.6 × 10⁻¹⁰ m² s⁻¹, 1.3 × 10⁻¹⁰ m² s⁻¹ < D(sand) < 2.3 × 10⁻¹⁰ m² s⁻¹). This new approach enables one to determine the diffusivity of paramagnetic molecules in homogeneous (aqueous solution) and porous media with basic NMR equipment, at low concentrations and in a noninvasive manner.

Quantum degenerate mixture of ytterbium and lithium atoms

We have produced a quantum degenerate mixture of fermionic alkali 6Li and bosonic spin-singlet 174Yb gases. This was achieved using sympathetic cooling of lithium atoms by evaporatively cooled ytterbium atoms in a far-off-resonant optical dipole trap. We observe co-existence of Bose condensed (T/T_c~0.8) 174Yb with 2.3*10^4 atoms and Fermi degenerate (T/T_F~0.3) 6Li with 1.2*10^4 atoms. Quasipure Bose-Einstein condensates of up to 3*10^4 174Yb atoms can be produced in single-species experiments. Our results mark a significant step toward studies of few and many-body physics with mixtures of alkali and alkaline-earth-like atoms, and for the production of paramagnetic polar molecules in the quantum regime. Our methods also establish a convenient scheme for producing quantum degenerate ytterbium atoms in a 1064nm optical dipole trap.

Real-Space Imaging of Kondo Screening in a Two-Dimensional O 2 Lattice

Kondo lattice systems can exhibit unusual many-body behaviors that result from the interplay between onsite Kondo screening and intersite coupling. We used scanning tunneling microscopy to image the Kondo resonance in a nonconventional Kondo lattice formed by self-assembled oxygen (O(2)) molecules, which are paramagnetic, on the gold reconstructed surface [Au(110)-1×2]. The interplay between the intermolecular coupling for molecules adsorbed along chains and the onsite Kondo effect leads to the coexistence of both local and nonlocal Kondo screening at the atomic level. The latter provides evidence for collective deconfinement of magnetization induced in Au, whereas the former shows local "hybridization" between the Kondo clouds of nearest-neighbor O(2) molecules.

Uridine-based paramagnetic supramolecular nanoaggregate with high relaxivity capable of detecting primitive liver tumor lesions

The water soluble uridine-based paramagnetic self-assembled amphiphilic molecules (LGd2-5) with DTTA binding site were synthesized and have been characterized in regard to their T1 magnetic resonance imaging (MRI) contrast agent (CA) properties. The water proton relaxivities have been measured in phosphate buffered saline (PBS) at 36 °C at 3 different magnetic fields. Among the self-assembled CAs, LGd3 showed unprecedented, high relaxivities of 30.3 and 23.4 mM-1 s−1 in PBS solution at 36 °C at 0.47 and 1.41 T, respectively. The non-covalent interactions between the new CAs and human serum albumin (HSA) have been investigated and the relaxivity was further increased by 135–215% depending on alkyl chain lengths. The chemically inertness of these complexes (LGd1, LGd2, LGd3, LGd4) against biologically most abundant metal ion (i.e. Zn2+) have shown within the range of commercial DTPA-based CAs. In vivo pharmacokinetics of the complex LGd3 showed highly specific for hepatocytes resulting in increase of contrast noise ratio by ∼240% in T1-weighted MR images of mouse liver 2 h after injection of the LGd3. It is capable to detect small hepatocellular carcinoma (HCC) with diameter of 1.5 mm.

Gd2@C79N: Isolation, Characterization, and Monoadduct Formation of a Very Stable Heterofullerene with a Magnetic Spin State of S = 15/2

The dimetallic endohedral heterofullerene (EHF), Gd(2)@C(79)N, was prepared and isolated in a relatively high yield when compared with the earlier reported heterofullerene, Y(2)@C(79)N. Computational (DFT), chemical reactivity, Raman, and electrochemical studies all suggest that the purified Gd(2)@C(79)N, with the heterofullerene cage, (C(79)N)(5-) has comparable stability with other better known isoelectronic metallofullerene (C(80))(6-) cage species (e.g., Gd(3)N@C(80)). These results describe an exceptionally stable paramagnetic molecule with low chemical reactivity with the unpaired electron spin density localized on the internal diatomic gadolinium cluster and not on the heterofullerene cage. EPR studies confirm that the spin state of Gd(2)@C(79)N is characterized by a half-integer spin quantum number of S = 15/2. The spin (S = ½) on the N atom of the fullerene cage and two octet spins (S = 7/2) of two encapsulated gadoliniums are coupled with each other in a ferromagnetic manner with a small zero-field splitting parameter D. Because the central line of Gd(2)@C(79)N is due to the Kramer's doublet with a half-integer spin quantum number of S = 15/2, this relatively sharp line is prominent and the anisotropic nature of the line is weak. Interestingly, in contrast with most Gd(3+) ion environments, the central EPR line (g = 1.978) is observable even at room temperature in a toluene solution. Finally, we report the first EHF derivative, a diethyl bromomalonate monoadduct of Gd(2)@C(79)N, which was prepared and isolated via a modified Bingel-Hirsch reaction.