Quantum tunneling of magnetization in a new [Mn18]2+ single-molecule magnet with s = 13

The reaction between 2-(hydroxyethyl)pyridine (hepH) and a 2:1 molar mixture of [Mn3O(O2CMe)6(py)3](ClO4) and [Mn3O(O2CMe)6(py)3](py) in MeCN leads to isolation of [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (1) in 10% yield. The complex is 2MnII,16MnIII and consists of a Mn4O6 central unit to either side of which is attached a Mn7O9 unit. Magnetization data collected in the 2.0-4.0 K and 20-50 kG ranges were fit to yield S = 13, g = 1.86, and D = -0.13 cm-1 = -0.19 K, where D is the axial zero-field splitting parameter. AC susceptibility studies in the 0.04-4.0 K range at frequencies up to 996 Hz display out-of-phase (chiM' ') signals, indicative of a single-molecule magnet (SMM). Magnetization vs applied DC field scans exhibit hysteresis at <1.0 K, confirming 1 to be a SMM. DC magnetization decay data were collected on both a microcrystalline sample and a single crystal, and the combined data were used to construct an Arrhenius plot. Between 3.50 and 0.50 K, the relaxation rate is temperature-dependent with an effective barrier to relaxation (Ueff) of 14.8 cm-1 = 21.3 K. Below ca. 0.25 K, the relaxation rate is temperature-independent at 1.3 x 10-8 s-1, indicative of quantum tunneling of magnetization (QTM) between the lowest energy Ms = +/-13 levels of the S = 13 state. Complex 1 is both the largest spin and highest nuclearity SMM to exhibit QTM.     

Lewis Base Catalyzed Aldol Reaction of Trimethylsilyl Enolates with Aldehydes

A new Lewis base catalyzed aldol reaction of trimethylsilyl enolates with aldehydes is established in DMF or pyridine solvent by using a Lewis base such as lithium diphenylamide (Tables 4 and 5) or lithium 2‐pyrrolidone (Tables 6–8). The effect of solvent suggests that this reaction proceeds via the pentacoordinated hypervalent silicate generated by the coordination of the above Lewis base to a trimethylsilyl enolate. Successive coordination of the solvent to the thus‐formed pentacoordinated silicate leads to an active enolate intermediate having hexacoordinated silicate, which, in turn, attacks carbonyl compounds to form the desired aldols (Scheme 5).     

Nuclear magnetism of sub-monolayer solid He on graphite

Nuclear magnetization has been measured with NMR down to temperatures around 0.1 mK for sub-monolayer solid ³He formed on graphite preplated with two layers of HD. The data for the commensurate phase show no evidence of a magnetic transition and a rather slow increase of the magnetization even at ultra-low temperature (0.15 mK), much lower than the antiferromagnetic exchange interaction of −6.3 mK.     

High temperature superconductor micro SQUID magnetometer for molecular-based magnets

We have developed a high temperature superconductor (HTS) micro SQUID magnetometer for molecular-based magnets. By employing the dipole approximation, we verified the flux value of 40Φ₀ from the saturation magnetic moment of the ferrimagnetic microcrystal of [Mn₂(H₂O)₂(CH₃COO)][W(CN)₈] · 2H₂O (15 × 15 × 13 μm). Considering the relative arrangement of the sample and the SQUID loop, including the influence of the film and grease, the calculated the flux values were Φ = 71Φ₀ and 31Φ₀ at distances of 20 and 30 μm between the sample bottom face and the SQUID plane, respectively. Thus, the experimentally obtained flux value is reasonable.     

A dinuclear MnIII-CuII single-molecule magnet

The reaction of 1/3 equivalent of CuCl2.2H2O with MnCl2.4H2O and 5-bromo-2-salicylideneamino-1-propanol (H(2)5-Br-sap) in methanol gave dark brown crystals of [MnIIICuIICl(5-Br-sap)2(MeOH)] (1). Complex 1 has an alkoxo-bridged dinuclear core of MnIII and CuII ions, which have elongated octahedral and square-planar coordination geometries, respectively. In dc magnetic susceptibility measurements, chi(m)T values increased as the temperature was lowered, followed by a sudden decrease below 20 K. This behavior is indicative of the occurrence of intramolecular ferromagnetic interactions, and fitting gave an S=5/2 spin ground state with an exchange coupling constant J(MnCu) of +78 cm(-1). Magnetization data collected as a function of temperature and applied magnetic field were analyzed by using a spin Hamiltonian with isotropic Zeeman and axial zero-field splitting (ZFS) terms, and a negative D(5/2) value (-1.86 cm(-1)) was obtained. A high-field EPR (HFEPR) spectrum (342.0 GHz) at 4.2 K was composed of four peaks, and two additional peaks at higher magnetic field appeared as the temperature was increased. The temperature dependences in the HFEPR spectra are indicative of a negative D(5/2) value, and fitting of the data gave D(5/2)=-1.81 cm(-1). In the ac magnetic susceptibility measurements, frequency dependent in-phase (chi(m)') and out-of-phase (chi(m)') signals with peak maxima at 0.7-1.5 K were observed and small peaks below 0.7 K appeared. The ac susceptibility data supports that 1 is a single-molecule magnet (SMM). Arrhenius plots for the chi(m)' peaks from 0.7-1.5 K gave the re-orientation energy barrier (DeltaE) of 10.5 K with a pre-exponential factor of 8.2x10(-8) s.     

Acoustic properties of aortic aneurysm obtained with scanning acoustic microscopy

In aortic aneurysm tissues, macrophages and their secretion of matrix metalloproteinases (MMPs) are playing important role for tissue degeneration. Some studies have shown that weakening of the mechanical properties of the degenerated tissues may progress the expansion of the aneurysm. However, actual measurement of the mechanical properties has not been investigated at microscopic level. The objective of the present study is to assess the mechanical properties of aortic aneurysm tissues by measuring acoustic properties by scanning acoustic microscopy (SAM). Twenty-one cases of aortic aneurysm including renal and common iliac aneurysm tissues were surgically excised. Each tissue was fixed by 4% formaldehyde and the specimens were treated as (1) picrosirius red staining for normal and polarized light microscopy, (2) CD68 staining for macrophage detection, and (3) no staining for acoustic microscopy. A specially developed SAM system operating in the frequency range of 100-200 MHz, was employed in the measurement. Images of amplitude and phase are obtained in a field of 2x2 mm. The intima was mainly consisted of degenerated collagen without polarization of picrosirius red staining. Macrophages stained by CD68 were observed near the degenerated collagen fibers. The sound speed was 1567 m/s in the intima, 1576 m/s in the media, 1640 m/s in the adventitia, respectively. Infiltration of macrophages showed higher values of attenuation and sound speed than the surrounding tissues. The sound speed of the intima was significantly lower than our previous measurement of atherosclerotic aorta without aneurismal change. As the tissue elasticity is closely correlated with the sound speed, the elasticity of the intima was considered to be lower in aneurysm tissues. This mechanical weakness may contribute to the expansion of the diameter of the aneurysm. Acoustic microscopy provided important data for assessing tissue mechanical properties of abdominal aneurysm.     

Variable-field aspect-ratio transformer with cascaded graded-index oval-core fibers

A new configuration is proposed for continuously transforming aspect ratios of field-intensity distributions in optical fibers. The field aspect ratio varies in proportion to the angle between the principal axes of two cascaded graded-index oval-core fibers. The highest aspect-ratio conversion is obtained at an angle of 90 degrees. The conversion effect is numerically and experimentally confirmed, showing that a circular field is successfully transformed into an elliptical one with an aspect ratio as high as 9 at a wavelength of 0.98 microm.     

Texture and molecular chain mobility in cold-deformed linear polyethylene

The fine texture and molecular motion in rolled (draw ratios λ = 2, λ = 5), cold-drawn (λ = 6), and hot-drawn (λ = 6), linear polyethylene (Sholex 6050) films were studied by small-angle x-ray scattering, wide-line nuclear magnetic resonance (NMR), and dynamic viscoelastic measurements. NMR spectra of the rolled (λ = 5) and cold-drawn samples show a small narrow component at room temperature. For these cold-deformed samples, a gradual decrease in the second moment is observed well below the α relaxation temperature and a new relaxation appears in the same temperature region in the logarithmic decrement versus temperature. This relaxation is absent in the original and hot-drawn samples. It is concluded that the new relaxation arises from molecular motion in intermediate regions where molecular chains are less mobile than in the amorphous state.