Spectral diffusion in the tunneling spectra of ligand-stabilized undecagold clusters

We report diffusion in the tunneling spectra of isolated, ligand-stabilized undecagold (Au11) clusters immobilized by attachment to alpha,omega-alkanedithiolate tethers inserted into alkanethiolate self-assembled monolayers. We use scanning tunneling microscopy and spectroscopy at cryogenic (UHV, 4 K) conditions to measure these clusters' conductance with complete control of their chemical and physical environment; additionally, thermal broadening of their electronic states as well as their mobility is minimized. At low temperature, the Au11 clusters demonstrate Coulomb blockade behavior, with zero-conductance gaps resulting from quantum size effects. Surprisingly, chemically identical and even single particles produced different families of tunneling spectra, comparable to previous results for heterogeneous distributions of particles. We hypothesize that, while these particles are chemically attached to the surface of the SAM for measurement, these assemblies may still be sufficiently dynamic to affect their transport properties significantly.     

Electronic Coupling between Two Amine Redox Sites through the 5,5′‐Positions of Metal‐Chelating 2,2′‐Bipyridines

Electron delocalization of new mixed‐valent (MV) systems with the aid of lateral metal chelation is reported. 2,2′‐Bipyridine (bpy) derivatives with one or two appended di‐p‐anisylamino groups on the 5,5′‐positions and a coordinated [Ru(bpy)₂] (bpy=2,2′‐bipyridine), [Re(CO)₃Cl], or [Ir(ppy)₂] (ppy=2‐phenylpyridine) component were prepared. The single‐crystal molecular structure of the bis‐amine ligand without metal chelation is presented. The electronic properties of these complexes were studied and compared by electrochemical and spectroscopic techniques and DFT/TDDFT calculations. Compounds with two di‐p‐anisylamino groups were oxidized by a chemical or electrochemical method and monitored by near‐infrared (NIR) absorption spectral changes. Marcus–Hush analysis of the resulting intervalence charge‐transfer transitions indicated that electron coupling of these mixed‐valent systems is enhanced by metal chelation and that the iridium complex has the largest coupling. TDDFT calculations were employed to interpret the NIR transitions of these MV systems.     

A short and stereoselective synthesis of (±)-aristeromycin

A new Stereoselective three-step total synthesis of (±)-aristeromycin starting from readily available 1-hydroxymethyl-3-cyclopentene is described.     

Novel pulsed electron spin resonance system and studies of phosphorus in natural silicon

An X-band pulsed electron spin resonance system has been designed and constructed specifically for studies of decoherence times of phosphorus donor electrons in silicon. The microwave electromagnetic field aspects of the structure have been analysed, and the probe head geometry optimised, through 3D electromagnetic simulations. Results for natural silicon samples at temperatures down to 6 K, are analysed and we obtain an estimate of the isolated spin decoherence time in ^natSi approximately an order of magnitude longer than previously measured.     

Pulsed fm NMR/on studies of single crystal54MnNi and125SbFe

Precision field shift studies of B_appl-8T using pulsed FM NMR/ON along hard directions (eg <100>⁵⁴MnNi, <112>¹²⁵SbFe and the easy axis (<111>⁵⁴MnNi, <100>¹²⁵SbFe) yield no measurable Knight shifts within the uncertainty allowed by current accuracy of the nuclear moments. This is in striking contrast to earlier measurements with the same apparatus that yielded K=+1.5 (4)% for⁶⁰CoFe. The modulation frequency dependences of the pulsed FMNMR/ON signals are investigated for a variety of rf parameters and compared with model predictions.     

A comparative TEM study of the 3D lattice of tellurium nanoclusters fabricated by different techniques in an opal host

An electron-microscope study of synthetic opals containing tellurium introduced into the regular void array either by melt injection or by evaporation from solution is reported. In the first case, a three-dimensional array of tellurium clusters interconnected by bridges was revealed. The clusters retain the same crystallographic orientation as one moves from one cluster to another. This suggests that cooling after inserting tellurium results in its directional crystallization, which is possibly controlled by the interconnecting channels. The second technique of tellurium introduction produces a nonuniform cluster array, with three-dimensional clusters growing not in all voids. The surface of most of the silica spheres is coated by a thin discontinuous layer of tellurium. This structure of the cluster array accounts for the nonlinear current-voltage characteristic of the object as a whole. A possibility of engineering cluster lattices differing in structural parameters has been demonstrated. Fiz. Tverd. Tela (St. Petersburg) 39, 2090–2095 (November 1997)     

Evaluation of a pseudoephedrine linker for asymmetric alkylations on solid phase

[reaction: see text] Immobilized pseudoephedrine amides have been conveniently prepared by attachment of pseudoephedrine to Merrifield resin and acylation on nitrogen. Deprotonation and alkylation of the resin bound amides proceeds smoothly. Products were cleaved from the resin to give ketones and alcohols in high enantiomeric excess and moderate to good overall yield.     

Chemistry of 2,4,6-trimercapto-1,3,5-triazine (TMT): acid dissociation constants and group 2 complexes

The acid dissociation constants of 2,4,6-trimercaptotriazine (H(3)TMT, 1) were determined and now can be employed in the preparation of complexes having specific M-TMT (M = divalent metal; TMT = 2,4,6-trimercapto-1,3,5-triazinide, C(3)N(3)S(3)(3-)) ratios. For example, the combination of H(3)TMT (1) with Mg(OH)(2) at pH 7.1 leads to the crystallization of Mg(H(2)TMT)(2).6H(2)O (4). With the appropriate pH adjustment, the contiguous series of compounds Ba(3)(TMT)(2).8H(2)O (3), Ba(H(2)TMT)(2).7H(2)O (5), and BaHTMT.3H(2)O (6) can be isolated. The compounds were characterized by mp, IR, TGA, elemental analysis, and, in the cases of 4, 5, and 6, crystallography. The comparison of 4 with 5 and 6 offers an interesting view of the difference in hard and soft bonding with TMT. In the saltlike Mg structure of 4, there is extensive hydrogen bonding, but in the Ba structures, 5 and 6, covalent Ba-S bonding dominates.     

Bicyclic and acyclic diamides: comparison of their aqueous phase binding constants with Nd(III), Am(III), Pu(IV), Np(V), Pu(VI), and U(VI)

This report describes affinity measurements for two, water-soluble, methyl-alkylated diamides incorporating the malonamide functionality, N,N,N',N' tetramethylmalonamide (TMMA) and a bicyclic diamide (1a), toward actinide metal cations (An) in acidic nitrate solutions. Ligand complexation to actinides possessing oxidation states ranging from +3 to +6 was monitored through optical absorbance spectroscopy, and formation constants were obtained from the refinement of the spectrophotometric titration data sets. Species analysis gives evidence for the formation of 1, 4, 1, and 2 spectrophotometrically observable complexes by TMMA to An(III, IV, V, and VI), respectively, while for 1a, the respective numbers are 3, 4, 2, and 2. Consistent with the preorganization of 1a toward actinide binding, a significant difference is found in the magnitudes of their respective formation constants at each complexation step. It has been found that the binding affinity for TMMA follows the well-established order An(V) < An(III) < An(VI) < An(IV). However, with 1a, Np(V) forms stronger complexes than Am(III). The complexation of 1a with Np(V) and Pu(VI) at an acidity of 1.0 M is followed by reduction to Np(IV) and Pu(IV), whereas TMMA does not perturb the initial oxidation state for these dioxocations. These measurements of diamide binding affinity mark the first time single-component optical absorbance spectra have been reported for a span of actinide-diamide complexes covering all common oxidation states in aqueous solution.