Toward self-assembled surface-mounted prismatic altitudinal rotors. A test case: trigonal and tetragonal prisms

[structure: see text] A self-assembly path toward prismatic molecular rotors based on transversely reactive terminally metalated molecular rods and pyridine-terminated star connectors has been extended. The concept has been tested on the assembly of trigonal and tetragonal prisms from the biphenyl rod, [Ph2P(CH2)3PPh2]Pt+ -C6H4-C6H4-Pt+ [Ph2P(CH2)3PPh2], and the star-shaped connectors, 1,3,5-tris(4-ethynylpyridyl)benzene and [tetrakis(4-pyridyl)cyclobutadiene]cyclopentadienylcobalt, respectively. The prisms have been fully characterized by NMR and MS, including diffusion-ordered NMR and collision-induced dissociation, and their chiral structures optimized by molecular mechanics are discussed.     

La8Br7Ni4: ribbons of Ni hexagons in condensed La6 trigonal prisms

A ternary lanthanum bromide La 8Br 7Ni 4 was synthesized from La, LaBr 3, and Ni under an Ar atmosphere at 830 degrees C. It crystallizes in space group C2/ m (No. 12) with lattice constants a = 29.528(4) A, b = 4.0249(6), c = 8.708(1) A, and beta = 94.515(2) degrees . The structure features condensed Ni-centered La 6 trigonal prisms. The Ni atoms are bonded to each other to form ribbons of Ni hexagons. Band structure, bonding, and physical properties of the compound have been investigated.     

Early solvent vapor exit in GC for coupled LC-GC involving concurrent eluent evaporation

Use of early solvent vapor exits for concurrent eluent evaporation with the loop-type interface has two purposes: protection of the GC detectors from large amounts of solvent vapors and more efficient discharge of the vapors to accelerate eluent evaporation and help avoiding broad solvent peaks. Use of a retaining pre-column after the uncoated pre-column can rule out losses of solute materials that form sharp peaks.     

Observation of a free rotation transient in hot stilbene vapor

The fluorescence anisotropy decay of collision-free trans-stilbene vapor at 463 K has been measured with 5 ps time resolution using fluorescence up-conversion. The anisotropy measured with 302 nm excitation shows a pulsewidth-limited decay attributed to free rotation followed by a constant value at longer times. The long-time anisotropy of 0.069 is close to the theoretical regular rotor value of 0.074, indicating minimal vibration-rotation energy transfer on a time scale of ≈ 50 ps. Comparison with the longtime anisotropies previously observed using shorter excitation wavelengths indicates that the rotational motion becomes more nearly statistical with increasing excess vibrational energy.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solution. Part IX. Spectral Evidence for a Homolytic Mechanism Involving Pyridine Complexes

The mechanism of dediazoniation of arenediazonium tetrafluoroborates in 2,2,2-trifluoroethanol (TFE) is strongly dependent on the concentration of added pyridine. The added base complexes with the diazonium ion and diverts it to a homolytic pathway. Complex formation is indicated by the disappearance of the \documentclass{article}\pagestyle{empty}\begin{document}$\raise1pt\hbox{---} \mathop {\rm N}\limits^ \oplus \equiv {\rm N}\raise1pt\hbox{---}$\end{document} stretching vibration and appearance of a new band at about 1640–1690 cm^?1 ascribed to the \documentclass{article}\pagestyle{empty}\begin{document}$\raise1pt\hbox{---} {\rm N}\raise1pt\hbox{=\kern-3.45pt=} {\rm N}\raise1pt\hbox{---} \mathop {\rm N}\limits^ \oplus {\rm C}_5 {\rm H}_5$\end{document} system. UV. and NMR. results support this conclusion. Chemically induced dynamic nuclear polarization (CIDNP) experiments clearly implicate a radical-pair as an important intermediate in the decomposition of these complexes.     

Dynamics of 4-benzylamino-7-nitrobenzofurazan in the 1-propanol/water binary solvent system. Evidence for composition-dependent solvent organization

We report on the fluorescence lifetime and rotational diffusion dynamics of 4-benzylamino-7-nitrobenzofurazan (BBD) in a series of 1-propanol/water binary solvent systems. The fluorescence lifetime of BBD increases monotonically with increasing 1-propanol concentration. The rotational diffusion dynamics of BBD also vary with solution 1-propanol content, but this variation is not monotonic. Comparison of the BBD rotational diffusion time constant to solution viscosity and 1-propanol composition reveals the presence of a solution composition dependence of solvent-solute interactions, with a relative decrease in solvent-solute interaction strength for solvent system compositions where the 1-propanol/water azeotrope is known to exist. These data point collectively to the existence of microscopic heterogeneity in these binary solvent systems.     

New cyclen derivative ligand for thorium(IV) separation by solvent extraction

A new N-containing ligand, 1,4,7,10-tetra-(4-nitrobenzyl)-1,4,7,10-tetraazacyclo-dodecane (L), was synthesized, and its structure was determined by ¹H NMR, high resolution mass spectrometry and X-ray diffraction. L crystallized in the monoclinic system (P2₁/n space group; a = 7.7895(2) Å, b = 22.9592(5) Å, c = 9.9204(2) Å; α = 90.00°, β = 105.481(3)°, γ = 90.00°; Z = 2). Slope analysis and the continuous variation method demonstrated that 1:2 complexes between Th(IV) and L are formed; furthermore, the XPS analysis suggested that two oxygen atoms might be provided by two water molecules and that eight nitrogen atoms might be provided by two L molecules to form a ten-coordinate compound with Th(IV). The extraction equilibrium constant for the complex formation between Th(IV) and L was logK _ex = 6.95 ± 0.15 (25 °C), and the Gibbs free energy, ΔG ^o (25 °C), of the 1:2 Th–L complex in dichloromethane was ?39.56 kJ/mol. The L ligand in dichloromethane only slightly extracted Th(IV) from HNO₃ solution at pH = 1–3; however, an extraction efficiency of E = 94.9 ± 0.3 % was observed at pH = 4.63. The selectivity of L for the Th(IV) cation over other cations (i.e., Cs(I), Sr(II), Y(III), La(III), Sm(III), Eu(III), U(VI), and ²⁴¹Am(III)) was evaluated. Furthermore, the stripping experiments showed that the stripping agent (0.5 mol/L Na₂CO₃ + 0.1 mol/L EDTA) could provide an optimal condition for stripping thorium, and thorium recovery was up to 91.6 ± 0.1 %.     

Electrochemical and spectroscopic studies of metal cyclam complexes with thiocyanate. Evidence for mixed ligand complex formation

Complexes of Ni^II, Co^II and Cu^II containing the macrocyclic ligand, 1,4,8,11-tetraazacyclotetradecane (cyclam), and their ability to form mixed ligand complexes with thiocyanate have been studied. These complexes in a 1:2 mole ratio, exhibit new absorption peaks at 450, 538 and 512 nm respectively. Addition of thiocyanate to the nickel–cyclam complex (1:2:5 mole ratio) led to the formation of a purple complex, exhibiting three distinct new absorption peaks at 330, 455 and 662 nm. A purple complex (1:2:10 mole ratio) separated, having absorption peaks at 352, 503 and 693 nm in CHCl₃. The Co^II–cyclam complex with thiocyanate in the same mole ratio exhibits two absorption peaks at 437 and 519 nm without appearance of any precipitate. The Cu^II–cyclam complex with thiocyanate did not form a mixed ligand complex. Electrochemical studies also confirmed the complex formation of Ni^II–cyclam with the thiocyanate with the appearance of two new oxidation peaks close to 1.25 and 1.60 V versus Ag/AgCl in H₂O and CHCl₃. The Co^II–cyclam complex with thiocyanate exhibited an oxidation peak at 1.2 V versus Ag/AgCl, while no peak was observed for the Cu^II–cyclam complex with thiocyanate. Based on spectroscopic and electrochemical studies the geometry of the complex has been evaluated.     

PTV vapor overflow — principles of a solvent evaporation technique for introducing large volumes in capillary GC

The concept of a GC solvent evaporation technique is outlined that involves a modified Programmed Temperature Vaporizing (PTV) injector. The vapor overflow technique is intended for introducing samples in large volumes of solvent by syringe injection of strongly diluted samples or by coupled LC-GC. The liquid is introduced into a packed vaporizing chamber kept above the solvent boiling point at a pressure which is near or below ambient. The carrier gas is essentially switched off. Evaporation and discharge of the solvent vapors occurs by expansion of the vapors, driven by the solvent vapor pressure. For transferring the sapmple into the column, the carrier gas is switched on again and the vaporizing chamber heated. Compared to PTV solvent split injection, vapor overflow offers the following advantages: It automatically optimizes operational parameters, therefore facilitating its application. Losses of volatile materials are minimized by a minimal flow rate through the injector. Vapor overflow is a promising technique for transferring watercontaining eluents in coupled LC-GC since no wettability is required and leaching of pre-column surfaces is avoided.     

A comparison between high-symmetry Mn12 single-molecule magnets in different ligand/solvent environments

We present angle-resolved high-frequency electron paramagnetic resonance (HFEPR) data collected for single-crystal samples of deuterated and undeuterated Mn₁₂-acetate. The spectra reveal fine structures associated with the various Mn₁₂ species corresponding to the different local solvent environments proposed by Cornia et al. [A. Cornia, R. Sessoli, L. Sorace, D. Gatteschi, A.L. Barra, C. Daiguebonne, Phys. Rev. Lett. 89 (2002) 257201], and recently confirmed by Hill et al. [S. Hill, R.S. Edwards, S.I. Jones, J.M. North, N.S. Dalal, Phys. Rev. Lett. 90 (2003) 217204] and del Barco et al. [E. del Barco, A.D. Kent, E.M. Rumberger, D.N. Hendrickson, G. Christou, Phys. Rev. Lett. 91 (2003) 047203]. Each of the fine structures exhibits a distinct dependence on the applied field orientation, thereby highlighting the discrete nature of the disorder. We then compare these data with spectra obtained for two recently discovered high-symmetry (S₄) analogs of Mn₁₂-Ac, differing only in their ligand and solvent structures. None of the highly reproducible fine-structures seen in the Mn₁₂-Ac samples are found for the new Mn₁₂ complexes, thus confirming the idea that the solvent structure significantly influences the QMT dynamics in Mn₁₂-Ac. Indeed, the HFEPR spectra for Mn₁₂-BrAc and Mn₁₂-tBuAc provide spectacular resolution, allowing for unique spectroscopic insights into high-symmetry giant spin SMMs.     

Characterization of electronic transition energies and trigonal distortion of the (FeO6)9- coordination complex in the Al2O3:Fe3+ system: a simple method for transition-metal ions in a trigonal ligand field

A theoretical method for studying the inter-relationships between electronic and molecular structure has been proposed on the basis of the complete energy matrices of electron-electron repulsion, the ligand field, and the spin-orbit coupling for the d5 configuration ion in a trigonal ligand field. As an application, the local distortion structure and temperature dependence of zero-field splitting for Fe3+ ions in the Al2O3:Fe3+ system have been investigated. Our results indicate that the local lattice structure of the (FeO6)(9-) octahedron in the Al2O3:Fe3+ system has an elongated distortion and the value of distortion is associated with the temperature. The elongated distortion may be attributed to the facts that the Fe3+ ion has an obviously larger ionic radius than the Al3+ ion and the Fe3+ ion will push the two oxygen triangles upward and downward, respectively, along the 3-fold axis. By diagonalizing the complete energy matrices, we found that the theoretical results of electronic transition energies and EPR spectra for Fe3+ ions in the Al2O3:Fe3+ system are in good agreement with the experimental findings. Moreover, to understand the detailed physical and chemical properties of the Al2O3, the theoretical values of the zero-field splitting parameters and the corresponding distortion parameters in the range 50 K 相似文献   

Electron bubbles in helium clusters. II. Probing superfluidity

In this paper we present calculations of electron tunneling times from the ground electronic state of excess electron bubbles in ((4)He)(N) clusters (N=6500-10(7), cluster radius R=41.5-478 A), where the equilibrium bubble radius varies in the range R(b)=13.5-17.0 A. For the bubble center located at a radial distance d from the cluster surface, the tunneling transition probability was expressed as A(0)phi(d,R)exp(-betad), where beta approximately 1 A(-1) is the exponential parameter, A(0) is the preexponential factor for the bubble located at the cluster center, and phi(d,R) is a correction factor which accounts for cluster curvature effects. Electron tunneling dynamics is grossly affected by the distinct mode of motion of the electron bubble in the image potential within the cluster, which is dissipative (i.e., tau(D)tau(0)) in superfluid ((4)He)(N) clusters, where tau(D) is the bubble motional damping time (tau(D) approximately 4 x 10(-12) s for normal fluid clusters and tau(D) approximately 10 s for superfluid clusters), while tau(0) approximately 10(-9)-10(-10) s is the bubble oscillatory time. Exceedingly long tunneling lifetimes, which cannot be experimentally observed, are manifested from bubbles damped to the center of the normal fluid cluster, while for superfluid clusters electron tunneling occurs from bubbles located in the vicinity of the initial distance d near the cluster boundary. Model calculations of the cluster size dependence of the electron tunneling time (for a fixed value of d=38-39 A), with lifetimes increasing in the range of 10(-3)-0.3 s for N=10(4)-10(7), account well for the experimental data [M. Farnik and J. P. Toennies, J. Chem. Phys. 118, 4176 (2003)], manifesting cluster curvature effects on electron tunneling dynamics. The minimal cluster size for the dynamic stability of the bubble was estimated to be N=3800, which represents the threshold cluster size for which the excess electron bubble in ((4)He)(N) (-) clusters is amenable to experimental observation.     

Complexity in the self-assembly of bifunctional molecules on HOPG: the influence of solvent functionality on self-assembled structures

Self-assembled monolayers of bifunctional molecules HOOC(CH2)nCOOH (n = 20, 18, 16, 14, 12, 10), HOOC(CH2)nCH2OH (n = 13, 14), and HOCH2(CH2)14CH2OH dissolved in octanoic acid were investigated using scanning tunneling microscopy, to understand the self-assembly of bifunctional molecules and the influence of a carboxylic acid solvent on the formation of self-assembled structures on HOPG. In the series of di-acids (HOOC(CH2)nCOOH), only HOOC(CH2)20COOH forms stable coadsorption structures with the solvent octanoic acid. The remaining di-acids form stable single-component monolayers and do not coadsorb with solvent octanoic acid. Coadsorption structures involving mixtures of di-acids were observed. This result suggests that coadsorption with acid solvent or with other di-acids occurs to maximize hydrogen-bond density in the overlayer. A quantitative model based on this concept is proposed. For hetero-bifunctional molecules HOOC(CH2)nCH2OH (n = 13, 14), the coadsorption of HOOC(CH2)14CH2OH and octanoic acid at the molecular level produces a microscopic mesh made of homogeneously arranged openings with a dimension of approximately 12.5 A x approximately 5.0 A x approximately 1.8 A. For the hetero-bifunctional molecule HOOC(CH2)13CH2OH, hydroxyl groups of two adjacent lamellae assemble to form a herringbone geometry, and the two carboxylic acid groups assemble with a straight head-to-head configuration. In addition, a new mixed hydrogen-bonding network of COOH...O-H was observed in another self-assembled structure of this molecule. The bifunctional molecule HOCH2(CH2)14CH2OH exhibits multiple packing patterns on HOPG via different hydrogen-bonding networks. HOCH2(CH2)14CH2OH self-assembles using the H-O...O-H network typical of the n-alcohol herringbone structure, forming an asymmetric adsorbate on HOPG. It also forms domains with another hydrogen-bonding network, in which molecules in adjacent lamellae are parallel to each other. This investigation demonstrates the complexity and diversity of self-assembled structures formed from bifunctional molecules on solid surfaces. It also indicates that a solvent with the same functional group as the solute can significantly impact the formation of the self-assembled structures of these bifunctional molecules.     

Evidence for glycosidic bond rotation in a nucleobase peroxyl radical and its effect on tandem lesion formation

Nucleobase peroxyl radicals are the major reactive intermediates formed in DNA when the biopolymer is exposed to gamma-radiolysis under aerobic conditions. The major reaction pathways for the peroxyl radical (1) derived from 5,6-dihydro-2'-deoxyuridin-6-yl involve pi-bond addition to or hydrogen atom abstraction from the adjacent nucleotides to produce tandem lesions. The ability to independently generate 1 at a defined site in DNA enabled us to probe its reactivity by varying the local DNA structure. The effect of DNA structure variation reveals that 1 reacts from its syn- and anti-conformations in competition with trapping by thiol. These experiments also reveal that tandem lesions will be produced as a mixture of diastereomers, which could impact their biological effects.     

Effects of solvent viscosity on ligand interconversion dynamics in the primary docking site of heme proteins

Interconversion dynamics of the ligand in the primary docking site of myoglobin (Mb) and hemoglobin (Hb) in trehalose and glycerol/D2O mixtures at 283 K was investigated by probing time-resolved vibrational spectra of CO photolyzed from these proteins. The interconversion dynamics in viscous media are similar to those in aqueous solution, indicating that it is minimally coupled to the solvent-coupled large-scale protein motion. Interconversion rates in the heme pocket of Hb in water solution are slower than those of Mb in trehalose glass, suggesting that the interconversion barrier in Hb is intrinsically higher than that in Mb and is not modified by the solvent viscosity.     

Study of maleated ethylene-propylene copolymers by fluorescence: Evidence for succinimide induced polar associations in an apolar solvent

A maleated ethylene-propylene copolymer (EP-MAH) was labelled with 1-naphthalene- and/or 1-pyrenemethylamine to yield an EP copolymer bearing succinimide pendants all labelled with a chromophore. The labelled EPs were reacted with LiAlH₄ so that the polar succinimide linker group between the EP backbone and the chromophore was converted into apolar pyrrolidine units. The resulting products were purified through a gel permeation chromatography column to remove the cleaved off chromophores. FT-IR spectroscopy revealed that after reduction, the peak assigned to the succinimide carbonyls was strongly diminished. UV-vis absorption and steady-state and time-resolved fluorescence measurements were performed in hexane and THF. The reduction of the succinimide carbonyls was found to have a significant effect on the luminescence properties of the labelled EPs. The polar associations taking place between the succinimide moieties in hexane were found to be dramatically decreased after reduction as shown by UV-vis absorption, steady-state excitation and emission fluorescence, time-resolved fluorescence, and fluorescence resonance energy transfer. These results demonstrate that the presence of pyrene aggregates for EP-MAH labelled with 1-pyrenemethylamine is due primarily to the polar succinimide moieties rather than the aromatic pyrene.