Revisiting the S-Au(111) interaction: static or dynamic?

The Au-S interaction is probably the most intensively studied interaction of Au surfaces with nonmetals, as, for example, it plays an important role in Au ore formation(1) and controls the structure and dynamics of thiol-based self-assembled monolayers (SAMs). Various S-induced surface structures on Au(111) were recently reported for different conditions and predominantly interpreted in terms of a static Au surface. Here, we demonstrate that the Au(111) surface exhibits a very dynamic character upon interaction with adsorbed sulfur: large-scale surface restructuring and incorporation of Au atoms into a growing 2D AuS phase were observed in situ. These results provide new insight into the Au-S surface chemistry.     

Probing the aromaticity of the [(H(t)Ac)3(μ2-H)6], [(H(t)Th)3(μ2-H)6],(+), and [(H(t)Pa)3(μ2-H)6] clusters

In this study we report about the aromaticity of the prototypical [(H(t)Ac)(3)(μ(2)-H)(6)], [(H(t)Th)(3)(μ(2)-H)(6)](+), and [(H(t)Pa)(3)(μ(2)-H)(6)] clusters via two magnetic criteria: nucleus-independent chemical shifts (NICS) and the magnetically induced current density. All-electron density functional theory calculations were carried out using the two-component zeroth-order regular approach and the four-component Dirac-Coulomb Hamiltonian, including scalar and spin-orbit relativistic effects. Four-component current density maps and the integration of induced ring-current susceptibilities clearly show that the clusters [(H(t)Ac)(3)(μ(2)-H)(6)] and [(H(t)Th)(3)(μ(2)-H)(6)](+) are non-aromatic whereas [(H(t)Pa)(3)(μ(2)-H)(6)] is anti-aromatic. However, for the thorium cluster we find a discrepancy between the current density plots and the classification through the NICS index. Our results also demonstrate the increasing influence of f orbitals, on bonding and magnetic properties, with increasing atomic number in these clusters. We think that the enhanced electron mobility in [(H(t)Pa)(3)(μ(2)-H)(6)] is due the significant 5f character of its valence shell. Also the participation of f orbitals in bonding is the reason why the protactinium cluster has the shortest bond lengths of the three clusters. This study provides another example showing that the magnetically induced current density approach can give more reliable results than the NICS index.     

Die Stabilit?t von Ammoniumpurpurat (Murexid)

Ohne Zusammenfassung     

Static and dynamic stereochemistry of 1,1′-bi(9,10-dihydro-9,10-ethano-anthryl)

Summary The title compound2 was prepared either by highpressure reaction of 1,1-bianthryl with ethylene or by coupling of 1-bromo-9,10-dihydro-9,10-ethanoanthracene (4). Both syntheses afforded a mixture of diastereoisomers (meso2a and racemate2b) in a ratio of 1.5:1 and 2.3:1, respectively. Configurational assignment was possible both from the¹H- and¹³C-NMR spectra and by coupling of laevorotatory4 (accessibly by enantioselective chromatography on triacetyl cellulose in ethanol) to laevorotatory2b. (+)-4 was tranformed into the dextrorotatory carboxylic acid (+)-5 of known configuration (9R) thus establishing the configuration of (+)-4 as (9R) too and hence the centrochirality in (–)-2b as (9S)(9S). The racemic form2b is a conformational (appr. 1.8:1) mixture of two rotamers.The rotational barrier was established as G ^#=92–95 kJ mol^–1 (depending on the temperature) both by¹H-NMR and CD kinetics (based on equilibration of the separated optically active rotamers ofracem.2). For the latter preferred conformations were assumed allowing the assignment of the axial chirality: e.g. (–)-(9S)(R)_a(9S) for the main rotamer of (–)-2 b [and (–)(9S)(S)_a(9S) for the underpopulated one].All assumptions were confirmed by X-ray crystal structure analyses of2 a and the main rotamer of2b with torsional angles around the 1,1-bonds of –111.1 and –121.2°, respectively.Dedicated to Prof. K. L. Komarek (Vienna) with cordial wishes on the occasion of the 65th anniversary of his birthday     

Measurement of Raman χ(3) and theoretical estimation of DOVE four wave mixing of hydrogen peroxide

Hydrogen peroxide has strong infrared (IR) transitions ν(6) and its combination band ν(2)+ν(6), which may provide a unique opportunity to implement doubly vibrationally enhanced (DOVE) four wave mixing (FWM) for directly measuring hydrogen peroxide in spectrally overcrowded mixtures. In this work, the magnitude of the DOVE third-order susceptibility χ(3) was theoretically estimated. By using a FWM interferometric method, one of the strongest Raman bands, O-O stretch ν(3) Raman χ(3) of 30 wt % H(2)O(2), was first measured to be 1.2 × 10(-14) esu. The Raman χ(3) of ν(2) was then determined to be 5.3 × 10(-15) esu based on their relative Raman intensities. The resulting Raman χ(3) of ν(2) was used to calculate the DOVE χ(3) of (ν(6), ν(2)+ν(6)), together with the dipolar moments of the two IR transitions determined from IR absorption measurement. The calculated value of DOVE-IR χ(3) was 1.1 × 10(-13) esu for pure H(2)O(2), about 1.5 times larger than that of the strong ring breathing Raman band of benzene. The large DOVE χ(3) suggests the feasibility of direct measurement of hydrogen peroxide in an aqueous environment using DOVE four wave mixing.     

Conformationally dynamic π-conjugation: probing structure-property relationships of fluorescent tris(N-salicylideneaniline)s

We recently reported the design and synthesis of a series of conformationally dynamic chromophores that are built on the C(3)-symmetric tris(N-salicylideneaniline) platform. This system utilizes cooperative structural folding-unfolding motions for fluorescence switching, which is driven by the assembly and disassembly of hydrogen bonds between the rigid core and rotatable peripheral part of the molecule. Here, we report detailed time-resolved spectroscopic studies to investigate the structure-property relationships of a series of functionalized tris(N-salicylideneaniline)s. Time-resolved fluorescence decay spectroscopy was applied to determine the main relaxation mechanisms of these π-extended fluorophores, and to address the effects of hydrogen bonding, steric constraints, and extension of the π-conjugation on their relaxation dynamics. Our results agree well with the conformational switching model that was previously suggested from steady-state experiments. Notably, extension of the π-conjugation from peripheral aryl groups resulted in the stabilization of the excited states, as evidenced by longer lifetimes and lower nonradiative decay constants. As a consequence, an increase in the fluorescence quantum yields was observed, which could be explained by the suppression of the torsional motions about the C-N bonds from an overall increase in the quinoid character of the excited states. A combination of time-resolved and steady-state techniques also revealed intermolecular interactions through π-π stacking at higher concentrations, which provide additional de-excitation pathways that become more pronounced in solid samples.     

Properties of χ-alumina as related to catalysis

-Alumina was synthesized by thermal decomposition of aluminium(III) isopropoxide, and its properties were investigated. The change in its surface area with increasing calcination temperature resembled that of -alumina. Both aluminas had the same acid strength, and their activities in isomerization of 1-butene were the same. -Alumina as a support for molybdenum catalyst in the epoxidation of allyl alcohol also behavedin the same manner as -alumina. Thus -alumina has almost the same catalytic properties as -alumina.     

Molecular precursors for the preparation of homogenous zirconia-silica materials by hydrolytic sol-gel process in organic media. Crystal structures of [Zr{OSi(O(t)Bu)3}4(H2O)2]·2H2O and [Ti(O(t)Bu){OSi(O(t)Bu)3}3

[Zr(OPr(i))(4)·Pr(i)OH] reacts with [HOSi(O(t)Bu)(3)] in anhydrous benzene in 1:1 and 1:2 molar ratios to afford alkoxy zirconosiloxane precursors of the types [Zr(OPr(i))(3){OSi(O(t)Bu)(3)}] (A) and [Zr(OPr(i))(2){OSi(O(t)Bu)(3)}(2)] (B), respectively. Further reactions of A or B with glycols in 1:1 molar ratio afforded six chemically modified precursors of the types [Zr(OPr(i))(OGO){OSi(O(t)Bu)(3)}] (1A-3A) and [Zr(OGO){OSi(O(t)Bu)(3)}(2)] (1B-3B), respectively [where G = (-CH(2)-)(2) (1A, 1B); (-CH(2)-)(3) (2A, 2B) and (-CH(2)CH(2)CH(CH(3)-)} (3A, 3B)]. The precursors A and B are viscous liquids, which solidify on ageing whereas the other products are all solids, soluble in common organic solvents. These were characterized by elemental analyses, molecular weight measurements, FAB mass, FTIR, (1)H, (13)C and (29)Si-NMR studies. Cryoscopic molecular weight measurements of all the products, as well as the FAB mass studies of 3A and 3B, indicate their monomeric nature. However, FAB mass spectrum of the solidified B suggests that it exists in dimeric form. Single crystal structure analysis of [Zr{OSi(O(t)Bu)(3)}(4)(H(2)O)(2)]·2H(2)O (3b) (R(fac) = 11.9%) as well as that of corresponding better quality crystals of [Ti(O(t)Bu){OSi(O(t)Bu)(3)}(3)] (4) (R(fac) = 5.97%) indicate the presence of a M-O-Si bond. TG analyses of 3A, B, and 3B indicate the formation of zirconia-silica materials of the type ZrO(2)·SiO(2) from 3A and ZrO(2)·2SiO(2) from B or 3B at low decomposition temperatures (≤200 °C). The desired homogenous nano-sized zirconia-silica materials [ZrO(2)·nSiO(2)] have been obtained easily from the precursors A and B as well as from the glycol modified precursors 3A and 3B by hydrolytic sol-gel process in organic media without using any acid or base catalyst, and these were characterized by powder XRD patterns, SEM images, EDX analyses and IR spectroscopy.     

与半衰期(t1/2)有关问题的探讨

从半衰期通式推导出t_(3/4),7_(7/8)……的通式及其共同表达式,并说明其物理意义: 各级反应半衰期(t_(1/2))的通式: t_(1/2)=k′C_0~(1-n) [1]我们可以根据此式,推导出t_(3/4),t_(4/8)……在各级反应中的通式。如对t_(3/4),可认为是初始浓度为C_0时的半衰期和初始浓度1/2C_0时的半衰期之和,可写为下式:     

Die Bildung und Stabilit?t von Tellur(VI)-Verbindungen

Ohne Zusammenfassung     

Die spektrophotometrische Bestimmung der Aktivit?t von Bakterienhyaluronidase (Hd)

Ohne Zusammenfassung     

Structures and dynamic solution behavior of cationic, two-coordinate gold(I)-π-allene complexes

A family of seven cationic gold complexes that contain both an alkyl substituted π-allene ligand and an electron-rich, sterically hindered supporting ligand was isolated in >90% yield and characterized by spectroscopy and, in three cases, by X-ray crystallography. Solution-phase and solid-state analysis of these complexes established preferential binding of gold to the less substituted C=C bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl C=C bond. Kinetic analysis of intermolecular allene exchange established two-term rate laws of the form rate=k(1)[complex]+k(2)[complex][allene] consistent with allene-independent and allene-dependent exchange pathways with energy barriers of ΔG(≠)(1)=17.4-18.8 and ΔG(≠)(2)=15.2-17.6 kcal mol(-1), respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (ΔG(≠)=8.9-11.4 kcal mol(-1)) intramolecular exchange of the allene π faces through η(1)-allene transition states and/or intermediates that retain a staggered arrangement of the allene substituents. VT NMR/spin saturation transfer analysis of [{P(tBu)(2)o-binaphthyl}Au(η(2)-4,5-nonadiene)](+)SbF(6)(-) (5), which contains elements of chirality in both the phosphine and allene ligands, revealed no epimerization of the allene ligand below the threshold for intermolecular allene exchange (ΔG(≠)(298K)=17.4 kcal mol(-1)), which ruled out the participation of a η(1)-allylic cation species in the low-energy π-face exchange process for this complex.     

Die Stabilit?t von Chelaten von 2(o-Oxyphenyl)-benzimidazol

Ohne Zusammenfassung     

Nichtklassische organolanthanoid(II)komplexe: Darstellung und reaktivität der ersten organoneodym(II)verbindung

NdCl₂(THF)₂ reacts with KCp to give [K(THF)_n]₂[Cp₂NdCl₂] (1) which represents the first divalent organoneodymium complex. Treatment of 1 with ^tBuCl, [Me₂NC(S)S]₂ or MesSeSeMes affords the organoneodymium(III) complexes Cp₂NdCl(THF) (2), Cp₂Nd(S₂CNMe₂) (3) and Cp₂NdSeMes(THF) (4) (Cp = η⁵- pentamethylcyclopentadienyl).     

Unsymmetrically substituted disilyne Dsi(2)(i)PrSi-Si≡Si-SiNpDsi(2) (Np = CH(2)(t)Bu): synthesis and characterization

The unsymmetrically substituted disilyne, Dsi(2)(i)PrSi-Si≡Si-SiNpDsi(2) (Np = CH(2)(t)Bu) 2, was synthesized and characterized by X-ray crystallography to show a trans-bent structure with a silicon-silicon triple bond length of 2.0569(12) ?. The (29)Si chemical shifts of the triply bonded silicon atoms of 2 are quite different, being observed at 62.6 ppm for the Dsi(2)(i)PrSi side and 106.3 ppm for the Dsi(2)NpSi side, indicating different hybridizations on the triply bonded silicon atoms at each site.     

Synthese und Reactivität von (2,6-iPr2Ph-dad)Ni(C2F4)

Upon warming the reaction mixture of Ni(cdt), C₂F₄, and 2,6-ⁱPr₂Ph-dad in THF from −78°C to room temperature the red-violet complex (2,6-ⁱPr₂Ph-dad)Ni(C₂F₄) (1) is obtained. 1 reacts with ethene already at −78°C by coupling of the olefinic ligands with the nickel atom to form the blue nickelatetrafluoro-cyclopentane compound (2,6-ⁱPr₂Ph-dad)Ni(C₂H₄C₂F₄) (2).     

A new perspective on transition states: χ1 separatrix

We present a new definition of the transition state for chemical reactions, named the χ(1) separatrix. In contrast to previous transition state definitions which depend on the choice of reaction coordinates, the χ(1) separatrix is defined by choosing a time scale for observation and is connected to exact rate constants in the high friction limit. We demonstrate that this separatrix appears in the isomerization of alanine dipeptide as a stationary population in quasi-equilibrium, without assuming a particular coordinate system or reactant and product surfaces.