Synthesis and stereochemical resolution of functional [5]pericyclynes

Three different kinds of ring carbo-mers of [5]cyclitol ethers were targeted as challenging examples of functional [5]pericyclynes. Three tertiary pentaaryl-carbo-[5]cyclitol methyl ethers were synthesized through a [11+4] ring-closing double addition of triphenyl- and tri-p-anisyl-undecatetrayn-diides to dibenzoylacetylene. These compounds, obtained as oily mixtures of stereoisomers, are stable and can behave as acetylenic ligands of one or two Co₂(CO)₆ units. NMR analysis reveals that the broad diasteroisomeric dispersity of a triether, is consistently reduced in the symmetrized pentaether. Three bis-secondary triaryl-carbo-[5]cyclitol methyl ethers with adjacent CH(OR) vertices were synthesized through a similar [11+4] ring-closing process, where the same tetrayn-diides add to both the carbaldehyde ends of the (η²-OCH-CC-CHO)Co₂(CO)₆ complex. Despite the possibility of tautomeric isomerization, the occurrence of two adjacent bis-propargylic carbinol vertices does not diminish the stability of the [5]pericyclyne framework. Finally, two bis-secondary carbo-[5]cyclitol methyl ethers with non-adjacent CH(OH) vertices were synthesized through an alternative [10+5] ring-closing process. The bis-secondary carbo-[5]cyclitols are regarded as isohypsic equivalents of the challenging [C,C]₅carbo-cyclopentadienyl cation. A diphenyl-hexaoxy-[5]pericyclyne with two non-adjacent secondary carbinol vertices was also prepared through a [10+5] ring-closing strategy: this molecule is an isohypsic equivalent of the previously calculated zwitterionic carbo-cyclopentadienone, which could be observed as a DCI/NH₃-MS fragment after treatment with SnCl₂/HCl. Analytical HPLC showed that the C₁₁ triphenyl-undecatetrayne precursor of the [11+4] strategy was obtained as a statistical 1:2:1 mixture of the three possible diastereoisomers. Semi-preparative HPLC allowed for the resolution of this mixture. The pure major diastereoisomer was employed to prepare a partly resolved sample of pentamethoxy-pentaphenyl-[5]pericyclyne. Analytical HPLC showed that the latter corresponds to the statistical distribution of the expected three residual diastereoisomers. Semi-preparative HPLC finally afforded samples of diastereoisomerically pure pentamethoxy-[5]pericyclyne as crystalline solids.     

Isotope shift and hyperfine structure measurements in titanium I

High accuracy measurements of hyperfine structure due to⁴⁷Ti and⁴⁹Ti in the 3d ² 4s ² a ³ F ₂?3d ² 4s4p z ⁵ D ₁ absorption line at σ=18482.772 cm^?1 have been performed by use of a Doppler-free experiment, where a beam of titanium atoms is crossed by a CW single mode tunable dye laser. They have allowed for the determination of isotope shifts between⁴⁶Ti,⁴⁷Ti,⁴⁸Ti,⁴⁹Ti and⁵⁰Ti. By use of accurate values of mean square nuclear charge radii for the even isotopes, it has been possible to separate mass shifts from field shifts and to determine accurate values for the mean square nuclear charge radii of⁴⁷Ti and⁴⁹Ti. The field shift presents a marked odd-even staggering.     

Intrinsic and extrinsic factors in anion electron-stimulated desorption: D- from deuterated hydrocarbons condensed on Kr and water ice films

The results of D(-) ion desorption induced by 3-20 eV electrons incident on condensed CD(4), C(2)D(6), C(3)D(8), C(2)D(4), and C(2)D(2) are presented. These compounds were deposited in submonolayer amounts on the surfaces of multilayer solid films of Kr and nonporous and porous amorphous ice. While desorption of the D(-) anions proceeds via well-known processes, i.e., dissociative electron attachment (DEA) and dipolar dissociation, significant perturbations of these processes due to presence of the different film substrates are observed. We have shown that it is possible to distinguish between the character and nature of these perturbations. The presence of the nonporous ice perturbs the D(-) desorption intensity by affecting the intrinsic properties of the intermediate anion states through which dissociation proceeds. On the other hand, the presence of the porous ice introduces extrinsic effects, which can affect electron energy losses prior to their interaction with the hydrocarbon molecule and/or the energies and intensities of the fragment species after dissociation. Simple mechanisms responsible for the observed variations in the intensities of desorbed anionic signals are proposed and discussed. Electron transfer from transient anion states to electron states of the substrate film or nearby hydrocarbon molecules appear as the most efficient mechanism to reduce the magnitude of the DEA process.     

Enantioseparations by capillary electro-chromatography: differences exhibited by normal- and reversed-phase versions of polysaccharide stationary phases

The influence of using normal-phase and reversed-phase versions of four commercial polysaccharide stationary phases on chiral separations was investigated with capillary electrochromatography (CEC). Both versions of the stationary phases, Chiralcel OD, OJ, and Chiralpak AD, AS were tested for the separation of two basic, two acidic, a bifunctional, and a neutral compound. Different background electrolytes were used, two at low pH for the acid, bifunctional and neutral substances, and three at high pH for the basic, bifunctional and neutral ones. This setup allowed evaluating differences between both stationary-phase versions and between mobile-phase compositions on a chiral separation. Duplicate CEC columns of each stationary phase were in-house prepared and tested, giving information about the intercolumn reproducibility. In general, reversed-phase versions of the current commercial polysaccharide stationary phases are found to be best for reversed-phase CEC, even though at high pH no significant differences were seen between both versions. Most differences were observed at low pH. For acidic compounds, it was seen that an ammonium formate electrolyte performed best, which is also an excellent electrolyte if coupling with mass spectrometry is desired. For basic, bifunctional and neutral compounds, no significant differences between the three tested electrolytes were observed at high pH. Here, a phosphate buffer is preferred as electrolyte because of its buffering capacities. However, if coupling to mass spectrometry is wanted, the more volatile ammonium bicarbonate electrolyte can be used as an alternative.     

Fuzzy identification of cutting acoustic emission with extended subtractive cluster analysis

This paper presents fuzzy acoustic emission identification in high precision hard turning process based on extended subtractive cluster analysis combined with the least-square estimation method. The fuzzy identification method provides a simple way to arrive at a definite conclusion based upon the information obtained with the difficulty in understanding the exact physics of the machining process. The experimental results prove that the proposed method is efficient and feasible.     

Perfect state transfer on weighted graphs of the Johnson scheme

Letters in Mathematical Physics - We characterize quantum perfect state transfer on real-weighted graphs of the Johnson scheme $${\mathcal {J}}(n,k)$$ , which represent spin networks with...     

Versatile post-functionalisation strategy for the formation of modular organic–inorganic polyoxometalate hybrids

Hybrid structures incorporating different organic and inorganic constituents are emerging as a very promising class of materials since they synergistically combine the complementary and diverse properties of the individual components. Hybrid materials based on polyoxometalate clusters (POMs) are particularly interesting due to their versatile catalytic, redox, electronic, and magnetic properties, yet the controlled incorporation of different clusters into a hybrid structure is challenging and has been scarcely reported. Herein we propose a novel and general strategy for combining multiple types of metal-oxo clusters in a single hybrid molecule. Two novel hybrid POM structures (HPOMs) bis-functionalised with dipentaerythritol (R–POM₁–R; R = (OCH₂)₃CCH₂OCH₂C(CH₂OH)) were synthesised as building-blocks for the formation of heterometallic hybrid triads (POM₂–R–POM₁–R–POM₂). Such a modular approach resulted in the formation of four novel heterometallic hybrids combing the Lindqvist {V₆}, Anderson–Evans {XMo₆} (X = Cr or Al) and trisubstituted Wells–Dawson {P₂V₃W₁₅} POM structures. Their formation was confirmed by multinuclear Nuclear Magnetic Resonance (NMR), infrared (IR) and UV-Vis spectroscopy, as well as Mass Spectrometry, Diffusion Ordered Spectroscopy (DOSY) and elemental analysis. The thermal stability of the hybrids was also examined by Thermogravimetric Analysis (TGA), which showed that the HPOM triads exhibit higher thermal stability than comparable hybrid structures containing only one type of POM. The one-pot synthesis of these novel compounds was achieved in high yields in aqueous and organic media under simple reflux conditions, without the need of any additives, and could be translated to create other hybrid materials based on a variety of metal-oxo cluster building-blocks.

A versatile modular approach has been developed for incorporating different metal-oxo nanoclusters with characteristic structures into a single hybrid molecule by covalently linking them with polyol ligands.