Stability of (C60)2 and epoxide dimers, (C60)2ON, and their anions

The PM3, AM1, and MINDO3 semiemperical methods are used to calculate the the energy difference between C60ON and C60ON- and the bond dissociation energy necessary to cleave neutral and negatively charged (C60)2 dimers and epoxide dimers, (C60)2ON, to their respective monomers C60, and C60ON/2. The results show that the anions of the dimers are significantly more stable than neutral dimers. This result may explain the higher thermal stability of the observed ferromagnetic phase in photolyzed C60. which has been attributed to epoxide dimers and oligomers. It also provides an explanation for the origin of unpaired electron spin necessary for ferromagnetism.     

Crystal structure and magnetic properties of the coupled spin dimer compound SrCu2(TeO3)2Cl2

Single crystals of the strontium copper tellurium oxochloride SrCu₂(TeO₃)₂Cl₂ were synthesized via solid-gas reactions in sealed evacuated silica tubes. The compound crystallizes in the monoclinic system, space group P2₁, a=7.215(2), b=7.2759(15), c=8.239(2) Å, β=96.56(4)°, Z=2. The building units are [SrO₆Cl₂] irregular polyhedra, [CuO₄] and [CuO₃Cl] square planes, [TeO₃E] tetrahedra and [TeO₃₊₁E] trigonal bipyramids; E being the 5s² lone pair of Te(IV). The Cu atoms can be regarded as forming a chain of weakly connected dimers. The magnetic susceptibility of the compound shows a broad maximum typical for antiferromagnetic spin fluctuations with a non-magnetic ground state. A Heisenberg spin model with coupled s=1/2 dimers leads to a satisfactory fitting of the experimental data.     

X-Ray Investigation of (E)-2-phenyl-1-chlorocyclopropane-1-carboxylic Acid — A Convenient Synthon for Transamine Synthesis

A method for the synthesis of (E,Z)-2-phenyl-1-chlorocyclopropane-1-carboxylic acid by the addition of ethyl trichloroacetate to styrene in the presence of copper(I) ammine complexes is suggested. The major isomer was isolated as the corresponding acid. The results of X-ray diffraction (XRD) analysis unequivocally prove that the benzene ring and the carboxylic group are in the cis-position and that the molecules form centrosymmetric dimers. (E)-2-phenyl-1-chlorocyclopropane-1-carboxylic acid can serve as a convenient synthon for transamine synthesis.     

Periodic trends in metal–metal bonding in edge-shared [M2Cl10] systems

Periodic trends in metal–metal interactions in edge-shared [M₂Cl₁₀]⁴⁻ systems, involving the transition metals from groups 4 through 8 and electronic configurations ranging from d¹d¹ through d⁵d⁵, have been investigated by calculating metal–metal bonding and spin-polarization (exchange) effects using density functional theory. The trends found in this study are compared with those for the analogous face-shared [M₂Cl₉]³⁻ systems reported in earlier work. Strong linear correlations between the metal–metal bonding and spin-polarization terms have been obtained for all groups considered. In general, spin polarization and electron localization are predominant in 3d–3d species whereas electron delocalization and metal–metal bonding are favoured in 5d–5d species, with more variable results observed for 4d–4d systems. As previously found for face-shared [M₂Cl₉]³⁻ systems, the strong correlations between the metal–metal bonding and spin polarization energy terms can be related to the fact that both properties appear to be similarly affected by the changes in the metal orbital properties and electron density occurring within the dⁿdⁿ groups. A significant difference between the face-shared and edge-shared systems is that while the 4d metals in the former show a strong tendency for delocalized metal–metal bonded structures, the edge-shared counterparts display much greater variation with both metal–metal bonded and weakly coupled complexes observed. The tendency for weaker metal–metal interactions can be traced to the inability of the edge-shared bridging structure to accommodate the smaller metal–metal distances required for strong metal–metal bonding.     

Synthesis of phthalocyanine dimers,trimers, and oligomers bridged via phenyl groups

Several phthalocyanine dimers, trimers, and oligomers bridged via aryl (phenyl) groups were prepared using the Suzuki–Miyaura cross coupling reaction of phthalocyanine-boronate ester and various halide derivatives under palladium catalyst reaction conditions. Photophysical data reveal energy transfer between the Pc moieties resulting in the appearance of new red-shifted Q-bands. The shift and the nature of Q-band depend on the number of phenyl groups, the number of Pc, and the position of attachment on the phenyl ring.     

STUDIES ON THE FRAGMENTATION OF DIALKYL DISULFIDES UPON ELECTRON IMPACT

Abstract

Recent studies on the fragmentation of some organic disulfides upon electron impact in a mass spectrometer have shown two main decomposition routes besides simple bond disconnection: skeletal rearrangements^1–3 and proton transfer⁴. Transposition of the molecular components takes place via loss of one or two sulfur atoms while the carbon skeleton is preserved. Such molecular rearrangements are particularly noticeable in small molecules like dimethyl disulfide. Larger systems show a different fragmentation pattern in which proton transfer to give alkyl hydro-disulfides becomes predominant. The transfer of protons has been shown to proceed by way of intramolecular 1,2 and 1,3 hydrogen shifts from α and β carbons respectively by means of selective deuterium labbeling of diethyl disulfide.⁴     

Liquid crystal dimers and the twist-bend nematic phase. The preparation and characterisation of the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates

Eleven members of the homologous series of liquid crystal dimers, the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates, have been synthesised and their transitional properties characterised. These dimers consist of two cyanobiphenyl units connected by an alkyl spacer attached via ester linkages. All eleven members exhibit exclusively nematic behaviour. The nematic–isotropic transition temperatures, T_NI, and associated entropy changes, ?S_NI/R, exhibit pronounced alternations as the length and parity of the spacer is varied; this is characteristic behaviour of liquid crystal dimers. The transitional properties of the ester-linked dimers are compared with the corresponding materials having either ether, methylene or carbonate linkages between the spacer and mesogenic units. For short spacer lengths and both odd- and even-membered dimers, the ester-linked materials show the highest values of T_NI and the methylene-linked the lowest. For longer spacer lengths, T_NI of the carbonate-linked dimers fall between those of the corresponding ester- and ether-linked dimers. The ether-linked materials show the largest alternation in ?S_NI/R on varying spacer length and the carbonate-linked dimers the lowest. This behaviour is interpreted in terms of the molecular geometry and it is suggested that the ether- and ester-linked odd-membered dimers have rather similar shapes. A phase diagram constructed using binary mixtures of the pentyl member of this ester-linked series and the known twist-bend nematogen, 1,7-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB), is presented. The twist-bend nematic–nematic transition temperature of the mixtures shows a striking convex curvature as the concentration of CB7CB is decreased, and so it is not possible to estimate a virtual twist-bend nematic–nematic transition temperature for the ester-linked material.     

Direct FTIR high resolution probe of small and medium size Arn(CO2)m van der Waals complexes formed in a slit supersonic expansion

A slit nozzle continuous expansion of argon seeded with carbon dioxide is probed using a Bruker IFS 120 HR FTIR spectrometer operating at 0.05 cm⁻¹ or 0.01 cm⁻¹ spectral resolution. About 250 individual rovibrational transitions are assigned which belong to the asymmetric stretch of the CO₂ moiety in Ar-CO₂ and (CO₂)₂ complexes. This made it possible to refine the set of spectroscopic constants characterizing these van der Waals species. Analysis of the observed spectral features allowed for evaluation of the number densities of complexes formed in a jet. The manifold of rovibrational lines of van der Waals complexes (along with the monomer lines) sits on an unresolved pedestal, the shape of which varies dramatically as a function of reservoir pressure and initial CO₂-Ar mixing ratios. Thorough examination of these variations allows for the observed features to be assigned to Ar_n(CO₂)_m clusters formed in CO₂ seeded Ar expansion.     

COPPER(II) COORDINATION COMPOUNDS: CLASSIFICATION AND ANALYSIS OF CRYSTALLOGRAPHIC AND STRUCTURAL DATA III. DIMERIC COMPOUNDS

Abstract

This review summarizes the data for over nine hundred dimeric Cu(II) coordination compounds. There are several types of the bridges, from which doubly bridges by far prevail. The most common ligands are O? and N? donors. From the stereochemical point of view, a square-pyramidal arrangement with different degrees of distortion about Cu(II), is the most common. Several relationships were found between the Cu-Cu distances and the Cu-L-Cu bridge angle and the type of bridging, between the intra-ligand L-Cu-L ring angles and coordination numbers.     

Two New Citrinin Dimers from a Volcano Ash‐Derived Fungus,Penicillium citrinum HGY1‐5

Two new citrinin dimers, penidicitrinin A ((2R,3S,5aS,9R,10S,12aR,12bR)‐2,3,5a,6,9,10,12a,12b‐octahydro‐7,12a‐dihydroxy‐12b‐methoxy‐2,3,4,9,10,11‐hexamethyl‐5H‐difuro[2,3‐b : 2′,3′‐h]xanthen‐5‐one; 1 ) and penidicitrinin B ((1S,3R,4S)‐1‐{2,6‐dihydroxy‐4‐[(1S,2R)‐2‐hydroxy‐1‐methylpropyl]‐3‐methylphenyl}‐3,4‐dihydro‐3,4,5‐trimethyl‐1H‐2‐benzopyran‐6,8‐diol; 2 ), together with three known citrinin monomers were isolated from a volcano ash‐derived fungus, Penicillium citrinum HGY1‐5. Their structures were established by spectroscopic methods, and they showed no cytotoxicity against two tumor cell lines.