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This paper describes the rheological effects observed after addition of (natural) polyols to several different types of heavy metal-free PVC formulations. It is found that addition of natural polyols, such as sorbitol, leads to changes in the rheology of the system comparable to the addition of external lubricants. Hence as suggested previously, addition of (natural) polyols may lead to the occurrence of undesirable plate-out phenomena. The magnitude of the effect on the rheology depends on the number of hydroxyl groups in the polyol and its propensity to undergo intramolecular cyclodehydration reactions. It is established that the undesirable rheological effects, which coincide with plate-out phenomena, can be suppressed by the addition of various types of inorganic as well as organic substances, most of which are known PVC additives. This will allow for the use of (natural) polyols as efficient and benign co-stabilisers in next generation stabiliser systems.

Calculation and visualisation of induced current density are important aids to the study of both aromaticity and observable molecular magnetic response properties. The ipsocentric method offers an accurate and economical approach to calculation of induced current density, and a physical interpretation in terms of occupied-orbital contributions. In monocyclic systems, these contributions allow rationalisation of the existence, sense, and strength of ring current using simple symmetry and node-counting arguments. Here we show maps computed with the model pseudo-pi version of the ipsocentric theory for large polycyclic aromatic hydrocarbons (PAH) of the Clar type. Maps are presented for D(6h)-symmetric systems with up to 438 carbon centres; a full ab initio calculation for an isomer of C114H30 confirms that the success of the pseudo-pi method for small PAH carries over to these larger systems. The computed maps follow a generic overall pattern that can be understood with a natural extension of the orbital arguments to a 'band theory' of totally resonant PAH. All show pi current densities that combine a Clar-sextet structure of localised benzenoid diatropic ring currents (contributed by just four HOMO electrons) with a global perimeter diatropic ring current contributed by the remaining pi electrons. Both currents are explained in the ipsocentric model: the localised currents arise from excitations from the HOMO; the perimeter current from excitations from the rest of the HOMO-band.

Valence bond wavefunctions are naturally geared to the chemist's idea of chemical bonding. In a structure one may distinguish different electron pair bonds and possible radical character. A structure may correspond to a covalent bond, where all electrons are equally divided over the atoms, or may describe an excess charge on a discrete part of the molecule, which indicates ionic bond character. From the weights of the structure in a variationally optimised multi-structure valence bond wavefunction one may derive the importance of the different bonding types. The individual structures could be considered to represent the different physical situations. We explore this concept for simple diatomic molecules and for polyatomics, and we discuss the relation to Lewis structures. We show that the assumption of individual properties for the individual structures leads to inconsistencies.

A procedure to obtain hollow colloidal particles has been developed using an emulsion templating technique. Monodisperse silicone oil droplets were prepared by hydrolysis and polymerization of dimethyldiethoxysilane monomer and incorporated in a solid shell using tetraethoxysilane. Hollow shells were obtained by exchange of the core. The formation of the oil droplets was investigated using static light scattering and 29Si solution NMR, and the hollow shells were characterized by electron microscopy and static light scattering. Details on the composition of the shell material were obtained from energy-dispersive X-ray analysis and 29Si solid state NMR, revealing that the shells consist of a hybrid cross-linked network of silica and siloxane units. Confocal microscopy was used to show that the shells are permeable to small dye molecules. The thickness of the coating can be easily varied from a few nanometers upward. Depending on the ratio of shell thickness to particle radius, three types of hollow shells can be distinguished depending on the way in which they buckle upon drying. We designate them as microspheres, microcapsules, and microballoons. As a result of their monodispersity, these particles can be used for making 3D-ordered materials.

Ring-current aromaticity of icosahedral C120 archimedene is probed at the ipsocentric/6-31G* level by direct mapping of the current density induced in its 4-, 6- and 10-sided faces by perpendicular magnetic fields. In contrast to planar phenylene analogues, the 4-faces (and only those faces) support global π ring currents: intense, paratropic (antiaromatic), stronger on the cage interior. The results validate a simple pseudo-π hydrogen-cluster model for curved carbon networks. Its application to [60]- and [70]-fullerenes shows dominant paratropic ring currents in pentagons militating against attribution of global aromaticity. Application to octahedral derivatives C60X12, shows eight ‘aromatised’ hexagonal rings. To cite this article: P.W. Fowler et al. C.R. Chimie 8(2005).

The propensity of (natural) polyols dispersed in heavy metal- and zinc-free PVC sheets to improve the initial colour, e.g. the colour during the first minutes of exposure to high processing temperatures (short-term stability), has been investigated. It is shown using W(Lab) values that the initial colour improves upon addition of polyols containing primary hydroxyl groups. The polyols act as HCl scavengers, presumably via an acid-catalysed SN2 substitution of the primary hydroxyl groups by chloride ions. In contrast, polyols with only secondary or tertiary hydroxyl groups accelerate the thermal degradation of PVC. Notwithstanding, the efficacy of the (natural) polyols containing primary hydroxyl groups will be reduced if the polyol is susceptible to competitive acid-catalysed intramolecular cyclodehydration reactions under the processing conditions. This is substantiated by a comparison of the behaviour of mannitol and 3,4-di-O-methyl-d-mannitol. The methylated derivative, which is less prone to undergo intramolecular cyclodehydration, improves the initial colour of heavy metal- and zinc-free PVC sheets more significantly than mannitol itself.

The long-term heat stabilisation efficiency of (natural) polyol additives in heavy metal- and zinc-free poly(vinyl chloride) (PVC) has been investigated. It is shown that polyols, such as sorbitol and xylitol, markedly reduce the dehydrochlorination rate and improve Congo Red values. Extraction experiments on unprocessed and ground-processed PVC-sorbitol (1.0 phr) mixtures after thermal degradation at 200 °C revealed that especially in the ground-processed PVC-sorbitol system, sorbitol is partly converted into its mono- and dianhydro-derivatives 1,4-sorbitan and isosorbide, respectively. Apparently, the HCl released during thermal degradation acts as the catalyst. Similar intramolecular cyclodehydration reactions also occur with the natural polyols, erythritol and xylitol, under these conditions. Scrutiny of the measured dehydrochlorination rates and the Congo Red values for ground-processed heavy metal- and zinc-free PVC–polyol mixtures show that in particular polyols containing primary hydroxyl groups exert long-term heat stabilisation and that they act as efficient HCl scavengers.

Direct computation of the π-current density, that is, the 'ring current', of anthranil (1) and its isomers 1,2-benzisoxazole (2) and benzoxazole (3) reveals different patterns of current flow: isomers 2 and 3 sustain strong benzene-like currents in the six-membered and bifurcated flow in the five-membered ring, whereas, in keeping with its lower thermodynamic stability, 1 has only a perimeter circulation without separate monocycle currents. Although the ring current criterion does not offer a sharp distinction between 2 and 3, their difference in thermodynamic stability is identical to that between isoxazole (4) and oxazole (5) suggesting an aromaticity order 1 < 2 ≈ 3. © 2005 Elsevier Ltd. All rights reserved.

Quenching/survival of ring currents in pi-clamped conjugated monocycles is controlled by the match or mismatch in parity between the frontier orbitals of the central pi-conjugated 4n+2/4n monocycle and those of the clamps. Changes in ring current are not primarily caused by bond alternation or 'Mills-Nixon' effects; current and geometry changes on clamping are both consequences of electronic structure.

Reported computational results for large [4n + 2]-annulenes indicate a falling off of aromaticity in D3h geometries but its retention in D6h geometries, as diagnosed on both energetic and magnetic criteria. Ipsocentric pseudo-π mapping of the current density induced by a perpendicular external magnetic field shows that ring currents follow this trend. Diatropic ring currents are quenched in D3h geometries but survive in D6h geometries of [4n + 2]-annulenes (4n + 2 = 30, 42, 54, 66). Ipsocentric orbital contributions explain this distinction in terms of the translational/diatropic, rotational/paratropic selection rules for current in monocycles, coupled with an account of differential angular-momentum mixing in D3h and D6h symmetries. The orbital model rationalises the differences between D6h and D3h geometries, accounts for the decay of aromaticity with ring size for D3h [4n + 2]-annulenes, and predicts trends for anti-aromatic [4n]-annulenes in the two symmetry groups.

Current-density maps are cal- culated at an ab initio level for the three symmetrical polycyclic aromatic hydro- carbons, circumcoronene (1 (D6h)), hex- abenzo(bc,ef,hi,kl,no,qr)coronene ( 2a (D6h) and 2b (D3d)), and hexaben- zo(a,d,g,j,m,p)coronene ( 3a (D6h), 3b (D6) and 3c (D3d)), all of which can be formally derived by annelation of ben- zene rings to a coronene core. Whilst 1 is planar, 2 has a non-planar minimum that is effectively isoenergetic with its planar form, and 3 has a well defined non- planar structure. The shape of the mo- lecular boundary rather than the planar- ity of the molecule plays the critical ro √ le in the character of the predicted cur- rents. Formal deletion of outer hexagons from circumcoronene (1) in two differ- ent ways produces either hexabenzocor- onene 2 with a prediction of disjoint local benzenoid diatropic currents linked by a global perimeter, or 3 with a giant diatropic perimeter current en- closinga weak paramagnetic circulation on the central hexagon. The current density map of 1 is effectively a super- position of those of 2 and 3. Its strong diatropic perimeter current subsumes the six weaker diatropic benzenoid cir- culations evident in 2, and bifurcates in the six outer benzenoid rings that form the corners of the giant hexagon; its benzene ™hub∫ sustains a diatropic cur- rent, as would be expected from the partial cancellation of the strongdia- tropic hub current of 2 by the weaker paratropic hub current of 3. The rela- tionship between the three molecules is rationalised by consideringorbital con- tributions to their current density maps.

The temperature dependence of the luminescence properties of nanocrystalline CdS/Mn 2+ particles is investigated. In addition to an orange Mn 2+ emission around 585 nm a red defect related emission around 700 nm is observed. The temperature quenching of both emissions is similar ( T q ≈100 K). For the defect emission the reduction in the lifetime follows the temperature dependence of the intensity. For the Mn 2+ emission however, the intensity decreases more rapidly than the lifetime with increasing temperature. To explain these observations a model is proposed in which the Mn 2+ ions are excited via an intermediate state involving shallowly trapped (≈40 meV) charge carriers.

This paper reviews reports of occurrences of fullerenes in circumstellar media, interstellar media, meteorites, interplanetary dust particles (IDPs), lunar rocks, hard terrestrial rocks from Shunga (Russia), Sudbury (Canada) and Mitov (Czech Republic), coal, terrestrial sediments from the Cretaceous-Tertiary-Boundary and Permian-Triassic-Boundary, fulgurite, ink sticks, dinosaur eggs, and a tree char. The occurrences are discussed in the context of known and postulated processes of fullerene formation, including the suggestion that some natural fullerenes might have formed from biological (algal) remains.

MALDI TOF-MS of tribenzo[l:1':1"]benzo[1,2-e:3,4-e':5,6-e"]triacephenanthrylene (1a, C60H30) gives C60.+ by multiple intramolecular cyclodehydrogenation reactions.

Direct visualization of the pi current density maps of highly strained annelated benzenes containing cyclopropa, cyclobuta, and cyclobutadieno clamps, alone and in combination, using a reliable distributed-origin, coupled Hartree-Fock method, shows the robustness of the classical benzene diatropic pi ring current. When only saturated clamps are used, the benzene ring current is essentially unchanged. In contrast, annelation with one or more cyclobutadieno clamps disrupts the benzene ring current. Analysis of orbital contributions to the current density maps gives a unified account of these observations in terms of the nature of the HOMO-LUMO transition.