T. Glass

A stable non-classical metallofullerene family.

In the evolving field of fullerenes, nanotubes and endohedral metallofullerenes, the isolated-pentagon rule (IPR) is sacrosanct — exceptions have been predicted, but no bare carbon cages with adjacent pentagons have been characterized. Small organic molecules with metal-stabilized fused five-membered rings (pentalenes) have been created, however, and here we describe a family of non-classical endohedral metallofullerenes with the general structure AxSc3−xN@C 68 (where x = 0–2, A is a rare-earth metal, Sc is scandium and N is nitrogen) that has a non-IPR cage of only 68 carbon atoms containing annelated five-membered rings. This internal ring network is metal-stabilized and is accessible for external organic reaction chemistry.

Synthesis and characterization of organosiloxane modified segmented polyether polyurethanes

Abstract Segmented polyurethanes derived from a 1000 M n hydroxyl terminated polytetramethylene oxide soft segment, 4,4′-methylene diphenyl diisocyanate, MDI, and 1,4-butanediol were modified with a 1200 M n secondary aminoalkyl functional polydimethylsiloxane (PDMS) oligomer via solution polymerization in tetrahydrofuran (THF)/dimethylacetamide (DMAC). Various compositions were studied using FTIR and NMR spectroscopy, thermal analysis, quantitative size exclusion chromatography (SEC), cone calorimetry, transmission electron microscopy, XPS and mechanical testing. The results suggest that, with as little as 15% of the polydimethylsiloxane one may reduce the cone calorimetry heat release rate by a factor of about 2/3 and hence improve fire resistance, while maintaining mechanical behavior. It is suggested that the low surface energy characteristics of PDMS promote migration to the air–polymer interface to form a predominately PDMS enriched surface. The latter is oxidized at elevated temperatures in air to a silicate-like material and this serves as a protective layer, which further reduces burning of the underlying polyurethane. Dynamic mechanical behavior and electron microscopy suggest that a complex mutiphase structure is produced, particularly at low PDMS weight fractions.

An isomer of the endohedral metallofullerene Sc3N@C80 with D5h symmetry

Abstract The discovery of a new isomer of the endohedral metallofullerene Sc3N@C80 is reported. 13C labeled (∼9%) Sc3N@C80 has been synthesized and has been characterized by HPLC, mass spectroscopy and 13C NMR data. The 13C NMR spectrum shows six peaks with a 1:2:2:1:1:1 ratio that are assigned to the D5h isomer. The results of theoretical calculations are also reported for the Ih and D5h isomers of Sc3N@C80.

The flow transfer of a bolus with 1H dynamic nuclear polarization from low to high magnetic fields

Abstract Studies involving a flowing bolus enhanced in a low magnetic field (0.34 T) with dynamic nuclear polarization (DNP), but monitored in a high magnetic field (4.7 T, 200 MHz 1 H NMR) are reported. The advantages of monitoring the NMR signal at high magnetic fields are increased chemical shift dispersion and greater magnetization flux at the NMR detector. For several different flowing fluids, the enhancement of the low field polarized signal exceeds the normal thermal Boltzmann magnetization for a high magnetic field system (4.7 T). Stable nitroxide radical systems immobilized on silica gel were utilized in these flow solid/liquid intermolecular transfer (SLIT) 1 H DNP experiments.

Investigation of the thermal curing chemistry of the phenylethynyl group using a model aryl ether imide

Abstract The high temperature curing reactions of oligomers bearing two phenylethynyl groups have previously been utilized to produce attractive new materials, but are not well understood. A model aryl ether imide compound with one such moiety was used to elucidate some of the chemistry occurring during these processes. It was demonstrated that oligomeric molecular weight soluble products were formed by thermal initiation at 375°C under nitrogen during these reactions, along with low molecular weight fragments derived from the model compound. These fragments were demonstrated to contain aliphatic protons in place of the phenylethynyl group and constitute about 5% of the overall material. Reactions of the model compound with deuterated biphenyl indicated that the reactive intermediates formed from the phenylethynyl moiety are able to abstract aromatic deuterium atoms in the presence of aromatic protons. However, these fragments appear to be side products not directly involved with the main curing reactions that occur after most of the carbon–carbon triple bonds have been reacted.

erratum: Small-bandgap endohedral metallofullerenes in high yield and purity

This corrects the article DOI: 43415

Transfer of1H and13C dynamic nuclear polarization from immobilized nitroxide radicals to flowing liquids

In this study,1H and13C dynamic nuclear polarization (DNP) was generated at a magnetic field strength of 0.33 T utilizing silica phase immobilized nitroxide (SPIN) samples. The polarization was subsequently transferred to flowing liquids and monitored at a magnetic field strength of 4.7 T. These solid/liquid intermolecular transfer (SLIT) experiments provide efficient polarization transfer without the necessity of the free radical system present in the monitoring fluid. Specifically, ultimate1H SLIT DNP Overhauser enhancements of −56 and −110 have been observed for benzene and chloroform in the presence of SPIN system 2, respectively.The13C SLIT DNP enhancement for benzene is dominated by three-spin effects and poor leakage factors (fc). However, a particularly favorable case is the chloroform/SPIN 2 system which exhibits a scalar dominated enhancement. For this case, positive enhancements 40–60 times the13C thermal Boltzmann magnetization at 4.7 T have been observed. The large scalar dominated13C DNP enhancement for this system represents one of the largest experimental enhancements reported to date. The13C DNP spectra for other samples which exhibit favorable scalar13C dominated enhancements (e.g., Freon 113) are also presented. Three different SPIN systems were also prepared and characterized in the present study.

Controlled molecular weight cresol-formaldehyde oligomers

Abstract Controlled molecular weight, linear, ortho- and para-cresol novolac oligomers have been synthesized by using calculated amounts of a monofunctional 2,6-dimethylphenol endcapping reagent. It was found that an excess of formaldehyde was needed to achieve the targeted molecular weights, thus suggesting that a dynamic equilibrium exists in these reactions whereby formaldehyde adds and eliminates from the cresol rings. Reaction progression was monitored by both 13C NMR and GPC. Number average molecular weights of these oligomers were confirmed using 13C NMR spectra and were found to be comparable to the targeted molecular weights. The glass transitions and viscosities of both the ortho- and para-cresol novolacs were compared at equivalent number average molecular weights. The Tgs increased as the molecular weights increased, but there were no observable differences between the Tgs of ortho- and para-cresol novolacs with similar molecular weights. The melt viscosities of ortho- and para-cresol novolcas with similar molecular weights were almost identical.

Phenylmaleimide encapped arylene ether imide oligomers

Abstract High temperature cure sites for polyimides and related high performance thermosetting systems are important and it has been demonstrated now by several groups that 4-phenylethynylphthalic anhydride and/or 3-phenylethynylaniline are attractive endcappers for polyimides that have wide curing windows and afford solvent resistant networks after 30–90 min at 350–400°C. The present paper has investigated phenylmaleic anhydride (PMA) as an alternate system which is thermally and possibly electron beam curable. Thus, PMA has been successfully utilized as a high temperature crosslinking endgroup for several high performance amorphous polyimides and initial results are described in this paper. Imide oligomers of controlled molecular weights were successfully synthesized by conducting either chemical imidization or by employing a two-step amine-terminated oligomer route followed by endcapping with PMA at about 140°C or lower. The more conventional higher temperature process was complicated by side reactions which are discussed herein. The molecular weights of the oligomers were successfully controlled from 2000 to 15 000 g mol −1 . The materials exhibited about at 200°C curing window between oligomer T g and cure exotherm. Glass transition temperatures increased by more than 50°C after curing and produced solvent resistant insoluble materials with about 85% gel content. Curing of the phenylmaleimide endgroups was most successful under a nitrogen atmosphere, but it appears that normal moulding operations are acceptable, wherein only moderate amounts of oxygen are introduced into the specimen. The thermosetting reaction appears to be more sensitive to radical initiation than the earlier studied phenylethynyl systems. Dynamic thermogravimetric analysis on the cured PMA functionalized oligomers showed good behaviour and 5% weight loss in air was only observed above 500°C. The systems are being evaluated as structural adhesives and matrix resins for composite materials and excellent titanium/titanium lapshear values of about 5000 psi (34 MPa) on the cured oligomers has been demonstrated.

The Encapsulation of Trimetallic Nitride Clusters in Fullerene Cages

The Kratschmer-Huffman electric-arc generator typically produces endohedral metallofullerenes in low yields with a wide array of different products, but the introduction of nitrogen leads to a new family of encapsulates. A family of endohedral metallofullerenes AnB3−nN@C2n (n=0–3, x=34, 39, and 40) where A and B are Group III and rare-earth metals is formed by a trimetallic nitride template (TNT) process in relatively high yields. The archetypal representative of this new class is the stable endohedral metallofullerene, Sc3N@C80 containing a triscandium nitride cluster encapsulated in an icosahedron (Ih), C80 cage. The Sc3N@C80 is formed in yields even exceeding empty-cage C84. Other prominent scandium TNT members are Sc3N@C68 and Sc3N@C78. The former Sc3N@C68 molecule represents an exception to the well known isolated pentagon rule (IPR). These new molecules were purified by chromatography with corresponding characterization by various spectroscopic approaches. In this paper we focus on the characterizat...