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PubMed2002

Synthesis and properties of 1,3,5-benzene periodic mesoporous organosilica (PMO): novel aromatic PMO with three point attachments and unique thermal transformations.

A new aromatic periodic mesoporous organosilica material containing benzene functional groups that are symmetrically integrated with three silicon atoms in an organosilica mesoporous framework is reported. The material has a high surface area, well-ordered mesoporous structure and thermally stable framework aromatic groups. The functional aromatic moieties were observed to undergo sequential thermal transformation from a three to two and then to a one point attachment within the framework upon continuous thermolysis under air before eventually being converted to periodic mesoporous silica devoid of aromatic groups at high temperatures and longer pyrolysis times. The mesoporosity of the material was characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and nitrogen porosimetry, whereas the presence and transformation of the aromatic groups in the walls of the materials were characterized by solid-state NMR spectroscopy, mass spectrometry, and thermogravimetric analysis. The attachment of a benzene ring symmetrically onto three siloxanes of the framework was used advantageously as a cross-linker to enhance the thermal stability of the organic group. Some of these properties are investigated in comparison with other aromatic PMOs that have only two point attachments and an amorphous phenylsilica gel that has only one point attachment. The successful synthesis of the first aromatic PMO with its organic group attached within the framework through more than two points is an important step toward the synthesis of PMOs having organic groups with more complex and multiple attachments within the framework.

Journal Article↗

PubMed2004

Large-scale synthesis of perpendicularly aligned helical carbon nanotubes.

Large-scale perpendicularly aligned helical carbon nanotube arrays were prepared by co-pyrolysis of Fe(CO)5 and pyridine onto the pristine quartz glass plates in a tube furnace at 900-1100 degrees C under a mixture flow of Ar and H2. The resultant aligned helical carbon nanotubes could not only facilitate the structure-property characterization for helical carbon nanotubes but also allow them to be effectively incorporated into devices for practical applications.

Journal Article↗

PubMed2003

A novel method for the templated synthesis of homogeneous samples of hollow carbon nanospheres from cellulose chars.

A novel method for the production of homogeneous samples of hollow carbon nanospheres is reported from cellulose, an inexpensive and renewable precursor. The nanospheres are of diameter 50 nm, graphitic wall thickness 5-10 nm, and can easily be produced in several hundred milligram batches. The nanospheres are derived from the laser pyrolysis of a nickel chloride templated cellulose char via open Ni-core shells.

Carbohydrate Sequence↗

PubMed2003

The stereochemistry of the thermal cheletropic decarbonylation of 3-cyclopentenone as determined by multiphoton infrared photolysis/thermolysis.

There are two allowed pathways for the thermal cheletropic decarbonylation of 3-cyclopentenone. The stereochemistry of decarbonylation of an unconstrained derivative (trans,trans-2,5-dimethyl-3-cyclopentenone, 4) has been determined for the first time. Under conventional pyrolysis conditions, thermal rearrangements of the initial product (trans,trans-2,4-hexadiene, 5) occur at the high temperatures required for the decarbonylation. However, by using multiphoton infrared photolysis/thermolysis to initiate decarbonylation, it was shown that the initial products from thermal decarbonylation of 4 are solely carbon monoxide and stereospecifically 5. The stereochemistry of decarbonylation is thus disrotatory, in accord with prior theoretical studies. A survey of crystal structures reveals ground-state distortions along this reaction coordinate as well.

Journal Article↗

PubMed2003

Comparison of "polynaphthalenes" prepared by two mechanistically distinct routes.

The Bergman cyclization has long been known to produce polymers as side products. More recently, this attribute has been harnessed for the production of conjugated materials. However, the structures of these polymers have not been established. To resolve this question, the metal-catalyzed polymerization of 1,4-dibromonaphthalene and thermal polymerization of o-diethynylbenzene were conducted. Two distinct polymers were obtained. Comparison of IR spectroscopy, MALDI-TOF MS, solid-state NMR spectroscopy, UV-vis reflectance spectroscopy, and pyrolysis GC-MS data indicates that only one of the polymers is consistent with poly(1,4-naphthalene).

Journal Article↗

PubMed2005

Bergman cyclization of sterically hindered substrates and observation of phenyl-shifted products.

Heating 1,2-bis(phenylethynyl)benzene in the presence of 1,4-cyclohexadiene at temperatures ranging from 260 to 360 degrees C yielded the expected Bergman product, 2,3-diphenylnaphthalene, as only a minor product (<3%) under all reaction conditions studied. The major products, resulting from one or more phenyl shifts, were 1,3- and 1,4-diphenylnaphthalene which formed in up to 16% and 11% yield, respectively. Although somewhat less efficiently, 1-ethynyl-2-(phenylethynyl)benzene and (Z)-1,6-diphenylhex-3-ene-2,5-diyne also yielded products resulting from phenyl shifts. These results are explained through computations that point to the role of steric repulsion and benzyne stability in driving these isomerizations. The computed barriers for phenyl shifting are dramatically higher than those observed in the case of sp3 radicals. However, these transformations are relevant in solution chemistry as well as in more extreme environments such as those encountered during combustion, pyrolysis, and electric discharge heating.

Journal Article↗

PubMed2005

Transmutation of fullerenes.

Fullerenes were pyrolyzed by subliming them into a stream of flowing argon gas and then passing them through an oven heated to approximately 1000 degrees C. C(76), C(78), and C(84) all readily lost carbons to form smaller fullerenes. In the case of C(78), some isomerization was seen. Pyrolysis of (3)He@C(76) showed that all or most of the (3)He was lost during the decomposition. C(60) passes through the apparatus with no decomposition and no loss of helium.

Journal Article↗

PubMed2004

Synthesis and structure of 2,5,8-triazido-s-heptazine: an energetic and luminescent precursor to nitrogen-rich carbon nitrides.

Derivatized s-triazine (C3N3) precursors have seen significant recent use in the production of carbon nitride materials. Larger polycyclic molecular precursors, such as those containing an s-heptazine core (C6N7 or tri-s-triazine), may improve stability and order in carbon nitride products. In this Communication, we describe the synthesis and crystal structure of 2,5,8-triazido-s-heptazine (2). Synthesis of 2 was achieved from melon, an oligomeric s-heptazine synthesized by the pyrolysis of NH4SCN. Melon was converted to molecular 2,5,8-trichloro-s-heptazine, which was then transformed to the triazide upon reaction with (CH3)3SiN3. The crystal structure of 2 verifies that the s-heptazine is planar and the azides adopt a pinwheel-like C3h arrangement around the periphery. The s-heptazine core shows pi delocalization in the C-N bonds around the periphery (av. 1.33 A), while the internal planar C-N bonds are longer (1.40 A). The heptazine units pack into parallel, but offset, layered sheets in the crystal. The triazide 2 exhibits photoluminescence at 430 nm and rapidly and exothermically decomposes upon heating at 185 degrees C to produce a tan thermally stable carbon nitride powder with a formula near C3N4.

Journal Article↗

PubMed2005

Long-range ordered thin films of block copolymers prepared by zone-casting and their thermal conversion into ordered nanostructured carbon.

Large-scale alignment of lamellae in thin films of diblock copolymers containing polyacrylonitrile and poly(n-butyl acrylate) was achieved by casting copolymer solution on a silicon substrate moved away at a constant speed from the casting nozzle (zone-casting). Grazing incidence small-angle X-ray scattering revealed that the lamellae, which were perpendicular to the substrate, were also aligned over macroscopic scale in the direction perpendicular to the casting direction. Such long-range ordered block copolymer films were then converted by pyrolysis into nanostructured carbons, with excellent preservation of lamellar morphology and orientation.

Acrylates↗

PubMed2005

Pyrogenic iron(III)-doped TiO2 nanopowders synthesized in RF thermal plasma: phase formation, defect structure, band gap, and magnetic properties.

Iron(III)-doped TiO(2) nanopowders, with controlled iron to titanium atomic ratios (R(Fe/Ti)) ranging from nominal 0 to 20%, were synthesized using oxidative pyrolysis of liquid-feed metallorganic precursors in a radiation-frequency (RF) thermal plasma. The valence of iron doped in the TiO(2), phase formation, defect structures, band gaps, and magnetic properties of the resultant nanopowders were systematically investigated using Mössbauer spectroscopy, XRD, Raman spectroscopy, TEM/HRTEM, UV-vis spectroscopy, and measurements of magnetic properties. The iron doped in TiO(2) was trivalent (3+) in a high-spin state as determined by the isomer shift and quadrupole splitting from the Mössbauer spectra. No other phases except anatase and rutile TiO(2) were identified in the resultant nanopowders. Interestingly, thermodynamically metastable anatase predominated in the undoped TiO(2) nanopowders, which can be explained from a kinetic point of view based on classical homogeneous nucleation theory. With iron doping, the formation of rutile was strongly promoted because rutile is more tolerant than anatase to the defects such as oxygen vacancies resulting from the substitution of Fe(3+) for Ti(4+) in TiO(2). The concentration of oxygen vacancies reached a maximum at R(Fe/Ti) = 2% above which excessive oxygen vacancies tended to concentrate. As a result of this concentration, an extended defect like crystallographic shear (CS) structure was established. With iron doping, red shift of the absorption edges occurred in addition to the d-d electron transition of iron in the visible light region. The as-prepared iron-doped TiO(2) nanopowders were paramagnetic in nature at room temperature.

Journal Article↗

PubMed2005

Binuclear cyclopentadienylcobalt carbonyls: comparison with binuclear iron carbonyls.

The binuclear cyclopentadienylcobalt carbonyls Cp2Co2(CO)n (n = 3, 2, 1; Cp = eta5-C5H5) are studied by density functional theory using the B3LYP and BP86 functionals. The experimentally known monobridged isomer Cp2Co2(CO)2(mu-CO) and the tribridged isomer Cp2Co2(mu-CO)3 of Cp2Co2(CO)3 with formal Co-Co single bonds are found to be similar in energy, with the precise relative energies of the two isomers depending on the functional chosen. For Cp2Co2(CO)2, the experimentally known coaxial isomer Cp2Co2(mu-CO)2 with two bridging CO groups and a formal Co=Co double bond (2.360 angstroms by B3LYP or 2.346 angstroms by BP86) is found to lie 38.2 (B3LYP) or 34.9 kcal/mol (BP86) below a perpendicular isomer perpendicular-Cp2Co2(CO)2. Similarly, for Cp2Co2(CO), the coaxial isomer Cp2Co2(mu-CO) with one bridging CO group and a formal CoCo triple bond (2.021 angstroms by B3LYP or 2.050 angstroms by BP86) is found to lie 9.36 (B3LYP) or 9.62 kcal/mol (BP86) below the corresponding perpendicular isomer perpendicular-Cp2Co2(CO). This coaxial isomer Cp2Co2(mu-CO) is a possible intermediate in the known pyrolysis of the trimer (eta5-C5H5)3Co3(mu-CO)3 to give the tetranuclear complex (eta5-C5H5)4Co4(mu3-CO)2. These optimized Cp2Co2(CO)n (n = 3, 2, 1) structures can be compared with the corresponding Fe2(CO)6+n structures since the CpCo and Fe(CO)3 groups are isolobal. In general, the metal-metal bonds are 0.09-0.22 angstroms shorter for the Cp2Co2(CO)n (n = 3, 2, 1) complexes than for the corresponding Fe2(CO)6+n complexes. For Fe2(CO)9, the experimentally well-known Fe2(CO)6(mu-CO)3 isomer is shown to be very close in energy to the unknown Fe2(CO)8(mu-CO) isomer, with the precise relative energies depending on the basis set used.

Carbon↗

PubMed2005

Interconversions of aryl radicals by 1,4-shifts of hydrogen atoms. A synthesis of benzo[a]corannulene.

Aryl radicals generated ortho to aryl substituents by flash vacuum pyrolysis (FVP) of the corresponding aryl chlorides are shown to be capable of transferring hydrogen atoms between the ortho and ortho' positions (1,4-shifts of hydrogen atoms). In the examples described here, the rearranged aryl radicals are trapped by subsequent radical cyclization reactions. For example, FVP of 2-(o-chlorophenyl)benzo[c]phenanthrene gives 1-phenylbenzo[ghi]fluoranthene as the major product by homolysis of the C-Cl bond, 1,4-shift of a hydrogen atom out of the sterically congested cove region to the radical center, cyclization of the rearranged radical, and rearomatization of the molecule by loss of the other cove region hydrogen. In a control experiment run under the same conditions, FVP of 2-phenylbenzo[c]phenanthrene, which lacks a radical precursor, gave primarily recovered starting material. When the FVP was repeated using 2-(2,6-dichlorophenyl)benzo[c]phenanthrene as the starting material, benzo[a]corannulene was obtained as the major product, presumably by the same cascade of events to produce 1-(o-chlorophenyl)benzo[ghi]fluoranthene, which then suffers a second radical cyclization spontaneously under the high-temperature conditions to give the geodesic polyarene.

Journal Article↗

PubMed2006

Ultralow-density nanostructured metal foams: combustion synthesis, morphology, and composition.

The synthesis of low-density, nanoporous materials has been an active area of study in chemistry and materials science dating back to the initial synthesis of aerogels. These materials, however, are most often limited to metal oxides, e.g., silica and alumina, and organic aerogels, e.g., resorcinol/formaldehyde, or carbon aerogels, produced from the pyrolysis of organic aerogels. The ability to form monolithic metallic nanocellular porous materials is difficult and sometimes elusive using conventional methodology. Here we report a relatively simple method to access unprecedented ultralow-density, nanostructured, monolithic, transition-metal foams, utilizing self-propagating combustion synthesis of novel transition-metal complexes containing high nitrogen energetic ligands. During the investigation of the decomposition behavior of the high-nitrogen transition metal complexes, it was discovered that nanostructured metal monolithic foams were formed in a post flame-front dynamic assembly having remarkably low densities down to 0.011 g cm(-3) and extremely high surface areas as high as 270 m(2) g(-1). We have produced monolithic nanoporous metal foams via this method of iron, cobalt, copper, and silver metals. We expect to be able to apply this to many other metals and to be able to tailor the resulting structure significantly.

Journal Article↗

PubMed2006

Translucent thin film Fe2O3 photoanodes for efficient water splitting by sunlight: nanostructure-directing effect of Si-doping.

Thin, silicon-doped nanocrystalline alpha-Fe2O3 films have been deposited on F-doped SnO2 substrates by ultrasonic spray pyrolysis and chemical vapor deposition at atmospheric pressure. The photocatalytic activity of these films with regard to photoelectrochemical water oxidation was measured at pH 13.6 under simulated AM 1.5 global sunlight. The photoanodes prepared by USP and APCVD gave 1.17 and 1.45 mA/cm2, respectively, at 1.23 V vs RHE. The morphology of the alpha-Fe2O3 was strongly influenced by the silicon doping, decreasing the feature size of the mesoscopic film. The silicon-doped alpha-Fe2O3 nano-leaflets show a preferred orientation with the (001) basal plane normal to the substrate. The best performing photoanode would yield a solar-to-chemical conversion efficiency of 2.1% in a tandem device using two dye-sensitized solar cells in series.

Journal Article↗

PubMed2006

Thermal formation of homochiral serine clusters and implications for the origin of homochirality.

Spontaneous assembly of amino acids into vapor-phase clusters occurs on heating the solid compounds in air. In comparison to the other amino acids, serine forms clusters to an unusual extent, showing a magic number octamer on sublimation; this octamer can be ionized and characterized by mass spectrometry. Two isomers of the vapor-phase serine octamer are generated, the minor one at 130 degrees C and the major at 220 degrees C. The higher temperature cluster shows a strong homochiral preference, as confirmed by isotopic labeling experiments. This serine cluster, like that generated earlier from solution in electrospray ionization experiments, undergoes gas-phase enantioselective substitution reactions with other amino acids. These reactions transfer the chirality of serine to the other amino acid through enantioselective incorporation into the octamer. Other serine pyrolysis products include alanine, glycine, ethanolamine, and small dipeptides, and many of these, too, are observed to be incorporated into the thermally formed serine octamers. Chiral chromatographic analysis confirmed that L-serine sublimation produced DL-alanine, glycine, and ethanolamine, while in the presence of hydrogen sulfide, L-serine yielded L-cysteine. The data demonstrate that sublimation of serine under relatively mild conditions yields chirally enriched serine octamers and that the chiral preference of the starting serine can be transferred to other compounds through cluster-forming chemical reactions.

Gas Chromatography-Mass Spectrometry↗

PubMed2006

Preparation of multilayered trimodal colloid crystals and binary inverse opals.

We report for the first time the preparation of multilayered trimodal colloid crystals (tCC) and their corresponding binary inverse opals (bIO), which present complex hierarchical structures that may have significant potential in photonics, phononics, separations, and catalysis, among others. A trimodal colloidal mixture of 465 nm polystyrene (PS), 84 nm poly(methyl methacrylate) (PMMA), and 6 nm silica particles in suspension was transferred onto a glass substrate and self-assembled into highly ordered trimodal crystal structures during vertical lifting deposition. Pyrolysis of the organic particles in the tCC resulted in silica bIO with interconnected meso- and macropores. Vis-NIR spectra of all structures were analyzed to reveal the internal architecture with each PS sphere correlating to 21-23 PMMA particles (a LI21-23 stoichiometry), which corresponded well with computer models.

Journal Article↗

PubMed2002

Authenticity assessment of estragole and methyl eugenol by on-line gas chromatography-isotope ratio mass spectrometry.

On-line capillary gas chromatography-isotope ratio mass spectrometry was used in the combustion (HRGC-C-IRMS) and the pyrolysis (HRGC-P-IRMS) modes to determine delta(13)C(PDB), delta(2)H(SMOW), and delta(18)O(SMOW) data of estragole (1) and methyl eugenol (2) originating from various sources. For 1, similar delta(13)C values, i.e., ranging from -35.4 to -29.9 per thousand and from -36.4 to -28.8 per thousand for the product of synthetic and natural origins, respectively, were found. The delta(2)H values ranged from -155 to -3 per thousand for synthetic 1 and from -193 to -105 per thousand for 1 from natural origin, whereas the determination of delta(18)O data gave values from +1.8 to +24.8 per thousand and from +2.7 to +18.7 per thousand for 1 from synthetic and natural origins, respectively. As synthetic 2 is produced by methylation of natural eugenol, the IRMS techniques did not allow differentiation of synthetic 2 from the product of natural origin. The recorded data ranges were nearly identical, i.e., delta(13)C = -37.4 to -35.0 per thousand and -41.1 to -32.2 per thousand; delta(2)H = -155 to -126 per thousand and -217 to -107 per thousand; delta(18)O = +5.5 to +6.6 per thousand and +2.7 to +6.9 per thousand, each for 2 from synthetic and natural origins, respectively.

Allylbenzene Derivatives↗

PubMed2002

Alkylpyridiniums. 1. Formation in model systems via thermal degradation of trigonelline.

Trigonelline is a well-known precursor of flavor/aroma compounds in coffee and undergoes significant degradation during roasting. This study investigates the major nonvolatile products that are procured after trigonelline has been subjected to mild pyrolysis conditions (220-250 degrees C) under atmospheric pressure. Various salt forms of trigonelline were also prepared and the thermally produced nonvolatiles analyzed by thin layer chromatography, liquid chromatography-electrospray ionization tandem mass spectrometry, and (1)H and (13)C nuclear magnetic resonance. Results revealed the decarboxylated derivative 1-methylpyridinium as a major product of certain salts, the formation of which is positively correlated to temperature from 220 to 245 degrees C. Moreover, trigonelline hydrochloride afforded far greater amounts of 1-methylpyridinium compared to the monohydrate over the temperature range studied. Investigations into other potential quaternary amine products of trigonelline also indicate nucleophilic substitution reactions that lead to dialkylpyridiniums, albeit at concentration levels approximately 100-fold lower than those recorded for 1-methylpyridinium.

Alkaloids↗