2-pyrazinylnitrene and 4-pyrimidylnitrene. Ring expansion to 1,3,5-triazacyclohepta-1,2,4,6-tetraene and ring opening to (2-isocyanovinyl)carbodiimide

Tetrazolo[1,5-a]pyrazine/2-azidopyrazine 9T/9A undergo photolysis in Ar matrix at cryogenic temperatures to yield 1,3,5-triazacyclohepta-1,2,4,6-tetraene 21 as the first observable intermediate, and 1-cyanoimidazole 11 and (2-isocyanovinyl)carbodiimide 22 as the final products. The latter tautomerizes to 2-(isocyanovinyl)cyanamide 23 on warming to 40 K. The same intermediate 21 and the same final products are obtained on matrix photolysis of the isomeric tetrazolo[1,5-c]pyrimidine/4-azidopyrimidine 24T/24A. These photolysis results as well as those of the previously reported thermal ring contraction of N-15-labeled 2-pyrazinyl- and 4-pyrimidylnitrenes to 1-cyanoimidazoles can all be rationalized in terms of selective ring opening of 21 or nitrine 10 to a nitrile ylide zwitterion 28 prior to formation of the final products, 11 and 22. The results are supported by high-level ab initio and DFT calculations (CASPT2-CASSCF(6,6), G3(MP2), and B3LYP/6-31+G*) of the energies and IR spectra of the intermediates and products.

2-cyano(2-pyridyl)ketene

Flash vacuum thermolysis of quinolizinones is a new way of generating ketenes. The title ketene is obtained from 1-cyano-2-hydroxyquinolizine-4-one and characterized by its Ar matrix infrared spectrum. (C) Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany 2002.

A theoretical study of the origin of rotational barriers in push-pull ethylenes

Using the B3LYP/6-31G* ab initio method, we have studied the rotation about the C=C bonds in 15 push-pull ethylenes of the general formula (X,Y)C=C(CHO)(2) [X, Y = NH2, NHCH3, N(CH3)(2), OCH3, SCH3] in the gas phase. Two stationary points (minimum and transition state) were located for all compounds. The geometry, dipole moments, natural bond orbital atomic charges, as well as the rotational barriers were examined. The torsion angle 0 depends essentially on the presence or absence of intramolecular hydrogen bonds, and the barrier is a function of the torsion angle. (C) 2002 Elsevier Science B.V. All rights reserved.

Prediction of absolute rate coefficients and product branching ratios for the C(P-3) plus allene reaction system

Complex chemical reactions in the gas phase can be decomposed into a network of elementary (e.g., unimolecular and bimolecular) steps which may involve multiple reactant channels, multiple intermediates, and multiple products. The modeling of such reactions involves describing the molecular species and their transformation by reaction at a detailed level. Here we focus on a detailed modeling of the C(P-3)+allene (C3H4) reaction, for which molecular beam experiments and theoretical calculations have previously been performed. In our previous calculations, product branching ratios for a nonrotating isomerizing unimolecular system were predicted. We extend the previous calculations to predict absolute unimolecular rate coefficients and branching ratios using microcanonical variational transition state theory (mu-VTST) with full energy and angular momentum resolution. Our calculation of the initial capture rate is facilitated by systematic ab initio potential energy surface calculations that describe the interaction potential between carbon and allene as a function of the angle of attack. Furthermore, the chemical kinetic scheme is enhanced to explicitly treat the entrance channels in terms of a predicted overall input flux and also to allow for the possibility of redissociation via the entrance channels. Thus, the computation of total bimolecular reaction rates and partial capture rates is now possible. (C) 2002 American Institute of Physics.

The role of Ti as a catalyst for the dissociation of hydrogen on a Mg(0001) surface

In this paper, the dissociative chemisorption of hydrogen on both pure and Ti-incorporated Mg(0001) surfaces are studied by ab initio density functional theory (DFT) calculations. The calculated dissociation barrier of hydrogen molecule on a pure Mg(0001) surface (1.05 eV) is in good agreement with comparable theoretical studies. For the Ti-incorporated Mg(0001) surface, the activated barrier decreases to 0.103 eV due to the strong interaction between the molecular orbital of hydrogen and the d metal state of Ti. This could explain the experimentally observed improvement in absorption kinetics of hydrogen when transition metals have been introduced into the magnesium materials.

Experimental and theoretical investigation of the structural, chemical, electronic, and high frequency dielectric properties of barium cadmium tantalate-based ceramics

Single-phase Ba(Cd1/3Ta2/3)O-3 powder was produced using conventional solid state reaction methods. Ba(Cd1/3Ta2/3)O-3 ceramics with 2 wt % ZnO as sintering additive sintered at 1550 degreesC exhibited a dielectric constant of similar to32 and loss tangent of 5x10(-5) at 2 GHz. X-ray diffraction and thermogravimetric measurements were used to characterize the structural and thermodynamic properties of the material. Ab initio electronic structure calculations were used to give insight into the unusual properties of Ba(Cd1/3Ta2/3)O-3, as well as a similar and more widely used material Ba(Zn1/3Ta2/3)O-3. While both compounds have a hexagonal Bravais lattice, the P321 space group of Ba(Cd1/3Ta2/3)O-3 is reduced from P (3) under bar m1 of Ba(Zn1/3Ta2/3)O-3 as a result of a distortion of oxygen away from the symmetric position between the Ta and Cd ions. Both of the compounds have a conduction band minimum and valence band maximum composed of mostly weakly itinerant Ta 5d and Zn 3d/Cd 4d levels, respectively. The covalent nature of the directional d-electron bonding in these high-Z oxides plays an important role in producing a more rigid lattice with higher melting points and enhanced phonon energies, and is suggested to play an important role in producing materials with a high dielectric constant and low microwave loss. (C) 2005 American Institute of Physics.

Mg-based nanocomposites with high capacity and fast kinetics for hydrogen storage

Magnesium and its alloys have shown a great potential in effective hydrogen storage due to their advantages of high volumetric/ gravimetric hydrogen storage capacity and low cost. However, the use of these materials in fuel cells for automotive applications at the present time is limited by high hydrogenation temperature and sluggish sorption kinetics. This paper presents the recent results of design and development of magnesium-based nanocomposites demonstrating the catalytic effects of carbon nanotubes and transition metals on hydrogen adsorption in these materials. The results are promising for the application of magnesium materials for hydrogen storage, with significantly reduced absorption temperatures and enhanced ab/desorption kinetics. High level Density Functional Theory calculations support the analysis of the hydrogenation mechanisms by revealing the detailed atomic and molecular interactions that underpin the catalytic roles of incorporated carbon and titanium, providing clear guidance for further design and development of such materials with better hydrogen storage properties.

Oxoketene-oxoketene, imidoylketene-imidoylketene and oxoketenimine-imidoylketene rearrangements. 1,3-shifts of phenyl groups

Dibenzoylketene 5 undergoes degenerate 1,3-shifts of the phenyl group between acyl and ketene carbon atoms, thus interconverting it with 6 and 7. This 1,3-shift takes place in the gas phase under flash vacuum thermolysis (FVT) conditions, but not in solution at 110-145 degrees C. Imidoyl(benzoyl)ketene 13 undergoes degenerate 1,3-shift of the phenyl group on FVT, thus interconverting it with 14, but the ketenimine isomer 15 is not formed, and none of these shifts take place in the solid state at 250 degrees C. Imidoyl(p-toluoyl)ketene 21 undergoes a 1,3-p-tolyl shift, interconverting it with ketene 22 but not with ketenimine 23. The imidoyl(p-toluoyl)ketene rotamer 25 cyclizes to 4-toluoyloxyquinoline 28 and 4-quinolone 29. The cyclization of imidoyl(benzoyl)ketene 13 to 4-benzoyloxyquinoline 18, and of 25 to 28 involves 1,3-C-to-O shifts of benzoyl (toluoyl) groups. Calculations of the transition states for the transformations at the B3LYP/6-31G** level of theory are in agreement with the observed reaction preferences.

First-principle study of adsorption of hydrogen on Ti-doped Mg(0001) surface

Ab initio density functional theory (DFT) calculations are performed to study the adsorption of H-2 molecules on a Ti-doped Mg(0001) surface. We find that two hydrogen molecules are able to dissociate on top of the Ti atom with very small activation barriers (0.103 and 0.145 eV for the first and second H-2 molecules, respectively). Additionally, a molecular adsorption state of H-2 above the Ti atom is observed for the first time and is attributed to the polarization of the H-2 molecule by the Ti cation. Our results parallel recent findings for H-2 adsorption on Ti-doped carbon nanotubes or fullerenes. They provide new insight into the preliminary stages of hydrogen adsorption onto Ti-incorporated Mg surfaces.

Formation of cumulenes, triple-bonded, and related compounds by flash vacuum thermolysis of five-membered heterocycles

Flash vacuum thermolysis of a large variety of heterocyclic compounds is a useful means of production of ketenes, ketenimines, thioketenes, allenes, iminopropadienones, bis(imino)propadienes, iminopropadienethiones, carbodiimides, isothiocyanates, acetylenes, fulminic acid, nitrile imines and nitrile ylides, nitriles, cyanamides, cyanates, and other compounds, often in preparatively useful yields.

Quinolizine-2,4-diones by reversible dimerisation 2-pyridylketenes

Quinolizine-2,4-diones 11 are obtained by ash vacuum thermolysis (FVT) of 3-acyl-1,2,3-triazolo[1,5-a]pyridines 7. The reaction takes place via methyl- and phenyl(2-pyridyl)ketenes 10, which are directly observable by infrared spectroscopy in low temperature matrices. FVT of 11 regenerates the ketenes 10.

Reversible luminescence thermochromism in dipotassiumsodium tris[dicyanoargentate(I)] and the role of structural phase transitions

As a function of temperature, the layered compound K2Na[Ag(CN)213 displays dramatic variations in luminescence thermochromism with major trend changes occurring around 80 K. In order to understand these interesting optical properties, high-resolution neutron diffraction investigations were performed on a polycrystalline sample of this material in the temperature range from 1.5 to 300 K, and previous synchrotron X-ray data of Larochelle et al. (Solid State Commun. 114, 155 (2000)) were reinterpreted. The corresponding significant structural changes were found to be continuous with an anomalous increase of the monoclinic c-lattice parameter with decreasing temperature, associated with slight reorientations of two inequivalent, approximately linear N-C-Ag-C-N units. In the whole temperature range, the crystal structure is monoclinic with the space group C2/m. Based on the structural results, the major luminescence thermochromism changes around 80 K are attributed to the dominance of a back energy transfer process from low- to high-energy excitons at high temperatures. (E) 2002 Elsevier Science (USA).

3-pyridylcarbene and 3-pyridylnitrene: Ring opening to nitrile ylides

Photolysis of 3-pyridyldiazomethane in an Ar matrix at 7-10 K gives 3-pyridylcarbene. Further photolysis causes ring opening to nitrile ylide 26 (formonitrile pent-2-en-4-ynylide) as the major reaction together with a minor amount of ring expansion to 1-azacyclohepta-1,3,4,6-tetraene, 27. Matrix photolysis of 3-azidopyridine leads to ring opening to formonitrile N-cyanovinylmethylide, 33.

Electronic structure methods applied to gas-carbon reactions

A review is given on the fundamental studies of gas-carbon reactions using electronic structure methods in the last several decades. The three types of electronic structure methods including semi-empirical, ab initio and density functional theory, methods are briefly introduced first, followed by the studies on carbon reactions with hydrogen and oxygen-containing gases (non-catalysed and catalysed). The problems yet to solve and possible promising directions are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.

Imidoylketene dimerization and rearrangement

FVT of pyrroledione 10 affords the NH-imidoylketene 11, which is characterized by its matrix isolation IR spectrum ( 2117 cm 1). On warming above 170 K, 11 dimerizes to the oxazinone 13, the X-ray crystal structure of which is reported. Imidoylketene 11 also undergoes a (reversible) 1,3-phenyl shift to afford the detectable alpha-oxoketenimine 16 (2062 cm(-1)) which at FVT temperatures above 400degreesC, isomerizes to 2-cyano-2-phenylacetophenone 18 (optimally at 700degreesC). Moreover, imidoylketene 11 can cyclize to azetinone 19, detectable at FVT temperatures up to 570degreesC, which undergoes cycloreversion to diphenylacetylene 20 and isocyanic acid (HNCO) 21. Energy profiles calculated at the B3LYP/6-31G** level for the unsubstituted imidoylketene, the diphenylimidoylketene 11 and the N-tert-butylimidoylketene are also reported.