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Brendan Twamley

Publications and source records attributed to Brendan Twamley.

At least 19 recordsLinked to original sources
PubMed2006

Energetic bicyclic azolium salts.

Bicyclic azoles, 2-methyl-5-(imidazol-1-yl)-2H-tetrazole (1), 2-methyl-5-(1,2,4-triazol-1-yl)-2H-tetrazole (4), 1-methyl-5-(imidazol-1-yl)-1H-tetrazole (7), 1-methyl-5-(1,2,4-triazol-1-yl)-1H-tetrazole (10), 1-methyl-4-nitro-2-(imidazol-1-yl)-1H-imidazole (13), and 1-methyl-4-nitro-2-(1,2,4-triazol-1-yl)-1H-imidazole (16) were prepared. Their thermally stable azolium salts, 3, 6, 9, 12, 15, and 18-21, with densities ranging between 1.519-1.674 g cm-3, were synthesized by quaternization with nitric or perchloric acid or with iodomethane followed by metathesis reactions with silver nitrate and silver perchlorate. The structures of 12 b and 21 b were confirmed by single-crystal X-ray analysis. The standard enthalpies of formation for some of the new salts were calculated by using the computationally feasible DFT(B3LYP) and MP2 methods in conjunction with an empirical approach based on densities of salts. The calculated values range from DeltaHdegreef=209.9 (21 a) to 412.3 (12 b) kJ mol-1 in which the experimental densities are >1.515 g cm-3.

Journal Article↗
PubMed2006

The nature of halogen...halogen synthons: crystallographic and theoretical studies.

A study of the halogen...halogen contacts in organic compounds using ab initio calculations and the results of previously reported crystallographic studies show that these interactions are controlled by electrostatics. These contacts can be represented by the geometric parameters of the C--X1...X2--C moieties (where theta1=C--X1...X2 and theta2=X1...X2--C; ri=X1...X2 distance). The distributions of the contacts within the sum of van der Waals radii (rvdW) versus thetai (theta1=theta2) show a maximum at theta approximately 150 degrees for X=Cl, Br, and I. This maximum is not seen in the distribution of F...F contacts. These results are in good agreement with our ab initio calculations. The theoretical results show that the position of the maximum depends on three factors: 1) The type of halogen atom, 2) the hybridization of the ipso carbon atom, and 3) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom. Calculations show that the strength of these contacts decreases in the following order: I...I>Br...Br>Cl...Cl. Their relative strengths decrease as a function of the hybridization of the ipso carbon atom in the following order: sp2>sp>sp3. Attaching an electronegative atom to the carbon atom strengthens the halogen...halogen contacts. An electrostatic model is proposed based on two assumptions: 1) The presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the halogen atom.

Algorithms↗
PubMed2006

Dimethylammonium trichlorocuprate(II): structural transition, low-temperature crystal structure, and unusual two-magnetic chain structure dictated by nonbonding chloride-chloride contacts.

Catena(dimethylammonium-bis(mu2-chloro)-chlorocuprate), (CH3)2NH2CuCl3, forms chains of Cu2Cl6(2-) bifold dimers linked along the structural chain axis by terminal chlorides forming long semicoordinate bonds to adjacent dimers. The structural chains are separated by dimethylammonium ions that hydrogen bond to chloride ions of the dimers. A structural phase transition below room temperature removes disorder in the hydrogen bonding, leaving adjacent dimers along the chain structurally and magnetically inequivalent, with alternating ferromagnetic and antiferromagnetic pairs. The coupled dimers are magnetically isolated from each other along the structural chain axis by the long semicoordinate Cu-Cl bond. However, the dimers couple to like counterparts on adjacent chains via nonbonding Cl...Cl contacts. The result is two independent magnetic chains, one an alternating antiferromagnetic chain and the other an antiferromagnetic chain of ferromagnetically coupled copper dimers, which run perpendicular to the structural chains. This magnetostructural analysis is used to fit unusual low-temperature (1.6 K) magnetization vs field data that display a two-step saturation. The structural phase transition is identified with neutron scattering and capacitance measurements, and the X-ray crystal structures are determined at room temperature and 84 K. The results appear to resolve long-standing confusion about the origins of the magnetic behavior of this compound and provide a compelling example of the importance of two-halide magnetic exchange.

Journal Article↗
PubMed2006

Syntheses and characterization of unsymmetric dicationic salts incorporating imidazolium and triazolium functionalities.

A series of both imidazolium- and triazolium-based unsymmetric dicationic salts with alkyl and polyfluoroalkyl substituents were prepared and characterized. Most of them can be classified as ionic liquids (MP < 100 degrees C). Key physical properties, such as melting point, thermal stability, density, and solubility in common solvents were determined and were compared with those of the related monocationic imidazolium- or triazolium-based salts. The effects of anions and substituents bonded to the triazolium and imidazolium cations on these properties were examined. 1-(3-Butyl-imidazolium-1-yl)methylene-(4-butyl-1,2,4-triazolium diiodide) (2d), the precursor of 1-(3-butylimidazolium-1-yl)methylene-(4-butyl-1,2,4-triazolium) bi[bis(trifluoromethanesulfonyl)amide] (3d), reacted with Pd(OAc)(2) at 120 degrees C to generate a binuclear palladium(II) dicarbene complex. The palladium(II) complex was characterized by single-crystal X-ray diffraction analysis and was used as a catalyst precursor for palladium-catalyzed Heck cross-coupling reactions in 3d. Preliminary results show that 3d could serve as both the solvent and catalyst support in the catalytic reactions.

Journal Article↗
PubMed2006

Coordination networks with fluorinated backbones.

Fluorine-containing ligands 2,3,5,6-tetrafluoro-1,4-bis(imidazol-1-yl-methyl)benzene (1) and 2,3,5,6-tetrafluoro-1,4-bis(2-methylimidazol-1-yl-methyl)benzene (2) were prepared and coordinated with AgNO3, Co(ClO4)2 x 6 H2O, and Cd(NO3)2 x 4 H2O, respectively, to form the following structures: 3D channel polymer [Ag2(1)2(NO3)2 x H2O x MeOH]n (3), 2D sheet polymer [Co(1)3(ClO4)2]n (4), 1D chain polymer [Cd(1)3(NO3)2 x 4 H2O]n (5), and a 2D herringbone sheet polymer [Ag(2)NO3 x 1.5 MeOH]n (6). The solid-state crystal structures of 3-6 were studied by single-crystal X-ray crystallography.

Crystallography, X-Ray↗
PubMed2006

Preparation of ultrafine chalcopyrite nanoparticles via the photochemical decomposition of molecular single-source precursors.

The synthesis and characterization of ultrafine CuInS2 nanoparticles are described. Ultraviolet irradiation was used to decompose a molecular single source precursor, yielding organic soluble approximately 2 nm sized nanoparticles with a narrow size distribution. UV-vis absorption, 1H and 31P{1H} NMR, and fluorescence spectroscopies and mass spectrometry were used to characterize decomposition of the precursors and nanoparticle formation. The nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy energy dispersive X-ray spectroscopy, powder X-ray diffraction (XRD), electron diffraction, inductively coupled plasma analysis, UV-vis absorption spectroscopy, and fluorescence spectroscopy. They have a wurzite-type crystal structure with a copper-rich composition. The hypsochromic shift in their emission band due to quantum confinement effects is consistent with the size of the nanocrystals indicated in the HRTEM and XRD analyses.

Chemical Fractionation↗
PubMed2005

Energetic quaternary salts containing bi(1,2,4-triazoles).

New energetic salts (2, 3, 9, 10, and 11) were synthesized via the protonation of 4,4'-bi(1,2,4-triazole) or N-4-(1,2,4-triazole)-N-3-(4-methyl-1,2,4-triazole)amine with nitric acid or perchloric acid or 5-nitro-tetrazole. The structures of 4,4'-bi(1,2,4-triazolium) nitrate (2), N,N-dimethyl-N'-(5-methyl-tetrazole)methanimidamide (8), and N-4-(1,2,4-triazole)-N-3-(4-methyl-1,2,4-triazolium)amine perchlorate (10) were confirmed by a single-crystal X-ray analysis. The physical properties and heats of combustion of the new ionic salts were measured, and the heats of formation were also determined.

Journal Article↗
PubMed2005

Energetic azolium azolate salts.

Energetic salts comprising substituted imidazolium and 1,2,4-triazolium cations and 4,5-dinitro-imidazolate and 5-nitrotetrazolate anions were synthesized and characterized. On the basis of experimentally obtained heats of combustion, the calculated heats of formation range from deltaHf(o) degrees = 80 (3) to 1071 kJ/mol (13). Imidazolate salts are more dense but have lower heats of formation than their tetrazolate analogues. Salts 4, 5, 8, and 11-14 fall into the ionic liquid class (mp < 100 degrees C). The structure of 1,2,4-triazolium 5-nitrotetrazolate (10) was confirmed by X-ray analysis.

Journal Article↗
PubMed2005

Guanidinium-based ionic liquids.

Cyclic (subset=N+<, subset = imidazolidine 3 and 4, hexahydro-pyrimidine 7 and 8, tetrahydro-1,3,5-oxadiazine 12, and triazoline 15 and 16) and acyclic [(R2N)2C=N+<, 19] guanidinium-based salts were synthesized via the quaternization of guanidine derivatives with nitric or perchloric acid or with iodomethane followed by metathesis reaction with silver nitrate, silver perchlorate, or ammonium dinitroamide. The structure of 15d was confirmed by single-crystal X-ray analysis. Most of the salts exhibited low melting points and good thermal stabilities. Their densities range between 1.2 and 1.5 g/cm3. Standard molar enthalpies of formation were calculated from experimentally determined constant-volume combustion energies obtained using an oxygen bomb calorimeter.

Journal Article↗
PubMed2005

13-methyl-2,6-dithia[7]metacyclophane: a useful molecule to connect VT NMR results and structure with calculations.

Ground state energies (DFT) and 1H and 13C NMR chemical shifts are calculated for the conformers of 13-methyl-2,6-dithia[7]metacyclophane (1), and the results are compared with X-ray structural data and variable-temperature NMR data, including the determination of the activation barrier. Calculations predict the correct low energy conformer with good agreement with chemical shifts, bond distances, and angles. VT NMR data for the 10-tert-butyl-substituted derivative 2 indicate that it undergoes the same conformational equilibria as 1. This paper should enhance the confidence that organic chemists have in calculations to satisfactorily predict conformer energies.

Journal Article↗
PubMed2004

Energetic salts from N-aminoazoles.

New energetic salts (2, 3, 5, 6, 13, 14, 21, 22) were synthesized via the quaternization of derivatives of N-aminoazoles with nitric or perchloric acid or with iodomethane followed by metathesis reaction with silver nitrate or silver perchlorate. The structure of 2-amino-4,5-dimethyltetrazolium nitrate (21) was confirmed by single-crystal X-ray analysis. Most of the salts exhibit good thermal stabilities and low melting points. By using experimentally determined constant volume combustion energies, the standard molar enthalpies of formation were derived on the basis of designed Hess thermochemical cycles.

Journal Article↗