Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 8 de 8
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Vis Exp ; (139)2018 09 18.
Artigo em Inglês | MEDLINE | ID: mdl-30295663

RESUMO

Boron carbide (B4C) is one of the hardest materials in existence. However, this attractive property also limits its machineability into complex shapes for high wear, high hardness, and lightweight material applications such as armors. To overcome this challenge, negative additive manufacturing (AM) is employed to produce complex geometries of boron carbides at various length scales. Negative AM first involves gelcasting a suspension into a 3D-printed plastic mold. The mold is then dissolved away, leaving behind a green body as a negative copy. Resorcinol-formaldehyde (RF) is used as a novel gelling agent because unlike traditional hydrogels, there is little to no shrinkage, which allows for extremely complex molds to be used. Furthermore, this gelling agent can be pyrolyzed to leave behind ~50 wt% carbon, which is a highly effective sintering aid for B4C. Due to this highly homogenous distribution of in situ carbon within the B4C matrix, less than 2% porosity can be achieved after sintering. This protocol highlights in detail the methodology for creating near fully dense boron carbide parts with highly complex geometries.


Assuntos
Compostos de Boro/química
2.
J Am Chem Soc ; 131(18): 6314-5, 2009 May 13.
Artigo em Inglês | MEDLINE | ID: mdl-19385621

RESUMO

The impact of temperature and charge on the conformation of tryptamine (Tryp) is examined in the gas phase by infrared laser-vibrational predissociation spectroscopy in the 2800-3800 cm(-1) region. Previous studies of neutral Tryp(H(2)O)(n) clusters showed preferential stabilization of specific tryptamine conformers through hydrogen bonding. When complexed with the biologically significant potassium ion, the only conformers found to form under these experimental conditions are built on hitherto unobserved neutral Tryp conformers. The electrostatic interaction between K(+), the tryptamine NH(2) lone pair, and the indole ring in K(+)(Tryp) favors the formation of these new conformers. The observed K(+)(Tryp)(H(2)O) conformers vary significantly from the previously reported neutral Tryp(H(2)O) structure. Using the argon tagging method, we show how variations in temperature impact the observed infrared spectra, demonstrating that different conformers are populated under the different experimental conditions. In addition, the presence of a high-energy conformer of K(+)(Tryp)(H(2)O), trapped by the argon evaporative cooling process, was identified. Exploring the conformational landscape of hydrated cluster ions bearing flexible biomolecules is now possible.


Assuntos
Elétrons , Temperatura , Triptaminas/química , Argônio , Gases , Íons , Potássio , Espectrofotometria Infravermelho/métodos
3.
J Am Chem Soc ; 130(46): 15393-404, 2008 Nov 19.
Artigo em Inglês | MEDLINE | ID: mdl-18939842

RESUMO

A delicate balance between competing and cooperating noncovalent interactions determines the three-dimensional structure of hydrated alkali-metal ion clusters. A critical factor influencing the balance reached is the internal energy content (or effective temperature) of the ion cluster. Cold cluster ions (approximately 50-150 K) have little internal energy, and enthalpic contributions have a greater influence on the relative population of low-lying minima. In clusters whose internal energy distributions correspond to temperatures approximately 250-500 K, entropic effects are expected to influence which structural isomers are present, favoring those where free energy has been minimized. Infrared photodissociation spectra of M(+)(H2O)(x=2-5) (approximately 250-500 K) are reported for M = Li, Na, K, and Cs to explore ion dependencies and entropic effects on the observed three-dimensional structure.

4.
J Am Chem Soc ; 130(46): 15381-92, 2008 Nov 19.
Artigo em Inglês | MEDLINE | ID: mdl-18939843

RESUMO

A delicate balance between competing and cooperating noncovalent interactions determines the three-dimensional structure of hydrated alkali-metal ion clusters. With a single water molecule hydrating an ion, the electrostatic ion...water interaction dominates. With more than one water molecule, however, water...water hydrogen-bonding interactions compete with the ion...water interactions to influence the three-dimensional structure. Infrared photodissociation spectra of M(+)(H2O)(x=2-5)Ar (with effective temperatures of approximately 50-150 K, depending on size and composition) are reported for M = Li, Na, K, and Cs, and dependencies on ion size and hydration number are explored.

5.
J Phys Chem A ; 111(49): 12409-16, 2007 Dec 13.
Artigo em Inglês | MEDLINE | ID: mdl-17696509

RESUMO

To properly understand the preferred structures and biological properties of proteins, it is important to understand how they are influenced by their immediate environment. Competitive intrapeptide, peptide...water, ion...water, and ion...peptide interactions, such as hydrogen bonding, play a key role in determining the structures, properties, and functionality of proteins. The primary types of hydrogen bonding involving proteins are intramolecular amide...amide (N-H...O=C) and intermolecular amide...water (O-H...O=C and H-O...H-N). n-Methylacetamide (NMA) is a convenient model for investigating these competitive interactions. An analysis of the IR photodissociation (IRPD) spectra of M+(n-methylacetamide)1(H2O)n=0-3 (M=Na and K) in the O-H and N-H spectral regions is presented. Ab initio calculations (MP2/cc-pVDZ) are used as a guide in identifying both the type and location of hydrogen bonds present. In larger clusters, where several structural isomers may be present in the molecular beam, ab initio calculations are also used to suggest assignments for the observed spectral features. The results presented offer insight to the nature of ion...NMA interactions in an aqueous environment and reveal how different ion...ligand pairwise interactions direct the extent of water...water and water...NMA hydrogen bonding observed.


Assuntos
Acetamidas/química , Modelos Moleculares , Proteínas/química , Análise Espectral/métodos , Ligação de Hidrogênio
6.
J Chem Phys ; 126(7): 074302, 2007 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-17328600

RESUMO

The design of cesium-selective ionophores must include the nature of cesium-water interactions. The authors have carried out extensive ab initio and density functional theory calculations of hydrated cesium cations to obtain reasonably accurate energetics, thermodynamic quantities, and IR spectra. An extensive search was made to find the most stable structures. Since water...water interactions are important in the aqua-Cs+ clusters, the authors investigated the vibrational frequency shifts as a function of the number of water molecules and the frequency characteristics with and without the presence of outer-shell water molecules. The predicted vibrational frequencies were then compared with the infrared photodissociation spectra of argon-tagged hydrated cesium cluster ions. This comparison allowed the identification of specific hydrogen-bonding structures present in the experimental spectra.


Assuntos
Césio/química , Modelos Químicos , Modelos Moleculares , Água/química , Transferência de Energia , Análise Espectral , Termodinâmica
7.
J Chem Phys ; 124(18): 184301, 2006 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-16709100

RESUMO

In recent years neutral indole(H2O)n clusters have been used to model the hydration of biomolecules containing an indole moiety. Both experimental and theoretical studies of the binary indole...OH2 system show NH...OH sigma hydrogen-bonding. By introducing a cation to the indole...OH2 model, cation...pi and ion...dipole electrostatic interactions are placed in direct competition with conventional indole...OH2 hydrogen-bonding. The effects, arising from a monovalent potassium cation on (indole)m(H2O)n clusters, were investigated using infrared photodissociation spectroscopy in the OH and NH stretching regions. In K+ (indole)1(H2O)(n < or = 4) and K+ (indole)2(H2O)(m < or = 3) clusters, the electrostatic ion...ligand interaction inhibits the formation of an indole NH...OH2 sigma hydrogen-bond. However, indole...H2O pi hydrogen-bonding via the five-membered indole ring is observed with three or more ligands around the ion.


Assuntos
Algoritmos , Indóis/química , Potássio/química , Água/química , Cátions , Ligação de Hidrogênio , Ligantes , Modelos Moleculares , Espectrofotometria Infravermelho , Eletricidade Estática
8.
J Chem Phys ; 124(2): 024319, 2006 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-16422595

RESUMO

The observed gas-phase coordination number of K+ in K+(H2O)m clusters is smaller than that observed in bulk solution, where the coordination number has been reported to be between 6 and 8. Both theoretical and gas-phase studies of K+(H2O)m cluster ions point to a coordination number closer to 4. In the gas phase, the coordination number is determined by a variety of factors-the most critical being the magnitude of the K+...ligand pairwise interaction. Decreasing the magnitude of the ion...ligand interaction allows more ligands to directly interact with the cation. One method for decreasing the ion...ligand interaction in K+(H2O)m clusters is to systematically substitute weakly bound ligands for the more strongly bound water molecules. The systematic introduction of para-difluorobenzene (DFB) to K+(H2O)m clusters was monitored using infrared photodissociation spectroscopy in the OH stretching region. By varying the ratio of DFB molecules to water molecules present in K+(H2O)m(DFB)n clusters, the observed coordination number of gas-phase K+ was increased to 8, similar to that reported for bulk solution.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA
...