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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Isomeric structures and structural transformations in small clusters

The temperature dependence of the energies of the isomers of a seven-particle system is studied with a view toward understanding ergodicity problems in Monte Carlo simulations. It is found that the phase space of particles in a cluster is not ergodic at lower temperatures.

Kaelberer, J. B.↗
NASA NTRS1990

Isomeric Trisaryloxycyclotriphosphazene Polymer Precursors

Substances useful for making heat-and fire-resistant polymers. Cyclotriphosphazene-based monomers and polymer precursors led to development of high-temperature materials. Cyclotriphosphazene-derived monomers, polymer precursors, and polymers becoming important from both industrial and scientific points of view. Presence of phosphazene moiety in cyclotriphosphazene-based monomers and polymer precursors expected to impact special properties in desired high-performance materials containing inorganic backbones for aerospace applications. Useful for obtaining heat-and fire-resistant polymers for composites, adhesives, molding powders, and coating laminates. Also used in structures (e.g. secondary structures in aircraft), in construction of spacecraft, and in electronics and computer industries.

St. Clair, Terry L.↗
NASA NTRS1952

Correlation of physical properties with molecular structure for some dicyclic hydrocarbons having high thermal-energy release per unit volume -- 2-alkylbiphenyl and the two isomeric 2-alkylbicyclohexyl series

Three homologous series of related dicyclic hydrocarbons are presented for comparison on the basis of their physical properties, which include net heat of combustion, density, melting point, boiling point, and kinematic viscosity. The three series investigated include the 2-n-alkylbiphenyl, 2-n-alkylbicyclohexyl (high boiling), and 2-n-alkylbiphenyls (low boiling) series through c sub 16, in addition to three branched-chain (isopropyl, sec-butyl, and isobutyl) 2-alkylbiphenyls and their corresponding 2-alkylbicyclohexyls. The physical properties of the low-boiling and high-boiling isomers of 2-sec-butylbicyclohexyl and 2-isobutylbicyclohexyl are reported herein for the first time.

Goodman, Irving A↗
NASA NTRS1994

Computed Minimum Energy Pathway for Isomerization in Ketene

A minimum energy pathway for interchange of the CH groups in ketene via a C2v structure has been obtained using complete active space self consistent field (CASSCF) derivative methods with a polarized valence double zeta basis set to define the reaction pathway followed by multi-reference internally contracted configuration interaction (ICCI) calculations with a [3s3p2d/3s2p] basis set to determine the energetics. Qualitatively, the C2v structure is found to be a shallow minimum on the potential energy surface separated from ketene by a small barrier (0.2 kcal/mol), a second minimum, and a larger barrier (3.0 kcal/mol). The minimum energy pathway leading from the C2v minimum to ketene starts by simultaneous rotation of the farther CH group out of the plane and away from the oxygen followed by increase of the CCO angle and subsequent 1,2-migration of the H of the nearer CH group toward the carbon of the farther CH group.

Walch, Stephen P.↗
NASA NTRS2004

Studying Biological Rhythms of Person's Skin-galvanic Reaction and Dynamics of Light Transmission by Isomeric Substance in Space Flight Conditions

Intensity and amplitude of human functional systems and human most important organs are wavelike, rhythmic by nature. These waves have constant periodicity, phase and amplitude. The mentioned characteristics can vary, however their variations have a pronounced reiteration in the course of time. This indicates a hashing of several wave processes and their interference. Stochastic changes in wave processes characteristics of a human organism are explained either by 'pulsations' associated with hashing (superposition) of several wave processes and their interference, or by single influence of environmental physical factors on a human organism. Human beings have respectively periods of higher and lower efficiency, state of health and so on, depending not only of environmental factors, but also of 'internal' rhythmic factor. Sometimes peaks and falls periodicity of some or other characteristics is broken. Disturbance of steady-state biological rhythms is usually accompanied by reduction of activity steadiness of the most important systems of a human organism. In its turn this has an effect on organism's adaptation to changing living conditions as well as on general condition and efficiency of a human being. The latter factor is very important for space medicine. Biological rhythmology is a special branch of biology and medicine, it studies rhythmic activity mechanisms of organs, their systems, individuals and species. Appropriate researches were also carried out in space medicine.

Glushko, Vladimir↗
NASA NTRS1990

Isomerization of 1-O-indol-3-ylacetyl-beta-D-glucose. Enzymatic hydrolysis of 1-O, 4-O, and 6-O-indol-3-ylacetyl-beta-D-glucose and the enzymatic synthesis of indole-3-acetyl glycerol by a hormone metabolizing complex

The first compound in the series of reactions leading to the ester conjugates of indole-3-acetic acid (IAA) in kernels of Zea mays sweet corn is the acyl alkyl acetal, 1-O-indol-3-ylacetyl-beta-D-glucose (1-O-IAGlu). The enzyme catalyzing the synthesis of this compound is UDP-glucose:indol-3-ylacetate glucosyl-transferase (IAGlu synthase). The IAA moiety of the high energy compound 1-O-IAGlu may be enzymatically transferred to myo-inositol or to glycerol or the 1-O-IAGlu may be enzymatically hydrolyzed. Alternatively, nonenzymatic acyl migration may occur to yield the 2-O, 4-O, and 6-O esters of IAA and glucose. The 4-O and 6-O esters may then be enzymatically hydrolyzed to yield free IAA and glucose. This work reports new enzymatic activities, the transfer of IAA from 1-O-IAGlu to glycerol, and the enzyme-catalyzed hydrolysis of 4-O and 6-O-IAGlu. Data is also presented on the rate of non-enzymatic acyl migration of IAA from the 1-O to the 4-O and 6-O positions of glucose. We also report that enzymes catalyzing the synthesis of 1-O-IAGlu and the hydrolysis of 1-O, 4-O, and 6-O-IAGlu fractionate as a hormone metabolizing complex. The association of synthetic and hydrolytic capabilities in enzymes which cofractionate may have physiological significance.

Non-NASA Center↗
NASA NTRS2005

Tetrahydrophthalic Anhydrides as Addition Curing Polyimide End Caps: Thermal Isomerization of Methylendianiline 3,6-Diphenyltetrahydrophthalic Bisimides

In depth NMR studies confirm that heating a 1:2 mixture of cis, cis, cis 3,6-diphenyltetrahydrophthalic anhydride (end cap 9c) with methylenedianiline at 316 C initially yields the corresponding highly congested cis, cis, cis 3,6-diphenyltetrahydrophthalic bisimide 11, which is converted at this temperature to the observed product, the less hindered trans, cis, trans isomer 12.

Frimer, Aryeh A.↗
NASA NTRS1986

Steranes and triterpanes generated from kerogen pyrolysis in the absence and presence of minerals

Changes in the degree of isomerization of steranes and triterpanes during kerogen pyrolysis in the absence or presence of minerals are described. Kerogen from a Monterey Formation diatomite core sample was pyrolyzed under reduced pressure at 300 C for 2-1000 hrs in the presence or absence of calcite, illite, and montmorillonite. It is observed that the extent of isomerization of biomakers in C-20 in 14 alpha(H), 17 alpha(H)-steranes, at C-22 in 17 alpha(H), 21 beta(H)-hopanes and of 17 beta(H), 21 beta(H)-hopanes correlates to early diagenetic stages in rock extracts from sedimentary basins, and faster isomerization for steranes and triterpanes in the bitumen then for those in the kerogen structure are detected. The effects of the minerals on the isomerization reactions are examined; calcite inhibits, and illite and montmorillonite catalyze the reactions.

Tannenbaum, E.↗
NASA NTRS1974

Thermal rearrangements in polypentenamer and polyoctenamer

Experiments were conducted to show that polypentenamer and polyoctenamer, in common with 1,4-polybutadiene, undergo anaerobic thermal cis-trans isomerization at temperatures as low as 200 degrees C. Their activation energies for isomerization are comparable to that of polybutadiene. The relative rate constants are found to decrease in the order: polybutadiene, polypentenamer, polyoctenamer. When heated in a vacuum at 200 to 270 degrees C, polypentenamer exhibits a loss of double bonds. This disappearance of unsaturation has no counterpart in polybutadiene or polyoctenamer. In polypentenamer, a mechanism for a thermally-induced ring closure that is different from the one shown for the cyclization-depolymerization of various unsaturated polymers is evident. The cyclization-depolymerization process does cause a loss of double bonds in polybutadiene and polyoctenamer at temperatures greater than or equal to 275 degrees C.

Golub, M. A.↗
NASA NTRS1975

Theoretical description of the diimide molecule

The N2H2 molecule is only metastable and as a consequence is not well characterized experimentally. Therefore, we have carried out extensive Hartree-Fock calculations in order to determine equilibrium geometries, one-electron properties, and the relative energies of the cis and trans isomers. In addition, Hartree-Fock and multiconfiguration calculations were carried out to determine the mechanism for isomerization. The trans isomer was found to be 6.6 kcal/mole more stable than the cis form using a basis set which included polarization functions. The lowest energy path found for isomerization occurred by inversion about one nitrogen (rather than rotation about the NN bond) with an activation energy of 47 kcal/mole. Excitation energies for the lowest singlet and triplet excited states are also presented.

Winter, N. W.↗
NASA NTRS1978

Thermal rearrangements of unsaturated polymers

Thermal anaerobic uncatalyzed cyclizations and cis-trans isomerizations observed in unsaturated hydrocarbon polymers are surveyed. Three main types of cyclizations are described. Type I is a radical reaction which is caused by chain rupture and gives rise to six-membered rings; this reaction occurs during pyrolysis of polymers with double bonds in a 1,5- or 1,6-diene configuration. Type II is a (2 + 2) thermal cycloaddition of double bonds in certain polymers with a 1,6-diene structure; bicycloheptane structures result. Type III is an intramolecular ene reaction. Many polymers containing a double bond linking CH units display thermal cis-trans isomerization. The common activation energy is approximately 130 kJ/mol, and the initial rate constants are lower, for homologous polymers, the greater the separation of the carbon-carbon double bonds.

Golub, M. A.↗
NASA NTRS1983

Characterization of geometric isomers of Norbornene end-capped imides

Three geometric isomers from the thermal isomerization of methylene-4,4' bis(endo-N-phenylbicyclo/2.2.1/hept-2-ene-5,6-di carboximide) (I) were chromatographically separated and isolated in order to investigate the thermal cure of norbornene end-capped imide oligomers, which display considerable promise for use in various aerospace adhesive and composite applications. Endo-endo (I), endo-exo (II), and exo-exo (III) configurations were assigned to each compound based on the results of NMR spectroscopy. Several chromatographic, spectroscopic, and thermal techniques were then used to characterize these three isomers which serve as model compounds for norbornene end-capped polyimides. It was found that each compound thermally isomerized to an equilibrium mixture of all three compounds prior to cure. It is proposed that these compounds react by different mechanisms in air and nitrogen.

Young, P. R.↗