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81.
The standard (p(o) = 0.1 MPa) molar enthalpy of formation of crystalline 2H-1,3-benzoxazine-2,4(3H)dione was measured, at T= 298.15 K, by static bomb calorimetry and the standard molar enthalpy of sublimation, at T= 298.15 K, was obtained using Calvet microcalorimetry. These values were used to derive the standard molar enthalpy of formation in the gaseous phase, T= 298.15 K, of -(401.0 +/- 3.5) kJ mol(-1). The standard molar enthalpy of sublimation of isatoic anhydride was recalculated, and our recommended experimental value for the standard molar enthalpy of formation in the gaseous phase, T= 298.15 K, is -(406.2 +/- 3.4) kJ mol(-1). Density functional calculations for the two isomers 2H-1,3-benzoxazine-2,4(3H)dione and isatoic anhydride, in which the ring nitrogen and oxygen have been transposed, confirm the experimental evidence of nearly identical thermochemical stability for these isomers.  相似文献   
82.
We report the supramolecular chemistry of several metal complexes of N-(4-pyridyl)benzamide (NPBA) with the general formula [Ma(NPBA)2AbSc], where M = Co2+, Ni2+, Zn2+, Mn2+, Cu2+, Ag+; A = NO3-, OAc-; S = MeOH, H2O; a = 0, 1, 2; b = 0, 1, 2, 4; and c = 0, 2. NPBA contains structural features that can engage in three modes of intermolecular interactions: (1) metal-ligand coordination, (2) hydrogen bonding, and (3) pi-pi stacking. NPBA forms one-dimensional (1-D) chains governed by hydrogen bonding, but when reacted with metal ions, it generates a wide variety of supramolecular scaffolds that control the arrangement of periodic nanostructures and form 1- (2-4), 2- (5), or 3-D (6-10) solid-state networks of hydrogen bonding and pi-pi stacking interactions in the crystal. Isostructural 7-9 exhibit a 2-D hydrogen bonding network that promotes topotaxial growth of single crystals of their isostructural family and generates crystal composites with two (11) and three (12) different components. Furthermore, 7-9 can also form crystalline solid solutions (M,M')(NPBA)2(NO3)2(MeOH)2 (M, M' = Co2+, Ni2+, or Zn2+, 13-16), where mixtures of Co2+, Ni2+, and Zn2+ share the same crystal lattice in different proportions to allow the formation of materials with modulated magnetic moments. Finally, we report the effects that multidimensional noncovalent networks exert on the magnetic moments between 2 and 300 K of 1-D (4), 2-D (5), and 3-D (7, 8, 10, and 13-16) paramagnetic networks.  相似文献   
83.
Density functional theory calculations were employed to study the relative contribution of resonance versus inductive effects toward the 37 kcal/mol enhanced gas-phase acidity (DeltaH degrees (acid)) of formic acid (1) over methanol (2). The gas-phase acidities of formic acid, methanol, vinyl alcohol (5), and their vinylogues (6, 8, and 9) were calculated at the B3LYP/6-31+G level of theory. Additionally, acidities were calculated for the formic acid and vinyl alcohol vinylogues in which the formyl group and the vinyl group, respectively, were perpendicular to the rest of the conjugated system. Comparisons among these calculated acidities suggest that inductive effects are the predominant effects responsible for the enhanced acidity of formic acid over methanol, accounting for between roughly 62% and 65% of the total enhanced acidity; the remaining 38% to 35% of the acidity enhancement appears to be due to resonance effects. Further comparisons suggest that resonance effects are between roughly 58% and 65% of the 26 kcal/mol calculated acidity enhancement of vinyl alcohol over methanol, and the remaining 42% to 35% are due to inductive effects.  相似文献   
84.
We have derived a new multipolar expansion for intermolecular potential-energy functions with applications in molecular physics, theoretical chemistry, and mathematical physics. The new formulation employs a separation of radial and angular terms with a simple index structure that leads to computational efficiency and ease of physical interpretation. For the case of the Coulomb interaction, we compare the present formulation with two conventional multipole expansions: the Cartesian tensor and the irreducible spherical tensor expansions. The new formalism leads to efficient numerical algorithms that are useful for general applications beyond intermolecular potentials. In addition to the electrostatic Coulomb interaction, we illustrate the formalism with applications to special function theory and a bipolar expansion involved in potential theory.  相似文献   
85.
Several electrophilic lactonization procedures have been explored as a means of functionalizing olefinic carboxylic acids with relative asymmetric induction, Iodolactonization of δ,? -unsaturated acids under conditions of thermodynamic control exhibits good 1,2- and 1,3-, but not 1,4-induction in the formation of δ-lactones. Mercurilactonization proceeds with good stereocontrol in the formation of both γ- and δ-lactones (1,2-induction), but suffers from the difficulty of elimination during reductive demercuration; phenylselenolactonization with N-(phenylseleno)phthalimide is apparently kinetically controlled, affording high induction with 10, a strongly sterically biased substrate leading to a δ-lactone, but not with 16, which leads to a γ-lactone. In contrast, hydroxymethyllactonization proceeds with good stereocontrol in the case of 26, the ester of 10, but not with the analogous ester of 16. The lactones resulting from cyclization of 10 and 13 were converted in stereospecific fashion into each of the stereoisomers of(±)-serricornin.  相似文献   
86.
A thin chip polymer-based microsprayer has been coupled to a hybrid quadrupole time-of-flight mass spectrometer (QTOF MS) and introduced in carbohydrate research. The feasibility of the approach is demonstrated for mapping, sequencing and structural elucidation of glycoconjugates originating from human body fluids and tissues such as a glycopeptide mixture from normal human urine and an isolated and purified GT1 ganglioside fraction from normal adult human brain. The optimization procedure required by each glycoconjugate category is described and the advantages of the system in terms of flexibility and adaptability to QTOF MS, stability of the ESI MS signal, carbohydrate ionization and sequencing, sensitivity, speed of analysis and sample consumption are discussed.  相似文献   
87.
Several bifunctionally reactive linkers containing halide or sulfonate ester groups were prepared. The linkers were used to quaternize 5-(4-methoxyphenyl)-2-(4-pyridyl)oxazole and 2-(6-chromanyl)-5-(4-pyridyl)oxazole to produce fluorescent stains that contained a reactive group such as an isothiocyanate, an N-hydroxysuccinimidyl ester, a maleimide, or an oxirane. The stains were derivatized with either 1-propylamine, 1-propanethiol, or piperidine, as appropriate, to help in characterization. The stains may serve as more photostable alternatives to fluoresceins or coumarins.  相似文献   
88.
In the recently discovered (2+1)-dimensional relativistic Chern-Simons model, self-duality can be achieved when the Higgs potential density assumes a special form for which both the asymmetric and symmetric vacua are ground state solutions. This important feature may imply the coexistence of static topological and non-topological vortex-like solutions inR 2 but the latter have been rather elusive to a rigorous construction. Our main purpose in this paper is to prove the existence of non-topological radially symmetricN-vortex solutions in the self-dual Chern-Simons model. By a shooting method, we obtain a continuous family of gauge-distinctN-vortex solutions. Moreover, we are also able to classify all possible bare (or 0-vortex) solutions.  相似文献   
89.
We consider a one-dimensional model of a system in contact with a heat bath: A particle (the system ormolecule) of massM, confined to the unit interval [0, 1], is surrounded by an infinite ideal gas (thebath of atoms) of point particles of massm with which it interacts via elastic collisions. The atoms are not affected by the walls at 0 and at 1. We obtain convergence to equilibrium for the molecule, from essentially any initial distribution on its position and velocity. The infinite composite system of molecule and bath has very good ergodic properties: it is a Bernoulli system.Supported in part by NSF Grants PHY 78-03816 and PHY 78-15920  相似文献   
90.
Bioassay-guided fractionation of the methanol extract of Erylus lendenfeldi using engineered strains of budding yeast (Saccharomyces cerevisiae) has resulted in the isolation of the known compound eryloside A (1) and two new compounds, erylosides K (2) and L (3). The structures were established based mainly on 1D and 2D NMR data. The absolute stereochemistry of eryloside A, which had never been fully characterized, was determined using the modified Mosher's method. The absolute stereochemistry of eryloside K was determined by comparison with tetrahydroeryloside A. Compounds 1-3 exhibited selective cytotoxicity against a yeast strain (Δrad50) deficient in double strand break (DSB) repair.  相似文献   
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