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排序方式: 共有215条查询结果,搜索用时 31 毫秒
1.
A.L. Greer 《Thermochimica Acta》1980,42(2):193-222
The subject of metallic glasses is introduced. These materials have technological potential, but only if their stability is adequate. It is, therefore, important to study their transformations on annealing, especially structural relaxation. The ferromagnetic Curie temperature, Tc, is a particularly useful parameter for monitoring structural relaxation and it can be measured by differential scanning calorimetry (DSC). The DSC method is convenient and accurate if temperature lags are corrected for. Ways of performing this correction in the DuPont DSC system are reviewed and a new method is proposed and tested. The experimental details of determining the Tc of metallic glasses are given. Finally, the relaxation behaviour of Fe80B20 glass, as studied by measuring its Tc, is reviewed. This behaviour is similar to that of conventional oxide glasses. 相似文献
2.
J. C. Greer R. Ahlrichs I. V. Hertel 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,18(4):413-426
A theoretical study of the energetics and intramolecular dynamics for ammonia cluster cations (NH3) 2 + and (NH3) 3 + is presented. The proton transfer mechanism after a vertical ionisation is followed in detail. Structural and energetic calculations are performed near the Hartree-Fock self consistent field (HF-SCF) limit; all open shell calculations are spin restricted (RHF). The calculations reconcile experimental results from thermochemical and photoionisation measurements: discrepancies in the energetics are shown to be due to the probing of different regions of the potential energy surface (PES). For the dynamics calculations, it is impractical to determine a large region of the multidimensional PES and then integrate Newton's equations. The calculations therefore incorporate a quantum mechanical determination of the electronic energy of the system after each time increment while nuclear degrees of freedom are handled classically. In this way, the classical reaction path across the Born-Oppenheimer surface is obtained. 相似文献
3.
4.
Greer A Vassilikogiannakis G Lee KC Koffas TS Nahm K Foote CS 《The Journal of organic chemistry》2000,65(21):6876-6878
We report the effects of added acid in the reaction of singlet oxygen with trans-4-propenylanisole (1). We provide evidence that solvent acidity modifies the behavior of the transient intermediates. Relative to reactions in aprotic solvent, enhanced dioxetane concentrations are observed in MeOH and in nonprotic solvents with acid. We suggest a new mechanism that invokes a proton transfer from MeOH and benzoic acid to perepoxide (2) and zwitterion (3) intermediates. 相似文献
5.
[formula: see text] Hexamethylbipyrazine-N,N',N",N"'-tetraoxide (1) is synthesized in racemic form and cocrystallized with tetracyanoethylene to give a donor-acceptor (DA) networked lattice. The resulting DA2 cocrystal contains homochiral 2-D DA arrays (layers) cohered by 2.7 A NO...C=C DA bonds with periplanar O p-orbital/C=C orientations. Layer formation is stereoselective, while interlayer relations yield a racemic lattice. 相似文献
6.
J. C. Greer C. Hüglin I. V. Hertel R. Ahlrichs 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1994,30(1):69-75
Geometric structures and the energies for the ground and several excited electronic states of a sodium atom bound with one or two ammonia molecules are presented. All self consistent field (SCF) calculations are performed with extended basis sets. Geometry optimization and one electron properties have been performed within the SCF approximation. Excited states have been calculated with the multi-configuration SCF (MCSCF) technique. This system may be viewed as a precursor to solvation in a macroscopic system. The excited state calculations provide important information for spectroscopic studies of these complexes. 相似文献
7.
The N-oxide and N,N'-dioxide (1ao and 1ado) of trans-1,1'-azonorbornane (1) associate with TCNE in solution and in the solid state. The solution complexes are characterized by charge-transfer optical absorptions at lambda(m) 364 and 540 nm in CH(2)Cl(2) for 1ao/TCNE and 1ado/TCNE, respectively. These complexes are weakly bound, with K(f) values of 0.5-3.0 M(-)(1). Crystals of (1ao)(2)/TCNE and 1ado/TCNE are isolable, and their structures have been determined by X-ray diffraction. Local donor-acceptor (DA) interactions between the pi dipolar donors and the electron poor TCNE are found. Crystals of (1ao)(2)/TCNE are composed of discrete D-A-D triads in which the 1ao oxygen approaches one of the olefinic C atoms (C(1), 2.86 ?) of TCNE more closely than the other (C(2), 3.07 ?). The O-C(1)-C(2) angle is 87 degrees, and the azooxide molecular plane lies perpendicular to the molecular plane of TCNE. 1ado/TCNE crystals are composed of extended -D-A-D-A- strands in which both oxygen poles of the azodioxide simultaneously interact with alternate TCNE acceptors. The D-A geometry here is structurally analogous to that in the (1ao)(2)/TCNE crystal. PM3 calculations of 1:1 and 1:2 trans HN(O)NH(O)/TCNE complexes constrained to have D-A (O-C) distances of 2.88 ?, but which are otherwise geometry optimized, reproduce the DA topology observed in the crystalline samples. 相似文献
8.
9.
In this paper,we study the surface instability of a cylindrical pore in the absence of stress. This instability is called the Rayleigh-Plateau instabilty. We consider the model developed by Spencer et ... 相似文献
10.
Edyta M. Greer Christopher V. Cosgriff Olga Lavinda 《Journal of Physical Organic Chemistry》2012,25(12):1293-1298
Pentamethylene chain conformational effects for the Bergman cyclization of the 11‐membered ring enediyne, (3Z)‐3‐cycloundecene‐1,5‐diyne, 2, are examined theoretically with unrestricted Becke, three‐parameter, Lee–Yang–Parr/6‐31 G(d,p) calculations. A C1 symmetric enediyne conformation was found to be the global minimum, where its nonsymmetric pentamethylene chain prevented π‐orbital alignment of the acetylene groups for C–C σ bond product formation. The Bergman cyclization of 2 was found to be conformationally dependent. In a Curtin–Hammett type process, the C1 symmetric 2 inverts to one of the CS or C2 symmetric conformers required for the Bergman cyclization, which produced a CS or C2 symmetric 1,4‐diradical intermediate. The activation energy for the cyclization is slightly higher to reach the C2 symmetry diradical compared with the CS symmetry diradical. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献