The Diels-Alder reaction between cyclopentadiene and three dienophiles (acrolein, methyl acrylate and acrylonitrile) having different hydrogen bond acceptor abilities has been carried out in several ionic liquids and molecular solvents in order to obtain information about the factors affecting reactivity and selectivity. The solvent effects on these reactions are examined using multiparameter linear solvation energy relationships. The collected data provide evidence that the solvent effects are a function of both the solvent and the solute. For a solvent effect to be seen, the solute must have a complimentary character; selectivities and rates are determined by the solvent hydrogen bond donation ability (alpha) in the reactions of acrolein and methyl acrylate, but not of acrylonitrile. 相似文献
Ethylene is the simplest organic molecule containing a double bond and is the starting monomeric unit in the synthesis of polyethylene, one of the most largely produced polymers. Here we report a high pressure infrared study of ethylene at room temperature. A polymerization reaction is observed when the crystalline phase I is compressed above 3.0 GPa. The reaction kinetics was investigated at two different pressures, 3.6 and 5.4 GPa. The recovered product was identified in both cases as polyethylene, but while a conformationally disordered and branched low-density polymer is obtained at the highest pressure, a high-density crystalline polymer is obtained at 3.6 GPa. A reaction mechanism was proposed on the basis of the kinetic data and the structural information. 相似文献
Inclusive proton spectra have been measured for the reaction 197Au(16O, p)X at 315 MeV. The data, which are consistent with emission from a moving source, are compared with the fireball model and with models preequilibrium emission. 相似文献
The combined effect of high pressure and electronic photo‐excitation has been proven to be very efficient in activating extremely selective polymerisations of small unsaturated hydrocarbons in diamond anvil cells (DAC). Here we report an ambient temperature, large volume synthesis of high density polyethylene based only on high pressure (0.4–0.5 GPa) and photo‐excitation (~350 nm), without any solvent, catalyst or radical initiator. The reaction conditions are accessible to the current industrial technology and the laboratory scale pilot reactor can be scaled up to much larger dimensions for practical applications. FTIR and Raman spectroscopy, and X‐ray diffraction, indicate that the synthesised material is of comparable quality with respect to the outstanding crystalline material obtained in the DAC. The polydispersity index is comparable to that of IV generation Ziegler‐Natta catalysts. Moreover the crystalline quality of the synthesised material can be further enhanced by a thermal annealing at 373 K and ambient pressure.
We investigate low degree rational cohomology groups of smooth compactifications of moduli spaces of curves with level structures.
In particular, we determine
Hk([`(S)]g, \mathbb Q){H^k\left({\bar S}_{g}, {\mathbb Q}\right)} for g ≥ 2 and k ≤ 3, where [`(S)]g{{\bar S}_{g}} denotes the moduli space of spin curves of genus g. 相似文献
The motion of a massive test particle in a Schwarzschild spacetime surrounded by a perfect fluid with equation of state p0=wρ0 is investigated. Deviations from geodesic motion are analyzed as a function of the parameter w, ranging from w=1, which corresponds to the case of massive free scalar fields, down into the so-called “phantom” energy, with w<−1. It is found that the interaction with the fluid leads to capture (escape) of the particle trajectory in the case 1+w>0 (<0), respectively. Based on this result, it is argued that inspection of the trajectories of test particles in the vicinity
of a Schwarzschild black hole with matter around may offer a new means of gaining insights into the nature of cosmic matter. 相似文献
The Maxwell equations for the electromagnetic potential, supplemented by the Lorenz gauge condition, are decoupled and solved
exactly in de Sitter space–time studied in static spherical coordinates. There is no source besides the background. One component
of the vector field is expressed, in its radial part, through the solution of a fourth-order ordinary differential equation
obeying given initial conditions. The other components of the vector field are then found by acting with lower-order differential
operators on the solution of the fourth-order equation (while the transverse part is decoupled and solved exactly from the
beginning). The whole four-vector potential is eventually expressed through hypergeometric functions and spherical harmonics.
Its radial part is plotted for given choices of initial conditions. We have thus completely succeeded in solving the homogeneous
vector wave equation for Maxwell theory in the Lorenz gauge when a de Sitter space–time is considered, which is relevant both
for inflationary cosmology and gravitational wave theory. The decoupling technique and analytic formulae and plots are completely
original. This is an important step towards solving exactly the tensor wave equation in de Sitter space–time, which has important
applications to the theory of gravitational waves about curved backgrounds. 相似文献
In this paper, we describe authentication codes via (generalized) Gray images of suitable codes over Galois rings. Exponential
sums over these rings help determine—or bound—the parameters of such codes. 相似文献
Al-doped lithium manganese spinels, with starting composition Li1.02AlxMn1.98−xO4 (0.00<x≤0.06), are investigated to determine the influence of the Al3+ doping on the Jahn-Teller (J-T) cooperative transition temperature TJ-T. X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al3+ ions in the spinel phase. It is observed that Al3+ may distribute between the two cationic sublattices. The J-T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x>0.04 TJ-T it increases with increasing x, as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior. 相似文献