High-pressure vibrational properties of polyethylene

The pressure evolution of the vibrational spectrum of polyethylene was investigated up to 50 GPa along different isotherms by Fourier-transform infrared and Raman spectroscopy and at 0 K by density-functional theory calculations. The infrared data allow for the detection of the orthorhombic Pnam to monoclinic P2(1)∕m phase transition which is characterized by a strong hysteresis both on compression and decompression experiments. However, an upper and lower boundary for the transition pressure are identified. An even more pronounced hysteresis is observed for the higher-pressure transition to the monoclinic A2/m phase. The hysteresis does not allow in this case the determination of a well defined P-T transition line. The ambient structural properties of polyethylene are fully recovered after compression/decompression cycles indicating that the polymer is structurally and chemically stable up to 50 GPa. A phase diagram of polyethylene up to 50 GPa and 650 K is proposed. Analysis of the pressure evolution of the Davydov splittings and of the anomalous intensification with pressure of the IR active wagging mode provides insight about the nature of the intermolecular interactions in crystalline polyethylene.     

Self-adaptive gradient projection algorithms for variational inequalities involving non-Lipschitz continuous operators

Numerical Algorithms - In this paper, we introduce a self-adaptive inertial gradient projection algorithm for solving monotone or strongly pseudomonotone variational inequalities in real Hilbert...     

Large time behavior for some nonlinear degenerate parabolic equations

We study the asymptotic behavior of Lipschitz continuous solutions of nonlinear degenerate parabolic equations in the periodic setting. Our results apply to a large class of Hamilton–Jacobi–Bellman equations. Defining Σ as the set where the diffusion vanishes, i.e., where the equation is totally degenerate, we obtain the convergence when the equation is uniformly parabolic outside Σ and, on Σ, the Hamiltonian is either strictly convex or satisfies an assumption similar of the one introduced by Barles–Souganidis (2000) for first-order Hamilton–Jacobi equations. This latter assumption allows to deal with equations with nonconvex Hamiltonians. We can also release the uniform parabolic requirement outside Σ. As a consequence, we prove the convergence of some everywhere degenerate second-order equations.     

Some subgradient extragradient type algorithms for solving split feasibility and fixed point problems

In this paper, we consider the split feasibility problem (SFP) in infinite‐dimensional Hilbert spaces and propose some subgradient extragradient‐type algorithms for finding a common element of the fixed‐point set of a strict pseudocontraction mapping and the solution set of a split feasibility problem by adopting Armijo‐like stepsize rule. We derive convergence results under mild assumptions. Our results improve some known results from the literature. Copyright © 2016 John Wiley & Sons, Ltd.     

Local microstructure of silica glass

We present a microstructure study of different amorphous states of SiO₂, which are constructed by compress–decompress procedure. The local microstructure is analyzed through the coordination number, bond-angle statistic and the characteristic of void. The simulation reveals that the number of SiO₄, SiO₅ and SiO₆ units varies in different silica states, but their topology is identical. The densest model (highest density) is obtained upon compress pressure around 20 GPa. The change in void space for different silica states is also calculated and discussed here.     

Donor-state-enabling Er-related luminescence in silicon: direct identification and resonant excitation

We conclusively establish a direct link between formation of an Er-related donor gap state and the 1.5 microm emission of Er in Si. The experiment is performed on Si/Si:Er nanolayers where a single type of Er optical center dominates. We show that the Er emission can be resonantly induced by direct pumping into the bound exciton state of the identified donor. Using two-color spectroscopy with a free-electron laser we determine the ionization energy of the donor-state-enabling Er excitation as E(D) approximately 218 meV. We demonstrate quenching of the Er-related emission upon ionization of the donor.     

Pd-carbene catalyzed carbonylation reactions of aryl iodides

A series of carbene complexes [PdBr(2)((i)Pr(2)-bimy)L] (C2-C13) with different types of co-ligands (L) have been tested for their catalytic activities in the carbonylative annulation of 2-iodophenol with phenylacetylene in DMF to afford the respective flavone 2a. Complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides giving the desirable 2-substituted 4H-3,1-benzoxazin-4-ones (4) in good yields. Additionally, this Pd-NHC complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure. These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)-NHC complex in different types of carbonylations of aryl iodides under mild conditions.     

Acetyl Methyl Torsion in N‐Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry

The gas‐phase structures and parameters describing acetyl methyl torsion of N‐ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier‐transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C₁ symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm^?1 of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The ¹⁴N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values.