Vibrational and thermal properties of ScN and YN: quasi-harmonic approximation calculations and anharmonic effects

The structural, vibrational and thermal properties of rocksalt ScN and YN are investigated by using a first-principles plane-wave approach. The results are discussed in comparison with the similarly calculated results for rocksalt MgO and zincblende AlN. The thermal expansivity (α(V)) computed within the quasi-harmonic approximation shows that there are significant anharmonic effects in ScN and YN, which are comparable to those in MgO. Since no experimental results are available for α(V) of either ScN or YN, the anharmonic effects are accounted for by a variant of the very recently introduced effective semiempirical ansatz (Phys. Rev. B 2009 79 104304) for calculating anharmonic free energy, which does not require any input from experiment. The validity of this very simple approach is demonstrated first by applying it to MgO. For the considered phase of AlN, the quasi-harmonic approximation is valid up to very high temperatures, and the thus obtained α(V) is in good agreement with experiment. The values of α(V) for semiconductor transition metal nitrides that crystallize in the rocksalt phase are higher than those for the zincblende phase of group-IIIB nitrides, and a major part of these differences is due to the crystal structure.     

18 W single-stage single-frequency acoustically tailored Raman fiber amplifier

A single-mode polarization-maintaining fiber doped to increase the Raman gain while suppressing stimulated Brillouin scattering (SBS) was utilized in a single-stage counter-pumped Raman fiber amplifier. The SBS suppression was achieved through the acoustic tailoring of the core. A pump probe experiment was conducted to characterize the Brillouin gain and indicated the existence of multiple Brillouin peaks. When the amplifier was seeded with approximately 15 mW of 1178 nm light, 11.5 W of cw output power was obtained with a linewidth ≤2 MHz. The application of a thermal gradient to further mitigate the SBS process increased the output power to 18 W, thus providing a net amplifier gain >30 dB.     

Local Construction of Near-Optimal Power Spanners for Wireless Ad Hoc Networks

We present a local distributed algorithm that, given a wireless ad hoc network modeled as a unit disk graph U in the plane, constructs a planar power spanner of U whose degree is bounded by k and whose stretch factor is bounded by 1 + (2sin{frac{pi}{k}})^{p}, where k geq 10 is an integer parameter and p in [2, 5] is the power exponent constant. For the same degree bound k, the stretch factor of our algorithm significantly improves the previous best bounds by Song et al. We show that this bound is near-optimal by proving that the slightly smaller stretch factor of 1 + (2sin{frac{pi}{k + 1}})^{p} is unattainable for the same degree bound k. In contrast to previous algorithms for the problem, the presented algorithm is local. As a consequence, the algorithm is highly scalable and robust. Finally, while the algorithm is efficient and easy to implement in practice, it relies on deep insights on the geometry of unit disk graphs and novel techniques that are of independent interest.     

Toward the improvement of the tandem halide displacement/amide coupling spiro-cyclization as a new route to γ-lactam and pyrroloisoquinoline templates

Two efficient and rapid accesses to spiro-oxindole entities bearing an imide function were presented, and their performance was compared. The key components are N-substituted α-bromoacetamides to reach these derivatives in tandem process. The resulting spiro-imides from these methodologies were regioselectively reduced into corresponding N-acyliminium precursors, which were subsequently submitted to an intramolecular cyclization to provide pentacyclic spiro-oxindole architecture analogues to pteropodine and spirotryprostatin-B alkaloids with high diastereoselective control.     

Straightforward selective synthesis of linear 1-O-alkyl glycerol and di-glycerol monoethers

1-O-Alkyl glycerol and di-glycerol monoethers are, respectively, obtained in high yields and selectivity by catalytic reductive etherification of mono- and di-glycerol with linear aldehydes in the presence of 0.5 mol % of Pd/C under 10 bars of hydrogen using a Brønsted acid as co-catalyst.     

Performance Improving of OCDMA System Using 2-D Optical Codes With Optical SIC Receiver

In this paper, we have studied the probability of error of optical code division multiple access (OCDMA) system using 2-D optical codes, namely prime hop and hybrid codes with a probability distribution approach. In order to increase the OCDMA system performance and increase the number of simultaneous users in the system, we have investigated the system when two different receivers are employed, namely, OCDMA serial interference cancellation (SIC) receivers and OCDMA correlation conventional receiver (CCR). Comparison in terms of OCDMA system's performance between these two receivers has been reported. In order to obtain the optimum OCDMA system's performance, we have studied several optical SIC receiver structures. Optimum choice of the number of stages in the SIC receiver and the threshold value on each stage has been reported.     

First synthesis of 2'-deoxyfluoropuromycin analogues: experimental insight into the mechanism of the Staudinger reaction

The N(6),N(6)-dedimethyl-2'-deoxyfluoro analogue of puromycin (= 3'-deoxy-N(6),N(6)-dimethyl-3'-[O-methyltyrosylamido]adenosine), its 2',3'-regioisomer and a 3'-cytidyl-5'-(2'-deoxyfluoro)puromycyl dinucleotide analogue were synthesized following an approach involving i) the diastereospecific nitrite-assisted formation of a lyxo nucleosidic 2',3'-epoxide from an adenosine-2',3'-ditriflate derivative in a biphasic solvent mixture; ii) the regio- and stereoselective epoxide ring opening with sodium azide under mildly acidic aqueous conditions, iii) the stereospecific introduction of the fluor atom using DAST and iv) the reaction between the nucleosidyl or dinucleotidyl azide and an active ester of the N-protected amino acid using highly efficient solution conditions for the Staudinger-Vilarrasa coupling, to obtain the corresponding carboxamide directly from the in situ formed iminophosphorane. This coupling reaction furnished sterically quite demanding amides in 94 % isolated yields under very mild conditions and should therefore be of a more general value. Under certain reaction conditions we isolated (amino)acyltriazene derivatives from which dinitrogen was not eliminated. These secondary products are trapped and stabilized witnesses of the first intermediate of the Staudinger reaction, the phosphatriazenes (phosphazides, triazaphosphadienes) which usually eliminate dinitrogen in situ and rapidly rearrange into iminophosphoranes, unless they are derived from conjugated or sterically bulky azides and phosphines. The acyltriazenes could either be thermally decomposed or converted to the corresponding N-alkyl carboxamides through proton-assisted elimination of dinitrogen. All compounds were carefully characterized through MS spectrometry, (1)H, (19)F, (31)P and (13)C NMR spectroscopy.     

Vertex Cover: Further Observations and Further Improvements

Recently, there has been increasing interest and progress in lowering the worst-case time complexity for well-known NP-hard problems, particularly for the Cover problem. In this paper, new properties for the problem are indicated, and several simple and new techniques are introduced, which lead to an improved algorithm of time O(kn + 1.2852^k) for the problem. Our algorithm also induces improvement on previous algorithms for the problem on graphs of small degree.     

Joint channel estimation and data detection for OFDM based cooperative system

Telecommunication Systems - Orthogonal frequency division multiplexing based cooperative system using Alamouti space–time block coding at relay node represents an alternative solution to...     

(E) Enol ethers from the stereoselective reduction of α-alkoxy-β-ketophosphonates and Wittig type reaction

When α-alkoxy-β-ketophosphonates, prepared by the Rh(II) mediated insertion reaction of α-diazo-β-ketophosphonates into the OH bond of primary alcohols, were reduced either by NaBH₄ in the presence of CaCl₂ or by DIBAL, they respectively gave the corresponding anti or syn stereomeric hydroxyphosphonates with pronounced to complete stereoselectivity. Submitted to the action of potassium tert-butoxyde, syn isomers led to the corresponding pure (E) enol ethers in moderate to good yields. Under the same conditions anti isomers led to a mixture of (Z) and (E) enol ethers in rather poor yields. The sequence was applied to the preparation of some allyl-vinyl ethers with a (E) configuration for the vinylic double bond.