Experimental evaluation of capacity statistics for short VDSL loops

We assess the capacity potential of very short very-high data-rate digital subscriber line loops using full-binder channel measurements collected by France Telecom R&D. Key statistics are provided for both uncoordinated and vectored systems employing coordinated transmitters and coordinated receivers. The vectoring benefit is evaluated under the assumption of transmit precompensation for the elimination of self-far-end crosstalk, and echo cancellation of self-near-end crosstalk. The results provide useful bounds for developers and providers alike.     

Mechanism of a chemical classic: quantum chemical investigation of the autocatalyzed reaction of the serendipitous wöhler synthesis of urea

The detailed reaction pathways for the ammonium cyanate transformation into urea (W?hler's reaction) in the gas phase, in solution, and in the solid state have exhaustively been explored by means of first-principles quantum chemical calculations at the B3LYP level of theory using the 6-31G(d,p) basis set. This serendipitous synthesis of urea is predicted to proceed in two steps; the first step involves the decomposition of the ammonium cyanate to ammonia and isocyanic or cyanic acid, and the second one, which is the main reaction step (and probably the rate-determining step), involves the interaction of NH(3) with either isocyanic or cyanic acid. Several alternative pathways were envisaged for the main reaction step of W?hler's reaction in a vacuum involving the formation of "four-center" transition states. Modeling W?hler's reaction in aqueous solution and in the solid state, it was found that the addition of NH(3) to both acids is assisted (autocatalyzed) by the active participation of extra H(2)O and/or NH(3) molecules, through a preassociative, cooperative, and hydrogen-transfer relay mechanism involving the formation of "six-center" or even "eight-center" transition states. The most energetically economic path of the rate-determining step of W?hler's reaction is that of the addition of NH(3) to the C=N double bond of isocyanic acid, directly affording urea. An alternative pathway corresponding to the anti-addition of ammonia to the Ctbd1;N triple bond of cyanic acid, yielding urea's tautomer HN=C(OH)NH(2), seems to be another possibility. In the last case, urea is formed through a prototropic tautomerization of its enolic form. The energies of the reactants, products, and all intermediates along with the barrier heights for each reaction path have been calculated at the B3LYP/6-31G(d,p) level of theory. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurfaces was performed at the same level of theory.     

Thermal stability of zincite single crystals

The effect of temperature on the stability of synthetic ZnO crystals in neutral (N₂), oxidative (O₂), and reductive (95% Ar-5% H₂) media and in a low vacuum has been studied. In the first and second cases, zincite does not undergo changes up to 1000°C. After an 8-h exposure in oxygen at 1100°C, a decrease in the size of hexagonal growth hillocks on the monohedron face (0001) and smoothing of its relief on the whole have been observed. In the presence of hydrogen, etching traces manifest themselves on the crystal surface even at 760°C; further heating leads to their sublimation. The effect of annealing on the luminescence properties of zincite has been demonstrated.     

The dramatic effect of NH3 co-ligation on the Fe+-assisted activation of carbon dioxide in the gas phase: from bare metal ions to complexes

The catalytic efficiency of Fe(+) ion over the CO(2) decomposition in the gas phase has been extensively investigated with the help of electronic structure calculation methods. Potential-energy profiles for the activation process Fe(+) + CO(2) --> CO + FeO(+) along two rival potential reaction paths, namely the insertion and addition pathways, originating from the end-on kappa(1)-O and kappa(2)-O,O coordination modes of CO(2) with the metal ion, respectively, have been explored by DFT calculations. For each pathway the potential energy surfaces of the high-spin sextet (S = 5/2) and the intermediate-spin quartet (S = 3/2) spin-states have been explored. The complete energy reaction profile calculated by a combination of ab initio and density functional theory (DFT) computational techniques reveals a two-state reactivity, involving two spin inversions, for the decomposition process and accounts well for the experimentally observed inertness of bare Fe(+) ions towards CO(2) activation. Furthermore, the coordination of up to three extra ancillary NH(3) ligands with the Fe(+) metal ion has been explored and the geometric and energetic reaction profiles of the CO(2) activation processes Fe(+) + n x NH(3) + CO(2) --> [Fe(NH(3))(n)(CO(2))](+) --> [Fe(NH(3))(n)(O)(CO)](+) --> CO + [Fe(O)(NH(3))(n)](+) (n = 1, 2 or 3) have thoroughly been scrutinized for both the insertion and the addition mechanisms. Inter alia, the geometries and energies of the various states of the [Fe(NH(3))(n)(CO(2))](+) and [Fe(NH(3))(n)(O)(CO)](+) complexes are explored and compared. Finally, a detailed analysis of the coordination modes of CO(2) in the cationic [Fe(NH(3))(n)(CO(2))](+) (n = 0, 1, 2 and 3) complexes is presented.     

Deciphering the bonding mode of the trihalide ligands in a series of halogen carrier homo- and hetero-trihalide Cu(II) Schiff base complexes

Electronic structure calculation techniques (DFT) have been used to decipher the bonding of the trihalide ligands in a series of homo- and hetero-trihalide Cu(II) Schiff base complexes formulated as [Cu(RdienR)(X)(XY₂)] (RdienR = Schiff base; R = furan, thiophene or pyrrol; X = Cl or Br; Y = Cl, Br or I). The association of the incoming Y₂ halogen molecule with one of the halide X ligands of the precursor [Cu(RdienR)(X)₂] complexes alters their distorted trigonal bipyramidal stereochemistry which is transformed to a distorted square pyramidal geometry. The bonding mechanism between the halogen Y₂ molecule and the halide X ligand was thoroughly explored by means of various electronic parameters and charge decomposition analysis techniques. The bond dissociation energy of the Cu–XY₂ bond, BDECu–XY₂${\text{BDE}}_{\text{Cu}''–''{\text{XY}}_2}$, was estimated in the range of 61.9–68.4 kcal/mol, while the bond dissociation energy of the X–Y₂ bond, BDECu–XY₂${\text{BDE}}_{\text{Cu}''–''{\text{XY}}_2}$, was found in the range of 10.6–12.5 kcal/mol. It was found that the X?Y₂ interactions correspond to weak hyperconjugative donor–acceptor interactions between a non-bonding n(X) molecular orbital (donor orbital) localized on the coordinated halide X ligand and an antibonding σ^∗(Y–Y) molecular orbital (acceptor orbital) localized on the Y₂ halogen molecule. The n(X) → σ^∗(Y–Y) donor–acceptor interactions are associated with a second-order perturbation stabilization energy, ΔE(2) of 34.5–52.5 kcal/mol. The loose association of the halogen molecules with the coordinated halide ligand renders the [Cu(RdienR)(X)(XY₂)] complexes good halogen carrier molecules.     

Selfishness and altruism on the MISO interference channel: the case of partial transmitter CSI

We study the achievable ergodic rate region of the two-user multiple-input single-output interference channel, under the assumptions that the receivers treat interference as additive Gaussian noise and the transmitters only have statistical channel knowledge. Initially, we provide a closed-form expression for the ergodic rates and derive the Nash-equilibrium and zero-forcing transmit beamforming strategies. Then, we show that combinations of the aforementioned selfish and altruistic, respectively, strategies achieve Pareto-optimal rate pairs.     

Mechanistic insights into the Bazarov synthesis of urea from NH3 and CO2 using electronic structure calculation methods

The mechanism of the noncatalyzed and reagent-catalyzed Bazarov synthesis of urea has extensively been investigated in the gas phase by means of density functional (B3LYP/6-31G(d,p)) and high quality ab initio (CBS-QB3) computational techniques. It was found that the first step of urea formation from NH3(g) and CO2(g) corresponds to a simple addition reaction leading to the carbamic acid intermediate, a process being slightly endothermic. Among the three possible reaction mechanisms considered, the addition-elimination-addition (AEA) and the double addition-elimination (DAE) mechanisms are almost equally favored, while the concerted (C) one was predicted kinetically forbidden. The second step involves the formation of loose adducts between NH3 and carbamic acid corresponding to an ammonium carbamate intermediate, which subsequently dehydrates to urea. The formation of "ammonium carbamate" corresponds to an almost thermoneutral process, whereas its dehydration, which is the rate-determining step, is highly endothermic. The Bazarov synthesis of urea is strongly assisted by the active participation of extra NH3 or H2O molecules (autocatalysis). For all reaction pathways studied the entire geometric and energetic profiles were computed and thoroughly analyzed. The reaction scheme described herein can be related with the formation of both isocyanic acid, H-N=C=O, and carbamic acid, H2N-COOH, as key intermediates in the initial formation of organic molecules, such as urea, under prebiotic conditions.     

Far-Field Multicast Beamforming for Uniform Linear Antenna Arrays

The problem of transmit beamforming to multiple cochannel multicast groups is considered for the important special case when the channel vectors are Vandermonde. This arises when a uniform linear antenna antenna (ULA) array is used at the transmitter under far-field line-of-sight propagation conditions, as provisioned in 802.16e and related wireless backhaul scenarios. Two design approaches are pursued: (i) minimizing the total transmitted power subject to providing at least a prescribed received signal-to-interference-plus-noise-ratio (SINR) to each intended receiver; and (ii) maximizing the minimum received SINR under a total transmit power budget. Whereas these problems have been recently shown to be NP-hard, in general, it is proven here that for Vandermonde channel vectors, it is possible to recast the optimization in terms of the autocorrelation sequences of the sought beam vectors, yielding an equivalent convex reformulation. This affords efficient optimal solution using modern interior point methods. The optimal beam vectors can then be recovered using spectral factorization. Robust extensions for the case of partial channel state information, where the direction of each receiver is known to lie in an interval, are also developed. Interestingly, these also admit convex reformulation. The various optimal designs are illustrated and contrasted in a suite of pertinent numerical experiments.     

Structure and energetics of gaseous HZnCl

The HZnCl molecule has recently been observed both in the gas phase and in argon matrixes, and the complexity of its spectrum has created big discrepancies between experimental and theoretically predicted Zn-H bond distances. In an attempt to resolve the questionable H-ZnCl bond length issue, as well as to study its stability relative to bond breakage, we have investigated at a very high ab initio level its geometric and energetic parameters. At the coupled cluster level of theory [CCSD(T)] with the new correlation-consistent basis sets for Zn by Balabanov and Peterson, and with the inclusion of core/valence and one-electron Douglas-Kroll-Hess relativistic effects, the basis set extrapolated H-ZnCl and HZn-Cl bond lengths have been calculated to be 1.499 and 2.079 A, compared with the experimental estimates of 1.596-1.789 and 2.08346 A, respectively. With the same procedure, the experimentally unknown bond length of ground-state ZnCl has been found to be equal to 2.122 A. As in the free diatomics ZnCl and ZnH, the HZn-Cl bond is stronger than the H-ZnCl bond by about 30 kcal/mol.