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911.
Bendjaballah S Kahlal S Costuas K Bévillon E Saillard JY 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(7):2048-2065
DFT calculations with full geometry optimization have been carried out on a series of real and hypothetical compounds of the type [CpM(C8H6)], [(CO)3M(C8H6)], [M(C8H6)2], [(CpM)2(C8H6)], [[(CO)3M]2(C8H6)], and [M2(C8H6)2] (M = transition metal). The bonding in all the currently known compounds is rationalized, as well as in the (so far) hypothetical stable complexes. Depending on the electron count and the nature of the metal(s), eta2 (predicted), eta3, eta5, eta8, or intermediate coordination modes can be adopted. In the case of the mononuclear species, the most favored closed-shell electron counts are 18 and 16 metal valence electrons (MVE). In the case of the dinuclear species, an electron count of 34 MVEs is most favored. However, other electron counts can be stabilized, especially in the case of dinuclear complexes. Coordinated pentalene should most often be considered as formally being a dianion, but sometimes as a neutral ligand. In the former case it can behave as an aromatic species made of two equivalent fused rings, as a C5 aromatic ring connected to an allylic anion, or even as two allylic anions bridged by a C7=C8 double bond. In the latter case, it can behave as a bond-alternating cyclic polyene or as a C5 aromatic ring connected to an allylic cation. 相似文献
912.
Marcus V. P. dos Santos Eduardo C. Aguiar João Bosco P. da Silva Ricardo L. Longo 《Journal of computational chemistry》2013,34(8):611-621
A new approach Procedure for Investigating Categories of Vibrations (PICVib) for estimating vibrational frequencies of selected modes using only the structure and energy calculations at a more demanding computational level is presented and explored. The PICVib has an excellent performance at only a small fraction of the computational demand required for a complete analytical calculation. The errors are smaller than ca. 0.5% when DFT functionals are combined with high level ab initio methods. The approach is general because it can use any quantum chemical program and electronic structure method. It is very robust because it was validated for a wide range of frequency values (ca. 20–4800 cm–1) and systems: XH3 (D3h) with X = B, Al, Ga, N, P, As, O, S, and Se, YH4 (D4h) with Y = C, Si, and Ge, conformers of RDX, SN2 and E2 reactions, [W(dppe)2(NNC5H10)] complex, carbon nanotubes, and hydrogen‐bonded complexes including guanine‐cytosine pair. © 2012 Wiley Periodicals, Inc. 相似文献
913.
A method for calibration and validation subset partitioning 总被引:13,自引:0,他引:13
This paper proposes a new method to divide a pool of samples into calibration and validation subsets for multivariate modelling. The proposed method is of value for analytical applications involving complex matrices, in which the composition variability of real samples cannot be easily reproduced by optimized experimental designs. A stepwise procedure is employed to select samples according to their differences in both x (instrumental responses) and y (predicted parameter) spaces. The proposed technique is illustrated in a case study involving the prediction of three quality parameters (specific mass and distillation temperatures at which 10 and 90% of the sample has evaporated) of diesel by NIR spectrometry and PLS modelling. For comparison, PLS models are also constructed by full cross-validation, as well as by using the Kennard-Stone and random sampling methods for calibration and validation subset partitioning. The obtained models are compared in terms of prediction performance by employing an independent set of samples not used for calibration or validation. The results of F-tests at 95% confidence level reveal that the proposed technique may be an advantageous alternative to the other three strategies. 相似文献
914.
Camargo Emerson R. Longo Elson Leite Edson R. 《Journal of Sol-Gel Science and Technology》2000,17(2):111-121
Pure MgNb2O6 powders with high specific surface area and high crystallinity have been successfully synthesized by the Polymerized Complex (PC) Method. A solution of water, citric acid, ethylene glycol and niobium and magnesium ions, was polymerized at 130°C by the water elimination and was calcined at 400°C for 2 h. The precursor formed was calcined at temperatures from 700°C to 1000°C in air to obtain the columbite phase. Thermal analysis, Raman spectroscopy, x-ray diffraction, adsorption/desorption hysteresis and scanning electron microscopy were used to investigate the polymer decomposition and the columbite phase evolution. No evidence for phase separation of crystalline MgCO3 and Nb2O5 in Raman spectra and x-ray diffraction pattern as distinct intermediates were found, and the MgNb2O6 probably is formed through a single-step decomposition of oxycarbonate intermediate, as other double oxide synthesized by the PC Method, as indicated by the thermal decomposition of the precursor. 相似文献
915.
Silva BV Violante FA Pinto AC Santos LS 《Rapid communications in mass spectrometry : RCM》2011,25(3):423-428
Using electrospray ionization (tandem) mass spectrometry (ESI-MS(/MS)) spectrometric experiments, the Sandmeyer reaction was monitored on-line, and key intermediates were intercepted and characterized for the first time. The mechanistic information provided by on-line ESI-MS(/MS) is in accordance with Sandmeyer's proposal, and was made possible by coupling a microreactor on-line to the ESI ion source, which allowed reactions to be screened from 0.7-2.0 s, identifying and characterizing all intermediates that were formed and consumed during the reaction. 相似文献
916.
Philip S. H. Wong Barbara J. Antonio David V. Dearden 《Journal of the American Society for Mass Spectrometry》1994,5(7):632-637
Reactions of laser-desorbed Na+, K+, Rb+, and Cs+ with thermally vaporized valinomycin generate metal-ligand complexes in a Fourier transform ion cyclotron resonance trapping cell, proving that complexes can form via gas-phase ion-molecule reactions. Although desorption of intact pre-formed complexes cannot be ruled out, this route appears minor. Relative rate constants for the complexation reactions show strong dependence on the charge densities of the cations. Competition experiments between valinomycin and the synthetic ionophores 18-crown-6 (18C6) and [2.2.2]-cryptand ([2.2.2]) show that valinomycin has a higher intrinsic alkali metal cation affinity than either 18C6 or [2.2.2], in contrast to the complex formation constants observed in methanol, where K+ affinities are in the order [2.2.2] > 18C6 > valinomycin. 相似文献
917.
Marie‐Claire Giel Christopher J. Smedley Emily R. R. Mackie Taijie Guo Jiajia Dong Tatiana P. Soares da Costa John E. Moses 《Angewandte Chemie (International ed. in English)》2020,59(3):1181-1186
The boom in growth of 1,4‐disubstituted triazole products, in particular, since the early 2000’s, can be largely attributed to the birth of click chemistry and the discovery of the CuI‐catalyzed azide–alkyne cycloaddition (CuAAC). Yet the synthesis of relatively simple, albeit important, 1‐substituted‐1,2,3‐triazoles has been surprisingly more challenging. Reported here is a straightforward and scalable click‐inspired protocol for the synthesis of 1‐substituted‐1,2,3‐triazoles from organic azides and the bench stable acetylene surrogate ethenesulfonyl fluoride (ESF). The new transformation tolerates a wide selection of substrates and proceeds smoothly under metal‐free conditions to give the products in excellent yield. Under controlled acidic conditions, the 1‐substituted‐1,2,3‐triazole products undergo a Michael addition reaction with a second equivalent of ESF to give the unprecedented 1‐substituted triazolium sulfonyl fluoride salts. 相似文献
918.
The data clustering problem consists in dividing a data set into prescribed groups of homogeneous data. This is an NP-hard problem that can be relaxed in the spectral graph theory, where the optimal cuts of a graph are related to the eigenvalues of graph 1-Laplacian. In this paper, we first give new notations to describe the paths, among critical eigenvectors of the graph 1-Laplacian, realizing sets with prescribed genus. We introduce the pseudo-orthogonality to characterize m3(G), a special eigenvalue for the graph 1-Laplacian. Furthermore, we use it to give an upper bound for the third graph Cheeger constant h3(G), that is, h3(G) 6 m3(G). This is a first step for proving that the k-th Cheeger constant is the minimum of the 1-Laplacian Raylegh quotient among vectors that are pseudo-orthogonal to the vectors realizing the previous k - 1 Cheeger constants. Eventually, we apply these results to give a method and a numerical algorithm to compute m3(G), based on a generalized inverse power method. 相似文献
919.
Examination of time-reversal acoustics in shallow water and applications to noncoherent underwater communications 总被引:7,自引:0,他引:7
Smith KB Abrantes AA Larraza A 《The Journal of the Acoustical Society of America》2003,113(6):3095-3110
The shallow water acoustic communication channel is characterized by strong signal degradation caused by multipath propagation and high spatial and temporal variability of the channel conditions. At the receiver, multipath propagation causes intersymbol interference and is considered the most important of the channel distortions. This paper examines the application of time-reversal acoustic (TRA) arrays, i.e., phase-conjugated arrays (PCAs), that generate a spatio-temporal focus of acoustic energy at the receiver location, eliminating distortions introduced by channel propagation. This technique is self-adaptive and automatically compensates for environmental effects and array imperfections without the need to explicitly characterize the environment. An attempt is made to characterize the influences of a PCA design on its focusing properties with particular attention given to applications in noncoherent underwater acoustic communication systems. Due to the PCA spatial diversity focusing properties, PC arrays may have an important role in an acoustic local area network. Each array is able to simultaneously transmit different messages that will focus only at the destination receiver node. 相似文献
920.
A. L. Stepanov C. Marques E. Alves R. C. da Silva M. R. Silva R. A. Ganeev A. I. Ryasnyansky T. Usmanov 《Technical Physics》2006,51(11):1474-1481
Single-crystal Al2O3(0001) and Al2O3(1120) substrates are implanted by 160-keV Au+ ions with doses from 1015 to 1017 cm?2. Some of the implanted samples are air-annealed at 800–1200°C. The properties of the synthesized composite layers are studied by Rutherford backscattering and linear optical reflection measurements, and their nonlinear optical characteristics are examined by RZ-scanning using a picosecond Nd: YAG laser operating at a wavelength of 1064 nm. The Rutherford backscattering spectra indicate that the implanted impurity concentrates near the surface of the Al2O3. The formation of gold nanoparticles in the Al2O3 can be judged from the characteristic optical plasmon resonance band in the reflectance spectra of the samples irradiated to a dose higher than 6.0 × 1016 cm?2. The synthesized particles are shown to be responsible for nonlinear optical refraction in the samples. The nonlinear refractive index, n 2, and the real part of the third-order susceptibility, Rex(3), of the composite layers are determined. 相似文献