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Vibration Frequencies (vibration + frequency)
Selected AbstractsOptical, Magnetic and Structural Properties of the Spin-Crossover Complex [Fe(btr)2(NCS)2]·H2O in the Light-Induced and Thermally Quenched Metastable StatesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 36 2007Vincent Legrand Abstract [Fe(btr)2(NCS)2]·H2O [btr = 4,4,-bis(1,2,4-triazole)] is thearchetype of highly cooperative and low-dimensional spin-crossover complexes, which exhibit low-spin (LS) to high-spin (HS) light-induced conversion at very low temperature. The structural reorganizations related to the light-induced and thermally induced LS,HS transitions were characterized by single-crystal X-ray diffraction below the relaxation temperature (T = 15 K < TLIESST) and at 130 K within the thermal hysteresis loop. We show that the LIESST and thermal spin transitions lead to the same structural variations, namely an elongation of the Fe,N bonds by 0.18 Å (Fe,NNCS) and 0.20 Å (Fe,Nbtr), on going from LS to HS, together with a reorientation of the NCS group by nearly 13°. The atomic displacement amplitudes, derived from the crystal structures, indicate lattice vibration modes of larger amplitudes and correlatively lower vibration frequencies in the HS state. The deformation of the crystal lattice as a function of temperature and laser excitation was quantitatively analyzed in terms of the HS and LS thermal-expansion (,HS and ,LS) and spin-transition spontaneous-strain (,) tensors. The eigendirections and eigenvalues of the , and , tensors correlate well with the weak and strong interactions in the solid and are responsible for the high cooperativity and low-dimensional behaviour. Magnetic and spectroscopic measurements were performed in all the different spin states and related to the structural findings. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] DSC-Ritz method for high-mode frequency analysis of thick shallow shellsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2005C. W. Lim Abstract This paper addresses a challenging problem in computational mechanics,the analysis of thick shallow shells vibrating at high modes. Existing methods encounter significant difficulties for such a problem due to numerical instability. A new numerical approach, DSC-Ritz method, is developed by taking the advantages of both the discrete singular convolution (DSC) wavelet kernels of the Dirichlet type and the Ritz method for the numerical solution of thick shells with all possible combinations of commonly occurred boundary conditions. As wavelets are localized in both frequency and co-ordinate domains, they give rise to numerical schemes with optimal accurate, stability and flexibility. Numerical examples are considered for Mindlin plates and shells with various edge supports. Benchmark solutions are obtained and analyzed in detail. Experimental results validate the convergence, stability, accuracy and reliability of the proposed approach. In particular, with a reasonable number of grid points, the new DSC-Ritz method is capable of producing highly accurate numerical results for high-mode vibration frequencies, which are hitherto unavailable to engineers. Moreover, the capability of predicting high modes endows us the privilege to reveal a discrepancy between natural higher-order vibration modes of a Mindlin plate and those calculated via an analytical relationship linking Kirchhoff and Mindlin plates. Copyright © 2004 John Wiley & Sons, Ltd. [source] Computational determination of effects of electric fields upon "trigger linkages" of prototypical energetic moleculesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2009Peter Politzer Abstract For five prototypical energetic molecules (nitrobenzene, methyl azide, methyl nitrate, nitromethane, and dimethylnitramine), we examine computationally the effects of external electric fields upon their "trigger linkage" bonds, the breaking of which is believed to play a key role in detonation initiation. The bonds are, respectively, CNO2, NN2, ONO2, CNO2, and NNO2. The calculations are at the B3PW91/6,31G** level. We find that fields along these bonds that reinforce the molecules' intrinsic polarities also lower their energies and increase the bonds' stretching vibration frequencies. This suggests a strengthening of the bonds. Fields in the opposite direction do the reverse. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Theory of chemical bonds in metalloenzymes III: Full geometry optimization and vibration analysis of ferredoxin-type [2Fe,2S] clusterINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2007Mitsuo Shoji Abstract The nature of chemical bonds in a ferredoxin-type [2Fe,2S] cluster has been investigated on the basis of natural orbitals and several bond indices developed in Parts I and II of this study. The broken-symmetry hybrid density functional theory (BS-HDFT) with spin projection approach has been applied to elucidate the natural orbitals and occupation numbers for a model compound [Fe2S2(SCH3)4] (1), which is used to calculate the indices. The molecular structure, vibration frequencies, electronic structures, and magnetic properties in both oxidized and reduced forms of 1 have been calculated and compared with the experimental values. The optimized molecular structures after approximate spin projection have been in good agreement with experimental data. The structure changes upon one-electron reduction have been slight (<0.1 Å) and only limited around one side of the Fe atom. Raman and infrared (IR) spectra have been calculated, and their vibration modes have been assigned using the bridging 34S isotope substitution. Their magnetic properties have been examined in terms of spin Hamiltonians that contain exchange interactions and double exchange interactions. The BS-HDFT methods have provided the magnetic parameters; i.e., effective exchange integral (J) values and valence delocalization (B) values, which agree with the experimental results. It is found that large charge transfer (CT) from the bridging sulfur to the iron atoms has led to the strong antiferromagnetic interactions between iron atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Accurate prediction of thermodynamic properties of alkyl peroxides by combining density functional theory calculation with least-square calibrationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2009Cun-Xi Liu Abstract Owing to the significance in kinetic modeling of the oxidation and combustion mechanisms of hydrocarbons, a fast and relatively accurate method was developed for the prediction of ,fH of alkyl peroxides. By this method, a raw ,fH value was calculated from the optimized geometry and vibration frequencies at B3LYP/6-31G(d,p) level and then an accurate ,fH value was obtained by a least-square procedure. The least-square procedure is a six-parameter linear equation and is validated by a leave-one out technique, giving a cross-validation squared correlation coefficient q2 of 0.97 and a squared correlation coefficient of 0.98 for the final model. Calculated results demonstrated that the least-square calibration leads to a remarkable reduction of error and to the accurate ,fH values within the chemical accuracy of 8 kJ mol,1 except (CH3)2CHCH2CH2CH2OOH which has an error of 8.69 kJ mol,1. Comparison of the results by CBS-Q, CBS-QB3, G2, and G3 revealed that B3LYP/6-31G(d,p) in combination with a least-square calibration is reliable in the accurate prediction of the standard enthalpies of formation for alkyl peroxides. Standard entropies at 298 K and heat capacities in the temperature range of 300,1500 K for alkyl peroxides were also calculated using the rigid rotor-harmonic oscillator approximation. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Effects of entropy on the gas-phase pyrolysis of ethyl N,N -dimethylcarbamateJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2007Chang K. Kim Abstract In this study, we examined the gas-phase pyrolysis of ethyl N,N -dimethylcarbamate theoretically at various theoretical levels. The reaction consists of a two-step mechanism, with N,N -dimethylcarbamic acid and ethylene as reaction intermediates. In the first step, the reaction proceeds via a six-membered cyclic transition state (TS), which is more favorable than that via a four-membered cyclic TS. Here, the contribution of entropy to the overall potential energy surface was found to play an important role in determining the rate-limiting step, which was found to be the second step when viewed in terms of the enthalpy of activation (,H,), but the first step when entropy changes (,T,S,) were considered. These results are consistent with experimental findings. Moreover, the experimental activation entropy can be reproduced by using the hindered rotor approximation, which converts some low vibration frequencies that correspond to internal rotational modes into hindered rotors. © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 625,631, 2007 [source] Calculation of the vibration frequencies of ,-quartz: The effect of Hamiltonian and basis setJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2004C. M. Zicovich-Wilson Abstract The central-zone vibrational spectrum of ,-quartz (SiO2) is calculated by building the Hessian matrix numerically from the analytical gradients of the energy with respect to the atomic coordinates. The nonanalytical part is obtained with a finite field supercell approach for the high-frequency dielectric constant and a Wannier function scheme for the evaluation of Born charges. The results obtained with four different Hamiltonians, namely Hartree,Fock, DFT in its local (LDA) and nonlocal gradient corrected (PBE) approximation, and hybrid B3LYP, are discussed, showing that B3LYP performs far better than LDA and PBE, which in turn provide better results than HF, as the mean absolute difference from experimental frequencies is 6, 18, 21, and 44 cm,1, respectively, when a split valence basis set containing two sets of polarization functions is used. For the LDA results, comparison is possible with previous calculations based on the Density Functional Perturbation Theory and usage of a plane-wave basis set. The effects associated with the use of basis sets of increasing size are also investigated. It turns out that a split valence plus a single set of d polarization functions provides frequencies that differ from the ones obtained with a double set of d functions and a set of f functions on all atoms by on average less than 5 cm,1. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1873,1881, 2004 [source] Negative mass sound shielding structures: Early resultsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2009Emilio P. Calius Abstract Elastic meta-materials or meta-composites can be designed to have a negative effective mass density at certain vibration frequencies, thus blocking wave propagation through the material within that frequency band. The negative mass behaviour is generated by resonant structures within the material that oscillate 180° out of phase with the acoustic pressure waves applied to the surface. As this research is in its initial stages this paper describes work in progress in both the experimental and numerical domains, and some early results from solids containing geometrically simple spring,mass resonant structures. Behaviour is characterized experimentally by dynamic tests of individual resonators as well as impedance tube measurements of panel-like structures containing multiple resonant elements. The experimental results exhibit clear evidence of the expected resonances, and partial band gap behaviour. Finite element models of both single resonant elements and impedance tube specimens are being developed, and the current status of these models is described. Their results to date show good agreement with the mass law and qualitative agreement with the experimental results. [source] Comparative theoretical study of small Rhn nanoparticles (2 , n , 8) using DFT methodsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2010V. Bertin Abstract This work is aimed at identifying some key characteristics (energy, geometry, and spin) concerning Rhn particles (2 = n , 8) to perform further studies on adsorption and coadsorption sites of pollutants (CO and NO). The DFT methods of the Gaussian 03 program with the LANL2DZ basis set and the LANL2 potential are used. With the purpose to obtain a better nanoparticles definition, five different functionals were tested: B3LYP, O3LYP, BPW91, BP86, and HCTH; and the corresponding results are used to determine which of them best describes distances, spin, and gives acceptable highest vibration frequency and binding energy values, by comparing these results with values measured or calculated by many other authors. For the structure optimization process of the particles, the initial geometric shape was taken mainly from the literature, using the Rh,Rh distance: 2.67 Å, known for the bulk; and doing a complete optimization. We also considered flat nanoparticles structures, which most of them display three-dimensional structures after the optimization process. The few flat shapes are mainly higher in energy than those of three-dimensional structure. For some Rhn particles for different n values, the spin of the ground state present degeneration. In some cases, the optimization process changes the initial geometry, but in most cases, there are only minor changes in bonds and geometry. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Modification of polypropylene by melt vibration blending with ultra high molecular weight polyethyleneADVANCES IN POLYMER TECHNOLOGY, Issue 3 2002Kejian Wang Abstract A novel vibration internal mixer was used to prepare polypropylene/ultra high molecular weight polyethylene PP/UHMWPE blends with two additional adjustable processing parameters (vibration frequency and vibration amplitude) as compared with those prepared in the steady mode. Microscopy, mechanical tests, and differential scanning calorimetry showed that vibration influenced the blend morphology and the product properties. The good phase homogeneity of the blends might be due to the variation of shear rate either spatially or temporally in blending. Additionally, the vibration internal mixer could be used to analyze the dependency of viscosity on the shear rate. Vibration enhanced the interpenetration of UHMWPE into PP and vice versa. Subsequently, the formed crystals of two components were connected, and there was epitaxy between PP and UHMWPE crystals. Moreover, the crystalline aggregates, with the amorphous UHMWPE, formed a complex network-like continuous structure, which improved the elongation ratio at the break and the yield strength. The higher the vibration frequency and/or the larger the vibration amplitude at a fixed average rotation speed of the mixer, the more significant these effects were. The larger amount of the connected crystals, especially of , form of PP in the bulk , form PP as well as with the continuous phase structure, led to a higher tensile properties of PP/UHMWPE vibration blended. © 2002 Wiley Periodicals, Inc. Adv Polym Techn 21: 164,176, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/adv.10020 [source] Rheological behavior of a polymer melt under the impact of a vibration force fieldJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2007Guang-sheng Zeng Abstract A model for the molecular motion of a polymer melt under the impact of a vibration force field was developed. From complicated theory deduction, an expression of the relaxation time and dynamic apparent viscosity were obtained. The effect of a vibration frequency and amplitude on the melt's dynamic apparent viscosity is explained in terms of shear-thinning and untie-tangle criteria. The model is supplemented by a calculation sample and experiment, which show that dynamic apparent viscosity of a melt will tend to decrease as the vibration frequency or amplitude increases. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 [source] Photo-initiated energy transfer in nanostructured complexes observed by near-field optical microscopyJOURNAL OF MICROSCOPY, Issue 3 2003G. A. Wurtz Summary We report an apertureless near-field optical study on nanostructured objects formed by J-aggregates adsorbed on silver (Ag) nanoparticles. Near-field images reveal that the enhanced near-field from the dressed particle's (DP) resonantly excited plasmon oscillation is efficiently absorbed by the J-aggregates. The sensitivity of the near-field images recorded at the harmonics of the probe vibration frequency suggests that the DP is releasing part of the absorbed energy radiatively upon interaction with the probe. The role of the probe in providing this new radiative relaxation channel is further confirmed as fluorescence from the J-aggregates on the particle is detected on the particle location only. We based the interpretation of our results on the near-field optical response from a bare Ag particle excited at the plasmon resonance as well as on far-field emission and transient absorption experiments. [source] Internal friction investigation of reverse martensitic transformation in oil-quenched Ni64Al36 alloyPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 12 2008Z. C. Zhou Abstract Low-frequency internal friction investigation of reverse martensitic transformation in oil-quenched Ni64Al36 alloy has been carried out using a multifunctional internal friction apparatus from room temperature to 400 °C and additionally differential scanning calorimetry and X-ray diffraction experiments were also completed. It has been shown that an internal friction peak presents at about 220 °C in the internal friction,temperature curve during heating for the oil-quenched Ni64Al36 alloy but not for the furnace-cooled Ni64Al36 alloy. The peak still appears during cooling and the peak temperature shifts to lower temperature. The changes of the peak temperature positions cannot be visibly observed when the vibration frequency is changed. The peak heights increase with decreasing vibration frequency and increasing heating rate, being linearly directly proportional to It has been suggested that the internal friction peak results from reverse martensitic transformation of L10 , , during heating and originates from martensitic transformation of , , L10 during the subsequent cooling process. The influence of the thermal cycles on the transformation is not observed for the limited thermal cycles. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||