Successive heterolytic cleavages of H2 achieve N2 splitting on silica-supported tantalum hydrides: a DFT proposed mechanism

DFT(B3PW91) calculations have been carried out to propose a pathway for the N(2) cleavage by H(2) in the presence of silica-supported tantalum hydride complexes [(≡SiO)(2)TaH(x)] that forms [(≡SiO)(2)Ta(NH)(NH(2))] (Science 2007, 317, 1056). The calculations, performed on the cluster models {μ-O[(HO)(2)SiO](2)}TaH(1) and {μ-O[(HO)(2)SiO](2)}TaH(3), labelled as (≡SiO)(2)TaH(x) (x = 1, 3), show that the direct hydride transfers to coordinated N-based ligands in (≡SiO)(2)TaH(η(2)-N(2)) and (≡SiO)(2)TaH(η(2)-HNNH) have high energy barrier barriers. These high energy barriers are due in part to a lack of energetically accessible empty orbitals in the negatively charged N-based ligands. It is shown that a succession of proton transfers and reduction steps (hydride transfer or 2 electron reduction by way of dihydride reductive coupling) to the nitrogen-based ligands leads to more energetically accessible pathways. These proton transfers, which occur by way of heterolytic activation of H(2), increase the electrophilicity of the resulting ligand (diazenido, N(2)H(-), and hydrazido, NHNH(2)(-), respectively) that can thus accept a hydride with a moderate energy barrier. In the case of (≡SiO)(2)TaH(η(2)-HNNH), the H(2) molecule that is adding across the Ta-N bond is released after the hydride transfer step by heterolytic elimination from (≡SiO)(2)TaH(NH(2))(2), suggesting that dihydrogen has a key role in assisting the final steps of the reaction without itself being consumed in the process. This partly accounts for the experimental observation that the addition of H(2) is needed to convert an intermediate, identified as a diazenido complex [(≡SiO)(2)TaH(η(2)-HNNH)] from its ν(N-H) stretching frequency of 3400 cm(-1), to the final product. Throughout the proposed mechanism, the tantalum remains in its preferred high oxidation state and avoids redox-type reactions, which are more energetically demanding.     

Numerical Study on Depinning Dynamics of Fluids Interacting with Disorder

We investigate the depinning of two-dlmensional fluids interacting with quenched disorder, based on Langevin simulations. For weak disorder the fluids depin elastically and flow in an ordered state. A power-law scaling fit between velocity and driving force can be obtained for the onset of motion in the elastic regime. This is in good agreement with that of colloid, charge density wave, and superconducting vortex systems. With an increasing strength of the disorder, we find a sharp crossover to plastic depinning, accompanied by a substantial increase in the depinning force. The scaling fit obtained in the elastic regime becomes invalid when plastic flow occurs. In the plastic regime, the fluids flow in channels and the hexatic order decays exponentially with drives.     

High-frequency acoustic emissions generated by a 20 kHz sonochemical horn processor detected using a novel broadband acoustic sensor: a preliminary study

This paper describes the application of a novel broadband acoustic sensor to evaluating the acoustic emissions from cavitation produced by a typical commercial 20 kHz sonochemical horn processor. Investigations of the reproducibility of the processor, and of the variation in cavitation emissions as a function of output setting and sensor location are described, and resulting trends discussed in terms of the broadband integrated power in the megahertz frequency range. Companion studies with a conventional membrane hydrophone have illustrated for the first time that cavitation emissions produced by a sonochemical horn processor can extend to frequencies beyond 20 MHz, and the sensor shows that significant nonlinearity can be seen in measured cavitation activity with increasing nominal output power.     

低温GaAs被动调Q锁模Nd∶Gd_(0.42)Y_(0.58)VO_4混晶激光器特性研究

采用低温生长GaAs晶体作为被动饱和吸收体兼输出镜,实现了Nd∶Gd0.42Y0.58VO4混晶激光器的调Q锁模运转。研究了Nd∶Gd0.42Y0.58VO4激光器的基频运转特性。在输出镜透射率T=10%、腔长L=40 mm的情况下,当抽运功率为8.6 W时,获得激光输出功率3.78 W,光光转换效率为43.9%。并测量了Nd∶Gd0.42Y0.58VO4混晶被动调Q激光器的输出特性。实验结果表明激光器调Q运转阈值为2 W,当抽运功率为3.7 W时,激光器出现调Q锁模行为;当抽运功率为8.6 W时,激光器调Q锁模深度达70%以上,对应的脉冲包络重复频率为670 kHz,半峰全宽为180 ns,平均输出功率为1.35 W,光光转换效率为15.7%。