Highly selective tritium-from-deuterium isotope separation by pulsed NH3 laser multiple-photon dissociation of chloroform

Infrared multiple-photon dissociation of CTCl₃ was investigated using a pulsed CO₂ laser-pumped NH₃ laser at 12.08 μm. No evidence of any CDCl₃ depletion or decomposition product was observed in photolyzed CTCl₃/CDCl₃ mixtures. A lower limit of the single-step T/D enrichment factor, β, was found to be ≈ 165, based on the sensitivity in measuring CDCl₃ depletion. The low-fluene CTCl₃/CDCl₃ optical selectivity in absorptions is > 9000 at the 835 cm^?1 CTCl₃ ν₄ peak.     

Arene C−H Activation at Aluminium(I): meta Selectivity Driven by the Electronics of SNAr Chemistry

The reactivity of the electron-rich anionic Al^I aluminyl compound K₂[(NON)Al]₂ (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) towards mono- and disubstituted arenes is reported. C−H activation chemistry with n-butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer-type transition state. Selectivity is therefore based on similar electronic factors to classical S_NAr chemistry, which implies the destabilisation of transition states featuring electron-donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C−H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene C−H activation at a site which is neither ortho nor para to a methyl substituent.     

Equations for finite-difference, time-domain simulation of sound propagation in moving inhomogeneous media and numerical implementation

Finite-difference, time-domain (FDTD) calculations are typically performed with partial differential equations that are first order in time. Equation sets appropriate for FDTD calculations in a moving inhomogeneous medium (with an emphasis on the atmosphere) are derived and discussed in this paper. Two candidate equation sets, both derived from linearized equations of fluid dynamics, are proposed. The first, which contains three coupled equations for the sound pressure, vector acoustic velocity, and acoustic density, is obtained without any approximations. The second, which contains two coupled equations for the sound pressure and vector acoustic velocity, is derived by ignoring terms proportional to the divergence of the medium velocity and the gradient of the ambient pressure. It is shown that the second set has the same or a wider range of applicability than equations for the sound pressure that have been previously used for analytical and numerical studies of sound propagation in a moving atmosphere. Practical FDTD implementation of the second set of equations is discussed. Results show good agreement with theoretical predictions of the sound pressure due to a point monochromatic source in a uniform, high Mach number flow and with Fast Field Program calculations of sound propagation in a stratified moving atmosphere.     

Time-domain calculations of sound interactions with outdoor ground surfaces

A time-domain formulation for sound propagation in rigid-frame porous media, including waveform attenuation and dispersion, is developed. The new formulation is based on inversion of the relaxation functions from a previous model [Wilson DK, Ostashev VE, Collier SL. J Acoust Soc Am 2004;116:1889-92], thereby casting the convolution integrals in a form amenable to numerical implementation. Numerical techniques are developed that accurately implement the relaxational equations and transparently reduce to previous results in low- and high-frequency limits. The techniques are demonstrated on calculations of outdoor sound propagation involving hills, barriers, and ground surfaces with various material properties. We also compare the relaxation formulation to a widely applied phenomenological model developed by Zwikker and Kosten. The two models can be made equivalent if the resistance constant, structure constant, and compression modulus in the ZK model are allowed to be weakly frequency dependent. But if the ZK parameters are taken to be constant, as is typically the case, the relaxation model provides more accurate calculations of attenuation by acoustically soft porous materials such as snow, gravel, and forest litter.