Microwave determination of the conformation and dipole moment of 1,3-difluoroacetone

The microwave spectra of the ground state and several low-lying vibrational modes of 1,3-difluoroacetone have been assigned and analyzed. The assigned form has a molecular conformation in which one fluorine atom lies cis and the other trans to the oxygen atom. The rotational constants of the ground state species were determined using a centrifugal distortion analysis: A = 6024.843 ± 0.006 MHz, B = 2454.414 ± 0.001 MHz, C = 1783.897 ± 0.001 MHz. The molecular dipole moment components of the ground state species lie along the a and b principal axes with μ_a = 2.38 ± 0.03 D, μ_b = 0.89 ± 0.03 D, and μ_T = 2.54 ± 0.03 D. Comparative intensity measurements with OCS microwave lines indicate that the assigned form constitutes only 20% to 30% of the total gas mixture, the remainder presumably consisting of one or more other conformers, perhaps the gauche-gauche form. The lowest vibrational frequency (82 ± 12 cm^?1) is attributed to the trans-CH₂F torsion, while the next-higher vibrational frequency (127 ± 15 cm^?1) is believed to be the cis-torsion. A low-frequency in-plane bending motion is found at 285 ± 25 cm^?1.     

An experimental study of a two-dimensional plane turbulent wall jet

 Laser-Doppler measurements were conducted in a plane turbulent wall jet at a Reynolds number based on inlet velocity, Re ₀, of 9600. The initial development as well as the fully developed flow was studied. Special attention was given to the near-wall region, including the use of small measuring volumes and the application of specific near-wall data corrections, so that wall shear stresses were determined directly from the mean velocity gradient at the wall using only data below y ⁺=4. It was possible to resolve the inner peak in the streamwise turbulence intensity as well as the inner (negative) peak in the shear stress. Limiting values of (u′)⁺ and uv ⁺ were determined. Turbulence data from the outer region of the flow were compared to earlier hot wire measurements and large differences in the normal turbulence intensity and the shear stress were found. These differences can be attributed to high turbulence intensity effects on the hot-wires. Received: 17 October 1996 / Accepted: 8 December 1997     

Boundaries and random walks on finitely generated infinite groups

We prove that almost every path of a random walk on a finitely generated nonamenable group converges in the compactification of the group introduced by W. J. Floyd. In fact, we consider the more general setting of ergodic cocycles of some semigroup of one-Lipschitz maps of a complete metric space with a boundary constructed following Gromov. We obtain in addition that when the Floyd boundary of a finitely generated group is non-trivial, then it is in fact maximal in the sense that it can be identified with the Poisson boundary of the group with reasonable measures. The proof relies on works of Kaimanovich together with visibility properties of Floyd boundaries. Furthermore, we discuss mean proximality of ϖΓ and a conjecture of McMullen. Lastly, related statements about the convergence of certain sequences of points, for example quasigeodesic rays or orbits of one-Lipschitz maps, are obtained.     

Sulfophenylation of Polysulfones for Proton‐Conducting Fuel Cell Membranes

Polysulfone has been sulfophenylated by lithiation and anionic reaction with 2‐sulfobenzoic acid cyclic anhydride. This provides a new convenient method to modify polysulfones by attaching pendant sulfonated phenyl groups via ketone links. Membranes of the sulfophenylated polysulfones show promise for use in proton‐exchange‐membrane fuel cells. For example, a membrane with 0.9 sulfophenyl units per repeating polysulfone unit and 30 wt.‐% water was found to have a proton conductivity of 32 mS/cm at 60°C.