High selectivity for primary C-H bond cleavage of propane σ-complexes on the PdO(101) surface

We investigated regioselectivity in the initial C-H bond activation of propane σ-complexes on the PdO(101) surface using temperature programmed reaction spectroscopy (TPRS) experiments. We observe a significant kinetic isotope effect (KIE) in the initial C-H(D) bond cleavage of propane on PdO(101) such that the dissociation yield of C(3)H(8) is 2.7 times higher than that of C(3)D(8) at temperatures between 150 and 200 K. Measurements of the reactivity of (CH(3))(2)CD(2) and (CD(3))(2)CH(2) show that deuteration of the methyl groups is primarily responsible for the lower reactivity of C(3)D(8) relative to C(3)H(8), and thus that 1° C-H bond cleavage is the preferred pathway for propane activation on PdO(101). By analyzing the rate data within the context of a kinetic model for precursor-mediated dissociation, we estimate that 90% of the propane σ-complexes which dissociate on PdO(101) during TPRS do so by 1° C-H bond cleavage.     

Individual and collective behavior of vibrating motors interacting through a resonant plate

We report on experiments of many small motors—cell phone vibrators—glued to and interacting through a resonant elastic plate. We find that the motors tend to avoid frequencies that are just higher than the resonances of a plate, preferring instead frequencies just below those resonances. As a result, motors interacting through a resonant plate exhibit hysteresis in their frequency versus driving voltage. We also find that the stability of a single motor near a resonance is different from the stability of a group of motors near a resonance. When the driving voltage is constant and the transient behavior of the system has passed, we find that the average frequency of all the motors is constant.     

From finitary to infinitary second‐order logic

A back and forth condition on interpretations for those second‐order languages without functional variables whose non‐logical vocabulary is finite and excludes functional constants is presented. It is shown that this condition is necessary and sufficient for the interpretations to be equivalent in the language. When applied to second‐order languages with an infinite non‐logical vocabulary, excluding functional constants, the back and forth condition is sufficient but not necessary. It is shown that there is a class of infinitary second‐order languages whose non‐logical vocabulary is infinite for which the back and forth condition is both necessary and sufficient. It is also shown that some applications of the back and forth construction for second‐order languages can be extended to the infinitary second‐order languages. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Shielding against noise interfering with quantitation of insect infestations by acoustic detection systems in grain elevators

Acoustic devices used to detect hidden insect infestations must be shielded from noise in most practical applications. One device developed specifically for use in a noisy environment, the Acoustic Location ‘Fingerprinting’ Insect Detector (ALFID), counts the numbers of insects present in grain samples from shipments being graded for export at commercial grain elevators. This report considers the performance of ALFID's noise-shielding components, which include an enclosure for passive reduction of ambient noise, and an electronic system for active detection and masking of sounds originating outside the grain sample container. Sound pressure levels (SPLs) of ambient noise are reduced inside the enclosure by 60–90 dB at frequencies between 1 and 10 kHz, with a reduction of ~6.5 dB per octave (frequency doubling). The active noise-masking system protects ALFID from loud ambient sounds not sufficiently attenuated by the enclosure. If the output from one of four sensors mounted on the outside of the grain sample container rises above a preset amplitude threshold, a signal is triggered that inhibits acquisition of insect sound data from sensors inside the container. In tests of the complete ALFID system at a grain elevator with ambient noise of 73 ± W dB re 20 μPa SPL, the mean rate of noise-mask triggering was 5.5 s⁻¹, inhibiting acquisition of insect sounds for only 3.9% of the total testing period. This level of performance is sufficient to enable successful operation of ALFID under such noise conditions.     

Electrosorptive detection in liquid chromatography of simple charged organic compounds

Summary The applicability of electrosorptive (ES) detection in liquid chromatography to charged organic species, specifically amines, alkanolamines, and alkylsulfonates is demonstrated. Separations can be carried out using ion-pair chromatography, and detection acieved by measuring changes in differential double-layer capacitance (DLC), C_d, brought about by analyte adsorption at potentials either well negative or positive of the potential of zero charge (pzc), depending on whether cationic or anionic forms, respectively, are being determined. The C_d-analyte concentration response curves are linear over a limited range. A welcome feature of the technique is that organic modifiers may be used in the mobile phase without deleterious consequence to the detection. Attempts were also made to detect these charged species as ion pairs by DLC at potentials close to the pzc. This approach, however, appears to have less practical utility.     

The hearing aid feedback path: mathematical simulations and experimental verification

Acoustic feedback in hearing aids has received little attention in the literature. Feedback occurs when stability conditions of the open-loop transfer function of an in situ hearing aid are violated. Solving the feedback problem will first require knowledge of the open-loop transfer function. Included in the open-loop transfer function is the acoustical path by which sound emanating from the earmold vent returns to the microphone (i.e., the feedback path). Reported herein are two different mathematical procedures for simulating transfer functions of the feedback path of an eyeglass-type hearing aid. In one procedure the vent exit was modeled as a point source of sound located on a flat plane, while it was treated as a point source on a sphere in the other. Results of laboratory experiments indicate that the mathematical models accurately predict those acoustic phenomena for which they were intended: point sources on plane and spherical baffles. Results of manikin experiments showed both models to be less accurate for simulating the feedback path around the human head. The maximum difference between experiment and theory was 6 dB at one frequency. Surprisingly, the flat-baffle model produced better agreement with experimental results than did the sphere model.     

Limited field of view spin echo MR imaging

Several groups have reported using a method of limiting the field of view (FOV) where the slices excited by the 90 and 180 degree pulses are perpendicular. However, only one slice can be excited during each repetition time, so multislice imaging is not possible. We present a modification of this method that allows multislice imaging. The slices excited by the 90 degrees and 180 degrees pulses are at a small angle; the field of view is limited and multislice imaging is possible. The modifications also allow the center of the FOV to be offset to any position. We describe the conditions that yield optimal images for the given FOV, slice thickness, and interslice gap. Representative images demonstrating the features of the technique are presented. The technique can be used to reduce the number of phase-encoding steps resulting in reduced imaging time, or it can be used to increase the spatial resolution without increasing the imaging time.     

Temperature and volume dependence of some magnetic properties of Pr-pnictides

Several experiments relating to the volume dependence of crystal field interactions in rare earth pnictides have been carried out. These experiments include measurements of 1) the temperature dependence of the susceptibility of PrN, 2) the room temperature compressibilities of PrN and PrAs, 3) the NMR frequency shifts for ¹⁴¹Pr in PrN and PrSb, and 4) the low-temperature pressure dependence of the ¹⁴¹Pr frequency shift in PrN and PrSb. Although electrostatic screening effects on crystal field interactions in PrN are smaller than in the heavier Pr- pnictides, the anomalous volume dependence of the ¹⁴¹Pr NMR shift is essentially the same in all these compounds.     

Electronic structure of C84 and KxC84: Comparison to C60 and graphite

X ray photoemission spectra show ～10 valence band features in C₆₀ and C₈₄ films, although they are less distinct in C₈₄ because of the increased number of valence electrons and the decreased molecular symmetry. The presence of inequivalent carbon atoms in C₈₄ is reflected by the fact that the C 1s main line is ～0.2 eV wider than in C₆₀. The C 1s satellite region of C₈₄ exhibits four features within 15 eV of the main line and shows that the HOMO-LUMO π-π* excitation energy of C₈₄ is ～0.5 eV smaller than in C₆₀. Potassium 2p spectra for K-doped C₈₄ films suggest the formation of a phase with composition K₃C₈₄ with two spectroscopically-distinguishable K sites. A new spectral feature emerges when the K content is increased beyond K₃C₈₄, suggesting a phase transition and a saturated composition of K₆C₈₄, as for K_xC₆₀. These similarities are intriguing since K_xC₈₄ is insulating for all x while K₃C₆₀ is metallic and superconducting.