Focusing of dispersive phonons in GaAs

The ballistic propagation of highly dispersive phonons in GaAs in analyzed with both a bond-charge model (BCM) and a shell model, and compared to imaging experiments. When scattering is neglected, both models predict dispersive focusing pattern which depend strongly on the selected group velocities. In contrast, velocity-selected phonon images obtained with a PbBi tunnel-junction detector on a 0.7-mm thick crystal, display well-defined caustic lines whose angular positions do not shift at longer delay times. When isotope scattering is considered in the calculation, however, the shell model reproduces the experimental focusing structures remarkably well.For highly dispersive phonons, the bond-charge and shell models predict markedly different phonon-focusing patterns. In particular, a concentration of heat flux along [100] is predicted by BCM forv=1.7–2.0 THz slow-transverse (ST) phonons., but this effect is absent in the shell model. Both models predict new focusing structures in the fasttransverse (FT) mode at high frequencies. Our calculations which include isotope scattering indicate that to observe these new structures considerable technical advances are required in the phonon-imaging experiments.     

Effects related to Stevenson's rule in mass spectral intensity correlations

According to Stevenson's rule, there is a possibility that the chloromethyl and bromomethyl ions in the spectra of alkylhalides may not be formed from the molecular ion by processes described by electronic ground states alone. Since ion intensities at 75 eV are generally considered to be governed by appearance potentials, the operation of Stevenson's rule should influence correlations of intensity data according to the kinetic approach to mass spectra. Deviations from linearity were obtained in correlations of CH₂Br⁺ intensities, but not in those of CH₂CL⁺. Stevenson's rule is of help in explaining the deviations, but by its nature it is not absolute in predicting their occurrence with certainty. Consequently caution must be exercised in the interpretation of new data, particularly for aliphatic compounds.     

Activation du methyl-2 benzothiazole par metallation

The α-metalation of 2-methylbenzothiazole is studied with the aim of obtaining quantitatively an anionic intermediate which is able to react fully with benzaldehyde. The relative importance of the factors which influence the formation and reactivity of this intermediate are examined: the nature of the metalating agent, the solvent, the temperature and the reaction time.Butyllithium and phenyllithium, in the presence of linear or cyclic ethers at ?78°C, allow quantitative transformation into 2-benzothiazolylmethyllithium, a highly reactive agent at low temperatures (?78 to ?35°C), but non-reactive at 0°C. The addition of complexing bases raises the maximum temperature at which the reaction is possible. This method is applicable to 2-benzylbenzothiazole and induces a stereoselective attack on benzaldehyde.     

Evaluating Re-order Levels and Probabilities of Stockout during a Lead Time for Some Stock-Control Models

When several suppliers are concurrently used to replenish one stock item, it is often of interest to compute the mean and the variance of the effective lead time, i.e., the minimum of all lead times. However, in many cases, especially with non-identical lead time distributions, these quantities do not have closed form expressions and their calculation can be complicated. Difficulty in computation also arises in evaluating demand during lead time, even if there is only one supplier. This paper presents a numerical integration method using Gaussian quadrature to evaluate the effective lead time mean and variance, re-order level and the probability of stockout during the effective lead time. The Gaussian quadrature rules are easy to implement and several data sets are used in the paper to demonstrate the method.     

Computational investigation of rules governing selective adsorption intensities

Rules previously derived by Wolfe and Weare governing selective adsorption scattering intensities are investigated computationally via the attractive corrugated wall (ACW) model of Harvie and Weare. These rules, which correlate features of the scattering intensity with the form of the atom surface potential, correctly predict intensity features seen in the ACW calculation.     

Characterization of proton NMR relaxation times in normal and pathological tissues by correlation with other tissue parameters

To help understand which tissue parameters best account for the water proton NMR relaxation times, the longitudinal relaxation time (T₂), the transverse relaxation time (T₂), and the water content of 16 tissues from normal adult rats were measured at 10.7 MHz and 29°C. Regression analyses between the above and other tissue parameters were performed. These other tissue parameters included: the amounts of various organic and inorganic components, protein synthetic rate, oxygen consumption rate, and morphological composition. In addition, the differences in T₁, T₂, and water content values between normal liver and malignant tumor (Morris #7777 a transplantable hepatoma) were studied to help understand how a disease state can be detected and characterized by NMR spectroscopy. The results of this study and information from the literature allow the following generalizations to be made about tissue T₁ and T₂ values: (1) Each normal tissue has rather consistent and characteristic T₁ and T₂ relaxation times which are always shorter than the T₁ and T₂ of bulk water; (2) tissues with higher water content tend to have longer T₁ relaxation times; (3) tissue T₂ values are not, however, as well correlated with water content as T₁ values; (4) tissues with shorter T₁ values have higher calculated hydration fractions, greater amounts of rough endoplasmic reticulum, and a greater rate of protein synthetic activity; (5) tissues with higher lipid content, associated with intracellular non-membrane bounded lipid droplets, tend to have longer T₂ values; (6) tissues with greater overall surface area, whether in the form of cellular membranes or intracellular or extracellular fibrillar macromolecules, tend to have shorter T₂ values; (7) the differences between T₁ and T₂ values between tumor and normal tissues correlated with differences in the volume fraction (amounts) of extracellular fluid volumes and in the amounts of membrane and fibrillar surface area in the cells. The above generalizations should be useful in predicting T₁ and T₂ changes associated with specific tissue pathologies.     

Studies in nuclear magnetic resonance—XII: Complete analysis of the proton spectra of tert-butylcyohexane-3,3,4,4,5,5-d6 and -3,3,5,5-d4

Accurate parameters for the two title compounds have been obtained by the aid of a spectral subtraction technique in conjunction with analysis by LAOCN3. It is observed that J_{1eq, 6eq} and J_{1ax, 2ax} are abnormally large and small, respectively, and the significance of these and other coupling constants are discussed in terms of the probable geometry of tert-butylcyclohexane. The tert-butyl group affects the chemical shifts of the 4-protons, indicating that tert-butylcyclohexyl derivatives are poor models for fixed conformers of monosubstituted cyclohexanes. The deuterium isotope effects on proton chemical shifts appear to be influenced by the same factors as proton-proton coupling constants.