Delbrück scattering

The various calculations of the Delbrück scattering amplitude are presented. This is followed by some generalities on the elastic scattering of photons by heavy atoms and finally by a review of the experiments together with a comparison with the calculations.     

Positronium formation in nonpolar solvents. A comparative study of the influence of chlorinated ethanes,ethenes and cyclohexanes on the yield of o-Ps

Inhibition of positronium formation caused by different chlorocyclohexanes, chloroethanes and chloroethenes used as additives in cyclohexane, 2,2,4-trimethylpentane, benzene and n-hexane has been investigated by means of positron lifetime measurements. Chlorocyclohexanes and chloroethanes cause increased inhibition with increasing chlorine substitution. Comparisons with earlier results of chlorobenzenes show that the behaviour of the actual additives is quite different from that of the chlorobenzenes. The results are discussed in terms of the “hot Ps-model” and the “spur-model”.     

Mass spectrometry of onium compounds. XXIII—ionisation potentials in the structural assignment of [M – N2] ions from diazo-oxides

The ionisation potential shows that fulven-6one is formed pyrolytically from o-benzo-diazo-oxide. Appearance potential measurements show increasing pyrolytic contribution to the [M – N₂] species with increase in ion source temperature.     

The depolarization of light scattered from rough metal surfaces

To describe the depolarization of the electromagnetic field scattered from rough metal surfaces, a finite, non-vanishing value of the conductivity has to be considered. To reveal the modifications imposed by a complex dielectric constant, the Beckmann formula is expanded in power series of slope terms. We then arrive at an analytic expression which is convenient for evaluation as a function of the parameters involved. With this expression, plots of the cross-polarization ratio vs. angle of incidence and vs. slope are presented.     

An experimental investigation of the motion of long bubbles in inclined tubes

The relative motion of single long air bubbles suspended in a constant liquid flow in inclined tubes has been studied experimentally. Specially designed instrumentation, based on the difference in refractive properties of air and liquid with respect to infrared light, has been constructed and applied to measure bubble propagation rates.A series of experiments were performed to determine the effect of tube inclination on bubble motion with liquid Reynolds and Froude numbers, and tube diameter as the most important parameters.Particular aspects of the flow are described theoretically, and model predictions were found to compare well with observations. A correlation of bubble and average liquid velocities, based on a least squares fit to the data is suggested. Comparisons with other relevant models and data are also presented.     

Electrochemical reduction of α,α′-Dibromo-dibenzylsulfone, -sulfoxide,and -sulfide

The electrochemical reduction of the title compounds in dimethylformamide affords in all cases stilbene as a major product, leading to the conclusion that cyclization to a three-membered ring containing sulfur is a common feature in these reductions. Other products isolated in the reductions help to shed light upon the course of the reactions.     

Regioselectivity in reactions of alkynylmetal complexes with pyrimidinones

Regioselectivity is observed in $\text{1}\text{1}$-adduct formation between phenylethynyltriisopropoxytitanium and 2(1/H)-pyrimidinones; the new carbon-carbon bond is formed at C(4). In contrast the 3,4- and 3,6-dihydro products are formed together from the corresponding magnesium and lithium reagents, but from the magnesium compound the major product is the 3,6-dihydro isomer. Ethynylmagnesium bromide gives equimolar amounts of the 3,4- and 3,6-dihydro isomers. Dehydrogenation of the products gives the alkynylated aromatic heterocycles.     

Rank estimates in a class of semiparametric two-sample models

We consider a two-sample semiparametric model involving a real parameter and a nuisance parameter F which is a distribution function. This model includes the proportional hazard, proportional odds, linear transformation and Harrington-Fleming models (1982, Biometrika, 69, 533–546). We propose two types of estimates based on ranks. The first is a rank approximation to Huber's M-estimates (1981, Robust Statistics, Wiley) and the second is a Hodges-Lehmann type rank inversion estimate (1963, Ann. Math. Statist., 34, 598–611). We obtain asymptotic normality and efficiency results. The estimates are consistent and asymptotically normal generally but fully efficient only for special cases.Research partially supported by National Science Foundation Grant DMS-86-02083 and National Institute of General Medical Sciences Grant SSS-Y1RO1GM35416-01     

Total angular momentum conservation in statistical unimolecular theory should be represented in energy space

A representation of total angular momentum conservation in energy space is described. Several advantages of such a representation are discussed, primarily in connection with statistical unimolecular theory, experimental branching ratios and product energy distributions. Specific examples are given for the systems K + CsF and O + I₂.     

Determination of ethane,pentane and isoprene in exhaled air using a multi-bed adsorbent and end-cut gas-solid chromatography

A method for the determination of exhaled ethane, pentane and isoprene was developed and validated. The method was based on pre-concentration of the analytes on a multi-bed solid adsorbent tube containing Tenax TA, Carboxen 569 and Carboxen 1000, thermal desorption and gas chromatography (GC) with flame ionisation detection (FID). A pre-column in an end-cut GC system was used to avoid problems with water and strongly retained substances. The detection limits were 5, 2 and 6 pmol per sample for ethane, pentane and isoprene, respectively, using a sample volume of 500 ml. The linearity was good for all analytes with correlation coefficients exceeding 0.999. The repeatability for exhaled air samples was 7, 10 and 12% for ethane, pentane and isoprene, respectively. Analysis of a certified reference material of ethane and pentane did not differ significantly from the certified values. Ethane and pentane levels were stable up to six days of storage in sample tubes. Isoprene levels were not stable during storage in the sample tubes used here, but using Carbopack X instead of Carboxen 569, levels were stable up to two days. The levels of exhaled ethane, pentane and isoprene in healthy subjects (n = 4) were 8.1+/-5.8 pmol l(-1), 11+/-5.8 pmol l(-1) and 2.4+/-0.90 mnol l(-1), respectively. The method could, with minor modifications, be used to determine other low-molecular hydrocarbons in exhaled air as well.