Quenching of fluorescence from Na(32P) and K(42P) atoms following photodissociation of NaCl and KCl at 193 nm

 Quenching of fluorescence from Na(3² P) and K(4² P) atoms by various collision partners was studied at 973 and 1273K. Excited alkali atoms were produced photolytically by excimer laser light at 193nm. For each collision pair, the appropriate relative velocity was computed and used to evaluate the quenching cross-section from the measured rate constants. Cross sections for CO₂, O₂ and N₂ are large (10–60Ų) while for Ar, the values are <1 Ų. The results are compared with those of previous investigations as a function of relative velocity. Finally, implications for combustion diagnostics are briefly discussed. Received: 29 March 1996     

Fe2P2O7 and Fe2P4O12 studied between 5–800 K

Mössbauer spectra of triclinic Fe₂P₂O₇ indicate the existence of two crystallographic metal positions in the structure. In the paramagnetic region the two Mössbauer doublets are closely overlapping. The magnetic transition takes place at ≈ 21 K and the saturated fields are around 12 tesla for the two positions. In monoclinic Fe₂P₄O₁₂ the two octahedrally coordinated metal positions give quite different quadrupole splittings (1.5 and 3 mm/s at room temperature) and hyperfine field values (42 and 12.5 Tesla at 5 K). The transition temperature is at ≈ 18.5 K.     

Micelle-sequestered dissociation of cationic DNA-intercalated drugs: unexpected surfactant-induced rate enhancement

Detergent sequestration using micelles as a hydrophobic sink for dissociated drug molecules is an established technique for determination of dissociation rates. The anionic surfactant molecules are generally assumed not to interact with the anionic DNA and thereby not to affect the rate of dissociation. By contrast, we here demonstrate that the surfactant molecules sodium dodecyl sulfate (SDS), sodium decyl sulfate, and sodium octyl sulfate all induce substantial rate enhancements of the dissociation of intercalators from DNA. Four different cationic DNA intercalators are studied with respect to surfactant-induced dissociation. Except for the smallest intercalator, ethidium, the dissociation rate constants increase monotonically with surfactant concentration both below cmc and (more strongly) above cmc, much more than expected from electrostatic effects of increased counterion concentration. The rate enhancement, most pronounced for the bulky, multicationic, hydrophobic DNA ligands in this study, indicates a reduction of the activation energy for the ligand to pass out from a deeply penetrating intercalation site of DNA. The discovery that surfactants enhance the rate of dissociation of cationic DNA-intercalators implies that rate constants previously determined by micelle-sequestered dissociation may have been overestimated. As an alternative, more reliable method, we suggest instead the addition of excess of dummy DNA as an absorbent for dissociated ligand.     

Four levels of pattern recognition

Problems of pattern recognition in chemistry and other subjects can be divided conveniently into four different types depending on the level of scope of the problem.(1) Classification into one of a number of defined classes. As an example blood samples taken from persons known to be either controls or welders are considered. The problem is whether trace element concentrations in these samples contain information on whether or not a person is a welder.(2) Level 1 plus the possibility that an object is an outlier, i.e. does not belong to any of the defined classes. As an example, the üse of ¹³C-n.m.r. data to decide whether 2-substituted norbornanes have the exo or endo structure is discussed. (2A) Level 2, asymmetric. This situation occurs when one class does not have a systematic structure, but another class is homogeneous and can be described by a level 2 model. This occurs in the classification of materials or compounds as good or bad, active or inactive, and in binary classifications. As an example the use of trace element data to classify steel samples as having good or poor properties of strength is discussed.(3) Level 2 plus the ability to relate the variables measured to external properties of continuous character. As an example, the classification of a series of chemical compounds as β -receptor blockers, β -receptor stimulants, or neither, on the basis of their structural variables is discussed. In addition, relations between these structural variables and the measured biological activity are sought within each of the two classes.(4) Level 3 with the difference that several external property variables in the objects are measured. It may be desirable to use variables of the objects both for classification and for relations to several property variables: such examples are numerous in analytical chemistry.     

The end-on mechanism for lattice filling: Comparison with the conventional mechanism and application to the car-parking problem

We present the exact solution for the sequential, random, irreversible filling of one-dimensional lattices by linear n-mers using the end-on filling mechanism. The results are extrapolated to then limit (a variation on the car-parking problem) to yield a saturation coverage (packing density) of 0.7350. The end-on filling mechanism involves two steps for a single filling event. First, the landing site for one endpoint of the filling species is chosen and then the second endpoint is subsequently chosen (fromunfilled sites an appropriate distance from the first endpoint). We compare this mechanism to the conventional, one-step filling mechanism, where both endpoints of the filling species are chosen simultaneously. We present results detailing how the lattice saturation coverage varies for the two mechanisms. In addition, we extend our analysis to consider filling in the presence of a time-dependent, random distribution of inactive sites.     

Magnetic circular dichroism of heterocycles: thiophene

The existence of two transitions with opposite MCD in the 38000–45000 cm^?1 absorption of thiophene is demonstrated. The B-term for the lowest transition is negative, while it is positive for the higher one. Quantum mechanical calculations in the π-electron approximation resulted in energies, oscillator strengths and MCD terms in good agreement with the observed spectra.     

Extraction based on the flow-injection principle : Part 6. Film formation and dispersion in liquid—liquid segmented flow extraction systems

The dispersion mechanism in flow-injection extraction systems has been investigated. The phase with the highest affinity for the tubing material forms a thin film on the wall (e.g., 0.055 mm in a 0.7-mm i.d. PTFE tube with pentanol/water at a flow velocity of 11 cm s^?1). The film thickness increases linearly with increasing flow velocity and can be related to the viscosity/interfacial tension ratio in such a way that a low ratio indicates a thin film. The analyte is extracted into the film and into the adjacent segments. The film is stationary relative to the moving segments and this results in a backward transport of analyte molecules giving rise to dispersion. The thicker the film, the larger the dispersion. By decreasing the tube diameter and the flow velocity, lower dispersion results. Minimum dispersion is obtained for systems in which the phase carrying the analyte does not form the film.     

Random and cooperative sequential adsorption on infinite ladders and strips

We analyze various processes where particles are added irreversibly and sequentially at the sites of infinite ladders or broader strips (i.e., on terraces) of adsorption sites. For sufficiently narrow strips or ladders, exact solution in closed form is possible for a variety of processes. Often this is most naturally achieved by mapping the process onto an equivalent one-dimensional process typically involvingcompetitive adsorption. We demonstrate this procedure for sequential adsorption with nearest-neighbor exclusion on a 2× square ladder. For other select processes on strips slightly too broad for exact solution, almost exact analysis is possible exploiting an empty-site shielding property. In this way, we determine a jamming coverage of 0.91556671 for random sequential adsorption of dimers on a 2× square ladder. For broader strips, we note that the complexity of these problems quickly approaches that for × lattices.