Spanning point-line geometries in buildings of spherical type

We consider the point-line geometries that arise as a shadow space in a spherical building with a diagram of type A_n, B_n, C_n, D_n or E_n, and determine in which cases the geometry is spanned by the set of points on an apartment. It turns out that this happens precisely in the cases corresponding to a minimal weight.     

A remark on partial linear spaces of girth 5 with an application to strongly regular graphs

We derive a lower bound on the number of points of a partial linear space of girth 5. As an application, certain strongly regular graphs with=2 are ruled out by observing that the first subconstituents are partial linear spaces.     

Calorimetric,dilatometric and microscopic investigations of the system sodium stearate-stearic acid

Summary AT-x phase diagram of the system sodium stearate (NaSt)-stearic acid (HSt) has been determined by DTA. It shows the existence of five defined association compounds, three of them (2 NaSt · 3 HSt, NaSt · HSt and 3 NaSt · 2 HSt) with an incongruent melting point and the others (5 NaSt · 2 HSt and 5 NaSt · HSt) with a congruent melting point. The latter two compounds pass through various mesomorphic phases before melting. The components do not form solid solutions and are even non-miscible in the mesomorphic state below 200 °C. Mutual miscibility is only observed above this temperature in the subneat and neat region, occurring in the HSt composition range from 0 to 15 and from 0 to 5 mole% respectively. The existence of molecular associations in the liquid state is very probable, particularly in the composition range between 30 and 40 mole% stearic acid. This might result in a liquid structure similar to that present in the melt of anhydrous soaps.

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Zusammenfassung EinT-x Zustandsdiagramm des Natriumstearat (NaSt)- Stearins?ure (HSt)-Systems wurde mittels DTA bestimmt. Es weist fünf definierte assoziierte Verbindungen auf, drei (2 NaSt · 3 HSt, NaSt · HSt und 3 NaSt · 2 HSt) mit einem inkongruenten Schmelzpunkt und zwei (5 NaSt · 2 HSt und 5 NaSt · HSt) mit einem kongruenten Schmelzpunkt. Die zwei letzteren Verbindungen durchlaufen verschiedene mesomorphe Phasen, bevor sie schmelzen. Die Komponenten bilden keine festen L?sungen und sind im mesomorphen Zustand unter 200 °C nicht einmal miteinander mischbar. Nur bei h?heren Temperaturen sind sie v?llig mischbar, und zwar in der “subneat” und “neat” Region, die im HSt-Bereich von 0 bis 15 bzw. von 0 bis 5 Mol% reicht. Die Existenz molekularer Assoziate im flüssigen Zustand ist sehr wahrscheinlich, vor allem im Bereich zwischen 30 und 40 Mol% Stearins?ure. Dies k?nnte zur Bildung von Strukturen in der Flüssigkeit führen ?hnlich, wie dies bei der Schmelze wasserfreier Seifen der Fall ist.

     

Combinatorial configurations

Book Reviews

Combinatorial configurationsVladimir D. Tonchev: (Translated from Bulgarian to English by Robert A. Melter.) Pitman Monographs and Surveys in Pure and Applied Mathematics #40, Longman Scientific and Technical, Harlow, Essex, U.K., 1988, 189 pp.     

Optimization, standardization, and testing of a new NMR method for the determination of zeolite host-organic guest crystal structures

An optimized and automated protocol for determining the location of guest sorbate molecules in highly siliceous zeolites from (29)Si INADEQUATE and (1)H/(29)Si cross polarization (CP) magic-angle spinning (MAS) NMR experiments is described. With the peaks in the (29)Si MAS NMR spectrum assigned to the unique Si sites in the zeolite framework by a 2D (29)Si INADEQUATE experiment, the location of the sorbate molecule is found by systematically searching for sorbate locations for which the measured rates of (1)H/(29)Si cross polarization of the different Si sites correlate linearly with (1)H/(29)Si second moments calculated from H-Si distances. Due to the (1)H/(29)Si cross polarization being in the "slow CP regime" for many zeolite-sorbate complexes, it is proposed that the CP rate constants are best measured by (1)H/(29)Si cross polarization drain experiments, if possible, to avoid complications that may arise from fast (1)H and (29)Si T(1)rho relaxations. An algorithm for determining the sorbate molecule location is described in detail. A number of ways to effectively summarize and display the large number of solutions which typically result from a prediction of the structure from the CP MAS NMR data are presented, including estimates of the errors involved in the structure determinations. As a working example throughout this paper, the structure of the low loaded p-dichlorobenzene/ZSM-5 complex is determined under different conditions from solid-state (1)H/(29)Si CP MAS NMR data, and the solutions are shown to be in excellent agreement with the known single-crystal X-ray diffraction structure. This structure determination approach is shown to be quite insensitive to the use of relative rate constants rather than absolute values, to the detailed structure of the zeolite framework, and relatively insensitive to temperature and motions.     

Degenerate molecular shuttles with flexible and rigid spacers

The preparation and dynamic behavior of degenerate rotaxane molecular shuttles are described in which a benzylic amide macrocycle moves back and forth between two naphthalimide-glycine units along a diphenylethyne spacer or an aliphatic spacer consisting of a C(9), C(12), or C(26) alkyl chain. Subtle differences in the (1)H NMR spectra of the rotaxanes can be related to the presence of conformers in which the macrocycle interacts simultaneously with both glycines, especially in the case of the C(9) spacer. The kinetic data of the shuttling behavior in the C(26) rotaxane were obtained from dynamic NMR spectroscopy. The Eyring activation parameters were found to be ΔH(?) = 10 ± 1 kcal mol(-1), ΔS(?) = -6.5 ± 2.0 cal mol(-1) K(-1), ΔG(?)(298) = 11.9 ± 0.2 kcal mol(-1). For the systems with the shorter spacers, the shuttling rates were higher. Also in the diphenylethyne, rotaxane shuttling is rapid on the NMR time scale, indicating that the rigid unit does not impose a large barrier to the translocation of the macrocycle.     

Effect of molecular oxygen on the variable-temperature 29Si MAS NMR spectra of zeolite-sorbate complexes

Structure determinations of siliceous zeolite-sorbate host-guest complexes by solid-state NMR require highly resolved 29Si MAS NMR spectra. As the temperature is lowered, the 29Si MAS NMR spectra of many zeolite-sorbate complexes become broadened such that the resolution of the individual 29Si peaks is lost, limiting the application of solid-state NMR for structure determination. It is shown that the 29Si peak widths are related to the 29Si T2 relaxation times and that the source of the 29Si relaxation and the line broadening is paramagnetic molecular oxygen in the channels of the zeolite. Removal of the oxygen by purging the sample with nitrogen gas leads to a dramatic increase in the resolution of the 29Si MAS NMR spectrum of the p-dibromobenzene/ZSM-5 complex. An analysis of the individual 29Si T1 relaxation times reveals that the oxygen molecules are localized mainly in the zigzag channels of ZSM-5, suggesting that the p-dibromobenzene molecules are located in the channel intersections.     

A double quantum (129)Xe NMR experiment for probing xenon in multiply-occupied cavities of solid-state inclusion compounds

A method is presented for detecting multiple xenon atoms in cavities of solid-state inclusion compounds using (129)Xe double quantum NMR spectroscopy. Double quantum filtered (129)Xe NMR spectra, performed on the xenon clathrate of Dianin's compound were obtained under high-resolution Magic-Angle Spinning (MAS) conditions, by recoupling the weak (129)Xe-(129)Xe dipole-dipole couplings that exist between xenon atoms in close spatial proximity. Because the (129)Xe-(129)Xe dipole-dipole couplings are generally weak due to dynamics of the atoms and to large internuclear separations, and since the (129)Xe Chemical Shift Anisotropy (CSA) tends to be relatively large, a very robust dipolar recoupling sequence was necessary, with the symmetry-based SR26 dipolar recoupling sequence proving appropriate. We have also attempted to measure the (129)Xe-(129)Xe dipole-dipole coupling constant between xenon atoms in the cavities of the xenon-Dianin's compound clathrate and have found that the dynamics of the xenon atoms (as investigated with molecular dynamics simulations) as well as (129)Xe multiple spin effects complicate the analysis. The double quantum NMR method is useful for peak assignment in (129)Xe NMR spectra because peaks arising from different types of absorption/inclusion sites or from different levels of occupancy of single sites can be distinguished. The method can also help resolve ambiguities in diffraction experiments concerning the order/disorder in a material.     

ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

Improving the photochemical properties of molecular photoswitches is crucial for the development of light‐responsive systems in materials and life sciences. ortho‐Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ‐electron‐withdrawing F atoms ortho to the N?N unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para‐electron‐withdrawing groups (EWGs) work in concert with ortho‐F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho‐fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.