Synthesis and properties of stereoregular cyclic polysilanols: cis-[PhSi(O)OH](4), cis-[PhSi(O)OH](6), and tris-cis-tris-trans-[PhSi(O)OH](12)

New stereoregular cyclic polysilanols of the general formula [PhSi(O)OH]n (n = 6 and 12) have been selectively obtained in high yields by the reaction of cagelike oligophenylmetallasiloxanes with dilute solutions of hydrochloric acid at low temperatures. An alternative method was used to prepare cis-[PhSi(O)OH](4) from sodium phenylsiloxanolate, cis-[(Na(+))(4)[PhSi(O)O(-)](4)].(1-butanol)(x). All compounds were fully characterized by NMR and IR spectroscopy and molecular weight determinations. The structure of cis-[PhSi(O)OH](6) was confirmed by single-crystal X-ray analysis. Furthermore, a series of stereoregular cyclosiloxanes containing triorganylsiloxy groups at each silicon atom was prepared by the reactions of the cyclic polysilanols with triorganylchlorosilanes Me(3)SiCl, Me(2)ViSiCl, and Me(2)(CH(2)Cl)SiCl.     

One- and Two-Dimensional Torsion-Inversion Models for the Fluoral Molecule (CF3CHO) in Excited Electronic States

The structure of the conformationally nonrigid fluoral molecule (CF₃CHO) in the ground (S₀) and lowest excited triplet (T₁) and singlet (S₁) electronic states was studied by ab initio quantum-chemical methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecule in these electronic states were determined. The calculations demonstrated that the electronic excitation causes substantial changes in the molecular structure involving the rotation of the CF₃ top and the deviation of the CCHO carbonyl fragment from planarity. The quantum-mechanical problems for large-amplitude vibrations, namely, for the torsional vibration in the S₀ state and the torsional and inversion vibrations (nonplanar carbonyl fragment) in the T₁ and S₁ states, were solved in the one- and two-dimensional approximations. A comparison of the results of calculations revealed the correlation between the torsional and inversion motions.     

Reactivity of Mo3PdS4+ 4 Cluster: Evidence for New Ligands PhP(OH)2 and Ph2P(OH) and Structural Characterization of [Mo3(Pd(PPh3))S4(H2O)5Cl4]⋅0.5CH3OH⋅3H2O

Cuboidal cluster aqua complex [Mo₃(PdCl)S₄(H₂O)₉]³⁺ in 4M HCl causes isomerization of (HO)₂P(O)(H), (HO)P(O)(H)₂, PhP(O)(OH)(H), and Ph₂P(O)(H) into hydroxo tautomers P(OH)₃, HP(OH)₂, PhP(OH)₂, and Ph₂P(OH) which are stabilized by P coordination to the Pd atom in the cluster. The reactions were followed by ³¹P NMR and UV/Vis spectroscopy. Hypophosphorous acid H₂P(O)(OH) in the presence of the cluster is rapidly oxidized into phosphorous acid; the reaction can be made catalytic. Coordination of PPh₃ also takes place, giving [Mo₃(Pd(PPh₃))S₄(H₂O)₅Cl₄]0.5CH₃OH3H₂O, whose crystal structure was determined.     

Hadwiger Number and the Cartesian Product of Graphs

The Hadwiger number η(G) of a graph G is the largest integer n for which the complete graph K _n on n vertices is a minor of G. Hadwiger conjectured that for every graph G, η(G) ≥ χ(G), where χ(G) is the chromatic number of G. In this paper, we study the Hadwiger number of the Cartesian product of graphs. As the main result of this paper, we prove that for any two graphs G ₁ and G ₂ with η(G ₁) = h and η(G ₂) = l. We show that the above lower bound is asymptotically best possible when h ≥ l. This asymptotically settles a question of Z. Miller (1978). As consequences of our main result, we show the following:

Poly(arylene sulfide)s via nucleophilic aromatic substitution reactions of halogenated pyromellitic diimides

Nucleophilic aromatic substitution (SNAr) reactions are exploited to prepare poly(arylene sulfide)s (PAS's) via the reaction of bis-thiolates and dibrominated pyromellitic diimide (PMDI) derivatives. Small-molecule model studies reveal the reaction is well-defined and proceeds in quantitative yield in practical times at room temperature. Variation in comonomer feed ratios allowed some control over target polymer molecular weights in the step polymerization, but control was likely limited by the relatively poor polymer solubility in the dipolar aprotic solvents typically employed to promote SNAr reactions. One substitution pattern produces a steric “pocket” around the PMDI units, inducing a peculiar solubility trend in halogenated solvents; that is, greatly reduced solubility in CHCl₃ relative to CH₂Cl₂ and C₂H₂Cl₄. One example small-molecule readily dissolves in CHCl₃ at room temperature, then rapidly grows poorly soluble crystals revealed by single-crystal XRD to contain CHCl₃ molecules in the steric pockets. Finally, the recently demonstrated depolymerization of phthalonitrile-based PAS's via ipso substitution with monothiolates as chain scission agents yields quantitative molecular weight reduction to monomeric species from the polymers reported here.     

Synthesis of thieno[2,3-d]pyrimidin-2-ylmethanamine combinatorial library with four diversity points

The parallel solution-phase synthesis of more than 230 substituted thieno[2,3-d]pyrimidin-2-ylmethanamines has been accomplished. This strategy is based on the cyclization of 2-aminothiophen-3-carboxylates with chloroacetonitrile to construct the thieno[2,3-d]pyrimidine core with two diversity points. Derivatization of the active chlorine and functionalization of C-4 position of the pyrimidine ring allow the introduction of other diversity points. The products containing ester groups at the 6-position of the thieno[2,3-d]pyrimidine were used in amide synthesis. Simple manual techniques for parallel reactions, coupled with simple purification procedures, gave highly pure final products. The scope and limitations of the approach are discussed.     

Mode Converter Synthesis by the Particle Swarm Optimization

Particle Swarm Optimization (PSO) is an effective, simple and promising method intended for the fast search in multi-dimensional space [Kennedy and Eberhart, "Particle Swarm Optimization", Proc. of the 1995 IEEE International Conference on Neural Networks, 1995]. Besides special testing problems a number of engineering tasks of electrodynamics were solved by the PSO successfully [Robinson and Rahmat-Samii, "Particle Swarm Optimization in Electromagnetics", IEEE Trans. Antennas Propag., 2004; Jin and Rahmat-Samii, "Parallel Particle Swarm Optimization and Finite-Difference Time-Domain (PSO/FDTD) Algorithm for Multband and Wide-Band Patch Antenna Designs", IEEE Trans. Antennas Propag., 2005]. On the other hand, the scattering matrix technique is a fast and accurate method of mode converter analysis. We illustrate PSO by a number of converter designs developed for high-power microwaves control: a matching horn for output maser section, a corrugated converter of linear-polarized hybrid modes, a TE₀₁ mitre bend.     

Tire load rating to reduce soil compaction

Both experience and research warn that heavily loaded wheels of agricultural transport vehicles and heavy machinery may cause severe compaction damage to the farmland. A remedy consists of reducing both the wheel load and the contact pressure.Early in the 1990s, the author suggested an experimental examination of the problem of soil compaction under fully controlled conditions. The ensuing research program, which was sponsored by the Grant Agency of the Czech Republic, included a series of experiments with loaded wheels carried out in the experimental grounds of the Czech University of Agriculture and, subsequently, their physical modelling in the laboratory of the Department of Motor Vehicles, Technical faculty. This program has corroborated the idea that physical modelling under controlled conditions, complemented by an adequate evaluation procedure, has a promising potential to predict full-scale ground compaction and become a sound basis for practical measures. This paper describes the laboratory equipment, testing technique, and the way of evaluating the compaction potential of tires in terms of soil dry bulk density, leading to a Compaction Number (CN) rating of individual tires. Practically, the CN rating is supposed to be included in agricultural tire catalogues to complement the load capacity/inflation pressure values for hard ground (e.g., ETRTO specifications based on tire strength and wear).