Remote substituent effects on bond dissociation energies of para-substituted aromatic silanes

UB3LYP/6-31G(d) and ROMP2/6-311++G(d,2p) methods were used to calculate the Si-X bond dissociation energies (BDEs) of a number of para-substituted aromatic silanes (4-Y-C(6)H(4)-SiH(2)X, where X = H, F, Cl, or Li). It was found that the substituent effect on the Si-H BDE of 4-Y-C(6)H(4)-SiH(3) was small, as the slope (rho(+)()) of the BDE- regression was only 0.09 kJ/mol. In comparison, the substituent effect on the Si-F BDE of 4-Y-C(6)H(4)-SiH(2)F was much stronger, whose rho(+ )()value was -2.34 kJ/mol. The substituent effect on the Si-Cl BDE of 4-Y-C(6)H(4)-SiH(2)Cl was also found to be strong with a rho(+)() value of -1.70 kJ/mol. However, the substituent effect on the Si-Li BDE of 4-Y-C(6)H(4)-SiH(2)Li was found to have a large and positive slope (+9.12 kJ/mol) against. The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituent to stabilize or destabilize the starting material (4-Y-C(6)H(4)-SiH(2)X) as well as the product (4-Y-C(6)H(4)-SiH(2)* radical) of the homolysis. Therefore, the direction and magnitude of the effects of Y-substituents on the Z-X BDEs in compounds such as 4-YC(6)H(4)Z-X should have some important dependence on the polarity of the Z-X bond undergoing homolysis. This conclusion was in agreement with that from earlier studies (for example, J. Am. Chem. Soc. 1991, 113, 9363). However, it indicated that the proposal from a recent work (J. Am. Chem. Soc. 2001, 123, 5518) was unfortunately not justified.     

A New Chemo‐Enzymatic Route to Side‐Chain Liquid‐Crystalline Polymers: The Synthesis and Polymerization of 6‐(4‐Methoxybiphenyl‐4′‐oxy)hexyl Vinyl Hexanedioate

A novel synthetic method combining chemo and enzymatic synthesis strategies was employed to prepare a vinyl acetate type monomer, 6‐(4‐methoxybiphenyl‐4′‐oxy)hexyl vinyl hexanedioate (VA‐LC). Homo‐ and copolymers of VA‐LC with maleic anhydride (MAn) were prepared by conventional free radical polymerization using 2,2′‐azobisisobutyronitrile (AIBN) and 1,1′‐azobis (cyclohexane carbonitrile) (AHCN) as an initiator at 95 and 60 °C, respectively. The thermal properties of the generated polymeric material were investigated by differential scanning calorimetry (DSC), and the optical texture was inspected by polarizing optical microscopy (POM). While the monomer VA‐LC does not exhibit liquid‐crystalline properties, poly(VA‐LC), and the alternating copolymer of VA‐LC with maleic anhydride both displayed such properties.

     

Photochemical Reaction of Decacarbonyldirhenium with Thiophene and Crystal Structure of Tetradecacarbonylhydridotrirhenium

The photochemical reaction of Re₂(CO)₁₀ with thiophene in hexane solution was investigated under vacuum. Three rhenium clusters: H₂Re₃(CO)₁₂, HRe₃(CO)₁₄ and Re₃(CO)₁₄(OH)₄, were isolated. The structure of Fellmann-Kaesz cluster Complex HRe₃(CO)₁₄ was determined by use of the X-ray diffraction method. The three rhenium atoms form a plane of symmetry and L: Re₁Re₂Re₃ is 107°. The ten carbonyl groups bonded to the two terminal rhenium atoms Re₁ and Re₃, are staggered with respect to the central rhenium atoms. The bond lengths are 3.10 Å for Re₂-Re₃ and 3.34 Å for Re₁-Re₂. The bridging hydride is between Re₁ and Re₂.     

Experiments in Solving Mixed Integer Programming Problems on a Small Array of Transputers

The time-consuming process of solving large-scale Mixed Integer Programming problems using the branch-and-bound technique can be speeded up by introducing a degree of parallelism into the basic algorithm. This paper describes the development and implementation of a parallel branch-and-bound algorithm created by adapting a commercial MIP solver. Inherent in the design of this software are certain ad hoc methods, the use of which are necessary in the effective solution of real problems. The extent to which these ad hoc methods can successfully be transferred to a parallel environment, in this case an array of at most nine transputers, is discussed. Computational results on a variety of real integer programming problems are reported.     

Pheromones of coleoptera. Communication 6. Synthesis of R,Z-14-methyl-8-hexadecenal (cis-trogodermal)

Conclusions The simple synthesis of optically active R,Z-14-methyl-8-hexadecenal (cis-trogodermal), the principal components of the aggregation pheromone ofTrogoderma granarium, was accomplished. The synthesis was based on the use of R-1-bromo-4-methylhexane and Z-4-chloro-2-buten-1-ol as the sources of the chiral and olefin fragments.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 906–910, April, 1987.     

Charge transport in a mixed ionically/electronically conducting, cationic, polyacetylene ionomer between ion-blocking electrodes

The electrical behavior of the cationic, polyacetylene-based, conjugated ionomer, poly[(2-cyclooctatetraenylethyl)trimethylammonium trifluoromethanesulfonate], sandwiched between gold electrodes is reported. The steady-state current of this mixed ionically/electronically conducting system is assigned to be unipolar diffusive hole transport for voltages below approximately 1.4 V, giving way to bipolar migratory transport above approximately 1.4 V. In the low-voltage regime, a non-Faradaically controlled doping model is proposed where p-doping at the anode is balanced by the charging of an ionic double layer at the cathode. In the high-voltage regime, n- and p-type regions extend from the electrodes as the voltage becomes sufficient to drive disproportionation and the electric field required by the redistribution of ions begins to substantially influence carrier transport. The assignment of a transport mechanism is primarily based on analyzing the decay of the steady-state system under short-circuit and open-circuit conditions. First, it is shown that the power describing the power-law decay of the short-circuit current is characteristic of the steady-state carrier profile. Second, it is argued that a component of the time-dependent, open-circuit voltage decaying more rapidly than the time scale for ion motion is indicative of a substantial migratory component to steady-state transport, as observed in the high-voltage regime. The hole and electron mobilities are estimated to be on the order of 10(-7)-10(-6) cm(2) V(-1) s(-1).