Vibrational energy transfer in H2 + He

A semiclassical approach is developed to study vibrational energy transfer in H₂ + He by use of the a priori interaction potential including all nonzero impact parameter collisions. The calculated values of the rate coefficient are found to be in excellent agreement with experimental data which are available in the temperature ranges 60–450 K and 1350-3000 K. The temperature dependence is shown to seriously deviate from the Landau-Teller prediction below 1000 K. The calculation was carried out over the temperature range of 30 to 10000 K.     

In situ disorder-order transformation in synthetic gallosilicate zeolites with the NAT topology

Here, we report that synthetic gallosilicate molecular sieves with the NAT topology and Si/Ga ratios close to but slightly higher than 1.50 undergo an in situ transformation under their crystallization conditions. The materials have been studied ex situ by using powder X-ray diffraction, elemental and thermal analyses, and multinuclear MAS NMR. The transformation is characterized by a change in the distribution of Si and Ga of the NAT framework, from a quite (but not completely) disordered phase to a very highly (but not completely) ordered one, accompanied by a change from tetragonal to orthorhombic symmetry. During most of the solution-mediated transformation, no noticeable signs of fresh precipitation, phase segregation, or changes in the chemical composition are detected. Intermediate materials show variations in the degree of Si-Ga ordering and orthorhombic distortion and are not physical mixtures of the disordered and ordered phases. Ab initio calculations strongly suggest a preferential siting of Si in the tetrahedral sites involved in a smaller number of 4-rings in the NAT topology (i.e., the low multiplicity site). The cost of violations of Loewenstein's rule has also been calculated. For this topology and chemical composition the preferential siting and Loewenstein's rule drive together the system to the ordered configuration. A Monte Carlo sampling procedure affords a reasonable model for the initial, mainly disordered state, which fits well within the experimental disorder-order series.     

Lithium isotope separation by chemical exchange with polymer-bound azacrown compounds

The chromatographic separation of lithium isotopes was investigated by chemical exchange with the recently synthesized polymer-bound dibenzo pyridino diamide azacrown (DBPDA) and reduced dibenzo pyridino diamide azacrown (RDBPDA). Column chromatography was employed for the determination of the effect of solvents and ligand conformation on the separation coefficients. The maximum separation coefficients, , for the DBPDA and RDBPDA at 20.0±0.02°C with acetonitrile as eluent, were found to be 0.034±0.002 and 0.035±0.002, respectively. The isotope separation coefficient and adsorption capability of the lithium ion on the DBPDA and RDBPDA were only slightly dependent on ligand structure, but strongly dependent on the solvent. DBPDA and RDBPDA appeared to have almost the same value for the isotope separation coefficient of lithium.     

Self-assembly between silver(I) and di- and tri-2-pyridines with flexible spacer: formation of discrete metallocycle versus coordination polymer

The self-assembly of metallosupramolecules from reactions of flexible 2-pyridyl ligands and silver salts is described. When 1,3-bis(2-pyridyl)propane (L1), tris[(2-pyridyl)methyl]methane (L2), and 1,3-bis(2-pyridyl)-2-tolylpropane (L3) are used in combination with silver ions, novel discrete metallocyclic complexes are formed in crystals. Moreover, the self-assembly of 1,3-bis(2-pyridyl)-2-phenylpropane (L4) with silver nitrate yields a coordination polymer. The examination of its solution shows that this coordination polymer is formed via the solution-based discrete metallocyclic species.     

Hydrogen peroxide oxidation of di(hydroxo)platinum(II) species in carboxylic acids

A facile procedure for synthesizing the mono(hydroxo)tris(carboxylato)platinum(IV) species has been achieved. The reaction of [Pt^II(OH)₂(dmpda)] (dmpda=2,2-dimethyl-1,3-propanediamine) with a 30% aqueous solution of H₂O₂ in the presence of a carboxylic acid produces a stable [Pt^IV(OOCR)₃(OH)(dmpda)] (R=Me, Et) complex in high yield. The crystal structures of [Pt^IV(OOCMe)₃(OH)(dmpda)] . H₂O (triclinic P1 bar, a=8.761(2) Å, b=9.245(3) Å, c=10.659(2) Å, =106.25(2)°, =93.90(2)°, =98.92(2)°, V=813.1(3) ų, Z=2, R= 0.0474) and [Pt^IV(OOCEt)₃(OH)(dmpda)] (monoclinic P2₁/c, a=12.777(4) Å, b=10.514(2) Å, c=14.971(3) Å, =107.40(2)°, V=1919.2(8) ų, Z=4, R=0.0611) show that the hydroxyl group has been selectively positioned at an axial site. The intramolecular hydrogen bond between the OH and C=O moiety exists (O(H)...=C, 2.83 Å for [Pt^IV(OOCMe)₃(OH)(dmpda)] · H₂O; 2.72 Å for [Pt^IV(OOCEt)₃(OH)(dmpda)]. Formation of the axial-mono(hydroxo)tris(carboxylato)platinum(IV) species may be ascribed to a combination of `reactive-equatorial effects' with `cis-addition' in the carboxylic acid.     

catena‐Poly[[copper(II)‐μ‐2‐[bis­(2‐pyridylmeth­yl)amino]butyrato‐κ4N,N′,N′′,O:κO′] perchlorate]

The copper(II) ion in the syn–anti carboxyl­ate‐bridged one‐dimensional zigzag chain title complex, {[Cu(C₁₆H₁₈N₃O₂)]ClO₄}_n, exhibits a distorted trigonal–bipyramidal environment. Two N atoms and one carboxyl­ate O atom of the ligand form the basal plane, while the axial positions are filled by an N atom of the ligand and one O atom belonging to the carboxyl­ate group of an adjacent mol­ecule. The crystal packing is enhanced by C—H⋯O(perchlorate) hydrogen bonds.     

A ruthenium-catalyzed one-pot method for α-alkylation of ketones with aldehydes

Ketones react with an array of aldehydes in dioxane at 80 °C in the presence of a catalytic amount of RuCl₂(PPh₃)₃ along with KOH to give the corresponding α-alkylated ketones in moderate to good yields. A reaction pathway involving base-catalyzed cross-aldol reaction between ketones and aldehydes to form α,β-unsaturated ketones and regioselective reduction of carbon-carbon double bond of α,β-unsaturated ketones is proposed for this catalytic process.     

Regioselective coupling of pentafluorophenyl substituted alkynes: mechanistic insight into the zirconocene coupling of alkynes and a facile route to conjugated polymers bearing electron-withdrawing pentafluorophenyl substituents

The reaction of Cp(2)ZrCl(2) with 2 equiv of BuLi at -78 degrees C, followed by the addition of an unsymmetrical tetra- or pentafluorophenyl substituted alkyne R(1)C[triple bond]CAr(f) (R(1), Ar(f) = (CH(2))(4)Me, p-C(6)F(4)H; Me, p-C(6)F(4)H; Ph, C(6)F(5)), resulted in regioselective couplings of these alkynes to zirconacyclopentadienes in which the Ar(f) substituents preferentially adopt the 3,4-positions (beta beta) of the zirconacyclopentadiene ring. With Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) as the zirconocene reagent, the couplings could be carried out at room temperature; however, at higher temperatures significant quantities of the 2,4-fluoroaryl substituted (alpha beta) isomers were also formed. None of the conditions employed produced the 2,5-fluoroaryl substituted (alpha alpha) isomers. These fluoroaryl-substituted zirconacyclopentadienes were readily converted to butadienes via reactions with acids. The zirconacyclopentadiene Cp(2)ZrC(4)-2,5-Ph(2)-3,4-(C(6)F(5))(2), which resulted from the coupling of PhC[triple bond]C(C(6)F(5)), was converted to the corresponding thiophene by reaction with S(2)Cl(2), and to an arene by reaction with MeO(2)CC[triple bond]CCO(2)Me/CuCl. Mechanistic studies on zirconocene couplings of (p-CF(3)C(6)H(4))C[triple bond]C(p-MeC(6)H(4)) indicate that the observed regioselectivities are determined by an electronic factor that controls the orientation of at least one of the two alkynes as they are coupled. Additionally, these studies suggest an unsymmetrical transition state for the zirconocene coupling of alkynes, and this is supported by DFT calculations. The reaction of [(C(6)F(5))C[triple bond]CCH(2)](2)CH(2) with Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) resulted in a zirconacyclopentadiene in which the pentafluorophenyl substituents have been forced into the 2,5-positions (alpha alpha). Zirconocene coupling of the diyne (C(6)F(5))C[triple bond]C-1,4-C(6)H(4)-C[triple bond]C(C(6)F(5)) provided a route to conjugated polymers bearing electron-withdrawing pentafluorophenyl groups.     

Synthesis and characterization of polystyrene‐b‐poly (1,2‐isoprene‐ran‐3,4‐isoprene) block copolymers with azobenzene side groups

Polystyrene‐b‐poly(1,2‐isoprene‐ran‐3,4‐isoprene) block copolymers with azobenzene side groups were synthesized by the esterification of azobenzene acid chloride with polystyrene‐b‐hydroxylated poly(1,2‐isoprene‐ran‐3,4‐isopenre) block copolymers for creating new photochromic materials. The resulting block copolymers with azobenzene side groups were characterized for structural, thermal, and morphological properties. IR and NMR spectroscopies confirmed that the polymers obtained had the expected structures. Differential scanning calorimetric measurements by heating runs clearly showed the glass transitions of polystyrene and polyisoprene main chains and two distinct first‐order transitions at temperatures of azobenzene side groups around 48 and 83 °C. The microstructure of these block copolymer films was investigated using both transmission electron microscopy (TEM) and near‐field optical microscopy (NOM). TEM images revealed typical microphase‐separated morphologies such as sphere, cylinder, and lamellar structures. The domain spacing of microphase‐separated cylindrical morphology in the NOM image agreed with that of the TEM results. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2406–2414, 2002