Symmetry breaking in quantum chaotic systems

We show, using semiclassical methods, that as a symmetry is broken, the transition between universality classes for the spectral correlations of quantum chaotic systems is governed by the same parametrization as in the theory of random matrices. The theory is quantitatively verified for the kicked rotor quantum map. We also provide an explicit substantiation of the random matrix hypothesis, namely that in the symmetry-adapted basis the symmetry-violating operator is random.     

Preparation,spectral characterization,and single crystal X-ray structures of cis and trans isomers of 2,4,6-trifluoroethoxy-1,3,5-triethyl-1,3,5,2λ5, 4λ5, 6λ5-triazatriphosphorinane-2,4,6-trioxide, [ETNP(O)(OCH2CF3)]3

Oxidation of the cis isomer of the λ³-cyclotriphosphazane [EtNP(OCH₂CF₃)]₃ with trimethylamine-N-oxide (TMNO) gives the cis isomer of trioxo-λ⁵-cyclotriphosphazane [EtNP(O)(OCH₂CF₃)]₃; the trans isomer of [EtNP(O)(OCH₂CF₃)]₃ is obtained by the treatment of a cis and trans mixture of [EtNP(OCH₂CF₃)]₃ with aqueous H₂O₂. The two trioxocyclotriphosphazanes have been characterized by elemental analysis, IR, and NMR (¹H, ¹³C, ¹⁹F, and ³¹P) spectroscopy. The solid state structures of both the isomers have been determined by single crystal X-ray diffraction. The six-membered P₃N₃ ring in both the isomers exhibits a twist-boat conformation; in the cis isomer, the trifluoroethoxy substituents lie on the same side of the ring, whereas, in the trans isomer, two trifluoroethoxy groups are on one side of the ring and the third on the other side of the ring. Crystal data for cis-[EtNP(O)(OCH₂CF₃)]₃: monoclinic, P 2₁/ n , a = 13.593(3), b = 9.721(2), c = 17.539(3) Å, β = 99.49(2)°, V = 2286(1) ų, Z = 4, and Final R = 0.047. Crystal data for trans-[EtNP(O)(OCH₂CF₃)]₃: monoclinic, P 2₁/ n , a = 11.685(4), b = 15.115(5), c = 13.233(5) Å, - = 102.21(3)°, V = 2284(1) ų, Z = 4, and Final R = 0.078.     

Physical insights from a penetration depth model of optically pumped NMR

A model of optically pumped NMR (OPNMR) behavior in GaAs that connects the photon energy dependence of the OPNMR signal intensity for (69)Ga with different polarizations of light has been developed. Inputs to this model include experimental conditions--external magnetic field (B(0)), temperature (T), and optical pumping parameters (tau(L), laser helicity)--as well as parameters that arise from sample-specific characteristics--electron spin lifetime (T(1e)), electron lifetime (tau(e)), electron-nuclear correlation time (tau(c)), and sample thickness (z). These various inputs affect the profile of the OPNMR signal intensity as a function of photon energy (E) in a predictable manner. Therefore, the profile can serve as a composite fingerprint by which individual parameters can be inferred when not known. Characteristics of the profile include the photon energy for maximum OPNMR signal intensity and the intensity ratio between sigma(+) and sigma(-) light.     

Double cascade reactions based on the Barbas dienamine platform: highly stereoselective synthesis of functionalized cyclohexanes for cardiovascular agents

The amino acid proline catalyzed the three- and five-component cascade olefination-Diels-Alder-epimerization and olefination-Diels-Alder-epimerization-olefination-hydrogenation reactions of readily available precursors enones 1a-i, arylaldehydes 2a-k, alkyl cyanoacetates 3a-e and Hantzsch ester 9 to furnish highly substituted prochiral 1-cyano-4-oxo-2,6-diaryl-cyclohexanecarboxylic acid alkyl esters 6 and 1-cyano-4-(cyano-alkoxycarbonyl-methyl)-2,6-diaryl-cyclohexanecarboxylic acid alkyl esters 10 in a highly diastereoselective fashion with excellent yields. Prochiral cis-isomers 6 are excellent starting materials for the synthesis of cardiovascular agents and hypnotic active products.     

Structural diversity and supramolecular aggregation in calcium, strontium, and barium salicylates incorporating 1,10-phenanthroline and 4,4'-bipyridine: probing the softer side of group 2 metal ions with pyridinic ligands

Group 2 metal complexes [Ca(SA)2(phen)]n (1), [Sr2(SA)4(phen)4] (2), and [Ba(SA)2(phen)2]n (3) (SA = salicylate) have been obtained by the addition of 1,10-phenanthroline (phen) to the corresponding metal salicylates, while the bipyridine derivatives {[Ca3(SA)6(H2O)4](4bpy)2}n (4), {[Sr(SA)2(H2O)3](4bpy)1.5(H2O)}n (5), and {[Ba(SA)2(H2O)3]-(4bpy)1.5(H2O)}n (6) have been synthesized starting from the respective metal carbonates, salicylic acid (SA-H), and 4,4'-bipyridine (4bpy). The new compounds have been characterized by elemental analysis, pH measurements, thermal analysis, and spectroscopic measurements (IR, NMR, ultraviolet, and fluorescence). Molecular structure determination by single-crystal X-ray diffraction has been carried out for all the compounds. The thermal analysis studies indicate the loss of coordinated and/or lattice water molecules below 200 degrees C in 4-6 and the absence of any coordinated or uncoordinated water molecules in compounds 1-3. Compounds 1 and 3-6 exist as one-dimensional polymers while compound 2 crystallizes as a discrete dimer. Considerable variations have been observed in the molecular structures of 1-6 in terms of the geometry around the metal, the binding mode of salicylate, and the coordination behavior of the pyridine ligand. Calcium ion is hexacoordinated in 1, while in 4 both hexa- and heptacoordinated calcium ions are simultaneously present. Strontium exhibits coordination numbers of nine and eight in 2 and 5, respectively. The barium ions in 3 and 6 assume coordination numbers of eight and seven, respectively. While the OH group of the salicylate ligand does not directly bind the metal in 1-3, it coordinates to the metal ions in complexes in 4-6 in the un-ionized form. The 4bpy molecules show no direct ligation to the metal in 4-6; the phen ligands in 1-3, however, occupy one side of the coordination sphere around the metal. The presence of additional O-H...O, C-H...O, and N-H...O hydrogen bonding and pi-pi stacking in these compounds results in the formation of polymeric structures. The results obtained for the calcium complexes in this study have been compared with the available data in structural calcium chemistry with the aid of a detailed analysis of the Cambridge Structural Database.     

Facile ion-exchange synthesis of Gd-doped K2Ta2O6 photocatalysts with enhanced visible light activity

One of the well-known ways of increasing the visible light absorption capability of semiconducting materials is cation doping. This study aims to use Gd doping to tailor the bandgap energy of K₂Ta₂O₆ (KTO) for photocatalytic degradation of organic pollutants under visible light irradiation. Accordingly, the parent KTO and Gd-doped KTO with different Gd concentrations (K_2-3xGd_xTa₂O₆; x = 0.025, 0.05, 0.075 and 0.1 mol%) were synthesized by hydrothermal and facile ion-exchange methods, respectively. The powder XRD, FT-IR, SEM-EDS, TEM-SAED, N₂ adsorption-desorption, XPS, UV–Vis DRS, PL and ESR techniques were used to investigate the effect of Gd dopant concentration on the structural and photocatalytic properties of KTO. The photocatalytic activity of these samples was investigated for the photocatalytic degradation of methylene blue (MB) dye in an aqueous solution at room temperature under visible light irradiation. The experimental results show that all Gd-doped KTO samples exhibit enhanced photocatalytic activity compared with parent KTO toward MB degradation. In particular, Gd-KTO obtained by doping of 0.075 mol% shows the highest photocatalytic activity among the Gd-doped samples and the degradation efficiency of MB was 79% after 180 min of visible light irradiation, which is approximately 1.5 times as high as that by parent KTO (53%). In addition, trapping experiments and electron spin resonance (ESR) analysis demonstrated that the hydroxyl radicals (^?OH) have played a crucial role in the photocatalytic degradation of MB. The reusability and stability of Gd doped-KTO with a Gd content of 0.075 mol% against MB degradation were examined for five cycles. Based on the present study results, a visible light induced photocatalytic mechanism has been proposed for Gd0075-KTO sample.     

Computational studies on the structures and energies of the tautomers of 1-amino-3-nitrotriazol-5-one-2-oxide

Molecular orbital calculations at the DFT-B3LYP/aug-cc-pVDZ level were performed for the possible tautomers of 1-amino-3-nitro-1,2,4-triazol-5-one-2-oxide. We have examined the substitution effects of amino and nitro groups by comparing calculated geometries, relative energies, and electrostatic potentials of model molecules. The optimized structures, vibrational frequencies, and thermodynamic values for triazol-5-one-N-oxides were obtained in their ground state. The results show 1H, 4H tautomers to be most stable. Detonation velocity and detonation pressure were evaluated by the Kamlet and Jacob equations based on the predicted density and the calculated heat of explosion. Explosive properties appear to be promising compared with those of 1,3,5-trinitro-1,3,5-triazine (D = 8.75 km/s, P = 34.7 Gpa) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (D = 8.96 km/s, P = 35.96 Gpa), 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (D = 9.20 km/s, P = 42.0 Gpa) and octanitrocubane (D = 9.90 km/s, P = 48.45 GPa). The designed triazol-5-one-N-oxides satisfy the criteria of high energy density materials.