A comparison of psychophysical procedures for level-discrimination thresholds

Five different psychophysical procedures were used to measure level-discrimination (also called intensity discrimination) thresholds for 1-kHz tones at two levels (30 and 90 dB SPL) and two durations (10 and 500 ms). The procedures were the classic transformed up-down staircase method with a two-alternative forced-choice (2AFC) paradigm (UPD), 15- and 50-trial implementations of the method of maximum likelihood (MML) with a cued yes-no paradigm, and 18-trial implementations of ZEST using both cued yes-no and 2AFC paradigms. Results obtained from nine normal listeners show that estimates of level-discrimination thresholds for the four conditions are similar across all five procedures when different points of convergence are accounted for. The variance of threshold estimates within listener and condition was smallest for UPD, largest for the MML with 15 trials, and statistically indistinguishable among the others. The sweat factors ranged from 5.5 for MML with 50 trials to about 1.4 for UPD and ZEST. Simulations show that ideal performance of procedures may be far from real-life experience and that these deviations are likely to depend on complex interactions between listener behavior and parameter choices used for implementing the procedures. Therefore, empirical verification is important for judging the effectiveness of psychophysical procedures.     

X-ray crystal structure of 2,5-dioxo-4-imidazolidineethanesulfonamide, C5H9N3O4S

The crystal structure of 2,5-dioxo-4-imidazolidineethanesulfonamide or homocysteine sulfonamide hydantoin, C₅H₉N₃O₄S (1) was obtained by single-crystal X-ray diffraction. Crystallization of 1 occurs in the centrosymmetric monoclinic space group C2/c (No. 15) with a = 15.653(2), b = 9.6489(10), c = 11.066(2) Å, = 94.64(2), and Z = 8. Molecules are in an extended structure with a C–C–C–S torsion angle of –174.2(1). The imidazolidinedione ring is planar and the sulfonamide group has a distorted tetrahedral geometry. A three-dimensional network of intermolecular hydrogen bonding occurs within the crystal lattice involving both imidazolidinedione and sulfonamide functional groups.     

Polymer coatings to passivate calcite from acid attack: polyacrylic acid and polyacrylonitrile

The extent of passivation of calcite toward dissolution by aqueous acids arising from polymeric coatings based on polyacrylic acid or polyacrylonitrile is evaluated using a channel flow cell technique with microdisc electrode detection. In situ passivation with polyacrylic acid leads to a reduction in the reactivity of calcite toward acid attack with a reduction in the rate constant by up to an order of magnitude compared with untreated calcite. Ex situ passivation with polyacrylic acid for 24 h results in good coverage of the calcite by the polymer but it is shown to erode from the surface when exposed to an aqueous acid solution. In contrast, polyacrylonitrile is demonstrated to form a regular coating after exposure for just 1 h and offers robust potent protection from aqueous acid attack.     

Speciation and unusual reactivity in PuO2+x

Pu L(3) XAFS measurements show that the excess oxygen in single phase PuO(2+)(x)() occurs as oxo groups with Pu-O distances of 1.83-1.91 A. This distance and the energy of the edge (via comparison with a large number of related compounds) are more consistent with a Pu(IV/V) than a Pu(IV/VI) mixture. Analogous to Pu(IV) colloids, although the Pu-Pu pair distribution remains single site even when it shows substantial disorder, the Pu-O distribution can display a number of additional shells at specific distances up to 3.4 A even in high fired materials when no oxo groups are present, implying intrinsic H(+)/OH(-)(/H(2)O). The number of oxo atoms increases when samples are equilibrated with humid air at ambient temperature, indicating that the Pu reactivity in this solid system differs notably from that of isolated complexes and demonstrating the importance of nanoscale cooperative phenomena and total free energy in determining its chemical properties.     

Preparation of discrete oligoethers: synthesis of pentabutylene glycol and hexapropylene glycol by two complementary methods

Two experimentally facile methods for the preparation of discrete oligoethers are reported. The first involves an iterative sequence of oxidation, acetal formation, and reductive ring opening for the synthesis of penta-1,4-butylene glycol. The second method is also iterative, comprising phase-transfer etherification and end-group deprotection to form hexa-1,3-propylene glycol. These methods offer significantly improved yields and purification protocols over previously reported syntheses.     

Isolation and characterisation of transition and main group metal complexes supported by hydrogen-bonded zwitterionic polyphenolic ligands

Reaction of Zr(O(i)Pr)4 or Sn[N(SiMe3)2]2 with the tris-phenol amine ligand H3L(Me/Me) results in the formation of zirconium or tin complexes containing the new C3-symmetric zwitterionic ammonium-trisphenolate ligand HL2-, while increasing the steric bulk of the ligand results in the isolation of a zirconium complex containing the known trianionic ligand L3-.     

Synthesis, characterization, and X-ray crystal structure of di-(2,4-imidazolidinedione-5-ethyl)disulfide, C10H14N4O4S2

Di-(2,4-imidazolidinedione-5-ethyl)disulfide or homocystine hydantoin, C₁₀H₁₄N₄O₄S₂ (I), crystallizes in the non-centrosymmetric space group P2₁ (No. 4) with two molecules in the unit cell with a = 7.132(1), b = 9.282(2), c = 10.770(2) Å and = 105.68(1)°. The two imidazolidinedione rings are planar with a dihedral angle of 46.9°. The rings are joined by a diethyl disulfide bridge at chiral centers on the rings. The C-S-S-C torsion angle is –80°(–sc). The absolute stereochemistry of the chiral centers was determined to be (5S, 5S), = + 0.92 (11). Important bond distances include: S-S = 2.022(4); S-C (mean) = 1.809(8); and C=O (mean) = 1.224(7) Å. The compound is stabilized by a network of intermolecular carbonyl-to-amine hydrogen bonding and van der Waals cohesive forces.     

N2′-acetyl-6-diacetoxymethylpterin

Treatment of folic acid with bromine in 48% hydrobromic acid gives the hydrobromide salt of 6-formylpterin ( 1 ), and acetylation of 1 with recovered acetic anhydride leads to N^2′-acetyl-6-diacetoxymethylpterin ( 3 ).     

Biosynthetic pathways to isocyanides and isothiocyanates; precursor incorporation studies on terpene metabolites in the tropical marine sponges Amphimedon terpenensis and Axinyssa n.sp

The biosynthetic origins of the isocyanide and isothiocyanate functional groups in the marine sponge metabolites diisocyanoadociane (1), 9-isocyanopupukeanane (10) and 9-isothiocyanatopupukeanane (11) are probed by the use of [(14)C]-labelled precursor experiments. Incubation of the sponge Amphimedon terpenensis with [(14)C]-labelled thiocyanate resulted in radioactive diisocyanoadociane (1) in which the radiolabel is specifically associated with the isocyanide carbons. As expected, cyanide and thiocyanate were confirmed as precursors to the pupukeananes 10 and 11 in the sponge Axinyssa n.sp.; additionally these precursors labelled 2-thiocyanatoneopupukeanane (12) in this sponge. To probe whether isocyanide-isothiocyanate interconversions take place at the secondary metabolite level, the advanced precursor bisisothiocyanate 17 was supplied to A. terpenensis, but did not result in significant labelling in the natural product isocyanide 1. In contrast, in the sponge Axinyssa n.sp., feeding of [(14)C]-9-isocyanopupukeanane (10) resulted in isolation of radiolabelled 9-isothiocyanatopupukeanane 11, while the feeding of [(14)C]-11 resulted in labelled isocyanide 10. These results show conclusively that isocyanides and isothiocyanates are interconverted in the sponge Axinyssa n.sp., and confirm the central role that thiocyanate occupies in the terpene metabolism of this sponge.