Conformational landscape surfing induced by off-on pi-pi stacking in a porphyrin-quinone dyad

A covalently linked porphyrin-quinone dyad crystallizes with two orientations of the quinone, extended away from (off) and cofacial with the porphyrin macrocycle (on), which induce different conformations of the macrocycle and model the recently proposed structural effect of a nearby residue on the heme prosthetic group of a nitric oxide synthase.     

Use of basic dyes in the determination of anions, particularly as a means of determining antimony, thallium, and gallium

The use of xanthene, triphenylmethane and other basic dye cations as reagents for the determination of anions is reviewed. Elements and anions determined include Sb, Tl, Ga, Au, Te, In, Zn, Ta, Hg, Re, Sn, U, Os, B, Cr, Ag, ClO(4)(-), PO(4)(-), SO(4)(2-) and NO(3)(-). Difficulties encountered in developing satisfactory procedures involving basic dyes are discussed. Procedures for the purification and analysis of basic dye samples are outlined.     

Solvent effects on substitution reactions at complexes of the [Fe(CN)5L]3- type in binary aqueous mixtures

Summary The kinetics of replacement of 4,4-bipyridine, (4,4-bipy), and 4-cyanopyridine, (4-CNpy), by cyanide in [Fe(CN)₅-(4,4-bipy)]^3-. at 298 K have been studied in binary aqueous mixtures containing different amounts of t-butanol, methanol, glycerol, ethyleneglycol and sucrose. The plots of logarithms of the limiting rate constantsversus the mole fraction of water are linear over the entire composition range studied, showing the importance of solvation phenomena. A different straight line of log (k_L/s^–1)versus X_H2O is obtained for each mixture, indicating the influence of other solvent parameters on the reaction rate. A multiparameter regression of G _exp with A (acidity vector), B (basicity vector) and G^E is used for both reactions; plots of G _calc versus G _exp are linear with slopes of near unity.     

Nonaqueous suspensions of laponite and montmorillonite

It is shown how stable, nonaqueous suspensions of fully exfoliated smectite clays such as Laponite and montmorillonite can be obtained. Suspensions in toluene and in a branched aliphatic solvent (polydecene) were characterized using elemental analysis, rheometry, and small-angle X-ray scattering. For Laponite, stable suspensions were obtained by adsorbing a dichain poly(isobutylene) based stabilizer to the particles. This approach did not result in full exfoliation for montmorillonite particles, possibly because the stabilizer was able to connect individual clay sheets at the edges during the treatment process. Instead, a quaternary ammonium surfactant, dimethyl dioctadecylammonium bromide (DODAB), was first adsorbed to the clay. Subsequently, adsorption of the poly(isobutylene) based stabilizer to this pretreated clay resulted in fully exfoliated suspensions of montmorillonite. Suspensions of clay particles treated only with DODAB have been studied by several authors before and are included in this study for comparison. A detailed discussion of the suspension behavior of the different clay samples is given. The suspension routes presented here may enable further studies of the structure and flow behavior of suspensions of thin, flexible clay platelets as a function of aspect ratio.     

Atmospheric pressure laser desorption/chemical ionization mass spectrometry: a new ionization method based on existing themes

We have designed and constructed an atmospheric pressure laser desorption/chemical ionization (AP-LD/CI) source that utilizes a laser pulse to desorb intact neutral molecules, followed by chemical ionization via reagent ions produced by a corona discharge. This source employs a heated capillary atmospheric pressure inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. Preliminary results demonstrate that this technique provides approximately 150-fold increase in analyte ions compared to the ion population generated by atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI).     

Lewis-Base Adducts of Group 11 Metal(I) Compounds. LXXVI Structural Studies in the Copper(I) Halide : Norbornadiene System

The structural characterizations of some copper(I) halide (CuX) adducts with norbornadiene (nbd) are recorded. CuCl : nbd (1:1)₄ (a redetermination), (2:1)_2(|), are systems both based around Cu₄Cl₄ cubane-type cluster arrays. CuBr : nbd (7:3)_(|)( 0.5 MeOH), a complex polymer with 3-symmetry, is believed to be the complex previously described as an adduct of 2:1 stoichiometry. Attempts to obtain an iodide counterpart have resulted in the definition of an ephemeral adduct CuI : MeCN (3:2)_(|). 0.5 C₇H₈ in which, remarkably, the nbd is uncoordinated; the complex is a polymer, related to the [AgX(quinoline)]_(|) (X = Cl, Br) saddle polymer.     

The number oft-wise balanced designs

We prove that the number oft-wise balanced designs of ordern is asymptotically , provided that blocks of sizet are permitted. In the process, we prove that the number oft-profiles (multisets of block sizes) is bounded below by and above by for constants c₂>c₁>0.     

Interface interactions in modulated phases,and upsilon points

Certain features in Frenkel-Kontorova and other models of phases with a one-dimensional modulation can be analyzed by assuming parallel interfaces separating sets of lattice planes belonging to two different phases, and treating the free energy to create interfaces, as well as the interaction of two, three, or more interfaces, as phenomenological parameters. A strategy employed by Fisher and Szpilka for interacting defects can be extended to the case of interfaces, allowing a systematic study of the phase diagram by ignoring all interface interactions, and then successively taking into account pair, triple, and higher-order terms. The possible phase diagrams which can occur near the point where =0 include: various sorts of endpoints analogous to critical endpoints, an accumulation point of first-order transitions and triple points, and a self-similar structure which we call an upsilon point, which turns out to be an accumulation point of an infinite number of segments of first-order transition lines, each of which terminates in two upsilon points.