Reduction of iridium in solution

Iridium(IV) in solution may be reduced to metal via the intermediate Ir(I). Reduction of Ir(III) has not been detected. The current efficiency for metal deposition is low because: (1) The (IV)-(I) reduction is competing with the (IV) to (III) reduction, and hydrogen evolution in aqueous solution. (2) Ir(I) is very easily oxidised to (III), even by hydrogen. (3) Some iridium salts are not stable in the (IV) state (e.g. triaquotrichloroiridate in molar chloride).     

Role of counteranions in acid-induced aggregation of isomeric tetrapyridylporphyrins in organic solvents

The effect of adding various kinds of acids HX (X = Cl, Br, I, CF(3)COO, CF(3)SO(3), TFPB ((3,5-(CF(3))(2)C(6)H(3))(4)B) to isomeric tetra(2-pyridyl)porphyrin, tetra(3-pyridyl)porphyrin, and tetra(4-pyridyl)porphyrin (TpyP(2), TpyP(3), and TpyP(4)) in dichloromethane solution has been investigated through the combined use of UV/vis absorption, fluorescence emission, and resonance light scattering (RLS) techniques. The experimental evidence points to a marked dependence of the protonation and aggregation behavior on the nature of both acids and porphyrins. In general, three different trends can be recognized: (i) formation of a fully protonated species, followed by aggregation; (ii) formation of a tetraprotonated species, which aggregates and, on further addition of acid, disaggregates; and (iii) protonation of the four pyridyl moieties, leading to a tetraprotonated ion pair, in the unique case of the bulky TFPB(-) anion. In all cases, the protonated species and the resulting aggregates exhibit spectroscopic features that are markedly influenced by the nature of the counteranions. A model for J-aggregation has been proposed on the basis of an interplay of hydrogen bonding, electrostatic interactions, and dispersive interactions. Kinetic control of the aggregation process allows for a fine-tuning of the spectroscopic properties of the final aggregated species.     

Variation in the accumulation levels of N,N-dimethyltryptamine in micropropagated trees and in in vitro cultures of Mimosa tenuiflora

The present article reports the accumulation of N,N-dimethyltryptamine and its metabolic precursors (tryptophan, tryptamine) in different organs of micropropagated Mimosa tenuiflora trees (leaves, flowers and bark) subjected to seasonal variations (January and June), as well as in in vitro cultures (plantlets and calluses) of this plant species. The accumulation of all the tested compounds varied according to the organ, the month of collection, and age of the plant material. In all cases, the neurotoxic compound N,N-dimethyltryptamine (DMT) was detected with the lowest concentration 0.01% dry weight (DW) in flowers, and the highest 0.33% DW in bark. For the in vitro cultures, DMT was present in high yields in plantlets (0.1-0.2% DW), while in calluses this compound was initially detected but its concentration decreased significantly in the subsequent subcultures.     

Miscibility of gramicidin A-ethyl nonadecanoate in langmuir monolayers in the presence of salts dissolved in the subphase

The behavior of binary mixed Langmuir monolayers from gramicidin A (GA) and ethyl nonadecanoate (EN), spread on aqueous subphases containing NaCl and CaCl2, was investigated on the basis of the analysis of surface pressure-average area per molecule (pi-A) isotherms complemented with Brewster angle microscopy (BAM) images. Compression modulus versus surface pressure (C(S-1)-pi) curves indicate the existence of interactions in the GA-EN mixed monolayers at low surface pressures (below 5 mN m(-1)). However, for mixtures in which the ester is the predominant component, both GA and EN are miscible within regions from fully expanded to collapse. To examine the interactions between both components in the studied system, values of the mean molecular area per molecule (A12) were plotted as a function of molar fraction of gramicidin A (X(GA)). A12-X(GA) plots exhibit negative deviations from ideality at high surface pressures, wherein beta-helices of GA are vertically oriented in respect to the interface. However, at surface pressures below the plateau transition, which is due to reorientation of GA, the binary system obeys the additive rule. Brewster angle microscopy (BAM) was applied for a direct visualization of the monolayers morphologies. The obtained images prove that for molar ratios of GA > or = 0.3 and at surface pressures above 5 mN m(-1), both components are immiscible at the interface. The observed negative deviations from the additively rule were attributed to the formation of a three-dimensional phase in the mixed film, which provokes its contraction at the interface.     

On the role of solvent in acid-base equilibria of diuretics in acetonitrile-water mixed solvents

The dissociation constants of representative loop (furosemide), thiazide (chlorthiazide and trichlormethiazide) and potassium sparing (amiloride) diuretics in 10, 30, 40, 50 and 70% (w/w) acetonitrile-water mixtures at 298.15 K were determined, in accordance with IUPAC procedures. The variation in pK(a) values over the whole composition range can be explained by preferential solvation and the structural features in acetonitrile-water mixtures. Correlations between pK(a) values and various bulk and solvatochromic properties of the solvents were calculated. The linear solvation energy relationship (LSER) method was applied. The resulting equations allowed us to calculate pK(a) values for the diuretics studied in any acetonitrile-water mixture up to 70% (w/w) and should help to clarify the acid-base behaviour of diuretics in the widely-used acetonitrile-water mixed solvents.     

Interpretation of inorganic arsenic metabolism in humans in the light of observations made in vitro and in vivo in the rat

The toxicity of inorganic trivalent arsenic for living organisms is reduced by in vivo methylation of the element. In man, this biotransformation leads to the synthesis of monomethylarsonic (MMA) and dimethylarsinic (DMA) acids, which are efficiently eliminated in urine along with the unchanged form (As_i). In order to document the methylation process in humans, the kinetics of As_i, MMA and DMA elimination were studied in volunteers given a single dose of one of these three arsenicals or repeated doses of As_i. The arsenic methylation efficiency was also assessed in subjects acutely intoxicated with arsenic trioxide (As₂O₃) and in patients with liver diseases. Several observations in humans can be explained by the properties of the enzymic systems involved in the methylation process which we have characterized in vitro and in vivo in rats as follows: (1) production of As_i metabolites is catalyzed by an enzymic system whose activity is highest in liver cytosol; (2) different enzymic activities, using the same methyl group donor (S-adenosylmethionine), lead to the production of mono- and di-methylated derivatives which are excreted in urine as MMA and DMA; (3) dimethylating activity is highly sensitive to inhibition by excess of inorganic arsenic; (4) reduced glutathione concentration in liver moderates the arsenic methylation process through several mechanisms, e.g. stimulation of the first methylation reaction leading to MMA, facilitation of As_i uptake by hepatocytes, stimulation of the biliary excretion of the element, reduction of pentavalent forms before methylation, and protection of a reducing environment in the cells necessary to maintain the activity of the enzymic systems.     

Electric conductance of dispersions of metal oxides in solutions of weak acids in mixed dioxane-water solvents

The electric conductance of solutions of sulfuric, oxalic, benzoic, and salicylic acid (up to 0.02 M) in dioxane-water mixed solvents (90% and 93% dioxane by mass) has been studied in the presence and absence of TiO(2) and Al(2)O(3) (0.5-5% by mass). TiO(2) and Al(2)O(3) enhanced the conductance of solutions of organic acids in aqueous dioxane. The conductance is interpreted in terms of adsorption of acid in molecular form, dissolution of ceramic oxides in form of anionic complexes, and leaching of acidic impurities from ceramic oxides.     

Irregularities in the effect of potassium phosphate in ynamide synthesis

The yields of ynamides using Hsung's second generation protocol depend substantially on the quality of K(3)PO(4). Samples of K(3)PO(4) from different suppliers were investigated by various techniques, revealing that the use of pure and anhydrous K(3)PO(4) provides higher ynamide yields in comparison to samples contaminated with hydrates (K(3)PO(4) x 1.5 H(2)O and K(3)PO(4) x 7 H(2)O). With high quality K(3)PO(4), a number of ynamides were synthesized in yields of 52-91%. In addition, we report that ynamides can undergo regioselective hydroamination with carbamates.     

The role of group 14 element hydrides in the activation of C-h bonds in cyclic olefins

Formally, triple-bonded dimetallynes ArEEAr [E = Ge (1), Sn (2); Ar = C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2)] have been previously shown to activate aliphatic, allylic C-H bonds in cyclic olefins, cyclopentadiene (CpH), cyclopentene (c-C(5)H(8)) and 1,4-cyclohexadiene, with intriguing selectivity. In the case of the five-membered carbocycles, cyclopentadienyl species ArECp [E = Ge (3), Sn (4)] are formed. In this study, we examine the mechanisms for activation of CpH and c-C(5)H(8) using experimental methods and describe a new product found from the reaction between 1 and c-C(5)H(8), an asymmetrically substituted digermene ArGe(H)Ge(c-C(5)H(9))Ar (5), crystallized in 46% yield. This compound contains a hydrogenated cyclopentyl moiety and is found to be produced in a 3:2 ratio with 3, explaining the fate of the liberated H atoms following triple C-H activation. We show that when these C-H activation reactions are carried out in the presence of tert-butyl ethylene (excess), compounds {ArE(CH(2)CH(2)tBu)}(2) [E = Ge(8), Sn(9)] are obtained in addition to ArECp; in the case of CpH, the neohexyl complexes replace the production of H(2) gas, and for c-C(5)H(8) they displace cyclopentyl product 5 and account for all the hydrogen removed in the dehydroaromatization reactions. To confirm the source of 8 and 9, it was demonstrated that these molecules are formed cleanly between the reaction of (ArEH)(2) [E = Ge(6), Sn(7)] and tert-butyl ethylene, new examples of noncatalyzed hydro-germylation and -stannylation. Therefore, the presence of transient hydrides of the type 6 and 7 can be surmised to be reactive intermediates in the production of 3 and 4, along with H(2), from 1 and 2 and CpH (respectively), or the formation of 3 and 5 from 1. The reaction of 6 or 7 with CpH gave 3 or 4, respectively, with concomitant H(2) evolution, demonstrating the basic nature of these low-valent group 14 element hydrides and their key role in the 'cascade' of C-H activation steps. Additionally, during the course of these studies a new polycyclic compound (ArGe)(2)(C(7)H(12)) (10) was obtained in 60% yield from the reaction of 1,6-heptadiene and 1 via double [2 + 2] cycloaddition and gives evidence for a nonradical mechanism for these types of reactions.     

Amylose's recognition of chirality in polylactides on formation of inclusion complexes in vine-twining polymerization

Amylose selectively includes poly(L-lactide) (PLLA) among the poly(lactide)s (PLAs) to produce an inclusion complex when the phosphorylase-catalyzed polymerization of α-D-glucose 1-phosphate is performed in the presence of PLLA, poly(D-lactide) (PDLA), or poly(DL-lactide) (PDLLA) (vine-twining polymerization). This result indicates that amylose recognizes the chirality in PLAs on the formation of an inclusion complex in vine-twining polymerization. Modeling calculations support the amylose's chiral recognition in favor of PLLA and the atomistic details of the inclusion complex which involved the preferred orientation of the constituent molecular chains with respect to their fiber axis is proposed.     

Origin of Aldehydes in Sediments of Qinghai Lake

This paper has analysed the aldehyde fractions in sediments and living organisms of Qinghai Lake using GC-MS and discussed their origin, n-Aldehydes detected in the sediments ranged from C_(16) to C_(32) with a maximum at C_(22) C_(24) or C_(26) and showed a strong even over odd predominance. n-Aldehydes in the living organisms analysed ranged from C_(16) to C_(32) with a maximum at C_(26) or C_(28). The authors believed that aldehydes in modern sediments may directly derive from biosynthesis.     

Comparison of hexamethyldisiloxane dissociation processes in plasma

Different hexamethyldisiloxane (HMDSO) dissociation processes are investigated by means of absorption spectroscopy and mass spectrometry. All of these processes are expected to occur in plasma containing Ar-HMDSO gas mixture. We successively study interactions of the HMDSO molecule with electrons (energy ranges from 15 to 70 eV), with Ar((3)P(2)) metastable species (internal energy 11.55 eV) and with VUV photon (7.3 to 10.79 eV). The studies of HMDSO interactions with Ar((3)P(2)) and VUV photon provide new results concerning the dissociation pathways and the collision cross-sections. In the case of Ar((3)P(2)), the dissociation mechanisms result mainly in Si-C or Si-O bond breaking, producing SiMe(2,1) radicals. Less efficient mechanisms involve also Si-C and Si-O bond breaking producing Me, Si(2)Me(5)O, or SiMe(3), on one hand, and, on the other hand, Si-C and C-H bond breaking producing Si(2)Me(4)OH. In the case of photon interaction, the dissociation process is more selective and mainly produces Si(2)OMe(5) pentadisiloxane and methyl radicals due to Si-C bond breaking. Si-O bond breaking produces also SiMe(3) in a lower concentration. Dissociation cross-section values of HMDSO ranging from σ = 45 × 10(-20) m(2) to 180 × 10(-20) m(2) and from σ = 0.7 × 10(-22) m(2) to 18.3 × 10(-22) m(2), correspond to a global dissociation mechanism by Ar((3)P(2)) collision and to a selective dissociation mechanism (producing Si(2)OMe(5) and Me) by VUV photon interaction, respectively. All results are compared and discussed.     

Origin of Ketones in Sediments of Qinghai Lake

This paper analysed the ketone fractions in sediments and living organisms of Qinghai Lake using GC-MS and discussed their origin,n-Alkan-2-ones detected in the sediments ranged from C_(12) to C_(33) with a maximum at n-C_(25) or n-C_(29) and showed a strong odd predominance, n-Alkan-2-ones in the living organisms analysed ranged from C_(15) to C_(31) with a maximum at n-C_(27).The authors believed that ketones in modern sediments may directly derive from biosynthesis.     

Structure-reactivity relationships in the pyridinolysis of N-methyl-N-arylcarbamoyl chlorides in dimethyl sulfoxide.

Nucleophilic substitution reactions of N-methyl-N-arylcarbamoyl chlorides (YC(6)H(4)N(CH(3))COCl) with pyridines (XC(5)H(4)N) have been investigated in dimethyl sulfoxide at 45.0 degrees C. A striking trend in the selectivity parameters is that they are constant within experimental errors, rho(X) = -2.25 +/- 0.03, beta(X) = 0.42 +/- 0.01, and rho(Y) = 1.10 +/- 0.06, with changing reactivities of the electrophiles (deltasigma(Y)) and nucleophiles (deltasigma(X)), respectively, and this leads to a vanishingly small cross-interaction constant, rho(XY) approximately equals beta(XY) approximately equals 0. The rate data can be expressed in the Ritchie N(+) type equation. Based on this and other results, the mechanism of nucleophile (pyridine) addition to the resonance- stabilized carbocation is proposed. It has been shown from the definition of beta(XY) (and rho(XY)) together with the Marcus equation that the high intrinsic barrier, DeltaG(0), in the intrinsic-barrier controlled reaction series is a prerequisite for such reactions in which the cross-interaction vanishes and the N(+) relationship holds.     

Voltammetric determination of traces of Cr(VI) in the flow system in the presence of bipyridyne

The voltammetric method of Cr(VI) determinations in the flow system, based on the combination of selective accumulation of the product of Cr(VI) reduction on HMDE and a very sensitive method of chromium determination in the presence of bipyridine [Z. Gao K.S. Siow, Electroanalysis 8 (1996) 602] is proposed. The calibration graphs were linear from 3x10(-9) to 3x10(-8) and from 5x10(-10) to 5x10(-9) mol l(-1) for accumulation times 120 and 600 s, respectively. The relative standard deviation (R.S.D.) was 6.5% (n=5) for Cr(VI) concentration 1x10(-8) mol l(-1) and the accumulation time 120 s. The influence of foreign ions commonly present in water samples is presented. Validation of the method was made by comparison of the results of analyses of tap water by another electrochemical method and by recovery test for river and mineral water.     

Determination of trace concentrations of bromophenols in water using purge-and-trap after in situ acetylation

An analytical method that enables detection and quantification of bromophenols (BPs) at taste threshold concentrations (2,6-DBP: 0.5 ng/L) was developed. This method involves conversion of the BPs to their acetates, followed by isolation of the acetates by a modified purge-and-trap procedure, and analysis by gas chromatography-mass spectrometry (GC-MS) in selected ion monitoring (SIM) mode. Bromophenyl acetates were synthesized so that each of the two steps in the method could be developed and optimised in isolation to the other. Deuterated BPs (phenol-d5, 2-BP-d4, 4-BP-d2, 2,6-DBP-d3, 2,4-DBP-d3 and 2,4,6-TBP-d2) were synthesized to enable quantification of analytes using the deuterated analogues of analytes as internal standards. This method allowed quantification of BPs at concentrations ranging from the detection limits (3 ng/L for phenol and 0.1-0.5 ng/L for each of the BPs) to 1000 ng/L for each analyte, with repeatabilities of < or =14% (RSD) for concentrations of 1 ng/L and < or =9% (RSD) for concentrations of 10-1000 ng/L, with recoveries ranging from 91 to 97%.     

Nitrate ion photolysis in thin water films in the presence of bromide ions

Nitrate ions commonly coexist with halide ions in aged sea salt particles, as well as in the Arctic snowpack, where NO(3)(-) photochemistry is believed to be an important source of NO(y) (NO + NO(2) + HONO + ...). The effects of bromide ions on nitrate ion photochemistry were investigated at 298 ± 2 K in air using 311 nm photolysis lamps. Reactions were carried out using NaBr/NaNO(3) and KBr/KNO(3) deposited on the walls of a Teflon chamber. Gas phase halogen products and NO(2) were measured as a function of photolysis time using long path FTIR, NO(y) chemiluminescence and atmospheric pressure ionization mass spectrometry (API-MS). Irradiated NaBr/NaNO(3) mixtures show an enhancement in the rates of production of NO(2) and Br(2) as the bromide mole fraction (χ(NaBr)) increased. However, this was not the case for KBr/KNO(3) mixtures where the rates of production of NO(2) and Br(2) remained constant over all values of χ(KBr). Molecular dynamics (MD) simulations show that the presence of bromide in the NaBr solutions pulls sodium toward the solution surface, which in turn attracts nitrate to the interfacial region, allowing for more efficient escape of NO(2) than in the absence of halides. However, in the case of KBr/KNO(3), bromide ions do not appreciably affect the distribution of nitrate ions at the interface. Clustering of Br(-) with NO(3)(-) and H(2)O predicted by MD simulations for sodium salts may facilitate a direct intermolecular reaction, which could also contribute to higher rates of NO(2) production. Enhanced photochemistry in the presence of halide ions may be important for oxides of nitrogen production in field studies such as in polar snowpacks where the use of quantum yields from laboratory studies in the absence of halide ions would lead to a significant underestimate of the photolysis rates of nitrate ions.     

Luminescence of ion-pair states of halogens in liquid perfluorocarbons

The I(2)(D'-A') luminescence in liquid C(7)F(16), C(8)F(18), C(9)F(20), and C(12)F(26) has been observed following irradiation with visible (within the I(2) X → B band region) and UV (λ < 300 nm) lasers. The band is shifted by ~2000 cm(-1) to the red relative to the gas phase and is significantly broader (fwhm ≈ 3000 cm(-1) vs 500 cm(-1) in the gas phase). Two-color excitation of other halogens in C(n)F(2n+2) solvents revealed similarly shifted and strongly broadened D'-A' bands of Br(2), IBr, and BrCl. The stability of the ion-pair states of halogens in room temperature liquids opens new experimental opportunities for condensed phase studies and may be of applied interest considering the increasing industrial applications of perfluorocarbons.     

Electrophilicity of aromatic triflones in sigma-complexation processes

The kinetics of sigma-complexation of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole (4) have been investigated over a large pH range of 2-15.68 in methanol. Two competitive processes have been identified with the initial addition of MeO(-) at the unsubstituted 3-position of 4 to give a 1,3-dimethoxy adduct (4b-Me) and a subsequent and slow conversion of this species into the 1,1-dimethoxy isomer (4a-Me). Both 4a-Me and 4b-Me are more stable than the related adducts of 2,6-dinitro-4-trifluromethanesulfonylanisole, i.e.5a-Me and 5b-Me, and 2,4,6-trinitroanisole, i.e.6a-Me and 6b-Me, the latter compound being a conventional reference aromatic electrophile in Meisenheimer complex chemistry. The high thermodynamic stability of 4a-Me (pK(a) = 10.48) and 4b-Me (pK(a) = 12.23) relative to 5a-Me (pK(a) = 10.68) and 6a-Me (pK(a) = 12.56) or 5b-Me (pK(a) = 15.38) and 6b-Me (pK(a) = 16.46), is shown to derive from an especially high capacity of a para or an ortho SO(2)CF(3) group to stabilize a negative charge through Fpi-type polarization effects. From the kinetic data, it appears that the contribution of a methanol pathway to the formation of 4a-Me is much weaker than that found to operate in the formation of the 1,1-complex 5a-Me of 2,6-dinitro-4-trifluromethanesulfonylanisole, the experimental evidence suggesting that the reactivity of 4 and 5 is located just beyond the region defining the boundary between super- and normal-electrophilicity in methanol. Comparison of our results with available literature data show that this boundary corresponds to a pK(MeOH)(a) value of approximately 10, in agreement with our previous finding of a very effective solvent contribution to the sigma-complexation of 1,3,5-tris(trifluoromethanesulfonyl)benzene (13; pK(MeOH)(a) = 9.12) in methanol. Taking advantage of our observation that pK(MeOH)(a) and pK(H(2)O)(a) values for sigma-complexation at unsubstituted ring positions are related by a nice linear correlation, an approximate ranking of the electrophilicity of our aromatic triflones on the E scale developed by Mayr (Acc. Chem. Res. 2003, 36, 66) can be made.     

Change in rate-determining step in an E1cB mechanism during aminolysis of sulfamate esters in acetonitrile

The kinetics of the reactions of the nitrogen-sulfur(VI) esters 4-nitrophenyl N-methylsulfamate (NPMS) with a series of pyridines and a series of alicyclic amines and of 4-nitrophenyl N-benzylsulfamate (NPBS) with pyridines, alicyclic amines, and a series of quinuclidines have been investigated in acetonitrile (ACN) in the presence of excess amine at various temperatures. Pseudo-first-order rate constants (k(obsd)) have been obtained by monitoring the release of 4-nitrophenol/4-nitrophenoxide. From the slope of a plot of k(obsd) vs [amine], second-order rate constants (k'(2)) have been obtained for the pyridinolysis of NPMS, and a Br?nsted plot of log k'(2) vs pK(a) of pyridine gave a straight line with beta = 0.45. However, aminolysis with alicyclic amines of NPMS gave a biphasic Br?nsted plot (beta(1) = 0.6, beta(2) approximately equal to 0). Pyridinolysis and aminolysis with alicyclic amines and quinuclidines of NPBS also gave similar biphasic Br?nsted plots. This biphasic behavior has been explained in terms of a mechanistic change within the E1cB mechanism from an (E1cB)(irrev) (less basic amines) to an (E1cB)(rev) (more basic amines), and the change occurs at approximately the pK(a)'s (in ACN) of NPMS (17.94) and NPBS (17.68). The straight line Br?nsted plot for NPMS with pyridines occurs because the later bases are not strong enough to substantially remove the substrate proton and initiate the mechanistic change observed in the reaction of NPMS with the strong alicyclic amines and quinuclidines. An entropy study supports the change from a bimolecular to a unimolecular mechanism. This is the first clear demonstration of this E1cB mechanistic changeover involving a nitrogen acid substrate.