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In this contribution, a general expression is derived for the probability density of the time to the most recent common ancestor (TMRCA) of a simple birth–death tree, a widely used stochastic null-model of biological speciation and extinction, conditioned on the constant birth and death rates and number of extant lineages. This density is contrasted with a previous result which was obtained using a uniform prior for the time of origin. The new distribution is applied to two problems of phylogenetic interest. First, that of the probability of the number of taxa existing at any time in the past in a tree of a known number of extant species, and given birth and death rates, and second, that of determining the TMRCA of two randomly selected taxa in an unobserved tree that is produced by a simple birth-only, or Yule, process. In the latter case, it is assumed that only the rate of bifurcation (speciation) and the size, or number of tips, are known. This is shown to lead to a closed-form analytical expression for the probability distribution of this parameter, which is arrived at based on the known mathematical form of the age distribution of Yule trees of a given size and branching rate, which is derived here de novo, and a similar distribution which additionally is conditioned on tree age. The new distribution is the exact Yule prior for divergence times of pairs of taxa under the stated conditions and is potentially useful in statistical (Bayesian) inference studies of phylogenies. [ABSTRACT FROM AUTHOR]

Polarized-ion models—with and without dipole reaction fields and charge-quadrupole interaction—are evaluated in calculating atomization energies, bond angles, and bending force constants of groups 2 and 12 dihalides. A study of the sensitivity to changes in the input parameters reveals a strong dependence on dipole polarizabilities. The charge–quadrupole bending terms of a multipole expansion are important for quasi-linear molecules, where the preceding terms in the expansion tend to cancel each other. They normally stabilize the linear geometry, but enhance bent structures, if a certain criterion for the ratio of quadrupole polarizabilities of metal and halide ions is fulfilled. The models explain the bent alkaline earth halides, but fail spectacularly for the group 12 halides—HgBr[sub 2] and HgI[sub 2] are even calculated as unstable relative to the constituent atoms. The unexpected weakness of ionic bonds is related to the large increase of the ionization energies by relativistic effects and the lanthanide contraction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A thermodynamic treatment of the polarizable metal/electrolyte interface is presented and generalizes the classical electro-capillary equation, which interrelates variations in interfacial tension, cell potential, and solution composition, to include temperature changes. The result can be used in investigations of the structure of electrified interfaces; for example, in the calculation of entropy changes which occur in the solvent layer adjacent to the metal surface (inner or Stern layer) and, in particular, in the study of phase transitions in insoluble organic monolayers adsorbed at an electrochemical interface. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Electrochemical measurements on pincer ligands bis‐N‐(2,5‐dimethoxyphenyl)pyridine‐2,6‐dicarbothioamide (pdcta) and 6‐(4,7‐dimethoxy‐2‐benzothiazolyl)‐N‐(2,5‐dimethoxyphenyl)‐2‐pyridinecarbothioamide (pbcta) in N,N‐dimethylformamide (DMF) and dimethylsulfoxide (DMSO) on a glassy carbon working electrode revealed sulfur centred electrochemical processes. Spectroscopic, elemental and electrochemical analyses indicated that pdcta coordinated to Pd(II) via a κ3‐SNS mode to give 1 whereas pbcta coordinated via a κ3‐SNN mode to give 2. Electrochemical measurements in DMF indicated that the reduction of Pd(II)→Pd(0) proceeded in a concerted manner in the 2, whereas reduction was resolved into the Pd(II)→Pd(I)→Pd(0) in the case of 1. In the presence of p‐toluenesulfonic acid, both palladium complexes displayed electro‐catalytic proton reduction in DMF, with associated overpotentials calculated to be 97 mV and 156 mV for 1 and 2, respectively. The data suggests that the κ3‐SNS pincer ligand enhances the activity of the Pd(II) towards proton reduction relative to 1. [ABSTRACT FROM AUTHOR]

Analytical expressions describing the electrochemical impedance spectrum of a metallic electrode modified with an acid thiol monolayer, that undergoes a potential-induced proton transfer, have been derived from an electrostatic model of the interface. The frequency dispersion of the electrode impedance is described by an equivalent circuit that bears some similarities with that associated with a surface redox process. Good agreement is found between the theoretical predictions and the impedance of a Au(111) electrode modified with a 11-mercaptoundecanoic monolayer recorded as a function of ac frequency, dc potential and surface concentration of electrochemically active carboxylic groups. The same thermodynamic and kinetic parameter values are obtained from independent analysis of the voltammetric and impedimetric responses, except in the case of the reorganization energy for proton transfer, which seems to be affected by a slow potential-induced reorientation and dissociation of the external acid groups of the monolayer. [ABSTRACT FROM AUTHOR]

Efforts to reconstruct phylogenetic trees and understand evolutionary processes depend fundamentally on stochastic models of speciation and mutation. The simplest continuous-time model for speciation in phylogenetic trees is the Yule process, in which new species are “born” from existing lineages at a constant rate. Recent work has illuminated some of the structural properties of Yule trees, but it remains mostly unknown how these properties affect sequence and trait patterns observed at the tips of the phylogenetic tree. Understanding the interplay between speciation and mutation under simple models of evolution is essential for deriving valid phylogenetic inference methods and gives insight into the optimal design of phylogenetic studies. In this work, we derive the probability distribution of interspecies covariance under Brownian motion and Ornstein–Uhlenbeck models of phenotypic change on a Yule tree. We compute the probability distribution of the number of mutations shared between two randomly chosen taxa in a Yule tree under discrete Markov mutation models. Our results suggest summary measures of phylogenetic information content, illuminate the correlation between site patterns in sequences or traits of related organisms, and provide heuristics for experimental design and reconstruction of phylogenetic trees. [ABSTRACT FROM AUTHOR]

The structural evolution pathways leading to the conversion of some α-chlorinated carboxylic acids (Cl n H 3-n CCOOH, n = 0, 1, 2, 3) to their respective acid chlorides by thionyl chloride are investigated via density functional theoretical (DFT) modelling. For all compounds where n = 0–3, acid chloride formation occurs via two competing pathways, consisting of three activation barriers in both cases, all of which are enthalpy-controlled and moderate (ΔGǂ < 190 kJ mol−1). Though both pathways are not limited by the same step, they are both composed of only cyclic activated complexes. Rapid intra-molecular small molecule transfer (HCl) allows one pathway to be slight more productive than the other. Whereas all acids evolve via both competing pathways, the evolution of formic acid occurs exclusively via that which involves intramolecular HCl transfer where all the constituent transition states are formed quasi-synchronously. Results for both pathways are summarized in a detail kinetic model which, of-course, is based on the thermodynamic profiles. [Display omitted] • Chlorination of α-chlorinated carboxylic acids proceed via two competing pathways of near equal probabilities. • Chlorination of carboxylic acids occurs fastest via two-step process. • The chlorination rates for carboxylic acids are controlled by a combination steric and electronic factors. • All activated complexes in the transformation if carboxylic acids to acids chlorides are cyclic. • The mechanistic steps for the conversion of carboxylic acids to acid chlorides are enthalpy controlled. [ABSTRACT FROM AUTHOR]

By combining a description of the potential profile at electrodes coated with acid thiol monolayers with a quadratic relationship between activation energy and electrode potential, a rather simple expression for proton transfer voltammograms is derived. Our electrostatic analysis shows that proton transfer can only produce narrow voltammetric peaks when the immobilized acid groups lie close to the metal substrate. Quantitative fits of experimental voltammograms obtained with an Au(111) electrode modified with a 11-mercaptoundecanoic monolayer at pH 8.5 reveal that less than 1% of the carboxylic groups in the monolayer participate in the potential induced proton transfer process and that these groups lay close to the metal surface. A preliminary analysis of the kinetic parameters suggests that the interfacial electric field facilitates an intrinsically slow proton exchange between a proton donor and acceptor pair that are not in close contact with each other at the interface. [ABSTRACT FROM AUTHOR]

Abstract: The stationary birth-only, or Yule–Furry, process for rooted binary trees has been analysed with a view to developing explicit expressions for two fundamental statistical distributions: the probability that a randomly selected leaf is preceded by N nodes, or “ancestors”, and the probability that two randomly selected leaves are separated by N nodes. For continuous-time Yule processes, the first of these distributions is presented in closed analytical form as a function of time, with time being measured with respect to the moment of “birth” of the common ancestor (which is essentially inaccessible to phylogenetic analysis), or with respect to the instant at which the first bifurcation occurred. The second distribution is shown to follow in an iterative manner from a hierarchy of second-order ordinary differential equations. For Yule trees of a given number n of tips, expressions have been derived for the mean and variance for each of these distributions as functions of n, as well as for the distributions themselves. In addition, it is shown how the methods developed to obtain these distributions can be employed to find, with minor effort, expressions for the expectation values of two statistics on Yule trees, the Sackin index (sum over all root-to-leaf distances), and the sum over all leaf-to-leaf distances. [Copyright &y& Elsevier]

This study assessed the relative merits of the Derjaguin-Landau-Verwey-Overbeek and Sogami-Ise theories for the description of the electrostatic interaction between like-charged colloid particles in an electrolyte solution, which differ qualitatively in their predictions of the nature of this interaction. The former implies a repulsion at all distances with a screened-Coulomb (or Yukawa-type) dependence on distance, whereas the latter leads to an electrolyte-mediated, long-range attraction. The Sogami-Ise result for the interaction potential depends critically on the notion that the electrostatic parts of the Helmholtz and Gibbs energies of an electrolyte solution are different. By elaborating on the thermodynamic implications of this assumption, it was determined that it leads to a conflict with well-established results, in particular the relation between osmotic pressure and solvent chemical potential, already at the Debye-Hückel level. For this reason alone, the Sogami-Ise theory is untenable. The condensation and ordering phenomena observed with charged colloids in low-ionic.strength solutions, which the Sogami-Ise theory was originally designed to explain, can be interpreted without the need to resort to longrange electrostatic attraction, as has been demonstrated in previously published work. [ABSTRACT FROM AUTHOR]

Abstract: A theoretical treatment is presented of the relationship between absorption and emission maxima for (0–0) electronic transitions in transition metal complexes such as the ruthenium(II) polypyridyl chromophores used as dye sensitizers in photovoltaic cells, and the corresponding shift in standard redox (or half-wave) potential of these ionic complexes. It is shown that the potential shift is proportional to the average of absorption and emission wavenumbers for oxidized or reduced forms, a result that follows from a detailed analysis of medium relaxation effects, based on the Onsager cavity model, and attendant solvatochromic shifts. It is also shown how additional thermodynamic properties (reaction entropies and enthalpies) pertaining to the half reaction if the reactants are present in solution can be obtained from observation of the variation of spectra with temperature. For the case of reactants present in an insoluble monolayer, a general expression is derived for the Gibbs energy change that results from the promotion of all absorbing species to an excited state, and the changes in Gibbs energy resulting from the processes that return the system to ground-state equilibrium. [Copyright &y& Elsevier]

Abstract: Analytical expressions have been derived for the ionic surface excesses of a restricted primitive-model z–z electrolyte near a charged wall, by combination of a thermodynamic result with a recently derived equation for the surface potential as a function of salt concentration and surface charge density within the context of the Fawcett–Henderson method. This approximate result accounts for the finite size of ions and applies in the limit of low field strengths at the surface, corresponding to absolute surface charge densities not exceeding ca. 10 μCcm−2 and concentrations of at least 0.1 M. The theoretical expression has been tested via comparison with predictions based on Gouy–Chapman theory and data obtained with Monte Carlo simulations, and has been found to show a superior performance at high concentrations and large ion sizes in the case of 1–1 electrolytes and, to a lesser extent, 2–2 electrolytes. [Copyright &y& Elsevier]

The structure of dolomite, CaMg(CO3)2, was determined from 298 to 1466 K at a constant pressure of about 3 GPa using in situ synchrotron X-ray diffraction data to investigate the state of disorder. An order parameter s, defined as 2 xca - 1, varies from s = 1 (where xca = 1) for a completely ordered dolomite to s = 0 (where xca = 0.5) for a completely disordered dolomite. On heating, there is no measured change in s until the temperature is high enough to cause exchange of Ca2+ and Mg2+ cations. Significant disorder began to occur at about 1234 K [s = 0.83(1)] and increases along a smooth pathway to T = 1466 K [s = 0.12(5)]. The R3 ↔ R3c transition in dolomite is described by a modified Bragg-Williams thermodynamic model with the following molar free energy of disorder, Gd (T; s) = RTc[1 - s² + ½ a(s4- 1) - (T/Tc) {2 ln2 - (l+s) ln(1 + s) - (1 - s) ln(1 - s)}]. Using Tc = 1466 K and a = -0.29, this model provides an excellent agreement with experimental data. Moreover, the maximum enthalpy of disorder, Hd(s = 0) = RTc(1 - ½ a) ∼ 14 kJ/mol, agrees with published calorimetric data. A thermodynamic description of the aragonite + magnesite ↔ dolomite reaction boundary is also presented and it reproduces the main qualitative features correctly. [ABSTRACT FROM AUTHOR]

The theoretical basis for the calculation of acid dissociation constants in the lowest excited singlet or triplet state of organic compounds has been reexamined in light of a recent study on solvatochromism. A mathematical analysis based on the Onsager cavity model reveals that the absorption or emission frequency of an acid and its conjugate base, as they appear in the Fo¨rster equation, should be replaced with the averages of absorption and emission frequencies corresponding to the 0–0 transition of acid and base, respectively, in order to account for Franck–Condon effects on the free energy balance. The free energy of spontaneous medium relaxation is found to be the same for absorption and emission, and proportional to one-half of the Stokes shift.The main premise of the proposed method, besides the obvious requirement that both the acid and its conjugate base should fluoresce or phosphoresce, is that each excited state exists long enough for equilibrium with the medium to be established prior to emission. [Copyright &y& Elsevier]

• Two Co(II)-pincer ligands of coorcination modes κ3-SNS and κ3-ONS are reported. • The crystal structure of the pincer ligand pcta is reported. • Both complexes of Co(II) displayed electro-catalytic hydrogen evolution (HER). • The addition of co-ligands bpy or PPh 3 had varying effects on the HER. Bis- N -(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (pdcta) and N -(2,5- dimethoxyphenyl)-6-[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (pcta) were synthesized from a one pot mixture as pincer ligands. Crystals of pcta , grown from CDCl 3 were structurally characterized, and found to crystallize in the space group P 2 1 / c. The Co(II) complexes [Co(II)(κ3-SNS-pdcta)(CH 3 CO 2)(H 2 O)] (1) and [Co(II)(κ3-ONS-pcta)(CH 3 CO 2)(H 2 O)]·H 2 O (2) were prepared from the reaction of pdcta and pcta and Co(CH 3 CO 2) 2 ·4H 2 O in ethanol at reflux under argon atmosphere. The identities of 1 and 2 were confirmed from their elemental analyses, ESI MS and a series of spectroscopic measurements. Both complexes displayed electrocatalytic hydrogen evolution with p -toluene sulfonic acid monohydrate in the presence and absence of bpy or PPh 3 co-ligands. The hydrogen evolution occurred at moderate overpotentials (605–780) mV, with 1 giving better Faradaic efficiencies than 2 under all the conditions explored herein. Thermodynamic estimates, based on the reduction potentials, suggest that the homolytic pathway for the production of hydrogen from the CoIII-hydride species generated in the presence of the proton source, is more favourable than the heterolytic pathway in CH 3 CN. [ABSTRACT FROM AUTHOR]

An analysis of equilibrium size distributions of circular liquid-condensed (LC) domains in amphiphilic monolayers at the air/water interface, gleaned from microscopic images, is presented in terms of Gaussians fitted to a theoretical expression derived previously. It is demonstrated how, in principle, important properties of the monolayer, such as molecular dipole moments of surfactant head groups and line tension of the liquid-condensed/liquid-expanded phase boundary, can be obtained by combining the results of this simplified analysis with information from X-ray diffraction or surface pressure measurements. [Copyright &y& Elsevier]

We investigate the validity of employing a potential dependent polarizability in a model to determine atom in molecule (AIM) polarizabilities. It is found that the assumption of an electric potential dependent polarizability leads to a contravention of the principle of conservation of energy. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The synthesis and crystal structures of bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (dicarbothioamide I) and 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (L1) as well as the syntheses of the palladium(II) chloride and acetate pincer complexes are reported. The stability constant for the palladium complex formation at 25°C was found to be (2.04 ± 0.26) × 104 dm3 mol-1 and (2.30 ± 0.19) × 104 dm3 mol-1 with ΔfH = 8 ± 1 kJ mol-1, ΔfSθ = 108 ± 10 J K-1 mol-1, and ΔfH = 17 ± 4 kJmol-1 and ΔfSθ = 140 ± 20 JK-1mol-1 for the PdClL1 and Pd(OAc)L1, respectively. The kinetics of formation of the palladium(II) complexes were investigated and the mechanism is proposed to be associative in nature (ΔH‡1 = 34 ± 2 kJ mol-1 and AS‡1 = -113 ± 8JK-1 mol-1, and ΔH‡1 = 37 ± 3 kJ mol-1 and ΔS‡1 = -100 ± 8 JK-1mol-1 for the PdClL1 and Pd(OAc)L1 species, respectively). The electrochemical measurements of the acetonitrile solutions revealed irreversible electron transfers consistent with the electrochemical decomposition of the ligand and its coordination complexes. [ABSTRACT FROM AUTHOR]

Abstract: The reaction between ZnCl2 and di-2-pyridyl ketone benzoyl hydrazone (dpkbh) in acetonitrile under ultrasonic or reflux conditions gave [ZnCl2(η3-N,N,O-dpkbh)] in good yield. The identity of the compound was established from the results of its elemental analysis and a number of spectroscopic measurements. Solid-state infrared spectra of [ZnCl2(η3-N,N,O-dpkbh)] reveal the coordination of dpkbh, the presence of the amide proton and binding of the oxygen atom of dpkbh. 1H NMR spectra of [ZnCl2(η3-N,N,O-dpkbh)] show sensitivity to solvent and temperature variations and confirm the coordination of dpkbh. The electronic absorption spectra of [ZnCl2(η3-N,N,O-dpkbh)] in non-aqueous media show high sensitivity to changes in their surroundings and divulge two interlocked intra-ligand-charge transfer (ILCT) transitions of the donor–acceptor type between 300 and 500nm. Optical measurements show reversible inter-conversion between two forms of [ZnCl2(η3-N,N,O-dpkbh)] that may be due to complex–substrate interactions and thermodynamic analysis gave changes in enthalpy (ΔH θ ) of −23.3 and +14.1kJmol−1, entropy (ΔS θ ) of −51 and +31Jmol−1K−1, and free energy (ΔG θ ) of −8.16 and +4.54kJmol−1 at 298.15K in DMF and DMSO, respectively. Substrates in concentrations as low as 10−5 M can be detected and determined using [ZnCl2(η3-N,N,O-dpkbh)] in polar solvents. A provisional model for the binding of substrate to [ZnCl2(η3-N,N,O-dpkbh)] is developed and a binding constant in the range 6000–8000M−1 is obtained in the case of ZnCl2 in DMSO. Electrochemical measurements on [ZnCl2(η3-N,N,O-dpkbh)] in DMF show irreversible metal and ligand-based redox processes, and electrochemical reactions of dpkbh with ZnCl2 show facile coordination of ZnCl2 and formation of [ZnCl2(η3-N,N,O-dpkbh)]. X-ray structural analysis done on a crystal grown from a DMF solution of [ZnCl2(η3-N,N,O-dpkbh)] confirmed the identity of [ZnCl2(η3-N,N,O-dpkbh)] and shows an extensive network of non-covalent interactions that connects all molecules. [Copyright &y& Elsevier]

We undertook a palaeoecological evaluation of the development of ribs, flanges, tubes and pallial canals in recumbent rudist bivalves typified by the genus Titanosarcolites. We used an evidence based system to interpret potential functional significance of features. The dorsal position of the ancestral ligament and the antero-ventral-down orientation of recumbent Titanosarcolites indicates that a recumbent lifestyle was only possible after loss of a functional ligament. The presence of ribs on the ventral side of the shell in recumbents is interpreted as a means of lifting the commissure away from the seafloor to prevent fowling by sediments. Pallial canals, which invade the whole inner layer of the shell, contain tabulae, and therefore cannot be functional conduits for fluid flow, nor cavities for photosynthetic symbionts (as only some would receive light), and are best interpreted as features to allow rapid growth of large shells. Tubes in Titanosarcolites , and similar forms, lack tabulae, are open to the sea at the tips of the valves, and were formed by over-roofing and subsequent division; the presence of epibionts and geopetal structures indicates that they were not filled with sediment, and were conduits for water flow containing food. Mathematical modelling indicates that sucking water in through the tubes and expelling through the commissure was the most efficient mechanism. The presence of a posterior ventral groove is interpreted as a potential exhalent siphon and the presence of flanges on the shell is interpreted as a mechanism for keeping inhalant and exhalent waters separate. We suggest that large flanges also served the purpose of keeping competitive rudists at a distance and preventing overturning of recumbents during storm events. These large recumbent rudists seem to have developed their ecological strategies to cope with relatively low-food availability on late Cretaceous carbonate platforms. • Tube-bearing rudists fed by sucking in water through their tubes. • They expelling waste through a ventral posterior groove. • The presence of epibionts in tubes indicates that tubes were filled with water. • Flanges helped to lift the commissure above the sediment water-interface. • Flanges kept competitors away and directed currents during feeding. [ABSTRACT FROM AUTHOR]

Theory for interfacial proton transfer voltammetry of a molecular film containing any acid/base loading has been developed under equilibrium conditions. Diagnostic criteria to disentangle the interplay between diffuse layer and ionization effects are outlined. Easy-to-use analytical expressions for the voltammetric features are derived for the particular case of an invariant diffuse layer effect, which turn out to be entirely analogous to those for a surface redox conversion with Frumkin interactions. It is demonstrated that, regardless of the electrolyte concentration, significant ionization of the external acid groups located nearby the diffuse layer is sufficient for the fulfillment of this relevant particular case. A strategy is outlined to determine the amount, the intrinsic pKa, and the burial depth of the voltammetrically active groups from the surface concentration dependence of the main voltammetric features. Self-assembled monolayers of 11-mercaptoundecanoic acid deposited on Au(111), containing higher amounts of buried carboxylic groups than previously reported, have been studied to assess more critically the influence of electrostatic effects on the ionization process. Preliminary evidence suggests that the protonation/deprotonation voltammetric wave involves physisorbed rather than chemisorbed thiol molecules. Application of the present theoretical approach to this system reveals that the voltammetrically active carboxylic groups are located close to the electrode surface and become more acidic upon increasing their surface concentration. [ABSTRACT FROM AUTHOR]

When dpktch was reacted with ZnCl2 in refluxing acetonitrile in air [ZnCl2(η3-dpktch)] was isolated in good yield. Infrared spectra suggest weaker binding of dpktch in [ZnCl2(η3-dpktch)] than in [CdCl2(η3-dpktch)]. 1H-NMR studies in non-aqueous media show that [ZnCl2(η3-dpktch)] is sensitive to changes in its environment and exchanges its amide proton. Electronic absorption spectral measurements confirmed the sensitivity of [ZnCl2(η3-dpktch)] to changes in its surroundings and show inter-conversion between two intra-ligand-charge-transfer transitions (ILCT) at 330 ± 2 nm and 404 ± 2 associated with [ZnCl2(η3-dpktch)] and its conjugate base. Thermo-optical measurements in non-aqueous dmf and dmso show facile inter-conversion between [ZnCl2(η3-dpktch)] and its conjugate base, respectively. Also, it is shown that protonation of dmf by [ZnCl2(η3-dpktch)] is exothermic (standard enthalpy of protonation ΔHθ = -40.7 ± 1.8 kJ mol-1), but endothermic for dmso (ΔHθ = +8.3 ± 1.5 kJ mol-1). Chemical stimuli in concentrations as low as 5.0 × 10-7 M can be detected and determined using [ZnCl2(η3-dpktch)] in non-aqueous media. X-ray crystallographic studies on a monoclinic, P21/n single crystal of [ZnCl2(η3-dpktch)] confirmed the N,N,O-coordination of dpktch and revealed interdigitated units of [ZnCl2(η3-dpktch)] connected via a web of hydrogen bonds. [ABSTRACT FROM AUTHOR]

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH3)3+6 at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode, by using electrochemical techniques that operate at distinct time scales. Two limiting voltammetric behaviors are observed, consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH2)10-COOH and HS-(CH2)n-CH2OH (n 3, 5, and 10) SAMs. The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH3)63+ at hydroxyl-terminated SAMs is located outside the film I solution interface at short times. However, long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied, which takes the form of a chronoamperometric spike. Fledox probe permeation is shown to be a kinetically slow process, whose activation strongly depends on redox probe concentration, applied potential, and chemical composition of the intervening medium. The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM, whereas those of mercaptohexanol, mercaptoundecanol, and mercaptoundecanoic acid undergo an order/ disorder transition below a critical potential, which facilitates the approach of the redox probe toward the electrode surface. [ABSTRACT FROM AUTHOR]

Abstract: Potential of zero charge and interfacial capacitance values of ω-mercaptoalkanoic acid monolayers deposited on Au(111) are derived from immersion current transients, and are applied to the analysis of their acid properties. Potential of zero charge and capacitance titration curves extend over six pH units, and exhibit apparent pK1/2 values that vary with electrolyte concentration. Electrostatic adsorption of protons at the monolayer-solution boundary provides a quantitative explanation of these observations, and leads to a diffuse layer corrected pK a value of 4.3, which is independent of the thiol chain-length. [Copyright &y& Elsevier]

The article presents information on the structure of 7-Acetyl-12-methoxyhorminone. A horminone derivative with the systematic name 7-acetyl-12-methoxyabieta-8,12-diene-11,14- dione, C23H32O5, was isolated from Hyptis verticillata from Jamaica. The Hyptis genus has approximately 400 species and is a member of the Labiatae family growing mostly in the tropical Americas. A number of these species possess significant biological activities, including mycotoxinic, phytotoxinic, teratogenic, and antifertility properties. Hyptis verticillata is widely used in folklore medicine as a treatment for itching, insect stings and rheumatoid arthritis.

Mikanolide [systematic names: 1,10:2,3-diepoxy-6,8-dihydroxy-11-vinylgermacr-4-ene 12,14-di-γ-lactone and 7,10a-dimethyl-1a,1b,2a,6a,7,9a,10,10a-octahydro-4H-6,3-methenofuro[3,2-c]bisoxireno[f,h]oxacycloundecin-4,8(6H)-dione], C15H14O6, derived from a variety of Mikania micrantha growing in Portland, Jamaica, contains a methylcyclodecane ring fused to an unsaturated planar α,γ-lactone, an envelope-type near-planar vinyl-β,γ-lactone and two epoxide moieties. The crystal packing shows stacks of mikanolide molecules interlocked via a network of non-classical C-H…O hydrogen bonds between the lactone units. [ABSTRACT FROM AUTHOR]