Crystallization in poly(L-lactide)-b-poly(epsilon-caprolactone) double crystalline diblock copolymers: a study using X-ray scattering, differential scanning calorimetry, and polarized optical microscopy

The crystallization of well-defined poly(L-lactide)-b-poly(epsilon-caprolactone) diblock copolymers, PLLA-b-PCL, was investigated by time-resolved X-ray techniques, polarized optical microscopy (POM), and differential scanning calorimetry (DSC). Two compositions were studied that contained 44 and 60 wt % poly(L-lactide), PLLA (they are referred to as (L44C5614)-C-11 and (L60C409)-C-12, respectively, with the molecular weight of each block in kg/mol as superscript). The copolymers were found to be initially miscible in the melt according to small-angle X-ray scattering measurements (SAXS). Their thermal behavior was also indicative of samples whose crystallization proceeds from a mixed melt. Sequential isothermal crystallization from the melt at 100 degreesC (for 30 min) and then at 30 degreesC (for 15 min) was measured. At 100 degreesC only the PLLA block is capable of crystallization, and its crystallization kinetics was followed by both WAXS and DSC; comparable results were obtained that indicated an instantaneous nucleation with three-dimensional superstructures (Avrami index of approximately 3). The spherulitic nature of the superstructure was confirmed by POM. When the temperature was decreased to 30 degreesC, the PCL block was able to crystallize within the PLLA negative spherulites (with an Avrami index of 2, as opposed to 3 in homo-PCL), and its crystallization rate was much slower than an equivalent homo-PCL. Time-resolved SAXS experiments in (L60C409)-C-12 revealed an initial melt mixed morphology at 165 degreesC that upon cooling transformed into a transient microphase-separated lamellar structure prior to crystallization at 100 degreesC.

Melt structure and its transformation by sequential crystallization of the two blocks within poly(L-lactide)-block-poly(epsilon-caprolactone) double crystalline diblock copolymers

Sequential crystallization of poly(L-lactide) (PLLA) followed by poly(epsilon-caprolactone) (PCL) in double crystalline PLLA-b-PCL diblock copolymers is studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS). Three samples with different compositions are studied. The sample with the shortest PLLA block (32 wt.-% PLLA) crystallizes from a homogeneous melt, the other two (with 44 and 60% PLLA) from microphase separated structures. The microphase structure of the melt is changed as PLLA crystallizes at 122 degrees C (a temperature at which the PCL block is molten) forming spherulites regardless of composition, even with 32% PLLA. SAXS indicates that a lamellar structure with a different periodicity than that obtained in the melt forms (for melt segregated samples). Where PCL is the majority block, PCL crystallization at 42 degrees C following PLLA crystallization leads to rearrangement of the lamellar structure, as observed by SAXS, possibly due to local melting at the interphases between domains. POM results showed that PCL crystallizes within previously formed PLLA spherulites. WAXS data indicate that the PLLA unit cell is modified by crystallization of PCL, at least for the two majority PCL samples. The PCL minority sample did not crystallize at 42 degrees C (well below the PCL homopolymer crystallization temperature), pointing to the influence of pre-crystallization of PLLA on PCL crystallization, although it did crystallize at lower temperature. Crystallization kinetics were examined by DSC and WAXS, with good agreement in general. The crystallization rate of PLLA decreased with increase in PCL content in the copolymers. The crystallization rate of PCL decreased with increasing PLLA content. The Avrami exponents were in general depressed for both components in the block copolymers compared to the parent homopolymers. Polarized optical micrographs during isothermal crystalli zation of (a) homo-PLLA, (b) homo-PCL, (c) and (d) block copolymer after 30 min at 122 degrees C and after 15 min at 42 degrees C.

New polynuclear Mo-Fe complexes with ferrocenylamidobenzimidazole ligands

The reaction between [Mo(eta(3)-C3H5)(CO)(2)(NCMe)(2)Br] (1) and the ferrocenylamidobenzimidazole ligands FcCO(NH(2)benzim) (L1) and (FcCO)(2)(NHbenzim) (L2) led to a binuclear (2) and a trinuclear (3) Mo-Fe complex, respectively. The single-crystal X-ray structure of [Mo(eta(3)-C3H5)(CO)(2)(L2)Br] [L2 = {[(eta(5)-C5H5)Fe(eta(5)-C5H4CO)](2)(2-NH-benzimidazol-yl)}] shows that L2 is coordinated to the endo Mo(eta(3)-C3H5)(CO)(2) group in a kappa(2)-N,O-bidentate chelating fashion whereas the Mo-II centre displays a pseudooctahedral environment with Br occupying an equatorial position. Complex 2 was formulated as [MO(eta(3)-C3H5)(CO)(2)(L1)Br] on the basis of a combination of spectroscopic data, elemental analysis, conductivity and DFT calculations. L1 acts as a kappa(2)-N,N-bidentate ligand. In both L1 and L2, the HOMOs are mainly localised on iron while the C=O bond(s) contribute to the LUMO(s) and the next highest energy orbitals are Fe-allyl antibonding orbitals. When the ligands bind to Mo(eta(3)-C3H5)(CO)(2)Br, the greatest difference is that Mo becomes the strongest contributor to the HOMO. Electrochemical studies show that, in complex 2, no electronic interaction exists between the two ferrocenyl ligands and that the first electron has been removed from the Mo-II-centred HOMO. (c) Wiley-VCH Verlag GmbH & Co. KGaA.

Synthesis, spectroscopic and redox properties of some ruthenium(II) thiosemicarbazone complexes: structural description of four of these complexes

Sixteen neutral mixed ligand thiosemicarbazone complexes of ruthenium having general formula [Ru(PPh3)(2)L-2], where LH = 1-(arylidine)4-aryl thiosemicarbazones, have been synthesized and characterized. All complexes are diamagnetic and hence ruthenium is in the +2 oxidation state (low-spin d(6), S = 0). The complexes show several intense peaks in the visible region due to allowed metal to ligand charge transfer transitions. The structures of four of the complexes have been determined by single-crystal X-ray diffraction and they show that thiosemicarbazone ligands coordinate to the ruthenium center through the hydrazinic nitrogen and sulfur forming four-membered chelate rings with ruthenium in N2S2P2 coordination environment. In dichloromethane solution, the complexes show two quasi-reversible oxidative responses corresponding to loss of electron from HOMO and HOMO - 1. The E-0 values of the above two oxidations shows good linear relationship with Hammett substituents constant (sigma) as well as with the HOMO energy of the molecules calculated by the EHMO method. A DFT calculation on one representative complex suggests that there is appreciable contribution of the sulfur p-orbitals to the HOMO and HOMO - 1. Thus, assignment of the oxidation state of the metal in such complexes must be made with caution. (c) 2005 Elsevier B.V. All rights reserved.

Self-nucleation and crystallization kinetics of double crystalline poly(p-dioxanone)-b-poly(epsilon-caprolactone) diblock copolymers

The crystallization kinetics of each constituent of poly(p-dioxanone)-b-poly(epsilon-caprolactone) diblock copolymers (PPDX-b-PCL) has been determined in a wide composition range by differential scanning calorimetry and compared to that of the equivalent homopolymers. Spherulitic growth rates were also measured by polarized optical microscopy while atomic force microscopy was employed to reveal the morphology of one selected diblock copolymer. It was found that crystallization drives structure formation and both components form lamellae within mixed spherulitic superstructures. The overall isothermal crystallization kinetics of the PPDX block at high temperatures, where the PCL is molten, was determined by accelerating the kinetics through a previous self-nucleation procedure. The application of the Lauritzen and Ho. man theory to overall growth rate data yielded successful results for PPDX and the diblock copolymers. The theory was applied to isothermal overall crystallization of previously self-nucleated PPDX ( where growth should be the dominant factor if self-nucleation was effective) and the energetic parameters obtained were perfectly matched with those obtained from spherulitic growth rate data of neat PPDX. A quantitative estimate of the increase in the energy barrier for crystallization of the PPDX block, caused by the covalently bonded molten PCL as compared to homo-PPDX, was thus determined. This energy increase can dramatically reduce the crystallization rate of the PPDX block as compared to homo-PPDX. In the case of the PCL block, both the crystallization kinetics and the self-nucleation results indicate that the PPDX is able to nucleate the PCL within the copolymers and heterogeneous nucleation is always present regardless of composition. Finally, preliminary results on hydrolytic degradation showed that the presence of relatively small amounts of PCL within PPDX-bPCL copolymers substantially retards hydrolytic degradation of the material in comparison to homo-PPDX. This increased resistance to hydrolysis is a complex function of composition and its knowledge may allow future prediction of the lifetime of the material for biomedical applications.

Structural and spectroscopic properties of Ru(II) complexes of 4-(aryl)thiosemicarbazones of thiophen-2-carbaldehyde

Six Ru(II) complexes of formula [Ru(L)(2)(PPh3)(2)] have been prepared where LH = 4-(aryl)thiosemicarbazones of thiophen-2-carbaldehyde. X-ray crystal structures of five of the complexes are reported. In all the complexes ruthenium is six coordinate with a distorted octahedral cis-P-2, cis-N-2, trans-S-2 donor environment, and each of the two thiosemicarbazone ligands are coordinated in a bidentate fashion forming a four membered chelate ring. The complexes undergo a one-electron oxidation at similar to 0.5 V vs. Ag/AgCl. The EPR spectrum of the electrochemically oxidized solution at 100 K shows a rhombic signal, with transitions at g(1) = 2.27, g(2) = 2.00 and g(3) = 1.80. DFT calculations on one of the complexes suggest that there is 35% ruthenium and 17% sulfur orbital contribution to the HOMO. These results suggest that the assignment of metal atom oxidation states in these compounds is not unambiguous. (C) 2009 Elsevier Ltd. All rights reserved.

Synthesis of heterosulfamates. Search for structure-taste relationships

Thirty one new sodium heterosulfamates, RNHSO3Na, where the R portion contains mainly thiazole, benzothiazole, thiadiazole and pyridine ring structures, have been synthesized and their taste portfolios have been assessed. A database of 132 heterosulfamates ( both open-chain and cyclic) has been formed by combining these new compounds with an existing set of 101 heterosulfamates which were previously synthesized and for which taste data are available. Simple descriptors have been obtained using (i) measurements with Corey-Pauling-Koltun (CPK) space- filling models giving x, y and z dimensions and a volume VCPK, (ii) calculated first order molecular connectivities ((1)chi(v)) and (iii) the calculated Spartan program parameters to obtain HOMO, LUMO energies, the solvation energy E-solv and V-SPART AN. The techniques of linear (LDA) and quadratic (QDA) discriminant analysis and Tree analysis have then been employed to develop structure-taste relationships (SARs) that classify the sweet (S) and non-sweet (N) compounds into separate categories. In the LDA analysis 70% of the compounds were correctly classified ( this compares with 65% when the smaller data set of 101 compounds was used) and in the QDA analysis 68% were correctly classified ( compared to 80% previously). TheTree analysis correctly classified 81% ( compared to 86% previously). An alternative Tree analysis derived using the Cerius2 program and a set of physicochemical descriptors correctly classified only 54% of the compounds.

Human evolution, nutritional ecology and prebiotics in ancient diet

Modern studies of prebiotic non digestible carbohydrates continue to expand and demonstrate their colonic and systemic benefits. However, virtually nothing is known of their use among ancient populations. In this paper we discuss evidence for prebiotic use in the archaeological record from select areas of the world. It is suggested that members of our genus Homo would have had sufficient ecological opportunity to include prebiotic-bearing plants in diet as early as ~ 2 million years ago, but that significant dietary intake would not have taken place until the advent of technological advances that characterized the Upper Paleolithic of ~40,000 years ago. Throughout human evolution, hominid populations that diversified their diet to include prebiotic-bearing plants would have had a selective advantage over competitors.

Correlations for gas-phase reactions of NO3, OH and O3 with alkenes: an update

Methods are developed for predicting rate coefficients for reactions of initiators of tropospheric oxidation with unsaturated compounds that are abundant in the atmosphere; prognostic tools of this kind are essential for atmospheric chemists and modellers. To pursue the aim of exploring such tools, the kinetics of reactions of NO3, OH and O-3 with a series of alkenes are examined for correlations relating the logarithms of the rate coefficients to the energies of the highest occupied molecular orbitals (HOMOs) of the alkenes. A comparison of the values predicted by the correlations with experimental data (where the latter exist) allowed us to assess the reliability of our method. We used a series of theoretical methods to calculate the HOMO energies, and found that higher computational effort improves the agreement of the predicted rate coefficients with experimental values, especially for reactions of NO3 with alkenes that possess vinyllic halogen substituents. As a consequence, it is expedient to suggest new correlations to replace those presented by us and others that were based on the lower level of theory. We propose the following correlations for the reactions of NO3, OH and O-3 with alkenes: ln(k(NO3)/cm(3) molecule(-1) s(-1)) = 6.40(E-HOMO/eV) + 31.69, ln(k(OH)/cm(3) molecule(-1) s(-1)) = 1.21 (E-HOMO/eV)-12.34 and ln(k(O3)/cm(3) molecule(-1) s(-1)) = 3.28(E-HOMO/eV)-6.78. These new correlations have been developed using the larger experimental data sets now available, and the impact of the extended data on the quality of the correlations is examined in the paper. Atmospheric lifetimes have been calculated from both experimental and estimated rate coefficients to provide an overview of removal efficiencies for different classes of alkenes with respect to oxidative processes initiated by NO3, OH and O-3. A figure is presented to show the spatial scales over which alkenes may survive transport in competition with attack by NO3, OH and O-3. Removal by NO3 or OH is always more important than removal by O-3, and reactions with NO3 dominate for scales up to a few hundred metres.

Gas-phase rate coefficients for the reactions of O(3P), S(3P), Se(3P), and Te(3P) with alkenes: application of perturbation frontier molecular orbital theory, correlations, and structure-activity relations (SARs)

The kinetics of the reactions of the atoms O(P-3), S(P-3), Se(P-3), and Te((3)p) with a series of alkenes are examined for correlations relating the logarithms of the rate coefficients to the energies of the highest occupied molecular orbitals (HOMOs) of the alkenes. These correlations may be employed to predict rate coefficients from the calculated HOMO energy of any other alkene of interest. The rate coefficients obtained from the correlations were used to formulate structure-activity relations (SARs) for reactions of O((3)p), S(P-3), Se (P-3), and Te((3)p) with alkenes. A comparison of the values predicted by both the correlations and the SARs with experimental data where they exist allowed us to assess the reliability of our method. We demonstrate the applicability of perturbation frontier molecular orbital theory to gas-phase reactions of these atoms with alkenes. The correlations are apparently not applicable to reactions of C(P-3), Si(P-3), N(S-4), and Al(P-2) atoms with alkenes, a conclusion that could be explained in terms of a different mechanism for reaction of these atoms.

Electronic transitions and bonding properties in a series of five-coordinate “16-electron” complexes [Mn(CO)3(L2)]− (L2=chelating redox-active π-donor ligand)

In this article we present for the first time accurate density functional theory (DFT) and time-dependent (TD) DFT data for a series of electronically unsaturated five-coordinate complexes [Mn(CO)(3)(L-2)](-), where L-2 stands for a chelating strong pi-donor ligand represented by catecholate, dithiolate, amidothiolate, reduced alpha-diimine (1,4-dialkyl-1,4-diazabutadiene (R-DAB), 2,2'-bipyridine) and reduced 2,2'-biphosphinine types. The single-crystal X-ray structure of the unusual compound [Na(BPY)][Mn(CO)(3)(BPY)]center dot Et2O and the electronic absorption spectrum of the anion [Mn(CO)(3)(BPY)](-) are new in the literature. The nature of the bidentate ligand determines the bonding in the complexes, which varies between two limiting forms: from completely pi-delocalized diamagnetic {(CO)(3)Mn-L-2}(-) for L-2 = alpha-diimine or biphosphinine, to largely valence-trapped {(CO)(3)Mn-1-L-2(2-)}(-) for L-2(2-) = catecholate, where the formal oxidation states of Mn and L-2 can be assigned. The variable degree of the pi-delocalization in the Mn(L-2) chelate ring is indicated by experimental resonance Raman spectra of [Mn(CO)(3)(L-2)](-) (L-2=3,5-di-tBu-catecholate and iPr-DAB), where accurate assignments of the diagnostically important Raman bands have been aided by vibrational analysis. The L-2 = catecholate type of complexes is known to react with Lewis bases (CO substitution, formation of six-coordinate adducts) while the strongly pi-delocalized complexes are inert. The five-coordinate complexes adopt usually a distorted square pyramidal geometry in the solid state, even though transitions to a trigonal bipyramid are also not rare. The experimental structural data and the corresponding DFT-computed values of bond lengths and angles are in a very good agreement. TD-DFT calculations of electronic absorption spectra of the studied Mn complexes and the strongly pi-delocalized reference compound [Fe(CO)(3)(Me-DAB)] have reproduced qualitatively well the experimental spectra. Analyses of the computed electronic transitions in the visible spectroscopic region show that the lowest-energy absorption band always contains a dominant (in some cases almost exclusive) contribution from a pi(HOMO) -> pi*(LUMO) transition within the MnL2 metallacycle. The character of this optical excitation depends strongly on the composition of the frontier orbitals, varying from a partial L-2 -> Mn charge transfer (LMCT) through a fully delocalized pi(MnL2) -> pi*(MnL2) situation to a mixed (CO)Mn -> L-2 charge transfer (LLCT/MLCT). The latter character is most apparent in the case of the reference complex [Fe(CO)(3)(Me-DAB)]. The higher-lying, usually strongly mixed electronic transitions in the visible absorption region originate in the three lower-lying occupied orbitals, HOMO - 1 to HOMO - 3, with significant metal-d contributions. Assignment of these optical excitations to electronic transitions of a specific type is difficult. A partial LLCT/MLCT character is encountered most frequently. The electronic absorption spectra become more complex when the chelating ligand L-2, such as 2,2'-bipyridine, features two or more closely spaced low-lying empty pi* orbitals.

Syntheses, x-ray structures, photochemistry, redox properties, and DFT calculations of interconvertible fac- and mer-[Mn(SPS)(CO)3] isomers containing a flexible SPS-based pincer ligand

The lithium salt of the anionic SPS pincer ligand composed of a central hypervalent lambda(4)-phosphinine ring bearing two ortho-positioned diphenylphosphine sulfide side arms reacts with [Mn(CO)(5)Br] to give fac-[Mn(SPS)(CO)(3)], This isomer can be converted photochemicaily to mer-[Mn(SPS)(CO)(3)], with a very high quantum yield (0.80 +/- 0.05). The thermal backreaction is slow (taking ca. 8 h at room temperature), in contrast to rapid electrodecatalyzed mer-to-fac isomerization triggered by electrochemical reduction of mer-[Mn(SPS)(CO)(3)]. Both geometric isomers of [Mn(SPS)(CO)(3)] have been characterized by X-ray crystallography. Both isomers show luminescence from a low-lying (IL)-I-3 (SPS-based) excited state. The light emission of fac-[Mn(SPS)(CO)(3)] is largely quenched by the efficient photoisomerization occurring probably from a low-lying Mn-CO dissociative excited state. Density functional theory (DFT) and time-dependent DFT calculations describe the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of fac- and mer-[Mn(CO)(3)(SPS)] as ligand-centered orbitals, largely localized on the phosphinine ring of the SPS pincer ligand. In line with the ligand nature of its frontier orbitals, fac-[Mn(SPS)(CO)(3)] is electrochemically reversibly oxidized and reduced to the corresponding radical cation and anion, respectively. The spectroscopic (electron paramagnetic resonance, IR, and UV-vis) characterization of the radical species provides other evidence for the localization of the redox steps on the SIPS ligand. The smaller HOMO-LUMO energy difference in the case of mer-[Mn(CO)(3)(SPS)], reflected in the electronic absorption and emission spectra, corresponds with its lower oxidation potential compared to that of the fac isomer. The thermodynamic instability of mer-[Mn(CO)(3)(SPS)], confirmed by the DFT calculations, increases upon one-electron reduction and oxidation of the complex.

Strongly nucleophilic RhI centre in square-planar complexes with terdentate (κ3) 2,2′:6′,2′′-terpyridine ligands: crystallographic, electrochemical and density functional theoretical studies

Rh-I-terpyridine complexes have been unambiguously formed for the first time. The 2,21:6',2"-terpyridine (tpy), 4'-chloro-2,2':6',2"-terpyridine (4'-Cl-tpy) and 4'-(tert-butyldimethylsilyl-ortho-carboranyl)-2,2':6',2"-terpyridine (carboranyl-tpy) ligands were used for successful syntheses and characterisation of the corresponding Rh-I complexes with halide coligands, [Rh(X)(4'-Y-terpyridine)] (X = Cl, Y = H, Cl, carboranyl; X = Br, Y = H). All four neutral Rh-tpy complexes are square planar, with Rh-X bonds in the plane of the 4'-Y-terpyridine ligands. Full characterisation of these dark blue, highly air-sensitive compounds was hampered by their poor solubility in various organic solvents. This is mainly due to the formation of pi-stacked aggregates, as evidenced by the crystal structure of [Rh(Cl)(tpy)]; in addition, [Rh(Cl)(carboranyl-tpy)] merely forms discrete dimers. The (bonding) properties of the novel Rh-I-terpyridine complexes have been studied with single-crystal X-ray diffraction, (time-dependent) density functional theoretical (DFT) calculations, far-infrared spectroscopy, electronic absorption spectroscopy and cyclic voltammetry. From DFT calculations, the HOMO of the studied Rh-I-terpyridine complexes involves predominantly the metal centre, while the LUMO resides on the terpyridine ligand. Absorption bands of the studied complexes in the visible region (400-900 nm) can be assigned to MLCT and MLCT/XLCT transitions. The relatively low oxidation potentials of [Rh(X)(tpy)] (X = Cl, Br) point to a high electron density on the metal centre. This makes the Rh-I-terpyridine complexes strongly nucleophilic and (potentially) highly reactive towards various (small) substrate molecules containing carbon-halide bonds.

Rigid ferrocenophane and its metal complexes with transition and alkaline-earth metal ions

The rigid [6]ferrocenophane, L-1, was synthesised by condensation of 1,1'-ferrocene dicarbaldehyde with trans-1,2-diaminocyclohexane in high dilution at r.t. followed by reduction. When other experimental conditions were employed, the [6,6,6]ferrocenephane (L-2) was also obtained. Both compounds were characterised by single crystal X-ray crystallography. The protonation of L-1 and its metal complexation were evaluated by the effect on the electron-transfer process of the ferrocene (fc) unit of L-1 using cyclic voltammetry (CV) and square wave voltammetry (SWV) in anhydrous CH3CN solution and in 0.1 M (Bu4NPF6)-Bu-n as the supporting electrolyte. The electrochemical process of L-1 between 300 and 900 mV is complicated by amine oxidation. On the other hand, an anodic shift from the fc/fc(+) wave of L-1 of 249, 225, 81 and 61 mV was observed by formation of Zn2+, Ni2+, Pd2+ and Cu2+ complexes, respectively. Whereas Mg2+ and Ca2+ only have with L-1 weak interactions and they promote the acid-base equilibrium of L-1. This reveals that L-1 is an interesting molecular redox sensor for detection of Zn2+ and Ni2+, although the kinetics of the Zn2+ complex formation is much faster than that of the Ni2+ one. The X-ray crystal structure of [(PdLCl2)-Cl-1] was determined and showed a square-planar environment with Pd(II) and Fe(II) centres separated by 3.781(1) angstrom. The experimental anodic shifts were elucidated by DFT calculations on the [(MLCl2)-Cl-1] series and they are related to the nature of the HOMO of these complexes and a four-electron, two-orbital interaction.

Humid periods in southern Arabia: windows of opportunity for modern human dispersal.

Arabia is a key area for the dispersal of anatomically modern humans (AMH, Homo sapiens) out of Africa. Given its modern hostile environment, the question of the timing of dispersal is also a question of climatic conditions. Fresh water and food were crucial factors facilitating AMH expansions into Arabia. By dating relict lake deposits, four periods of lake formation were identified: one during the early Holocene and three during the late Pleistocene centered ca. 80, ca. 100, and ca. 125 ka. Favorable environmental conditions during these periods allowed AMH to migrate across southern Arabia. Between ca. 75 and 10.5 ka, arid conditions prevailed and turned southern Arabia into a natural barrier for human dispersal. Thus, expansion of AMH through the southern corridor into Asia must have taken place before 75 ka, possibly in multiple dispersals.