Sandwich Arrangements Research Articles

Interaction of manganese corroles with TCNQ and related acceptor molecules

Manganese corroles with the Mn atom in the oxidation states +III and +IV have been probed as donor moieties for supramolecular stacked donor–acceptor complexes with the typical acceptor units TCNQ (tetracyanoquinone), TCNP (tetracyanopyrazine), and TCNB (tetracyanobenzene). Four new compounds formed as single crystals from different co-crystallization attempts. In those cases where a Mn(III) corrole was used as the donor component, hydrolyzed and/or oxygenated compounds [(cor)Mn(TCNQ*)] and [(cor*)Mn(TCNP*)] were obtained as the exclusive products. With chloridomanganese(IV) corroles, sandwich-like 2:1 complexes [(cor)MnCl][Formula: see text][TCNQ] and [(cor)MnCl][Formula: see text][TCNB] form, with both components left intact. Crystallographic analyses reveal partial or complete charge transfer to unusual axial ligands and thus prove the high reactivity of Mn(III) corroles in the first two cases. For the sandwich arrangements, almost-unaltered geometric parameters of the Mn(IV) corroles are observed, pointing to negligible structural consequences for metal corroles when engaged in stacking interactions.

Synthesis and structures of tris(2-pyridyl)aluminate sandwich compounds [{RAl(2-py')2}2M] (py' = 2-pyridyl, M = Ca, Mn, Fe).

Reactions of the lithium salts [{RAl(2-py')3Li·THF] [2-py' = 2-py (2-pyridyl), R = (n)Bu (1), R = (sec)Bu (2); py' = 5-Me-2-py (5-methyl-2-pyridyl), R = Me (3); 6-Me-2-py (6-methyl-2-pyridyl), R = Et (4b)] with the corresponding metal(ii) halides give the new heterobimetallic sandwich compounds [{RAl(2-py)3}2M] [R = (n)Bu, M = Ca (5), Mn (6), Fe (7); R = (sec)Bu, M = Ca (8), Mn (9)], [{MeAl(5-Me-2-py)3}2Ca] (10) and [{EtAl(6-Me-2-py)3}2Ca] (11) and the co-complex [{EtAl(6-Me-2-py)3}Mn(μ-Cl)Li{(6-Me-2-py)3AlEt}] (12). While neither the bridgehead group (R) nor remote ring Me-groups have any impact on metal coordination in 5-10, the introduction of Me groups into the pyridyl substituent at the 6-position (i.e., adjacent to the donor pyridyl-N atoms) has a marked effect on the ability of the ligands to form sandwich arrangements, as seen in the distorted structure of the sandwich compound 11 and in the formation of the co-complex 12, consisting of a two half-sandwich arrangement linked by a μ-Cl ion. The syntheses and solid-state structures of the new precursor 4b and the new compounds 5-12 are reported.

Electronic characterization of all-thiophene conducting dendrimers: Molecules and assemblies

The molecular and electronic structure of all-thiophene dendrimers in both the neutral and oxidized states have been investigated performing quantum mechanical calculations on systems of up to 30 rings. Results evidenced that the repulsive steric interactions between the neighboring thiophene rings induce significant distortions from the planarity independently of the electronic state. On the other hand, the ionization potential per thiophene ring and the lowest π–π∗ transition energy decreases with the inverse of the longest α-conjugated chain of the dendrimer, i.e. when the generation increases. The lowest π–π∗ transition energy predicted for an infinite generation dendrimer is 2.08eV indicating that these materials are potential candidates to be used in optoelectronics. Additionally, Quantum mechanics/molecular mechanics calculations have been performed considering both the sandwich and T-shaped supramolecular arrangements. Results showed not only the stability of these aggregates but also the significant influence of the intermolecular electronic delocalization in the electronic properties of these materials.

Charge carrier transport anisotropy in ultrananocrystalline diamond films

The optoelectronic properties of ultrananocrystalline diamond films (UNCD) grown using N 2 = 0 and 5% in the deposition gas mixture, are investigated by transient photocurrent measurements under nanosecond light pulses, both in planar and sandwich contact arrangements. Independent of contact configuration and N 2% value, very similar characteristic times in the 6-7 ns range are detected in the nanosecond range, reflecting a homogeneous distribution of states responsible for such decay times. On a longer time scale, nitrogen addition appears to slow down carrier transport promoting trapping and detrapping processes responsible for single and two power law photocurrent decays in films deposited using N 2 = 5% for sandwich and planar contact arrangements, respectively. Such a result suggests a nitrogen induced transport anisotropy tentatively related to structural modifications occurring at relatively low N 2%.

Separation of proteins using sandwich membranes

The effect of sandwich configurations of paired ultrafiltration membranes on the transmission and rejection of proteins were evaluated using various sandwich arrangements. Data is presented for the fractionation of BSA and myoglobin, and for that of lysozyme and myoglobin. Two membranes were sandwiched together in several different configurations without spacer. The sandwich configurations were known as SS-sandwich, BS-sandwich, BB-sandwich and SB-sandwich, where S indicates that the skin layer faces the feed and B indicates that the support layer faces towards the feed. The result of single membrane transmission was compared with the transmission of the various sandwich arrangements. Interestingly not having a skin layer on top and in contact with the feed (as in BS and BB sandwich configurations) was useful for enhancing protein transmission whilst SS and SB configurations can be used to increase protein rejection. For the separation of lysozyme and myoglobin, a selectivity value of 9 was found for S, SS and SB configurations The effect of sandwich membranes varies not only with MWCO but also, to a small extent, with different types of membrane material. For example, the transmission of both proteins using single membranes is comparatively higher than the sandwich membrane for RC and PES membranes, but this was not found to be so for YM membranes.

Rejection amplification in the ultrafiltration of binary protein mixtures using sandwich configurations

Protein fractionation of lysozyme and myoglobin mixtures was investigated using sandwich arrangements of ultrafiltration membranes under controlled permeate flux. Both proteins have close molecular weights with values of 14300 g mol −1 for lysozyme and 17400 g mol −1 for myoglobin. The aim was to improve the selectivity of the separation by masking the pore size distribution of ultrafiltration membranes by having one membrane on top of another. Two membranes were arranged together in sandwich configurations. Two possible sandwich configurations were compared by assessing the transmission and selectivity for the protein mixture. For both sandwich configurations membrane fouling was reduced compared with a single membrane. Overall the S-sandwich with Biomax PES membranes was superior.

Effect of sandwich configuration of ultrafiltration membranes on protein fractionation

Protein fractionation using ultrafiltration is feasible after extensive process optimization, yet the poor selectivity of membranes is still one of the critical factors limiting the application of membrane systems to protein fractionation. Our present work on protein fractionation uses sandwich ultrafiltration membranes by exploiting the membrane pore size and the pore distribution characteristics of the membranes. The aim is to improve the selectivity of the separation by achieving a pure protein product in the permeate from a binary protein mixture. Two membranes of similar or different MWCO combination (30, 50 and 100 kDa) are stacked together in various sandwich arrangements. The proteins with molecular weight between 10 and 70 kDa (Lysozyme, Myoglobin and BSA) are used as model proteins in single and binary mixtures. The observed transmissions of Lysozyme with sandwich of the same membrane of 30/30, 50/50, and 100/100 kDa are 78, 85 and 97%, respectively; and these values are comparable to the transmission for the respective single membrane, being consistently slightly lower. Similar trends are being found for BSA.

Monte Carlo Studies of Isomers, Structures, and Properties in Benzene−Cyclohexane Clusters: Computation Strategy and Application to the Dimer and Trimer, (C6H6)(C6H12)n,n= 1−2

Low-temperature isomeric energies, structures, and properties of benzene-cyclohexane clusters are investigated via Monte Carlo simulations. The Monte Carlo strategy is first documented and then applied to (C(6)H(6))(C(6)H(12)) and (C(6)H(6))(C(6)H(12))(2) using four different potential energy surfaces. Results identify a single parallel-displaced dimer isomer. MP2 optimizations and frequency calculations support the Monte Carlo dimer structure and identify the van der Waals mode observed in vibronic spectra. Caloric simulations identify two temperatures where structural transitions occur and imply an experimental temperature below 10 K for dimers in cold supersonic expansions. The (C(6)H(6))(C(6)H(12))(2) studies identify eight independent trimer isomers: three form parallel-stacked (sandwich) arrangements with the two cyclohexane moieties related through a D(6)(h) transformation. The remaining five trimer isomers are trigonal, with no overall symmetry. Caloric studies indicate that the sandwich and trigonal isomeric classes coexist independently below 60 K, consistent with trimer vibronic spectra that contain two independent van der Waals progressions.

Sandwich‐artige Cyclopentadienylkomplexe von Hauptgruppenelementen

AbstractCyclopentadienyl (Cp) complexes of maingroup elements are classified and compared in terms of structural types, bonding features, and characteristic reactions. Halfsandwich, sandwich and multidecker sandwich arrangements of different type are observed including cluster species with weak or strong metal‐metal bonds. The Cp‐El bonding ranges from mainly covalent to purely ionic with important structure‐determining intermolecular contacts in the latter. π‐Complexation allows the stabilisation of compounds with low‐valent elements such as AI(I), Ga(I), and Si(I1). Important reaction principles and applications are shortly described.

Sieving characteristics of multilayer ultrafiltration membranes

Almost all commercial ultrafiltration membranes have an asymmetric structure. However, the unusual transport characteristics of these asymmetric, or multilayer, structures has not been widely appreciated. The general equations describing solute transport through a multilayer membrane are developed and used to examine the physical basis for the unusual transport properties of these membranes. Experimental data are obtained for the transport of polydisperse dextrans through asymmetric and symmetric ultrafiltration membranes, both alone and in sandwich (series) combinations. The data clearly show that the sieving characteristics of these multilayer membranes are dependent upon both the detailed properties and the orientation of the different layers. The sieving coefficients for traditional asymmetric membranes are shown to increase by well over an order of magnitude when used in the reverse (skin-side down) direction. Experiments with appropriate sandwich arrangements demonstrate that these composite structures can have higher selectivities than conventional membranes. The implications of these transport phenomena are examined in several different ultrafiltration applications.

Benzene Dimer: A Good Model for π−π Interactions in Proteins? A Comparison between the Benzene and the Toluene Dimers in the Gas Phase and in an Aqueous Solution

We have investigated the relative orientational preference of two benzene and two toluene molecules in a vacuum and in water, by means of free energy calculations. The gas-phase simulations reveal that, whereas the T-shaped benzene dimer is 0.78 kcal/mol lower in free energy than its stacked homologue, the sandwich arrangement of the toluene dimer is preferred over the T-shaped structure by 0.18 kcal/mol. MP2/TZP ab initio binding energies, evaluated for both dimers, were found to be consistent with the molecular mechanical estimates, hence suggesting that the van der Waals and the electrostatic contributions to the macromolecular force field employed herein are well balanced. We further note that our results agree quite nicely with the experimental binding energies of Neusser and Krause, obtained from breakdown measurements. The tendency witnessed in the gas phase is magnified in an aqueous solution, with differences in free energy between the T-shaped and the sandwich arrangements of the benzene and the toluene dimers equal to −1.47 and 1.12 kcal/mol, respectively. The calculated association constants and osmotic second virial coefficients also correlate reasonably well with the experimental data of Tucker and Christian. The conflict between the orientational preferences of the benzene and the toluene dimers is suggestive that trends in “π−π” interactions in proteins should be rationalized by other factors than simple electrostatic/dispersion considerations. The analysis of Phe−Phe pairs in protein crystallographic structures sheds light on the influence of both sterical hindrances and ancillary interactions between the aromatic moities and neighboring functional groups on the orientational preference of the phenyl rings.

Normal projectile penetration and perforation of layered targets

The response to normal impact of hard-steel blunt and conically-nosed projectiles on multi-layered plates of soft aluminum, both adjacent and spaced, as well as on adjacent disks of thin aluminum and polycarbonate, including sandwich arrangements, was studied experimentally in the subordnance velocity regime in the vicinity of the ballistic limit. A comparison of the penetration resistance of these targets with corresponding monolithic plates of aluminum or polycarbonate has been effected on the basis of aereal density. The ballistic resistance of the metallic monolithic target was found to be greater than that of several adjoining plates of the same thickness; this was considered to be due to the greater bending resistance of the former. On the other hand, a 15% greater ballistic resistance of the polycarbonate target was observed than for the corresponding aluminum disk. In several cases where an adjacent aluminum layer preceded the polymer, a terminal deflection of the aluminum plate in the direction opposite to that of the striker travel was observed, which was found to be due to the change in the penetration mode of the metal from petalling to extrusion. Elastic energy stored in the polycarbonate was returned to the metal cover, causing not only this rearward deflection, but also vibrations. When this occured at the ballistic limit, the period of these oscillations was found to correspond closely to that of a plate with a distributed mass with the projectile mass concentrated at its center. Wave propagation effects were not considered in the analysis, but also contribute to this reversed deflection. The experimental ballistic limits for monolithic targets of the two materials were compared to predictions of other investigators for both the blunt and conically-tipped strikers, derived from simple models of the process. The expected ballistic limit for combined targets was calculated from an energy approach with the aid of relations for this parameter based on the above models for a single material. Satisfactory correspondence was obtained between the test results and calculated values.

A New Type of Separator for High Temperature Water Electrolyzers

Perforated nickel sheets can be covered with polytetrafluoroethylene (PTFE Teflon) and used as diaphragms in water electrolyzers. The technique used consists of filling the sheet holes with camphor, spraying PTFE latex on them, and allowing the solid camphor to evaporate. Since the holes are left unclogged, the membrane has good ionic conductivity. However, electronic transport, because the nickel surface is covered by Teflon, is negligible. The hydrophobicity imparted by PTFE to the metal surface has a beneficial effect on gas bubble release during electrolysis. Insulating characteristics of the PTFE coating make this diaphragm adequate for use in sandwich arrangements for bipolar cells.

On the Limits of the Filter Concept for Color TV Screens

The image contrast of color television tubes is reduced by the diffuse reflection of ambient light at the tube face. A recent method used to compensate this effect makes use of color filters, which are transparent to the phosphor light and absorb the rest of the visible spectrum. The subject of this paper is an investigation of the fundamental optical capability of the color filter concept. The main emphasis is placed on the sandwich system, involving a filter layer between the phosphor layer and the screen glass. Experiments were performed on test samples, using a spectrophotometer for the reflection measurements and a scanning electron microscope for measuring the phosphor light output. They yield a gain in brightness contrast performance (BCP) of 15–28% for combinations of red or blue phosphors with commercially available inorganic filter pigments. The corresponding increase of brightness can be twice as high if the contrast is held constant. Model calculations, assuming isotropic light scattering within the powder layers and linear superposition of multiple reflections between adjacent layers, show good agreement with the experimental results. Ideal square filters yield a BCP gain of 100–120%, whereas a shift of real phosphor spectra or filter spectra along the wavelength axis only yields a gain of a few percent. The conclusion drawn is that the inorganic filter pigments used set a practical limit to the gain in BCP that can be achieved with the color filter concept. Some results on mixtures of phosphors and filters are included. The experimentally found BCP gain amounts to about 7–10%, i.e., only about one‐half of the sandwich gain. A simplified model leads to approximately the same values. The different optical behavior of a color pigment in sandwich and mixture arrangements is apparently due to differences in the effective optical path length for ambient light and phosphor light.

Electrochromism of merocyanine dyes as detector of internal charges in sandwich arrangements with molecular crystals

The photosensitized charge transfer reaction from a merocyanine dye laser to a p-chloranil single crystal is studied. It was found that the maximum of the action spectrum of the photocurrent shifts by several nanometers when the externally applied voltage or the illuminating light level is increased. A model is proposed which interprets these observations as being due to the induced electric field of a layer of ionized dye molecules at the very surface of the single crystal.

Positron lifetimes in solid and liquid mercury

Consistent and plausible values for positron lifetimed in solid and liquid mercury are obtained with both conventional sandwich and amalgam source-sample arrangements.