Syntheses, structures, and surface aromaticity of the new carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph, CH(2)SiMe(3)) and a stepwise functionalization of the inner and outer sphere of the cluster

The reaction of the acetylene RC triple bond CH (R = Ph, CH(2)SiMe(3)) with an excess of AlH(3).NMe(3) in boiling toluene leads to the carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph 1, CH(2)SiMe(3) 2) in good yield. Treatment of 2 with BCl(3) under varying conditions gives the chlorinated products [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] 3 and [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(2)Cl)(6)] 4, respectively. The latter clearly demonstrates that the cluster can be stepwise functionalized within the inner and outer sphere. The X-ray single-crystal structures of 1, 2, and 4 have been determined. All compounds have in common that the central core consists of a cluster having eight aluminum and six carbon atoms. The bonding properties in this cluster are described as a new manifestation of three-dimensional surface aromaticity. Each Al(4)C fragment of the cube is formed by four bonds with three electron pairs, thus leading to a strong delocalization of the electrons. A phenomenological modeling using a three-dimensional Hückel scheme with fitted parameters to reproduce the energies from ab initio calculations revealed that the orbital scheme localized at one Al(4)C fragment possesses an orbital sextet with a large HOMO-LUMO gap. This is in line with the criteria of aromaticity. The idea of aromaticity was sustained also by qualitative valence bond reasons enumerating the different resonance structures by means of graph theoretical methods.     

Photoelectron spectroscopy of isolated multiply negatively charged oligonucleotides

Ultraviolet photoelectron spectroscopy in an ion beam was used to investigate the electronic properties of isolated DNA oligonucleotides [dA(5)-4H](4-) and [dT(5)-4H](4-), carrying four excess negative charges. We find the fourth adiabatic electron affinity to be slightly negative for [dA(5)-4H](4-), while it is positive for [dT(5)-4H](4-). This implies a significant influence of the base composition on energetics, which is in turn relevant for analytic applications and also for charge transport properties.     

Total synthesis of dictyodendrin B

A concise total synthesis of dictyodendrin B (1) is reported, a scarce marine alkaloid endowed with promising telomerase inhibitory activity. Key steps of the chosen route are a reductive cyclization of ketoamide 11 to indole 12 mediated by low-valent titanium (from TiCl3 and KC8) followed by a photochemical 6pi-electrocyclization, which was performed in the presence of Pd/C and nitrobenzene to effect concomitant dehydrogenation/aromatization of the product initially formed. Regioselective bromination of the resulting pyrrolocarbazole 13 followed by lithium/bromine exchange and quenching of the resulting organolithium species with p-methoxybenzaldehyde installed the side chain at C2. Oxidation of the benzylic alcohol 15 thus obtained to ketone 17 was best achieved with catalytic amounts of tetra-n-propylammonium perruthenate (TPAP) and N-methylmorpholine-N-oxide (NMO) in dilute CH2Cl2 solution to avoid the formation of undue amounts of the unsymmetrical dimer 16. Ketone 17 was elaborated into the natural product by selective cleavage of the isopropyl ether with BCl3, introduction of the sulfate moiety with the aid of trichloroethyl chlorosulfuric acid ester, deprotection of all lateral methyl ether groups, and final reductive cleavage of the trichloroethyl ester moiety. The spectroscopic data of synthetic dictyodendrin B thus formed matched those of an authentic sample in all regards. Moreover, it was shown that global deprotection of the peripheral -OH groups in pyrrolo[2,3-c]carbazole 13 is accompanied by spontaneous air-oxidation to form the quinone core of dictyodendrin C.     

Protolytic surface reactions of a fluorapatite-maghemite mixture in aqueous suspension

Synthetically prepared maghemite and fluorapatite, characterized with BET, SEM, XRD, FT-IR, and FT-Raman, are used to investigate the protolytic properties and surface characteristics in a mixed system of maghemite and fluorapatite by means of potentiometric titrations and surface complex modeling. Titrations were performed in the pH range of 7.3-10.5 at 25 +/- 0.2 degrees C in ionic media of 0.10 mol dm(-3) NaNO3 with 0.0100 mol dm(-3) HNO3 and 0.0100 mol dm(-3) NaOH used as titrants. The constant capacitance model (CCM) was applied to interpret the titration data. Two models with different surface equilibria were tested. In the first model, the mixed system was treated as a one-component system with a total surface area of 40.04 +/- 5.2 m2 g(-1) without any consideration to the subsystems. The surface equilibria, triple bond XOH + H+ <==> triple bond XOH2+, beta(s)(-11)(int) = 6.74 +/- 0.07; XOH <==> triple bond XO- + H+, beta(s)(-11)(int) = -7.75 +/- 0.07, were found to represent an accurate model for the system, and the specific capacitance was optimized to 2.0 F m(-2). The number of active surface sites N(s) was found to be 1.2 sites nm(-2). This model has, however, no relation to the subsystems of maghemite and fluorapatite. The second model is related to the subsystems and displays the surface equilibria, triple bond S2OH<==> triple bond S2O- + H+, beta(s)(-101)(int) = -9.12 +/- 0.01; triple bond FeOH + H+<==> triple bond FeOH2+, lg beta(s)(-11)(int) = 6.80 +/- 0.01; triple bond FeOH<==>FeO- + H+, beta(s)(-11)(int) = -7.77 +/- 0.01, where S2OH is related to fluorapatite and FeOH is representing maghemite. Fluorapatite corresponds to the dominating active surface in the system. The specific capacitance was optimized to 18 F m(-2). The N(s) values were found to be 2.27 sites nm(-2) for fluorapatite and 0.80 sites nm(-2) for maghemite. The N(s) values together with evidence from the FT-Raman and SEM investigations reveal that interactions between maghemite and fluorapatite surfaces occur during the titration. The acid-base properties and surface characteristics of the subsystems maghemite-H+ and fluorapatite-OH- using the CCM have been published earlier.     

Conversion of sulphur dioxide in the atmosphere

Summary Pertinent, previous studies of the oxidation of SO₂ in the atmosphere are briefly reviewed. A project dealing with the conversion in the plume from an oil-fired power station is described in greater detail. Measurements were performed from an aircraft and included continuous registration of NO_x, SO₂ and ozone concentrations. The possibility of using NO_x as an internal tracer is discussed; also the use of the inert tracer SF₆ is treated and a special detector for the continuous registration of SF₆ in relative concentrations down to 10^–6 ppm is described. Preliminary results indicate a half-life for SO₂ in the plume of about half an hour.

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Umsetzung von Schwefeldioxid in der Atmosphäre

Zusammenfassung Einschlägige frühere Studien über die Oxidation von SO₂ in der Atmosphäre werden kurz diskutiert. Ein Projekt zum Studium der Umwandlung im Abrauch eines ölbefeuerten Kraftwerkes wird im Detail beschrieben. Die Messungen wurden mit Hilfe eines Flugzeuges vorgenommen und schließen die kontinuierliche Aufzeichnung von NO_x-, SO₂- und Ozon-Konzentrationen ein. Die Möglichkeit der Verwendung von NO_x als interner Indicator wird diskutiert. Außerdem wird der Gebrauch des inerten Indicators SF₆ behandelt und ein spezieller Detektor für die kontinuierliche Aufzeichnung von SF₆ mit relativen Konzentrationen bis herunter auf 10^–6 ppm beschrieben. Vorläufige Ergebnisse deuten auf eine Halbwertszeit von SO₂ im Abrauch von etwa 30 min hin.

Presented at the 6th Annual Symposium on Recent Advances in the Analytical Chemistry of Pollutants, April 21–23, 1976; Vienna, Austria     

Relaxometric study of copper [15]metallacrown-5 gadolinium complexes derived from alpha-aminohydroxamic acids

Proton nuclear magnetic relaxation dispersion (NMRD) profiles were recorded between 0.24 mT and 1.4 T for lanthanum(III)- and gadolinium(III)-containing [15]metallacrown-5 complexes derived from alpha-aminohydroxamic acids and with copper(II) as the ring metal. The influence of the different R-groups on the proton relaxivity was investigated, and a linear relationship between the relaxivity and the molecular mass of the metallacrown complex was found. The selectivity of the metallacrown complexes was tested by transmetalation experiments with zinc(II) ions. The crystal structure of the copper [15]metallacrown-5 gadolinium complex with glycine hydroximate ligands is reported.     

Theoretical study on solvation effects in chemical reactions: A vibrational coupling model

A vibrational coupling model to treat the solvation effects in chemical reaction rate calculations is proposed and applied to the intramolecular hydrogen transfer reaction CH₃O· → ·CH₂OH in the condensed phase. The effect of solvation is taken into account in two ways: (1) the solvent effect on the activation energy of the reaction is simulated by including 39 surrounding water molecules, represented by fractional charges at the assumed atomic positions, in the potential energy surface calculation; and (2) the vibrational couplings between the 10 nearest solvent molecules and the molecules constituting the reaction system are explicitly included in a vibrational frequency calculation. RRKM theory with Miller's tunneling correction included is employed to calculate the rate constants. The effect of solvation causes a significant change in the chemical reaction rate, mainly through a lowering of the activation energy. The vibrational coupling causes a slight increase of the rate constant in the tunneling region by perturbing the vibrational frequencies of the reactant and transition states, which appear in the rate-constant expression, but has little effect at higher temperatures.     

Chain-length dependence of the conformational order in self-assembled dialkylammonium monolayers on mica studied with soft X-ray absorption

Self-assembled alkyl chain based monolayers on mica are important for industrial and technological processes since they can be employed for an organic modification of the inorganic substrate. The conformational structure and orientational order of the films determine the interaction of the modified substrate with the environment and the chemical character and stability of its surface. We have studied the conformational order in ion exchanged dialkylammonium monolayers adsorbed on mica depending on the length of the alkyl chains systematically with near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In addition, films were characterized by water contact angle measurements. The experimentally determined average tilt angles of the chains are discussed in terms of the degree of order. It was found that the absolute number of gauche defects in the films increases with decreasing chain length.     

Self-assembling a molecular pegboard

This paper describes the design, construction, and application of a self-assembled, chemically addressable DNA nanogrid composed of DNA tiles of nanometer dimensions. This self-assembled structure permits precise placement of molecules at predetermined locations on a "molecular pegboard". We used the indexed DNA nanogrids to identify single molecules of DNA that hybridize at particular locations.     

Neutral and ionic aluminum,gallium, and indium compounds carrying two or three terminal ethynyl groups

The syntheses of the ionic compounds [Li(+).2 dioxane (2,6-iPr(2)C(6)H(3)N(SiMe(3))Al(C triplebond CSiMe(3))(3))(-)].0.75 dioxane (1), [(Li(+))(2).(dioxane)(7)](0.5) [2,6-iPr(2)C(6)H(3)N(SiMe(3))Ga(C triplebond CSiMe(3))(3)(-)].1.5 dioxane (2), and [(Li(+))(2).(dioxane)(7)](0.5) [2,6-iPr(2)C(6)H(3)N(SiMe(3))In(C triplebond CSiMe(3))(3)(-)].1.5 dioxane (3) by the reaction of the corresponding organo metal chloride with LiC triplebond CSiMe(3) are reported. The neutral ethynyl compounds Br-Al(C triplebond CtBu)(2).2 THF (4), Cl-Ga(C triplebond CtBu)(2).THF (5), Cl-In(C triplebond CtBu)(2).2 THF (6), Al(C triplebond CtBu)(3).C[N(Me)CMe](2) (7), Ga(C triplebond CtBu)(3).dioxane (8), and In(C triplebond CtBu)(3).NEt(3) (9) have been obtained in good yields from the reaction of AlBr(3), GaCl(3), and InCl(3) with LiC triplebond CtBu in the presence of a Lewis base. Compound 7 is the first heterocyclic carbene substituted ethynyl derivative. Aluminum and gallium compounds with three terminal ethynyl groups Al(C triplebond CPh)(3).NMe(3) (10) and Ga(C triplebond CPh)(3).NMe(3) (11) have been prepared by the reaction of AlH(3).NMe(3) or GaH(3).NMe(3) with three equivalents of phenylethyne. All the above-mentioned compounds have been structurally studied. In compound 1 the lithium ion is coordinated to the three terminal ethynyl groups, whereas in compounds 2 and 3 the lithium is coordinated to the solvent (dioxane). Compound 8 crystallizes as a coordination polymer with dioxane molecules bridging the individual gallium units.