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  • self-assembly  (23)
  • mass spectrometry  (13)
  • 1995-1999  (35)
  • 1
    ISSN: 0947-6539
    Keywords: gas-phase chemistry ; mass spectrometry ; periodic trends ; peroxides ; transition-metal ions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bimolecular gas-phase reactions of d-block transition-metal cations M+ with dimethyl peroxide were screened by means of Fourier transform ion cyclotron resonance mass spectrometry. The rich chemistry can be classified into four types of reactions: i) Oxygen-atom transfer to generate MO+, ii) elimination of radicals, mostly CH3O·, iii) intramolecular redox reaction of dimethyl peroxide to form CH3OH, CH2O and CO, and iv) charge transfer from the metal cation to produce CH3OOCH3+. Some general trends became apparent from this study. For example, the “early” transition metals almost exclusively induce oxygen transfer to generate MO+, in line with the notoriously high oxophilicities of these metals, and electron transfer is only observed for Zn+ and Hg+. Both the radical loss and the disproportionation reaction emerge from a rovibrationally highly excited insertion intermediate (CH3O)2M+, and for the first-row metals the branching ratio of the competing processes seems to be affected by the M+—OR bond strengths as well as the electronic groundstate configurations of M+. For the 4d and 5d cations Ru+—Ag+ and Pt+—Au+, respectively, products resulting from intramolecular redox reactions dominate; this probably reflects the higher propensity of these metal ions to facilitate β-hydrogen atom shifts.
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  • 2
    ISSN: 0947-6539
    Keywords: catenanes ; polycatenanes ; polyrotaxanes ; rotaxanes ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-assembly of three bis[2]catenanes and a bis[2]rotaxane, by two complementary strategies, is reported. A synthetic route to derivatives of bis-para-phenylene[34]crown-10 (BPP34C10) and 1,5-naphtho-para-phenylene[36]-crown-10 (1/5NPP36C10) containing a fused five-membered ring with a secondary amine function is described. These intermediate N-allylimido macrocyclic polyethers undergo template-directed reactions with 1,1′-[1,4-phenylenebis-(methylene)]bis-4,4′-bipyridinium bis-(hexafluorophosphate) and 1,4-bis(bromo-methyl)benzene to produce [2]catenanes containing an N-allyl functionality. The N-allylimido macrocyclic polyethers have also been reduced and deprotected to afford macrocycles possessing a free NH group, which are then linked through a 4,4′-biphenyldicarbonyl spacer to produce bis(crown ether)s, in which each crown ether moiety has two recognition sites. These ditopic BPP34C10 and 1/5NPP36C10 derivatives are capable of sustaining self-assembly reactions at both recognition sites to yield bis[2]catenanes. The self-assembly of a complementary bis[2]catenane, in which two tetracationic cyclophanes are linked together with a flexible hexyl chain, has also been achieved by treating 1,1′-[1,4-phenylenebis(methylene)]bis-4,4′-bipyridinium bis-(hexafluorophosphate) with a compound containing two linked 1,4-bis(bromomethyl)benzene units in the presence of BPP34C10. Replacing BPP34C10 with a dumbbell-shaped compound containing a linear polyether unit intercepted by a naphthalene residue and terminated by two bulky adamantoyl groups has led to the self-assembly of a bis[2]rotaxane. The X-ray crystal structures of one of the catenanes and its associated crown ether component are reported, together with solution state dynamic 1H NMR spectroscopic studies, showing that there is substantial degree of order characterizing the molecular structure of the catenanes.
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  • 3
    ISSN: 0947-6539
    Keywords: clusters ; gas-phase chemistry ; ionmolecule reactions ; iron complexes ; mass spectrometry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the gas phase reactions of small Fe+n clusters (n = 2-10) with O2, N2O and CO2 in an FT-ICR mass spectrometer. Under our experimental conditions, clusters of all sizes reacted readily with O2 and all but the dimer reacted with N2O. Only the smallest Fe+n clusters (n = 2-4) appeared to activate CO2. For all X-O molecules (X = O, N2, CO), reaction pathways were observed that include the transfer of O atoms. In addition, the reactions with O2 and N2O were accompanied by the loss of one or two Fe atoms. Thermochemical considerations based upon the well-known X-O bond energies were used to calculate Fen-O+ bond dissociation energies (BDEs) for sizes n = 2-6; these amount to roughly 550 kJ mol-1 and thus are considerably higher than the BDE of the Fe-O+ ion. All oxidation reactions of the Fe+n clusters (n = 2-6) studied in more detail were terminated by products of FexOx+ stoichiometry (x = 1-4). These “microoxides of iron” are not able to activate any further X-O bonds. Secondary reactions of FexOx+ clusters with C6H6, C2H4 and NH3 were investigated for two selected sizes (x = 2, 3) and compared with reactions of the naked Fe+n clusters.
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  • 4
    ISSN: 0947-6539
    Keywords: lactam ; macrocycles ; rotaxanes ; self-assembly ; template syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two new rotaxane types, the [2]rotaxanes 12a,b and the [3]rotaxane 13, have been synthesised by treating the “axle” 7 with the stopper component 9 in the presence of the “wheel” 8. A nonionic template intermediate of type 10 is proposed. The [3]rotaxane 13 was only obtained when the chain of the “axle” had a certain length: with n = 1 only [2]rotaxane 12 a was isolated, whereas with n = 2 the [3]rotaxane 13 was formed besides the [2]rotaxane 12 b. This suggests that more extended rotaxanes and polyrotaxanes can be synthesised by the template strategy.
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  • 5
    ISSN: 0947-6539
    Keywords: carbenes ; density functional calculations ; mass spectrometry ; thiamin ; ylides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 1, 2-hydrogen shift isomers of neutral (singlet and triplet) thiazole (1) and its radical cation have been investigated by a combination of mass spectro-metric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabilities of selected C3H3NS and C3H3NS.+ isomers and transition state structures. Although 3H-thiazole-2-ylidene (2) is less stable than 1, by 31.5 kcalmol-1, it is expected to be capable of independent existence, since the 1, 2-hydrogen shift from carbon to nitrogen involves a very large energy barrier of 72.4 kcalmol-1. The other 1, 2-hydrogen shift reaction from C(2) leads not to the expected cyclic 1H-thiazole-2-ylidene structure (3), which is apparently unstable, but rather to the ring-opened species HSCH=CHNC (4), which is 34.5 kcalmol-1 higher in energy than 1. The barrier in this case is lower but still large (54.9 kcalmol-1). The triplet ground states of 1, 2 and 4 are considerably destabilised (69.5, 63.2 and 58.7 kcalmol-1) relative to their singlet states. Interestingly, in addition to 2.+ and 4.+, the cyclic radical cation 3.+ is predicted to be stable although it is substantially higher in energy than ionised thiazole 1.+ (by 53.9 kcalmol-1), whereas 2.+ and 4.+ are much closer in energy (only 10.2 and 27.0 kcalmol-1 higher, respectively). Dissuading 2.+ and 3.+ from isomerising to 1.+ are energy barriers of 52.6 and 15.3 kcalmol-1, respectively. Experimentally, dissociative ionisation of 2-acetylthiazole enabled the generation of 2.+, which could be differentiated from 1.+ by collisional activation mass spectrometry. Reduction of the ylide ion 2.+ in neutralisation-reionisation mass spectrometry experiments yielded the corresponding neutral molecule 2. This direct observation of a thiazolium ylide provides support for postulates of such species as discrete intermediates in a variety of biochemical transformations.
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  • 6
    ISSN: 0947-6539
    Keywords: chelate ligands ; hydrogen bonds ; ligand design ; mass spectrometry ; stability constants ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of compounds containing the mononuclear complexes [M(tdci)2]3+ (tdci = 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol, M = Al, Fe, Ga, In) and [M(tdci)2]4+ (M = Ti, Ge, Sn) was prepared and characterized by elemental analysis, NMR spectroscopy, and FAB mass spectrometry. Characteristic fragmentation reactions in the mass spectra were elucidated. X-ray analysis of the AlIII, FeIII, GaIII, and InIII complexes revealed that two neutral, zwitterionic tdci ligands coordinate to the metal cation exclusively through deprotonated alkoxo groups. The six coordinated oxygen donors and the six N—H protons form a hydrophilic pocket, whereas the two cyclohexane rings and the twelve methyl groups form two hydrophobic shells. The hydrophilic pocket is filled with twelve water molecules, which are arranged as a second and a third coordination sphere around the metal cation. The reactivity in aqueous solution was investigated by potentiometric measurements. The bis complexes proved to be stable at pH 7. The evaluated formation constants show an increase of stability in the order AlIII〈InIII〈GaIII〈FeIII. The measurements established that tdci is one of the most effective tridentate ligands for small (r ≤ 0.8Å) and highly charged cations. The different chelating properties of tdci and of the unmethylated 1,3,5-triamino- 1,3,5-trideoxy-cis-inositol are discussed in terms of different steric requirements and different types of solvation of the corresponding complexes in aqueous solution.
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  • 7
    ISSN: 0947-6539
    Keywords: alkylthiols ; gold ; nanoparticles ; phase transitions ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Self-assembled monolayers (SAMs) of n-alkanethiolates on gold, silver, and copper have been intensively studied both as model organic surfaces and as modulators of metal surface properties. Sensitivity restrictions imposed by monolayer coverage and the low surface area of planar metal substrates, however, limit the characterization of these films in molecular terms to surface enhancement techniques. As a result, key aspects such as film dynamics and alkyl chain ordering remain ill-defined. The characterization of the thermal behaviour of SAMs is important not only for the design of stable, well-ordered organic superlattices, but also for the fundamental understanding of the factors that drive molecular interactions in two dimensions. Phase properties in SAMs have been addressed here through the synthesis of gold nanoparticles of 20-30 Å in diameter and fully covered with alkylthiol chains. These thiolmodified gold nanoparticles with large surface areas have enabled the monolayer film structure to be uniquely characterized by transmission FT-IR spectroscopy, NMR spectroscopy, and differential scanning calorimetry. Our studies reveal that for long-chain thiols (≥ C16), the alkyl chains exist predominantly in an extended, all-trans ordered conformation at 25°C. Furthermore, calorimetry, variable temperature transmission FT-IR spectroscopy, and solid-state 13C NMR studies have established that a cooperative chain melting process occurs in these alkylated metal colloids. How this arises is not immediately evident, given the relation between the extended chain conformation and the geometry of the spherical nanoparticles. Transmission electron microscopy (TEM) reveals that adjacent gold particles are separated by approximately one chain length; this suggests that chain ordering arises from an interdigitation of chains on neighboring particles. The thermotropic behavior is sensitive to the alkyl chain length and chain packing density. The alkylated nanoparticles can thus serve as a highly dispersed analogue to the much-studied planar SAMs.
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  • 8
    ISSN: 0947-6539
    Keywords: bridging ligands ; chelate ligands ; copper complexes ; self-assembly ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The “self-assembly” reaction between bischelating 2,2′-bipyrimidine, metallic copper, Cu2+, and two equivalents of bidentate Ph2P(CH2)nPPh2 produces crystallographically characterized dinuclear complexes with a [Cu1(μ-bpym)-Cu1]2+ core and chelating (n = 5; type I) or bridging diphosphine ligands (n = 6, 8; type II). Structures II may be designated as “inverse cryptate” arrangements. The structural data of the solids and the spectroscopic results for the dissolved species indicate a strained configuration in the case of complex [(μ-bpym)Cu2{μ-Ph2P-(CH2)n]2+ (6) with n = 6. Once the polymethylene chain length is reduced one notch further to n = 5 (5), the diphosphines can no longer bridge but help to form eight-membered chelate rings (structure I). In this complex there is a distortion towards a trigonal-pyramidal coordination geometry at the copper(I) centers, which is supported by the formation of an “organic sandwich” arrangement between the central bpym acceptor and two phosphino phenyl rings.
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  • 9
    ISSN: 0947-6539
    Keywords: catenanes ; macrocycles ; rotaxanes ; self-assembly ; tetrathiafulvalenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general stepwise approach is described for the preparation of tetrathiafulvalene (TTF)-based linear and monoand dimacrocyclic compounds incorporating one or two 1,4-dioxyphenylene, 9,10-dioxyanthrylene, or 1,5- or 2,6-dioxynaphthylene units from readily available starting materials. By utilizing the π-π stacking interactions of the TTF unit with the dipyridinium dication of 1,1′-[1,4-phenylenebis (methylene)] bis-4,4′-bipyridinium bis(hexafluorophosphate), a rotaxane and two [2]catenanes were synthesized starting from the linear and monomacrocyclic compounds, respectively. From the dioxyphenylene-based dimacrocycle, three [3]pseudocatenanes (trans, cis, and a mixture of cis/trans isomers) were obtained with the trans compound as the major product. From the dioxyanthrylene dimacrocycle, only the trans-[3]pseudocatenane was obtained. Catenane products were formed quantitatively from the 1,5-dioxynaphthylene dimacrocycle in a template-directed reaction, affording a trans-[3]pseudo-catenane together with a [4]pseudocatenane (mixture of cis/trans isomers). From the 2,6-dioxynaphthylene dimacrocycle, a cis-[3]pseudocatenane was obtained as the major product and a trans-[3]pseudocatenane as the minor one. For the [3]pseudocatenanes (i.e., both the cis and trans catenanes), in which the TTF units were clamped by the tetracationic macrocycle, isomerizations were completely prevented even in the presence of trifluoroacetic acid. All new rotaxanes and catenanes were characterized by electrospray mass spectrometry, and the cis- and trans- [3]pseudocatenanes were additionally investigated by 1H NMR spectroscopy. The electrochemical and spectral properties of the rotaxane and the catenanes are reported. Catenane formation increases the redox potentials of the TTF unit. The results demonstrate the versatility of TTF as a building block in the construction of supramolecular structures.
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  • 10
    ISSN: 0947-6539
    Keywords: crown ethers ; dialkylammonium salts ; hydrogen bonding ; molecular recognition ; pseudorotaxanes ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A very simple self-assembling system, which produces inclusion complexes with pseudorotaxane geometries, is described. The self-assembly of eight pseudorotaxanes with a range of stoichiometries-1:1, 1:2, 2:1, and 2:2 (host:guest)-has been achieved. These pseudorotaxanes self-assemble from readily available components-well-known crown ethers, such as dibenzo[24]crown-8 and bis-p-phenylene[34]crown-10, and secondary dialkylammonium hexafluorophosphate salts, such as (PhCH2)2NH+2PF-6 and (nBu)2NH+2PF-6-and have been characterized not only in the solid state, but also in solution and in the “gas phase”. The pseudorotaxanes are stabilized largely by hydrogen-bonding interactions and, in some instances, by aryl-aryl interactions.
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