Jahn—Teller effects in the mindo approximation: structures of the molecular cations of methane and the chloromethanes

The MINDO/3 technique gives geometries for (CH₄)⁺, (CCl₄)⁺ and the intermediate ions (CH_nCl_{4 ? n})⁺ (n = 1, 2, 3) which have symmetries in precise accord with the predictions of the Jahn—Teller effect. The ground state of (CH₄)⁺ has D_2d symmetry, with a C_3v structure ca. 45.6 kJ mol^?1 higher. (CCl₄)⁺ has a C_2v ground state, with a D_2d structure ca. 144 kJ mol^?1 higher: no bound state of C_3v symmetry could be found. (CH₃Cl)⁺ and (CHCl₃)⁺ both have C_s symmetry, and (CH₂Cl₂)⁺ has C_2v symmetry. The analogous fluoro ions are discussed briefly.     

Unnatural multidentate metal ligating α-amino acids

A family of penta- and hexadentate metal ligating α-amino acids, suitably protected for Fmoc solid-phase chemistry, has been prepared. These residues incorporate the mono-amides of ethanolaminetriacetic acid, ethylenediaminetriacetic acid, and ethylenediaminetetraacetic acid as side chains. Side chains are tethered varying distances (n) from the Cα-carbon to allow metal binding events to occur at distinct distances from the peptide backbone. These residues are designed to allow the facile installation of metal chelates along a peptide backbone.     

Voltammetric studies of microcrystalline C60 adhered to a carbon electrode surface and placed in contact with aqueous electrolyte containing potassium ions

The reduction of microcrystalline C₆₀ fullerene, adhered at a carbon electrode and immersed in aqueous electrolyte, has been studied under various voltammetric conditions. This work reports mainly the voltammetric studies carried out principally in electrolyte containing potassium ions. Comparison of adherence techniques, such as solvent casting and mechanical transfer methods, are made to assess if the type of adhered techniques has any significant influence on the observed electrochemistry. The solvent casting method is found to produce three peaks in the potential for C₆₀^0/n- redox couple as compared to a single and large peak produced when a mechanical transfer technique is employed. When the reduction potential of microcrystalline C₆₀ in the presence of K⁺ is compared with other cations, such as Li, Na, Rb and Cs, it is observed that the shift of reduction potential follows the change in the hydration energy in the order Cs>Rb>K>Na>Li. In a mixed electrolyte study of CsCl/KCl, the reduction potential and peak shape of C₆₀^0/n- redox couple during cyclic voltammetry is observed to change with concentration of the cations and the observed electrochemistry can be attributed to a cation-exchange mechanism. The reduction of C₆₀ is irreversible in aqueous electrolyte containing alkaline cations as the re-oxidation process does not produce any observed electro-activity. Evidence of the formation of a passive coating of K _n C₆₀ fulleride, which does not appear to undergo dissolution is obtained under cyclic voltametric conditions. This coating remains electrochemically active in the presence of tetrabutylammonium ions in acetonitrile. Scan rate, chronocoulometric, and scanning electron microscopic studies provide evidence of the presence of a surface process involving solid–solid transformation.     

Local and global chirality at surfaces: succinic acid versus tartaric acid on Cu110

A detailed comparison of tartaric acid (HOOC-CHOH-CHOH-COOH) and succinic acid (HOOC-CH(2)-CH(2)-COOH) molecules on a Cu(110) surface is presented with a view to elucidate how the two-dimensional chirality exhibited by such robust, chemisorbed systems is affected when both OH groups of the former molecule are replaced with H groups, a stereochemical change that leaves the metal-bonding functionalities of the molecule untouched but destroys both chiral centers. It is found that this change does not significantly affect the thermodynamically preferred chemical forms that are adopted, namely the doubly deprotonated bicarboxylate at low coverages (theta 相似文献   

Thermal latency of novel chelating borate catalysts as latent catalysts for epoxy–phenolic resins

Novel quaternary ammonium bis(2‐oxybenzoyloxy)borate salts ( 1a – 1c ) or quaternary ammonium bis(1,2‐benzenedioxy)borate salts ( 2a and 2b ) with tetra‐n‐butylammonium (TBA⁺), tetra‐n‐octylammonium (TOA⁺), or bis(triphenylphosphoranylidene)ammonium (PNP⁺) cations were synthesized as latent catalysts of epoxy/phenol–novolac resins by the complexation between boric acid and salicylic acid or catechol, followed by neutralization with quaternary ammonium hydroxide. Polyaddition reactions of diglycidyl ether of bisphenol A (DGEBA) and 4,4′‐bisphenol F (44BPF) or bisphenol F (BPF‐D) with the ammonium borates were investigated as model reactions of epoxy/phenol–novolac resin systems with respect to the thermal latency and storage stability of the catalyst. The polyaddition of DGEBA/44BPF with 1a – 1c in diglyme at 150 °C for 6 h proceeded up to 85–96% conversions and gave polymers with number‐average molecular weights of 4180–10,500, whereas the polyaddition at 80 °C for 6 h gave less than 8% conversions. However, the polyaddition with 2a containing TBA⁺ cation proceeded to only a 32% conversion at 150 °C for 6 h in diglyme and to a 64% conversion even at 180 °C for 6 h in triglyme and only gave low molecular weight oligomers, and no reaction proceeded in the polyaddition at 80 °C. However, polyaddition with 2b containing PNP⁺ cation proceeded up to a 96% conversion at 150 °C for 6 h in diglyme and gave a higher molecular weight polymer with a number‐average molecular weight of 8050, whereas the polyaddition at 80 °C for 6 h gave only a 5% conversion. The catalytic activity of ammonium borates 1a – 1c and 2a and 2b depended on the borate anion structure: 1a and 1c with bis(2‐oxybenzoyloxy)borate anion revealed higher activity than 2a and 2b with bis(1,2‐benzenedioxy)borate anion, respectively. In comparison with tetra‐n‐butylammonium bromide (TBAB) as a conventional ammonium salt or tetra‐n‐butylammonium tetrakis(benzoyloxy)borate (TBA‐TBB), 1a – 1c and 2b revealed better thermal latency. The catalytic activity of ammonium borates also depended on the bulkiness of the ammonium cation, and the order of activity was 1c (PNP⁺) > 1b (TOA⁺) ≧ 1a (TBA⁺) and 2b (PNP⁺) > 2a (TBA⁺). The storage stability of DGEBA/BPF‐D with the ammonium borate catalysts 1a – 1c and 2a and 2b in bulk at 40 °C was much better than that with TBAB and TBA‐TBB. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2702–2716, 2002     

Conversion of clavulanic acid into thiadeoxa nuclear analogues

1,4-Addition of sulphur nucleophiles to the diene ($\text{12}$) derived $\text{via}$ the pen-2-em ($\text{5}$) from clavulanic acid provides the thiadeoxa analogues ($\text{14}$–$\text{15}$). X-ray analysis of the ester ($\text{14}$) shows the thermodynamically stable isomers to have the same relative stereochemistry as clavulanic acid.     

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

Abstract— The aerobic dye-sensitized photooxygenation of monohydric phenols proceeds by way of singlet oxygen under the conditions studied. Various phenols give different proportions of reaction with and quenching of singlet oxygen. Para-substituted 2,6-di-t-butylphenols show a linear correlation between the log of the total rate of singlet oxygen removal and their halfwave oxidation potentials; the same correlation is given for certain phenol methyl ethers. A Hammett plot using s?⁺ gives ρ - 1.72 ± 0.12, consistent with development of some charge in the quenching step. Reaction of photo-chemically generated singlet oxygen with 2,4,6-triphenylphenol gives 2,4,6-triphenylphenoxy radical as an intermediate in singlet oxygen quenching, although no overall reaction occurs. Kinetic analysis indicates that the radical is derived exclusively from the interaction of 2,4,6-triphenylphenol with singlet oxygen. A charge-transfer mechanism for quenching of singlet oxygen by phenols is proposed.     

Nafion-PAN optical chemical sensor: optimization by FIA

A copper-sensitive optical chemical sensor (optrode) is described. The optrode is based on a Nafion membrane and an immobilized organic ligand coupled with a flow injection (FI) system. The FI system includes a flow-through removable measuring cell and a simple spectrophotometer. Owing to the miniature size of the system and the efficient use of optical fibers, this optrode is well suited for monitoring environmental water samples. The success of the described optrode system depends on the effectiveness of the FI reagent delivery system. Optimum contact time with the membrane (as determined by the reagent flow rates) and the injected sample volume are critical. Environmental water samples were analyzed for copper content using the optimized optrode system. To validate the optrode's performance, the same water samples were analyzed using the atomic absorption spectrophotometer.     

Screening of antagonists based on induced dissociation of a calmodulin-melittin interaction entrapped in a sol-gel derived matrix

Sol-gel derived materials offer a unique advantage for the development of sensing and screening platforms in that they allow for the entrapment of multiple species within a confined space. In this work, we show that it is possible to entrap an intact protein-peptide interaction, consisting of bovine calmodulin (bCaM) and melittin, into a sol-gel derived silicate material. Fluorescence emission data demonstrate that the entrapped complex behaves similarly to the complex in solution, and can undergo reversible dissociation upon introduction of the denaturant guanidine hydrochloride. Screening of antagonists of the bCaM-melittin complex was accomplished based on induced dissociation of the entrapped complex, which was followed by measuring the loss of sensitization of Tb(III) luminescence originating from energy transfer from the Trp of melittin to Tb(III) bound in the loops of bCaM. This study shows that entrapped protein-peptide complexes can be used as targets for drug screening or for fluorescence-based biosensing.     

Synthesis of a homoleptic niobium(V) thiolate complex and the preparation of niobium sulfide via thio "sol-gel" and vapor phase thin-film experiments

Reaction of [Nb(NMe(2))(5)] with 10 equiv of 2,6-Me(2)C(6)H(3)SH in toluene results in the formation of red crystals of [Nb(SC(6)H(3)Me(2)-2,6)(5)]. Crystal structure analysis of [Nb(SC(6)H(3)Me(2)-2,6)(5)] showed that the niobium center adopts a distorted trigonal bipyramidal geometry. Niobium disulfide, NbS(2), has been successfully prepared via a thio "sol-gel" process using [Nb(SC(6)H(3)Me(2)-2,6)(5)] as the metal source. In contrast, vapor phase thin-film studies revealed that [Nb(SC(6)H(3)Me(2)-2,6)(5)] functions as a single-source precursor to NbS films.