Three-dimensional aromaticity in deltahedral boranes and carboranes

Methods derived from topology and graph theory indicate that the deltahedral boranes B _n H _n ^2– and the corresponding carboranes C₂B_n–2H _n (6 n 12) may be regarded as three-dimensional delocalized aromatic systems in which surface bonding and core bonding correspond to -bonding and -bonding, respectively, in planar polygonal two-dimensional hydrocarbons CinnH _n ^(n–6)+ (n=5/7). The two extreme types of topologies which may be used to model core bonding in deltahedral boranes and carboranes are the deltahedral (D _n ) topology based on the skeleton of the underlying deltahedron and the complete (K _n ) topology based on the corresponding complete graph. Analyses of the Hoffmann-Lipscomb LCAO extended Hückel computations, the Armstrong—Perkins—Stewart self-consistent molecular orbital computations, and SCF MOab initio GAUSSIAN-82 computations on B₆H₆ ^2– indicate that the approximation of the atomic orbitals by the sum of the molecular orbitals, as is typical in modernab initio computations, leads to significantly weaker apparent core bonding approximated more closely by deltahedral (D _n ) topology than by complete (K _n ) topology.This work was presented at the Workshop The Modern Problems of Heteroorganic Chemistry sponsored by the A. N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences (May 8–13, 1993).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1353–1360, August, 1993.     

Polyimides containing oxyethylene units

Four aromatic dinitro compounds containing oxyethylene units 1,2-bis(4-nitrophenoxy)ethane, bis(2)-(4-nitrophenoxy)ethyl ether, 1,2-bis(2)-(4-nitrophenoxy)ethoxy)ethane, and bis(2-(2-(4-nitrophenoxy)ethoxy)ethyl) ether were prepared by an aromatic nucleophilic substitution reaction that involved 1-fluoro-4-nitrobenzene and the appropriate oxyethylene glycol. Each of the dinitro compounds could be reduced chemically or catalytically to produce the diamines 1,2-bis(4-aminophenoxy)ethane, bis(2-(4-amino-phenoxy)ethyl ether, 1,2-bis(2-(4-aminophenoxy)ethoxy)ethane, and bis(2-(2-(4-aminophenoxy)ethoxy)ethyl) ether. Polymerization of the diamines was carried out in m-cresol with toluene as an azeotroping agent. Polymides derived from 5,5′-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofuradione exhibited good solubility in chlorinated solvents, glass transition temperatures as low as 140°C, as evidenced by viscous flow, and good thermal stability; a 5% weight loss generally occured at greater than 450°C. Isothermal aging studies conducted at 600°F(315°C) for 200 h indicated that weight loss is directly related to oxyalkylene chain length–10% for the shortest and 50% for the longest chain.     

Experimental tests of chirality algebra

Most chiral molecules can be dissected into a collection of ligands attached to an underlying skeleton. Application of permutation group theory and group representation theory to such a model can lead to chirality functions which can be used to approximate pseudoscalar measurements such as optical rotation or circular dichroism. Such chirality functions have been tested experimentally for the following skeletons: (1) The polarized triangle of phosphines and phosphine oxides; (2) the tetrahedron of methane derivatives; (3) the disphenoid of allene and 2, 2-spirobiindane derivatives; (4) the polarized rectangle of [2, 2]-metacyclophanes; (5) the polarized pentagon of heterodisubstituted ferrocenes. The success of this method is fair to good for the polarized triangle, tetrahedron, and disphenoid skeletons but deteriorates rapidly for the polarized rectangle and polarized pentagon skeletons, in accord with the greater group-theoretical complexity of the latter skeletons.     

The effect of fluctuations on emission spectra: practical distinctions between raman and fluorescence spectra

By the use of stochastic theory it is clearly shown that two types of emission should to fluctuations such as would occur during interactions with their environment are resonantly driven by light. In our approach the electromagnetic field is treated in a classical manner and decay is introduced by a Wigner-Weisskopfansatz. The two types of emission should occur for a three-level system and relaxation to other levels is not involved in the mechanism. Also included is a discussion of operational definitions of fluorescence (and other spontaneous emissions) and of the relationship between absorption and scattering in terms of dephasing or T₂ processes, especially as this relates to conventional emission phenomena.     

Evaluation of a dual electrospray ionization/atmospheric pressure chemical ionization source at low flow rates (∼50 µL/min) for the analysis of both highly and weakly polar compounds

The atmospheric pressure ionization (API) source for a commercial mass spectrometer was modified to operate as a dual source in both the electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) techniques by simultaneously utilizing the electrospray probe and the corona discharge needle. A switching box was designed to operate in either manual or programmable modes to permit rapid switching between ionization techniques without changing sources, probes, or breaking vacuum. The source can be operated using the following ionization techniques: ESI only, APCI only, ESI/APCI simultaneously, and ESI/APCI alternatingly. The optimum operating conditions for these ionization techniques were similar to the manufacturer’s original specifications except that the APCI flow rate was lower (～50 µL/min versus 1000 µL/min) and externally heated nebulizing gas was found to be desirable. A four-component mixture, introduced by flow injection, was used to demonstrate the versatility of the dual ESI/APCI source.     

Organosilicon compounds XIII. Cleavage of the silicon-carbon bond of 2-trimethylsilylbenzothiazole

The high-yield synthesis of 2-trimcthylsilylbenzothiazole is described, and the labile nature of this silicon-helerocyclic bond is proved when cleavage is affected by aldehydes, acid halides, chloro-formates and anhydrides. Justification for this unusual reactivity and further insight into the reaction mechanism is offered.     

Duplex Stabilization of DNA: Oligonucleotides containing 7-substituted 7-deazaadenines

The oligonucleotide building blocks 4b–d derived from 7-bromo-, 7-chloro-, and 7-methyl-substituted 7-deaza-2′-deoxyadenosines 3b–d were prepared. They were employed in the solid-phase synthesis of the oligonucleotides 7–25 . The dA residues of the homomer d(A₁₂), the alternating d[(A-T)₆], and the palindromic d(G-T-A-G-A-A-T-T-C-T-A-C) were replaced by 3b–d as well as by the parent 7-deaza-2′-deoxyadenosine ( 3a ). The melting profiles and CD spectra of oligonucleotide duplexes, showing this major groove modification, were measured, and the T_m values as well as the thermodynamic data were determined. It was found that small substituents such as Br, Cl, or Me introduced in the 7-position of a 7-deazaadenine residue increase the duplex stability compared to oligonucleotides containing adenine.     

Reissert compound studies. XXXI. Emetine analogs based on the reaction of the reissert anion with 1,3,4,6,7,11b-hexahydro-9, 10-dimethoxybenzo[a] quinolizin-2-ones

Reaction of the Reissert anion with the carbonyl group of 1,3,4,6,7,11b-hexahydro-9,10-dimethoxybenzo[a]quinolizin-2-one and with 3-ethyl-1,2,3,4,6,7-hexahydro-9,10-dimethoxy-11bH-benzo[a]quinolizin-2-carboxaldehyde give emetine analogs. This anion does not react with the carbonyl group of 3-aIkyl-1,3,4,6,7,11b-hexahydro-9,10-dimethoxybenzo[a]quinolizin-2-one but instead gives a rearrangement product and the benzoquinolizinone cyanohydrin.     

Highly selective tritium-from-deuterium isotope separation by pulsed NH3 laser multiple-photon dissociation of chloroform

Infrared multiple-photon dissociation of CTCl₃ was investigated using a pulsed CO₂ laser-pumped NH₃ laser at 12.08 μm. No evidence of any CDCl₃ depletion or decomposition product was observed in photolyzed CTCl₃/CDCl₃ mixtures. A lower limit of the single-step T/D enrichment factor, β, was found to be ≈ 165, based on the sensitivity in measuring CDCl₃ depletion. The low-fluene CTCl₃/CDCl₃ optical selectivity in absorptions is > 9000 at the 835 cm^?1 CTCl₃ ν₄ peak.     

Direct immobilization of native yeast iso-1 cytochrome C on bare gold: fast electron relay to redox enzymes and zeptomole protein-film voltammetry

Cyclic voltammetry shows that yeast iso-1-cytochrome c (YCC), chemisorbed on a bare gold electrode via Cys102, exhibits fast, reversible interfacial electron transfer (k(0) = 1.8 x 10(3) s(-1)) and retains its native functionality. Vectorially immobilized YCC relays electrons to yeast cytochrome c peroxidase, and to both cytochrome cd(1) nitrite reductase (NIR) and nitric oxide reductase from Paracoccus denitrificans, thereby revealing the mechanistic properties of these enzymes. On a microelectrode, we measured nitrite turnover by approximately 80 zmol (49 000 molecules) of NIR, coadsorbed on 0.65 amol (390 000 molecules) of YCC.