Orthomodular Implication Algebras

In Boolean algebras the properties of the implication operation can be modeled by a so-called implication algebra that itself can be considered as a join-semilattice with 1 whose principal filters are Boolean algebras. This situation is generalized from Boolean algebras to orthomodular lattices.     

Structures and thermodynamic stabilities of the C2H4O isomers: Acetaldehyde,vinyl alcohol and ethylene oxide

Ab initio molecular orbital theory with a sequence of basis sets ranging from minimal to triple zeta plus polarization and with electron correlation incorporated using Möller-Plesset perturbation theory terminated at third order (MP3) is used to examine the structures and relative energies of the C₂H₄O isomers, acetaldehyde, vinyl alcohol and ethylene oxide. Acetaldehyde is indicated to be the most stable isomer with vinyl alcohol lying 45 kJ mol^–1 and ethylene oxide 114 kJ mol^–1 higher in energy. The theoretical structures and energies are in reasonable agreement with the best available experimental data.     

The effect of different atmospheres on structural changes of titanate nanotubes during heating

Thermal stability of titanate nanotubes (Ti-NT) was studied in temperature range from room temperature up to 1000 °C in three different atmospheres—in air, vacuum and helium—and differences were observed. In air, vacuum and helium, the release of adsorbed and crystal water was detected in temperatures up to 200 °C. Transformation of the initial structure to anatase form of TiO₂ was found independently of the atmosphere used. But in air the transformation to anatase occurred at lower temperature. The difference between these atmospheres was approximately 100 °C. However, only in air, the transformation to sodium hexatitanate and rutile form of TiO₂ occurred at higher temperatures. In vacuum, only the anatase phase was detected up to 800 °C. In helium, the transformation to rutile was observed but not to sodium hexatitanate. The changes of the initial structure of Ti-NT during the elevation of temperature were studied by combination of in situ powder X-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis and mass spectroscopy.     

Uncatalyzed transfer hydrogenation of quinones and related systems: a theoretical mechanistic study

Quantum chemistry calculations have been used to study the uncatalyzed transfer hydrogenation between a range of hydrogen donors and acceptors, in the gas phase and in solution. Our study shows in the first place that in order to obtain reliable condensed-phase transition structures, it is necessary to perform geometry optimization in the presence of a continuum. In addition, the use of a free energy of solvation obtained with the UB3-LYP/6-31+G(d,p)/IEF-PCM/UA0 combination, in conjunction with UMPWB1K/6-311+G(3df,2p)//B3-LYP/6-31+G(d,p) gas-phase energies, gives the best agreement with experimental barriers. In condensed phases, the geometries and energies of the transition structures are found to relate to one another in a manner consistent with the Hammond postulate. There is also a correlation between the barriers and the energies of the radical intermediates in accord with the Bell-Evans-Polanyi principle. We find that in the gas phase, all the transfer-hydrogenation reactions examined proceed via a radical pathway. In condensed phases, some of the reactions follow a radical mechanism regardless of the solvent. However, for some reactions there is a change from a radical mechanism to an ionic mechanism as the solvent becomes more polar. Our calculations indicate that the detection of radical adducts by EPR does not necessarily indicate a predominant radical mechanism, because of the possibility of a concurrent ionic reaction. We also find that the transition structures for these reactions do not necessarily have a strong resemblance to the intermediates, and therefore one should be cautious in utilizing the influence of polar effects on the rate of reaction as a means of determining the mechanism.     

Factors controlling the addition of carbon-centered radicals to alkenes

The addition rate constants of radicals to alkenes are strongly substituent dependent because of enthalpic, polar and steric effects. Recent absolute experimental and high level ab initio data for many prototype additions of small radicals are analyzed with the aid of the state correlation diagram. This leads to a unifying rationalization of the various effects and allows the prediction of rate constants to one order of magnitude or better. Propagation rate coefficients of homo- and copolymerizations and penultimate effects are also discussed.     

Planar-Tetracoordinate Carbon in a Neutral Saturated Hydrocarbon: Theoretical Design and Characterization

Exact planarity at the central carbon atom is achieved, according to molecular orbital calculations, in the strained polycyclic cage hydrocarbon dimethanospiro[2.2]octaplane (see structure). There are no glaringly long C−C bonds, which might have reflected inherent instability in this molecule that is yet to be synthesized.     

Development of a solid-phase extraction with capillary liquid chromatography tandem mass spectrometry for analysis of estrogens in environmental water samples

Capillary liquid chromatography (cLC) hyphenated with tandem mass spectrometry (MS-MS) was used to separate and quantitate trace concentrations of five estrogens in aqueous samples. New C(18)-based sorption materials bound to the silica support by monomeric and polymeric mechanisms were compared and tested for solid-phase extraction (SPE) of selected analytes with respect to optimization of their preconcentration yield. Application of an endcapped, monomer-bound preconcentration Discovery DSC-18Lt column under the optimized conditions provides yields in the range from 95 to 100% with a high repeatability (n=3, RSD≤7.2%). Using the electrospray ionization in the positive mode (ESI+), the cLC-MS-MS system (the Zorbax SB C18 capillary column and a binary mobile phase of acetonitrile and water containing 0.1% formic acid in both the components) was optimized to attain a sufficient retention of the early eluting estriol, a satisfactory resolution of the analytes and the maximum sensitivity of the determination. Both the isocratic and gradient elution were used and the optimized gradient method permitted analyses of aqueous environmental samples in 14 min within a linearity range from 6.1 to 25.0 (LOQ of analytes) to 500 ng/L and with a very good linearity (r>0.9981) for all the estrogens studied. The detection limits are in the range from 3.0 to 6.8 ng/L (1 μL injection volume). Six environmental water samples were analyzed and the studied estrogens were found in the Vltava river sample collected in Prague (13.2 ng/L for 17β-estradiol) and in the inlet to the wastewater treatment plant in Prague, at an overall concentration of 371.4 ng/L.     

Cyanovinyl radical: an illustration of the poor performance of unrestricted perturbation theory and density functional theory procedures in calculating radical stabilization energies

Stabilization energies for the 1-cyanovinyl radical (CH₂=CCN) have been calculated using a variety of conventional ab initio (M?ller–Plesset, quadratic configuration interaction and coupled-cluster) and density functional theory (B-LYP, B3-LYP) procedures, as well as with a range of compound methods. Compared with a high-level benchmark value (that predicts a stabilization energy of 17.1 kJ mol⁻¹), UMP2 and UMP4 give the wrong sign and magnitude of the stabilization energy (both methods predicting desta- bilization instead of stabilization), while B-LYP and B3-LYP overestimate the degree of stabilization. The RMP2, RMP4, QCISD(T) and CCSD(T) techniques, and several, but not all, variants of G2 and CBS theories give radical stabilization energies in good agreement with the benchmark value. Received: 15 June 1998 / Accepted: 19 August 1998 /  Published online: 15 February 1999     

An ONIOM investigation of the effect of conformation on bond dissociation energies in peptides

In the present study, we use the ONIOM strategy of Morokuma and coworkers to examine the various C H bond dissociation energies (BDEs) of a small peptide (2ONW) and compare these with values obtained for its component individual amino acid residues. To evaluate suitable methods for ONIOM-based geometry optimizations, we test an “internal consistency” approach against full B3-LYP//B3-LYP results, and find B3-LYP/6-31G(d):AM1 to be appropriate. We find that the BDEs at the α-carbon in 2ONW are generally larger than the corresponding values for the individual residues on their own. This is attributed to the constraints of the peptide backbone leading to conformations that are not ideal for captodative stabilization of the resulting α-radicals. At the more flexible β- and γ-positions, the differences between the BDEs in 2ONW and the individual residues are smaller. Overall, the α-BDEs are smaller than the β- and γ-BDEs in most cases. Thus, to rationalize the inertness of peptide backbones with respect to α-hydrogen abstraction that is frequently found experimentally, it is necessary to consider alternative protection mechanisms such as the polar effect. © 2018 Wiley Periodicals, Inc.