Accuracy of geometries: influence of basis set,exchange–correlation potential,inclusion of core electrons,and relativistic corrections

The geometries of a set of small molecules were optimized using eight different exchange–correlation (xc) potentials in a few different basis sets of Slater-type orbitals, ranging from a minimal basis (I) to a triple-zeta valence basis plus double polarization functions (VII). This enables a comparison of the accuracy of the xc potentials in a certain basis set, which can be related to the accuracies of wavefunction-based methods such as Hartree–Fock and coupled cluster. Four different checks are done on the accuracy by looking at the mean error, standard deviation, mean absolute error and maximum error. It is shown that the mean absolute error decreases with increasing basis set size, and reaches a basis set limit at basis VI. With this basis set, the mean absolute errors of the xc potentials are of the order of 0.7–1.3 pm. This is comparable to the accuracy obtained with CCSD and MP2/MP3 methods, but is still larger than the accuracy of the CCSD(T) method (0.2 pm). The best performing xc potentials are found to be Becke–Perdew, PBE and PW91, which perform as well as the hybrid B3LYP potential. In the second part of this paper, we report the optimization of the geometries of five metallocenes with the same potentials and basis sets, either in a nonrelativistic or a scalar relativistic calculation using the zeroth-order regular approximation approach. For the first-row transition-metal complexes, the relativistic corrections have a negligible effect on the optimized structures, but for ruthenocene they improve the optimized Ru–ring distance by some 1.4–2.2 pm. In the largest basis set used, the absolute mean error is again of the order of 1.0 pm. As the wavefunction-based methods either give a poor performance for metallocenes (Hartree–Fock, MP2), or the size of the system makes a treatment with accurate methods such as CCSD(T) in a reasonable basis set cumbersome, the good performance of density functional theory calculations for these molecules is very promising; even more so as density functional theory is an efficient method that can be used without problems on systems of this size, or larger.     

Reaction of CH acids with 2-arylidenecycloalkanones: Synthesis of β-keto acid anilide derivatives of naphthalene,indene, fluorene,and phenanthrene

Summary 2-Arylidene-cyclohexanone1, -cyclopentanone2, -1-indanone3 and-1-tetralone4 react with acetoacetanilide5 yielding 2-oxo-4-aryl-3-carboxylic acid anilides derivatives of naphthalene7, indene8, fluorene9 and phenanthrene10. Reaction of1 and3 with benzoylacetanilide6 yields the corresponding Michael adducts11 and12.

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Reaktion von CH-Säuren mit 2-Arylidencycloalkanonen. Synthese von -Ketosäureanilid-Derivaten von Naphthalin, Inden, Fluoren und Phenanthren

Zusammenfassung 2-Arylidencyclohexanone1, -cyclopentanone2, -1-indanone3 und -1-tetralone4 reagieren mit Acetoacetanilid5 unter Bildung von 2-Oxo-4-aryl-3-carbonsäureanilid-Derivaten von Naphthalin7, Inden8, Fluoren9 und Phenanthren10. Die Reaktion von1 und3 mit Benzoylacetanilid6 ergibt die entsprechenden Michael-Addukte11 und12.

     

Langmuir-Shäfer transfer of fullerenes and porphyrins: formation, deposition, and application of versatile films

Thin films consisting of a fulleropyrrolidine derivative 1 and a novel water-soluble porphyrin 2 were prepared by the Langmuir-Sh?fer (LS, horizontal lifting) method. In particular, a solution of 1 in chloroform and dimethyl sulfoxide was spread on the water surface, while porphyrin 2 (bearing peripheral anionic groups) was dissolved into the aqueous subphase. To the best of our knowledge, such a versatile method of film fabrication for fullerene/porphyrin mixed composite films has never been used before. Evidence of the effective interactions between the two moieties at the air-water interface was obtained from the analysis of the floating layers by means of surface pressure versus area per molecule Langmuir curves, Brewster angle microscopy and UV-visible reflection spectroscopy. The characterisation of the LS films by UV-visible spectroscopy reveals that the two constituents behave as discrete and weakly interacting pi systems. The use of polarised light suggests the existence of a preferential direction of the macrocyclic rings with an edge-on arrangement with respect to the substrate surface. Finally, photoaction spectra were recorded from films deposited by only one horizontal lifting onto indium-tin-oxide (ITO) electrodes and the observed photocurrent increased notably with increasing transfer surface pressure.     

Spectral characteristics of 2-amino-3-benzyloxy pyridine: Effects of solvents,pH, and β-cyclodextrin

Spectral characteristics of 2-amino-3-benzyloxypyridine (2ABP) has been studied in solvents of different polarity, pH, and β-cyclodextrin (β-CD) and compared with 2-amino pyridine (2AP). The inclusion complex of both amino pyridine (AP) molecules with β-CD are analysed by UV-visible, fluorimetry, FTIR, ¹H NMR, SEM and AM1 methods. The solvent studies shows no significant shift observed in absorption maxima between both AP molecules, but in the excited state a slight red shift is noticed in 2ABP than in 2AP which indicates that the addition of benzyloxy group in 2AP does not effectively increase the resonance interaction in 2ABP. The regular red shift observed in acidic pH solutions suggests intramolecular proton transfer (IPT) interaction in both molecules. β-CD studies shows that in pH ∼7, 2ABP forms 1: 2 inclusion complex from 1: 1 inclusion complex and 1: 1 inclusion complex is formed in pH ∼ 1. In pH ∼ 7, a red shift observed in 2ABP with lower β-CD concentration suggests aromatic ring encapsulated in the β-CD cavity and blue shift noticed at higher β-CD concentrations indicates pyridine ring encapsulated in the β-CD cavity.     

Micelle to vesicle transitions of N-dodecyl-1, ω-diaminoalkanes: effects of pH, temperature and salt

The self-assembly behavior of pH-sensitive amphiphiles N-dodecyl-1, 2-diaminoethane (C12N2N), N-dodecyl-1, 3-diaminopropane (C12N3N) and N-dodecyl-1, 4-diaminobutane (C12N4N) has been studied in aqueous solutions. Light scattering, viscosity and cryo-transmission electronic microscopy (cryo-TEM) results revealed that the aggregates transferred from spherical micelles to vesicles (MVT) via wormlike micelles as the pH was gradually varied from acidic to basic conditions. pH-dependent zeta potential and (1)H NMR studies confirmed these transitions. Interestingly, the formed wormlike micelles could transform into vesicles upon heating, which was studied by cryo-TEM, light scattering and viscosity techniques in detail. It is concluded that the pH and thermal MVT are a general phenomenon in all three amphiphiles investigated. Furthermore, NaCl induced a wormlike micelle to vesicle transition was also observed in C12N2N solution.     

Oxidation of fluoroalkyl-substituted allylic alcohols: a synthesis of fluorinated α,β-epoxyketones

Fluoroalkyl-substituted allylic alcohols are readily and selectively oxidized at the hydroxyl group with the Swern or Dess-Martin reagents to give ,-unsaturated ketones, and on treatment withm-chloroperbenzoic acid ortert-butyl hydroperoxide they are oxidized to the corresponding epoxyalcohols. The latter are oxidized by Swern or Dess-Martin reagents at the hydroxyl group to ,-epoxyketones, which form hydrates stable under the conditions of isolation.Translated fromIzvestiya Akadernii Nauk. Seriya Khimicheskaya, No. 3, pp. 680–683, March, 1996.     

Group 14 structural variations: perhalo derivatives of the “dimetallenes”: dicarbenes, disilenes, digermenes, distannenes, and diplumbenes

Trends in the structural variations of all perhalo derivatives of dicarbenes and their Group 14 analogues have been studied. This included all M₂X₄ molecules, where M = C, Si, Ge, Sn, or Pb, and X = F, Cl, Br, or I. Mapping the potential energy surface of all molecules has uncovered several isomers. The stability of these isomers depends on both the Group 14 atoms and the halogen ligands. Several isomers were found stable; the ones that are global minima include (with their symmetries and an example in parenthesis): the typical ethene structure X₂M=MX₂ (D _2h, F₂C=CF₂), an X₃M–MX structure (C _S, F₃Si–SiF, a trifluorosilyl–silylene), another X₃M–MX structure (C ₁, Cl₃Si–SiCl), one more X₃M–MX structure with a single halogen bridge (C ₁, I₂Si–μI–SiI), a trans double halogen bridged structure (D _2h, FSn–μF₂–SnF), and another trans double-bridged structure with puckered ring (C _S, IPb–μI₂–PbI). Some of the other structures that are stable but are not the global minima include: a trans-bent structure X₂M–MX₂ (C _2h, all X₂Si–SiX₂), cis double-bridged structure (C _2v with planar ring, FPb–μF₂–PbF, or with puckered ring, C _2v, IGe–μI₂–GeI), and even a square bipyramidal structure (D _4h, Sn–μF₄–Sn). The energy differences between some of the structures are small and the application of another computational method and using a different basis set might alter their relative stabilities. Reasons for the difference in the stabilities of isomers have been discussed.     

Enantioselective photodeconjugation of α,β-unsaturated esters: Effect of the nature of the chiral agent

A synergism exists between the effect of the amino and hydroxy groups of chiral amino alcohols on the enantiomeric excess of the photodeconjugation of α,β-unsaturated esters.     

Metal-catalyzed cycloetherification reactions of β,γ- and γ,δ-allendiols: chemo-, regio-, and stereocontrol in the synthesis of oxacycles

Versatile routes that lead to a variety of functionalized enantiopure tetrahydrofurans, dihydropyrans, and tetrahydrooxepines are based on chemo-, regio-, and stereocontrolled metal-catalyzed oxycyclization reactions of β,γ- and γ,δ-allendiols, which were readily prepared from (R)-2,3-O-isopropylideneglyceraldehyde. The application of Pd(II), Pt(II), Au(III), or La(III) salts as the catalysts gives controlled access to differently sized oxacycles in enantiopure form. Usually, chemoselective cyclization reactions occurred exclusively by attack of the secondary hydroxy group (except for the oxybromination of phenyl β,γ-allenic diols 3b and 3d) to an allenic carbon atom. Regio- and stereocontrol issues are mainly influenced by the nature of the metal catalysts and substituents.     

Reduction and alkylation of proteins in preparation of two-dimensional map analysis: why, when, and how?

The standard procedure adopted up to the present in proteome analysis calls for just reduction prior to the isoelectric focusing/immobilized pH gradient (IEF/IPG) step, followed by a second reduction/alkylation step in between the first and second dimension, in preparation for the sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) step. This protocol is far from being optimal. It is here demonstrated, by matrix assisted laser desorption/ionization-time of flight (MALDI-TOF)-mass spectrometry, that failure to reduce and alkylate proteins prior to any electrophoretic step (including the first dimension) results in a large number of spurious spots in the alkaline pH region, due to "scrambled" disulfide bridges among like and unlike chains. This series of artefactual spots comprises not only dimers, but an impressive series of oligomers (up to nonamers) in the case of simple polypeptides such as the human alpha- and beta-globin chains, which possess only one (alpha-) or two (beta-) -SH groups. As a result, misplaced spots are to be found in the resulting two-dimensional (2-D) map, if performed with the wrong protocol. The number of such artefactual spots can be impressively large. In the case of analysis of complex samples, such as human plasma, it is additionally shown that failure to alkylate proteins results in a substantial loss of spots in the alkaline gel region, possibly due to the fact that these proteins, at their pI, regenerate their disulfide bridges with concomitant formation of macroaggregates which become entangled with and trapped within the polyacrylamide gel fibers. This strongly quenches their transfer in the subsequent SDS-PAGE step.     

Insights into the β-phase of polyfluorene: characterization,regulation, and mechanism

Due to the unique blue-light properties, wide excited emission interval and so on, polyfluorenes (PFs) are among the most promising blue-light polymers that are widely used in organic lasers and organic light-emitting diodes. Usually, PFs exhibit two different phases: α-phase and β-phase, and the β-phase has higher photoluminescence efficiency and carrier mobility than the one of α-phase. Hence, improving the content of β-phase is very important to enhance the device performance. In this paper, we introduce the methods for characterizing the β-phase in PFs and summarize the recent research progress in regulating the β-phase content. Firstly, the differences between the α-phase and the β-phase of PFs in terms of structure, aggregation behavior, and optoelectronic properties are summarized. Then, the characterization for the β-phase, including ultraviolet–visible absorption spectrum (UV–vis), photoluminescence spectrum (PL), and Raman spectrum, transmission electron microscope (TEM), and X-ray diffraction (XRD) are introduced, and the calculating rules for the content of β-phase is also introduced. Furthermore, the methods to increase the content of β-phase are reviewed. Finally, a brief outlook on the challenges and future development prospects of β-phase in PFs are discussed.     

Carbohydrate hydration: heavy water complexes of α and β anomers of glucose, galactose, fucose and xylose

The singly and doubly hydrated complexes of the α and β anomers of a systematically varied set of monosaccharides, O-phenyl D-gluco-, D-galacto-, L-fuco- and D-xylopyranoside, have been generated in a cold molecular beam and probed through infrared-ultraviolet double resonance ion-dip (IRID) spectroscopy coupled with quantum mechanical calculations. A new 'twist' has been introduced by isotopic substitution, replacing H(2)O by D(2)O to separate the carbohydrate (OH) and hydrate (OD) vibrational signatures and also to relieve spectral congestion. The new spectroscopic and computational results have exposed subtle aspects of the intermolecular interactions which influence the finer details of their preferred structures, including the competing controls exerted by co-operative hydrogen bonding, bi-furcated and OH-π hydrogen bonding, stereoelectronic changes associated with the anomeric effect, and dispersion interactions. They also reassert the operation of general 'working rules' governing conformational change and regioselectivity in both singly and doubly hydrated monosaccharides.     

Reactivity of a σ,σ,σ,σ-tetraradical: the 2,4,6-tridehydropyridine radical cation

The 2,4,6-tridehydropyridine radical cation, an analogue of the elusive 1,2,3,5-tetradehydrobenzene, was generated in the gas phase and its reactivity examined. Surprisingly, the tetraradical was found not to undergo radical reactions. This behavior is rationalized by resonance structures hindering fast radical reactions. This makes the cation highly electrophilic, and it rapidly reacts with many nucleophiles by quenching the N-C ortho-benzyne moiety, thereby generating a relatively unreactive meta-benzyne analogue.     

Chemistry, commentary, and community: discussion of “an examination of the vaporization enthalpies and vapor pressures of pyrazine, pyrimidine, pyridazine, and 1,3,5-triazine” by Lipkind and Chickos

Vaporization enthalpies for the isomeric diazines were discussed within the context of recent measurements and estimation techniques. It is suggested that pyridazine shows enigmatic behavior.

Reactions of dibenzylphosphine oxide with α,β-unsaturated nitriles: synthesis of aminophospholene oxides

The reactions of dibenzylphosphine oxide with ,-unsaturated nitriles in the presence of two equivalents of NaH in Bu^tOMe afforded substituted aminophospholene oxides in 20—83% yields. In the presence of bulky substituents at the and positions of unsaturated nitriles, cyclization of the initially formed adducts proceeds with high stereoselectivity to give the single stereoisomer of aminophospholene oxide.     

Electrochemical reduction of dehydroamino acids: synthesis and photophysical properties of β,β-diarylalanines

Several β,β-disubstituted dehydroalanines were prepared from β,β-dibromo or β-bromo, β-substituted dehydroalanines and aryl boronic acids using a Suzuki-Miyaura cross-coupling reaction. The electrochemical behaviour of these compounds was studied by cyclic voltammetry. All compounds studied showed similar reduction potentials and these were similar to the peak potential of the methyl ester of N-tert-butoxycarbonyl dehydrophenylalanine. Thus, the presence of a second aryl moiety in the dehydroalanine scaffold does not significantly change the reduction potential.Controlled potential electrolyses were performed at the cathodic peak potential in the presence of triethylammonium chloride as proton donor. The only products isolated in good to high yields were the corresponding β,β-diarylalanines. This reaction was also carried out using a dipeptide containing a β,β-diaryldehydroalanine to give a 1:1 diastereomeric mixture of the reduction product.The photophysical properties of two of the β,β-diaryldehydroalanines and of the corresponding β,β-diarylalanines were studied in three solvents of different polarity. The β,β-diaryldehydroalanines show low fluorescent quantum yields (Φ_F<9%) due to the conjugation of the aromatic moieties with the α,β-double bond and with the carbonyl group, which favours the non-radiative deactivation pathways. The absence of conjugation in the reduction products leads to a significant increase in the fluorescence quantum yields. These results show that the β,β-disubstituted alanines could be used as fluorescent markers.