bluuebunny/crossref_metadata_2025_split huggingface dataset:
Visualization of Tight‐Binding Calculations–The Electronic Structure and Electron Localization of the Si(100) Surface
Authors: Fässler Thomas F. , Häussermann Ulrich , Nesper Reinhard 
Published: July 2007

AbstractThe advantage of computer graphics in the visualization of tight‐binding calculations is highlighted in a model study of the reconstruction of the Si(100) surface. Three different surface models—the unreconstructed surface Si(100)‐(1 × 1), and symmetric and asymmetric pairing of surface atoms Si(100)‐(2 × 1)—are investigated on the basis of density of states (DOS), local (projected) density of states (LDOS) and crystal orbital‐overlap population (COOP) analysis. For the visualization of the real‐space properties of tight‐binding calculations, two‐ and three‐dimensional images of the total (TED) and partial electron densities (PED) are shown. The PED calculated near the Fermi level are compared to densities of HOMOs and LUMOs in molecular systems and used to analyse constant current mode STM images, obtained by applying bias voltages of different sign. They show excellent agreement with STM experiments. The electron‐localization function (ELF) has been shown to describe chemical bonds in molecules and solids surprisingly well. Here, the ELF is calculated for surfaces. In order to visualize the shape of the “dangling” surface bonds and bonds connecting surface atoms, two‐ and three‐dimensional representations of the ELF are discussed. Using the reconstruction of the Si(100) surface as an example, we show that combining methods for extracting information from quantum mechanical calculations, such as PED, TED and ELF, leads to a more comprehensive description of the electronic surface structure. With the help of computer graphics, chemical concepts routinely used for describing local properties of molecules can be transferred very effectively to extended systems.
Ligand‐Field Control and Hydrogen Bonding as Design Elements in the Assembly and Crystallization of Poly(azolyl)borate‐Metal Complexes: Chelate Complexes Versus Coordination Polymers and Symmetrical Versus Distorted Grid Sheets
Authors: Janiak Christoph , Scharmann Tobias G. , Günther Wulf , Girgsdies Frank , Hemling Holger , Hinrichs Winfried , Lentz Dieter 
Published: July 2007

AbstractThe 1‐ and 2‐D coordination polymers [Mn{HB(C2H2N3)3}2(H2O)2].4H2O (11) and [Ni{H2B(CHN4)2}2‐(NH3)2] (13), respectively, and the chelate complex [Ni{H2B(C2H2N3)2}2(H2O)2] · 2H2O (12) were synthesized and structurally characterized. The compounds contain ambidentate poly(azolyl)borato ligands (azolyl = triazolyl or tetrazolyl), which can chelate or bridge metal centers. The metal–ligand structures in 11–13 differ from the known coordination modes of the poly(azolyl)borates towards other metal centers. We describe how a change in the metal and/or the conditions of crystallization affects the ligand‐field stabilization energy and favors one type of nitrogen donor atom over the other for the poly(triazolyl)borato ligands. The crystal structures of 11 and 12 contain additional water of crystallization; this leads to hydrogen‐bonded solvent substructures. In the case of the bis(triazolyl)borato ligands. The crystal structures of 11 and 12 contain additional water of crystallization; this leads to hydrogen‐bonded solvent substructures. In the case of the bis(tetrazolyl)borato ligand the water substructure is shown to function as a “reinforcing bar” that symmetrizes the metal–ligand grid sheet.
Stereodivergent Hetero‐Diels‐Alder Reactions of Chiral 1‐Oxa‐1,3‐butadienes through a Conformational Switch Induced by Lewis Acids
Authors: Tietze Lutz F. , Schneider Christoph , Grote Andrea 
Published: July 2007

AbstractThe stereodivergent asymmetric hetero‐Diels‐Alder reaction of achiral and chiral 1‐oxa‐1,3‐butadienes carrying an oxazolidine moiety with various enol ethers in the presence of different Lewis acids is described as a highly stereoselective and efficient approach to dihydropyrans, which can be used for the synthesis of carbohydrates. In the cycloaddition of the achiral oxabutadiene very good endo/exo selectivity was possible, and with the chiral oxabutadienes excellent 1,6‐asymmetric induction was additionally observed. In the processes a reversal of facial selectivity occurs by changing the Lewis acid, allowing the synthesis of both enantiomers of the dihydropyrans with the same auxiliary. Thus, cycloaddition of 1 to 2 in the presence of Me2 AlCl gives predominantly the endo product 3 (3:4 = 10:1), whereas with SnCl4 the exo product 4 is obtained (3:4 = 1:15). The reaction of 7 and 1 a in the presence of Me2AlCl as promoter nearly exclusively yields the endo‐I adduct 16 a (16 a+17 a:18 a+19 a = > 50:1; 16 a:17 a = 60:1), whereas with TMS‐OTf the endo‐II‐product 17 a was obtained as the main component (16 a+17 a:18 a+19 a = > 50:1; 16 a:17 a = 1:7.9). The use of SnCl4 leads to a mixture of endo and exo, again, however, with excellent induced selectivity. A similarly good induction was obtained with the oxabutadiene 9 containing the new auxiliary 8. Also, other enol others 1 b–g were used, some of which afforded excellent induction. Mechanistic considerations are used to explain the results.
Chirality of Large Random Supramolecular Structures
Authors: Katzenelson Omer , Hel‐Or Hagit Zabrodsky , Avnir David 
Published: July 2007

AbstractThe structural chirality of large, random supramolecular structures, spiral diffusion‐limited aggregates, is analyzed and studied in detail by using a continuous chirality measure. It is found that classical definitions and terminologies of chirality are too restrictive for the description of such complex objects. A refined methodology and a conceptual vocabulary are developed, along with a generalized definition of chirality. Their application is demonstrated in detail on these large structures. The classical definition of chirality, tailored for small, nonrandom species, is a limiting case of the generalized viewpoint we propose.
Tricarbonyl(η6‐1,2‐dioxobenzocyclobutene)chromium(0): Preparation, Nucleophilic Addition Studies and Syntheses of cmplex Polycyclic Systems by Dianionic Oxy‐Cope Rearrangement
Authors: Brands Michael , Wey Hans G. , Bruckmann Joachim , Krüger Carl , Butenschön Holger 
Published: July 2007

AbstractThe tricarbonylchromium complex 3 of 1,2‐dioxobenzocyclobutene (1) is easily prepared by hydrolysis of diacetal 4. Complex 3 stereoselectively undergoes both single and double nucleophilic addition reactions at the keto groups. As a consequence, addition of excess alkenyllithium to 3 gives rise to a dianionic oxy‐Cope rearrangement yielding benzocyclooctenedione complexes with good diastereoselectivity. Trapping of the bis(enolate) intermediate with chlorotrimethylsilane gives bis(enolether) 23. Subsequent addition of two different alkenyllithium derivatives yields the asymmetrically substituted rearrangement product 15. To some extent, dependent on the method of hydrolysis, an intramolecular aldol addition follows the dianionic oxy‐Cope rearrangement. This results in the formation of benzoanellated bicyclo[3.3.0]octane derivatives and, in the case of 1‐cyclopentenyllithium as the alkenylmetal, in the tetraquinane system 19. The trans‐anellation of one of the cyclopentane rings in trans‐20 has been verified by an X‐ray structural analysis. The highly substituted rearrangement product 21/22 is formed only in small yield; in solution the bis(enol) tautomeric form 22 is favored over diketone 21.
Diphosphonio Dihydrophosphetide and 1,2‐Diphospholide Cations
Authors: Jochem Georg , Schmidpeter Alfred , Nöth Heinrich 
Published: July 2007

AbstractCondensation of a 1,3‐diphosphoniopropenide cation at its reactive 1,3‐positions with a dichlorophosphine in the presence of triethylamine provides a route to 2,4‐diphosphoniodihydrophosphetide cations. An excess of dichlorophosphine in the presence of an additional reducing agent results in a ring expansion and yields 3,5‐diphosphoniodihydro‐1,2‐diphospholide cations. The chlorosubstituted cation derived from PCl3 can be further reduced to the hydrolytically stable 3,5‐diphosphonio‐1,2‐diphospholide cation. It adds halogen to the PP bond and can easily be regained from the halogen adduct. Structural comparison of the 1,2‐diphenyl‐ and 1,2‐dichlorodihydro‐1,2‐diphospholide cation with the 1,2‐diphospholide cation shows three stages of interaction of the C3 and the P2 entities of the ring: no conjugation in the first case, hyperconjugative extension of the allylic system to include the phosphorus atoms in the second case and cyclic π conjugation in the third.
Mechanism of an Alkyl‐Dependent Photochemical Homolysis of the Re–Alkyl Bond in [Re(R)(CO)3(α‐diimine)] Complexes via a Reactive σπ* Excited State
Authors: Rossenaar Brenda D. , Kleverlaan Cornelis J. , Van De Ven Maartje C. E. , Stufkens Derk J. , VlčEk Antonín 
Published: July 2007

AbstractMLCT excitation of the complexes [Re(R)(CO)3(α‐diimine)] (R = Me, Et, benzyl (Bz); α‐diimine = iPr‐PyCa, R′‐DAB) results in the homolysis of the Re‐R bond leading to the formation of radicals R. and [Re(CO)3(α‐diimine)]. as primary photoproducts. The quantum yield of this photoprocess is dependent on the alkyl group used. For R = Me, the quantum yield is low (10−2) and depends on the temperature and excitation wave‐length, whereas for R = Et and Bz the quantum yield is near unity and independent of T and λexc. The reaction is shown to proceed via a σ(Re‐R)π* excited state that is rapidly (< 20 ps) populated by a nonradiative transition from the optically excited MLCT state. Time‐resolved IR and UV/Vis absorption spectra studied in the ns‐μs and ps‐μs time domains, respectively, show that the σπ* excited state is rather long‐lived (τ ≈ 250 ns) in noncoordinating solvents; the dissociation of the Re‐R bond from this state is strongly accelerated by polar or coordinating solvents (τ< 20 ps). The σπ* excited state is spectroscopically characterized by a (presumably σπ* → MLCT) transition at approximately 500 nm and by CO stretching frequencies closely resembling their ground‐state values. The relative energies of the MLCT and reactive σπ* states, controlled by the nature of the alkyl lig‐and, determine the photoreactivity of the complexes.
Design and Synthesis of a “Starburst”‐Type Nonadiazo Compound and Magnetic Characterization of Its Photoproduct
Authors: Matsuda Kenji , Nakamura Nobuo , Inoue Katsuya , Koga Noboru , Iwamura Hiizu 
Published: July 2007

AbstractA “starburst”‐type nonadiazo compound was designed and synthesized by using the cyclotrimerization of 3,5‐dibenzylphenyl ethynyl ketone as a key step. The diazo compound was photolyzed in methyltetrahydrofuran solid solution at cryogenic temperatures and analyzed by means of Faraday magnetometry and EPR spectroscopy. While the m‐phenylene‐connected nonacarbene was predicted to have a nonadecet (S = 9) ground state, the magnetic data of the photoproduct was more consistent with a pentadecet (S = 7) species. The result was interpreted in terms of the intramolecular cross‐linking between the carbene centers by the determination of both the amount and the multiplicity of the spin. In the “starburst”‐type polycarbenes with extended branching, the reactive carbene centers are able to approach one another and thus appear to readily recombine. This study highlights one of the limitations of the extension of the carbene network by way of flexible “starburst”‐type structures.
Gas‐Phase Acid‐Induced SN2′ versus SN2 Mechanism in Allylic Alcohols
Authors: Renzi Gabriele , Lombardozzi Antonietta , Dezi Emanuela , Pizzabiocca Adriano , Speranza Maurizio 
Published: July 2007

AbstractA first demonstration of the existence of the concerted SN2′ mechanism in the gas phase was obtained by establishing the regioselectivity of the attack of a neutral nucleophile, such as MeOH, on several allylic oxonium ions. These were generated in the gas phase by the reaction of radiolytically formed GA+ acids (GA+CnH+5 (n = 1, 2), iC3H+7, and (CH3)2F+) with trans‐ (1) and cis‐2‐buten‐1‐ol (2) as well as with 1‐buten‐3‐ol (3). Firm evidence in favor of the concerted SN2′ pathway accompanying the classical SN2 one in these systems was obtained after careful evaluation of the extent of conceivable intramolecular isomerization both of the primary oxonium ions from GA+ attack on 1–3 before nucleophilic displacement by MeOH and of their substituted intermediates before neutralization. The intermediacy of free allylic ions in the nucleophilic substitution was ruled out by generating the ions by protonation of 1,3‐butadiene and by investigating their behavior in exactly the same media employed in the substitution reactions. The regioselectivity of MeOH with the ionic substrates investigated showed the occurrence of nearly equally extensive SN2′ and SN2 pathways in the oxonium ions from 1 (SN2′ (57 ± 2%) and SN2 (43 ± 2%)) and 3 (SN2′ (54 ± 2%) and SN2 (46 ± 2%)), whereas, with 2, the SN2 (66 ± 2%) reaction prevailed over the SN2′ one (34 ± 2%). The role of intrinsic structural factors in determining the SN2′/SN2 branching in the selected oxonium ions is discussed.
The photochemical Formation of Fulvene from Benzene via Prefulvene–a Theoretical Study
Authors: Dreyer Jens , Klessinger Martin 
Published: July 2007

AbstractResults of semiempirical MN‐DOC‐CI and ab initio CASSCF calculations reveal that fulvene is a primary product of the photolysis of benzene. The photochemical step is assumed to lead to prefulvene, and both a synchronous conversion and a two‐step process via bicyclo[3.1.0]hexa‐1,3‐diene (isofulvene) could be ruled out as possible pathways from prefulvene to benzene. The most probable mechanism for the photochemical isomerization of benzene to fulvene involves the intermediate structures prefulvene and 1,3‐cyclopentadienylcarbene and has to compete with the almost barrierless formation of benzvalene and rearomatization to benzene. The short‐lived intermediates are rather flexible structures with negligible barriers to inversion at the radical center in the threemembered ring of prefulvene and prebenzvalene and to rotation around the exocyclic single bond in 1,3‐cyclopentadienylcarbene.
NMR Chemical‐Shift Anomaly and Bonding in Piano‐Stool Carbonyl and Related Complexes–an Ab Initio ECP/DFT Study
Authors: Kaupp Martin 
Published: July 2007

AbstractThe origin of the unusually large carbonyl 13C shifts and of unusual periodic trends in four‐legged piano‐stool complexes [M(n5‐C5H5)(CO)4]− (MTi, Zr, Hf) and in related species has been investigated by using a combination of ab initio effective‐core potentials (ECPs) and density‐functional theory (DFT). The ECP/SOS‐DFPT(IGLO) calculations indicate a considerable reduction in the anisotropy of the 13C(CO) chemical shift tensors compared to terminal carbonyl ligands in “normal” complexes. This is due to large paramagnetic contributions from metal d AO type (dz2, dxy) orbitals to the parallel component, σ33, of the shielding tensors of the carbonyl carbon atoms. The neutral d4 Group 5 and 6 complexes [M(n5‐C5H5)(CO)4] (MV, Nb, Ta) and [M(n5‐C5H5)(CO)3CH3] (MCr, Mo, W) exhibit successively smaller but still significant paramagnetic d‐orbital contributions to σ33, consistent with the observed less dramatic deshielding. The three‐legged d6 piano‐stool complexes [M(n5‐C5H5)(CO)3] (MMn, Tc, Re) do not exhibit these reductions of the shielding anisotropy, but have carbonyl 13C shift tensors comparable to regular octahedral carbonyl complexes. The special situation for the four‐legged complexes is related to the presence of high‐lying occupied metal d orbitals, and particularly to the favorable spatial arrangement of these d orbitals with respect to the carbonyl ligands. Bent‐sandwich d2 complexes like [Zr(n5‐C5H5)2(CO)2] exhibit comparable deshielding contributions from an occupied metal d orbital. For similar reasons, the 17O resonances for these piano‐stool and bent‐sandwich complexes are also predicted to be at unusually high frequencies, with low shift anisotropy. NMR shifts for the (n5‐C5H5)‐ligand atoms and the structures of the complexes are also discussed.
Self‐Assembled Monolayers on Gold Nanoparticles
Authors: Badia Antonella , Singh Shanti , Demers Linette , Cuccia Louis , Brown G. Ronald , Lennox R. Bruce 
Published: July 2007

AbstractSelf‐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.
A Modified Cyclodextrin with a Fully Encapsulated Dansyl Group: Self‐Inclusion in the Solid State and in Solution
Authors: Corradini Roberto , Dossena Arnaldo , Marchelli Rosangela , Panagia Anna , Sartor Giorgio , Saviano Michele , Lombardi Angela , Pavone Vincenzo 
Published: July 2007

AbstractA monofunctionalized β‐cyclodextrin containing a dansyl moiety, 6‐ deoxy‐ 6 ‐N‐ (N′‐ (5‐ dimethylamino ‐ 1 ‐ naphthalenesulfonyl)diaminoethane) ‐ β‐cyclodextrin (CD‐en‐DNS,2), was synthesized and its crystal structure determined. It was shown that the dansyl group is fully encapsulated within the cyclodextrin cavity, with the dimethylamino and sulfonyl groups emerging from opposite sides. The shape of the cavity is considerably flattened, since O(4)–O(4) distances parallel to the naphtalene ring were found to be longer than the others. The conformation of the diaminoethane linker was found to be determined by the inclusion of the dansyl group and by a hydrogen bond between the sulfonamide NH and one of the O(6)–H groups on the cyclodextrin rim. The self‐inclusion features of the aromatic moiety were found to be consistent with the solution data:1H NMR ROESY spectra suggested that the orientation of the dansyl moiety observed in the solid state was retained in aqueous solution; the circular dichroism spectrum was consistent with an axial complexation model. Fluorescence spectra showed that the inclusion of the dansyl group in the cyclodextrin cavity considerably increases the quantum yield: time‐resolved fluorescence experiments showed the presence of a long‐lifetime component (16.1 ns), which was attributed to the included fluorophore. The ability of2to act as a fluorescence sensor was evaluated by the addition of several guests of different shape: fluorescence intensity was lowered, especially upon addition of adamantanecarboxylic acid. All the data obtained were consistent with the model of the in‐out movement of the dansyl group from the self‐included conformation observed in the solid state to a position more exposed to the bulk solvent. Copper(II) was shown to enhance the difference in the fluorescence of2in the presence of guests by additional static quenching.
Synthesis, Structure, and Stereoselective Reaction of a Chiral Hydroxy‐Stabilized Metal‐Free Enolate
Authors: Reetz Manfred T. , Hütte Stephan , Goddard Richard , Robyr Chantal 
Published: July 2007

AbstractThe reaction of acetophenone with tetrabutylammonium hydroxide affords the tetrabutylammonium enolate of phenyl (2‐hydroxy‐2‐phenyl)propyl ketone. The crystal structure of this chiral enolate shows intramolecular hydrogen bonding between the hydroxyl group and the enolate oxygen atom. Furthermore, the α‐methylene units of the ammonium counterion form hydrogen bonds to the basic enolate C and O atoms and to the O atom of the hydroxy group. This three‐point bonding occurs selectively on the Re,Re side, a phenomenon which may be responsible for the direction of diastereo‐selectivity in the epoxide‐forming reaction of the enolate with N‐bromosuccinimide.
A New Experimental Method to Determine the Mutual Orientation of Helices in Coiled‐Coil Proteins: Structural Information about the Dimeric Interface of cJun, cFos, GCN4, and gp41
Authors: Kazmierski Wieslaw M. , McDermed John , Aulabaugh Ann 
Published: July 2007

AbstractDisruption of protein dimers interacting by a leucine zipper motif represents a new potential pharmaceutical target. However, structural information concerning the exact nature of the interacting helices is usually not available. Towards this end, we have developed a disulfide‐trapping approach capable of distinguishing between the ad and gd modes of dimerization (Fig. 1), thus providing information useful in the design of small molecules that interfere with helix–helix interactions. We designed and synthesized nine cysteine‐substituted peptide fragments: GCN 4(g), GCN 4(a), GCN 4(d), cFos(g), cFos(a), cFos(d), cJun(g), cJun(a), and cJun(d), and evaluated the covalent crosslinking rates for them and their binary mixtures. Neither homogeneous cJun nor cFos dimerized and crosslinked, but their binary mixtures did with t1/2 of formation a > d > g, indicating to cFos–cJun heterodimerization according to ad mode (Fig. 1 a). Similarly, GCN 4 dimerized and crosslinked in the ad fashion; this result was in excellent agreement with the published X‐ray structure. Next, we investigated the mode of gp 41 dimerization, which appears critical for HIV‐1 replication The gp41 cysteinesubstituted fragments gp 41(g), gp 41(a), and gp 41(d) also dimerized and crosslinked, but with a different order of t1/2 of formation g > d > a, thus providing evidence that gp41 dimerizes in the gd mode (Fig. 1 b). Thus, the crosslinking experiments allow rapid elucidation of structural details of macromolecular interactions in aqueous media. These findings should prove useful in the design of compounds that inhibit macromolecular association.