bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synthesis of Rigid Receptors Based on Triphenylene Ketals
Authors: Schopohl Matthias C. , Faust Andreas , Mirk Daniela , Fröhlich Roland , Kataeva Olga , Waldvogel Siegfried R. 
Published: July 2005

AbstractThe synthesis of rigid receptors based on triphenylene ketals, including some improved procedures, is described in detail. Since chemical transformations are strongly influenced by the rigid character and steric bulkiness of the receptors, the construction of a subunit allowing quicker synthetic development is also reported. The preparation of these receptor structures involves an oxidative trimerization and a subsequent repeated isomerization procedure. The scope for the attachment of sterically demanding affinity groups on the receptor scaffold and the corresponding subunit is discussed, and the molecular structures and available space for molecular recognition of the chirally modified systems are outlined by some crystal structures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Liquid Crystals Based on Hypervalent Sulfur Fluorides: Exploring the Steric Effects of ortho‐Fluorine Substituents
Authors: Kirsch Peer , Hahn Alexander 
Published: July 2005

AbstractLiquid crystals that have ortho‐fluorinated pentafluoro‐λ6‐sulfanyl terminal groups are the most polar materials that are still compatible with current active matrix LCD technology. The synthesis of the first examples of this class of substance has been achieved by direct fluorination. The physico‐chemical characterization, supported by DFT calculations, revealed some surprising features of these new compounds: the bulky SF5 group is easily deformed and responds in a quite unusual way to the steric strain exerted by its ortho substituents. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Novel Chiral (Salen)MnIII Complexes Containing a Calix[4]arene Unit as Catalysts for Enantioselective Epoxidation Reactions of (Z)‐Aryl Alkenes
Authors: Amato Maria E. , Ballistreri Francesco P. , Pappalardo Andrea , Tomaselli Gaetano A. , Toscano Rosa M. , Williams David J. 
Published: July 2005

AbstractNew asymmetric (salen)MnIII and UO2 complexes containing a calix[4]arene unit in the ligand framework were synthesized. The UO2 complexes were characterized by 1H‐, 13C‐, 2D TOCSY and T‐ROESY NMR spectroscopy. Furthermore, the structure of one UO2 complex was determined by single‐crystal X‐ray analysis. The data showed that UO2 complexes, which can be considered in first approximation models of the Mn=O oxidant active species, possess a chiral pocket and adopt relevant conformations for the selectivity of the oxygen transfer process. Epoxidation data of model alkenes with the MnIII complexes showed moderate ee values and were not conclusive in indicating that the calix[4]arene unit might be able to influence the selectivity by a molecular recognition mechanism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Asymmetric γ‐Deprotonation and Substitution Reactions of (Z)‐1,3‐Diphenyl‐1‐propenyl N,N‐Diisopropylcarbamate
Authors: Reuber Jenny , Fröhlich Roland , Hoppe Dieter 
Published: July 2005

Abstract(Z)‐1,3‐Diphenyl‐1‐propenyl N,N‐diisopropylcarbamate (6d) is deprotonated by n‐butyllithium/(–)‐sparteine (5) with a high degree of enantiotopic differentiation in the γ‐position to form the enantiomerically enriched allyllithium derivative 7d. Despite of its high mesomeric stabilization it shows a high degree of configurational stability. Trapping with carbonyl electrophiles proceeds exclusively in a syn‐SE' substitution as shown by direct assignments and stereochemical correlations. Thus, the involvement of a η3 complex is suggested. After transfer to the analogous allyltitanates 16 enantiomerically and diastereomerically pure homoaldol products 17 are furnished. Evidence for a competing retro‐homoaldol reaction is reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Multiple Vinylation of Tribenzotriquinacenes and Fenestrindanes at Their Aromatic Peripheries by Use of Nájera’s Catalyst
Authors: Cao Xiao‐Ping , Barth Dieter , Kuck Dietmar 
Published: July 2005

AbstractHighly efficient six‐ and eightfold Heck cross‐coupling reactions were carried out with two related convex‐concave and saddle‐shaped hydrocarbons bearing several ortho‐dibromo functionalities at the molecular periphery, viz. the hexabromotribenzotriquinacene 3a and the octabromofenestrindane 4a. In contrast to conventional palladium catalysts, use of Nájera’s oxime‐derived palladacycle catalyst 7 effected exhaustive vinylation with styrene or methyl acrylate in high yield and without concomitant formation of partial cross coupling or reduction products. The hexavinyltribenzotriquinacenes and octavinylfenestrindanes thus obtained are of interest as potential building blocks for novel organic networks. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
p‐Nitrobenzyloxycarbonyl (pNZ) as a Temporary Nα‐Protecting Group in Orthogonal Solid‐Phase Peptide Synthesis – Avoiding Diketopiperazine and Aspartimide Formation
Authors: Isidro‐Llobet Albert , Guasch‐Camell Judit , Álvarez Mercedes , Albericio Fernando 
Published: July 2005

Abstractp‐Nitrobenzyloxycarbonyl (pNZ) was used as a temporary protecting group for α‐amino functionalities in solid‐phase peptide synthesis. The corresponding derivatives are readily synthesized solids that perform well on solid phase. The pNZ moiety is orthogonal with the most common protecting groups used in peptide chemistry, and is removed under neutral conditions in the presence of catalytic amounts of acid. The use of pNZ derivatives in conjunction with Fmoc chemistry circumvents typical side reactions associated with the use of piperidine, such as DKP and aspartimide formation. The flexibility of pNZ can be exploited for the preparation of libraries of small organic molecules. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Preparation of Cyclopropane Analogues of the Natural AntibioticTAN 1057 A/B
Authors: Kordes Markus , Brands Michael , Es‐Sayed Mazen , de Meijere Armin 
Published: July 2005

AbstractThe two new analogues 2 and 3 of the natural dipeptide antibiotic TAN 1057 A/B (1), both with cyclopropyl containing β‐amino acid side chains, were prepared. The synthesis of the corresponding β‐amino acids in appropriately protected form (4‐Z3 and 5‐Z2, respectively) from the correspondingly protected (hydroxyethyl)‐ (7a) and (hydroxymethyl)‐substituted cyclopropylideneacetate (7b), prepared in two steps each from homoallyl and allyl benzyl ether, respectively, was achieved in nine and seven steps, respectively, with overall yields of 17 (4‐Z3) and 9.3 % (5‐Z2), respectively. Coupling with the N‐methyldihydropyrimidinone 22 and deprotection gave the compounds 2 and 3, which turned out to be active against methicillin resistant strains of Staphylococcus aureus, albeit to a lesser extent than TAN 1057 itself. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Introduction of Aromatic and Heteroaromatic Groups in the 2‐ and 8‐Positions of the Tröger’s Base Core by Suzuki, Stille and Negishi Cross‐Coupling Reactions – A Comparative Study
Authors: Solano Carlos , Svensson Daniel , Olomi Zarina , Jensen Jacob , Wendt Ola F. , Wärnmark Kenneth 
Published: July 2005

AbstractA comparative study on the bis(functionalization) of the Tröger’s base core by aromatic and heteroaromatic groups using palladium‐catalyzed cross‐coupling reactions is presented. Three different reactions, the Suzuki, Stille, and Negishi couplings, were investigated using Tröger’s base analogues equally substituted in the 2,8‐positions with (HO)2B, Bu3Sn and ZnCl groups, respectively, as the metallated component. Six aryl halides with different electronic and steric properties were employed as coupling partners. The presence of the bulky and electron‐rich phosphane P(tBu)3 as co‐catalyst was found to play an important role. In addition, the palladium source, [Pd(PPh3)4] or [Pd2(dba)3], was also found to be an important factor for the yield of the reactions. The Suzuki coupling was found to be the best method in general, giving excellent yields for most aryl halides, whereas the Stille and Negishi couplings gave moderate to good yields. Finally, the crystal structures of the 4‐nitrophenyl‐ and the 2‐pyridyl‐appended analogues of Tröger’s base, 7d and 7f, are presented in the Supporting Information. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Solid‐Phase Synthesis of Cyclic C‐Glycoside/Amino Acid Hybrids by Carbamate Coupling Chemistry and On‐Support Cyclization
Authors: Katajisto Johanna , Lönnberg Harri 
Published: July 2005

AbstractA solid‐supported synthesis of cyclic C‐glycoside/amino acid conjugates is described. For this purpose, N‐(tert‐butoxycarbonyl)‐[6‐O‐(p‐nitrophenoxycarbonyl)‐2,3,4‐tri‐O‐(p‐toluoyl)‐β‐D‐glycopyranosyl]methylamines derived from galactose and glucose were prepared and used as activated monomers together with appropriately protected amino acids. The solid phase assembly of the linear precursor was conducted by alternating peptide and carbamate coupling, using glutamate immobilized through its γ‐carboxy function to a SCAL linker as a handle. On‐support cyclization followed by cleavage from the support gave the desired conjugates. Removal of the p‐toluoyl protections was carried out in solution phase by methoxide ion‐catalyzed transesterification in methanol. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Mapping single‐molecule SERRS from Langmuir–Blodgett monolayers on nanostructured silver island films
Authors: Goulet Paul J. G. , Pieczonka Nicholas P. W. , Aroca Ricardo F. 
Published: July 2005

AbstractTo explore the breakdown of ensemble averaging as the single molecule regime is approached, spatial mapping of surface‐enhanced resonance Raman scattering (SERRS) intensities was employed in the study of mixed dye–fatty acid Langmuir–Blodgett (LB) monolayers deposited on nanostructured Ag island films. By variation of the ratio of the two components in the films, the effects of dye concentration, on both SERRS spectra and LB monolayer architecture, were explored down to the single‐molecule level. Insight was gained into the nature of areas of intense local electromagnetic field strength (i.e. ‘hot spots’) that provide enormous enhancement of Raman signals, and their wavenumber of occurrence on nanostructured Ag island films. These enhancing films were further characterized by UV–visible surface plasmon absorbance and atomic force microscopy. The target analyte employed in this work, n‐pentyl‐5‐salicylimidoperylene, was dispersed in monolayers of arachidic acid on Ag nanostructured films for single point and 2D mapping SERRS experiments. The optical characterization of this dye was completed with solution‐phase molecular absorption and fluorescence, in addition to monolayer resonance Raman scattering (RRS). Single‐molecule SERRS mapping results suggest that electromagnetic hot spots on Ag island films are in fact highly localized on the nanoscale, corresponding to relatively few molecular sites, and hence also stress the rarity of coincidence between hot spots and single isolated molecules as a key consideration in ultrasensitive SERRS measurements of this type. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced hyper Raman scattering in silver colloidal solutions
Authors: Kneipp Harald , Kneipp Katrin 
Published: July 2005

AbstractWe report surface‐enhanced hyper Raman scattering (SEHRS) from molecules in silver colloidal solutions using high repetition rate mode‐locked picosecond excitation at 1064 nm. SEHRS was is studied under non‐resonant and resonant conditions by choosing adenine and crystal violet as test molecules. Compared with its SERS spectrum, the SEHRS spectrum of adenine displays strong ‘new’ lines, which, in agreement with theory, in most cases also show up in the infrared absorption spectrum. We also explore the capability of SEHRS for trace detection and demonstrate detection and identification of crystal violet based on its SEHRS signals down to nanomolar concentrations. Copyright © 2005 John Wiley & Sons, Ltd.
Electronic effects in SERS by liquid water
Authors: Chen Yan‐Xia , Otto Andreas 
Published: July 2005

AbstractBy exploiting the attenuated total reflection (ATR)–Otto configuration, surface‐enhanced Raman scattering of water at monocrystalline copper and silver electrodes in 0.1 M Na2SO4 was observed at cathodic electrode potentials with hydrogen evolution at a laser wavelength of 647.1 nm. The proposed enhancement model is based on the condition that the upshift of the Fermi level of the electrode and the absorption of one laser photon allow an electron of the electrode at the Fermi level to reach the lower level of the energy band of delocalized electrons in liquid water. The electron interaction with the water molecules in the first hydration shell of cations in the Helmholtz plane is so‐called impulse scattering, which is the short‐time, non‐resonant excursion to the anionic ground state of molecules known for a long time in low‐energy electron scattering from free molecules. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced resonance Raman scattering using pulsed and continuous‐wave laser excitation
Authors: Littleford Rachael E. , Cunningham Dale , Matousek Pavel , Towrie Michael , Parker Anthony W. , Khan Imran , McComb David , Ewen Smith W. 
Published: July 2005

AbstractPulsed and continuous‐wave (CW) lasers were compared as excitation sources for surface‐enhanced resonance Raman scattering (SERRS). CW excitation provided SERRS spectra with a greater signal‐to‐noise ratio and more sensitive detection by a factor of ∼50 compared with the high peak power, low repetition rate pulsed configuration used. The SERRS intensity using a pulsed laser produced a non‐linear response with respect to changes in power of the laser. At powers of less than ∼0.012 mW, the absolute intensity under the peaks of the CW and pulsed SERRS spectra converged, suggesting that lower peak power, high repetition rate systems may be more effective excitation sources for SERRS. Transmission electron microscopy of pulsed laser‐irradiated silver particles showed significant sample damage and morphological changes. This problem was overcome with the use of a recirculating large‐volume flow cell system, providing a fresh sample for each measurement. A picosecond‐resolved time delay experiment found that SERRS intensity decreased by ∼60% when exposed to a 400 nm pump pulse and probed with a 529 nm pulse. As the time delay between pump and probe increased the system recovered gradually to ∼60% of the original SERRS intensity after 50 ps, where it remained constant. This suggests that the surface bonding between the silver and the dye is significantly perturbed, with some nanoscale diffusion occurring of the dye away from the metal surface. Hence chemical enhancement is temporarily prevented and electromagnetic enhancement is reduced as a function of 1/r3 as the dye moves away from the surface. Additionally, transient heating of the colloidal particles caused by the pulsed laser may also lead to plasmon shifts and changes in absorption intensity. Other factors such as surface annealing or decomposition of the silver particle or dye due the extreme temperature conditions may account for the permanent loss in SERRS intensity. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced Raman scattering from oxazine 720 adsorbed on scratched gold films
Authors: Brolo Alexandre G. , Addison Christopher J. 
Published: July 2005

AbstractSurface‐enhanced Raman scattering (SERS) and surface‐enhanced resonance Raman scattering (SERRS) from oxazine 720 (oxa) dye adsorbed on scratched gold films are reported. The SERS‐active surface was prepared by performing a series of scratches in a 100 nm thick gold film deposited in glass. Atomic force microscopic imaging revealed a sub‐structure within the scratches containing a set of parallel gold wires of different sizes and shapes. The 1‐D order imposed by the parallelism between these wires is responsible for an interesting polarization effect observed in forward scattering experiments. It is shown that the maximum enhanced signal is observed when the polarization of the incident field is perpendicular to the direction of the scratches. This polarization discrimination may be useful in the design of SERS applications in chemical sensing and optical switching. Moreover, we also show that these scratched gold surfaces can be used as ordinary SERS substrates for experiments in backscattering using a common Raman microscope in non‐resonance conditions with the excitation energy. This was accomplished by obtaining the electrochemical SERS of oxa in situ (under electrochemical control). The potential dependence of the SERS from oxa adsorbed on scratched Au is compared with previous results obtained with Ag electrodes. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced Raman spectroscopy with ultraviolet excitation
Authors: Lin Xu‐Feng , Ren Bin , Yang Zhi‐Lin , Liu Guo‐Kun , Tian Zhong‐Qun 
Published: July 2005

AbstractSurface‐enhanced Raman scattering (SERS) spectroscopy excited with a UV laser was successfully developed and the UV–SER spectra of various adsorbates, including pyridine and SCN−, on different transition metal electrodes were obtained. The experimental requirements for obtaining UV–SERS in an electrochemical system are given. The surface enhancement factor of a roughened Rh electrode covered with thiocyanate as a model molecule was estimated to be about two orders of magnitude in the UV region, consistent with our preliminary theoretical calculation based on the electromagnetic model. The investigation of SERS in the UV region will improve the understanding of the SERS enhancement mechanism and broaden the research field of SERS in areas such as surface science and the life sciences. Copyright © 2005 John Wiley & Sons, Ltd.
Investigations of chemisorption and reaction at non‐aqueous electrochemical interfaces by in situ surface‐enhanced Raman spectroscopy
Authors: Cao Peigen , Sun Yuhua , Gu Renao 
Published: July 2005

AbstractSurface‐enhanced Raman scattering (SERS) spectroscopy has been successfully extended to transition metal/non‐aqueous electrochemical interfaces recently in our laboratory. Some interesting and new findings are presented with the objective of illustrating the virtues of SERS as a versatile technique to examine not only aqueous but also non‐aqueous interfaces. Emphasis is placed on highly catalytic roughened platinum metal surfaces. Static interfacial structure information of Pt (Ag, Au)/acetonitrile was extracted from the relevant SERS spectral features of the surface species, including interfacial trace water, the production of dissociation reactions of acetonitrile (CN−), and some effects on these spectra exerted by interfacial inorganic ions. The simultaneous competitive adsorption and reorganization processes between these interfacial species and some intentionally added small organic molecules were also systematically investigated. The important aspect of the corrosion inhibition mechanism relevant to non‐aqueous media was then probed based on the success of observing SERS from non‐aqueous interfaces. These included mainly widely used N‐ and S‐containing inhibitors for iron, nickel, silver and gold. Finally, the archetypal surface catalytic oxidation mechanism of C1 molecules (formic acid and methanol) on Pt group metals was investigated by using SERS combined with traditional electrochemical techniques. The non‐linear vibrational Stark tuning effect of the poisoning intermediate CO and its oxidation processes are also described in comparison with experimental observations from aqueous solution and other metal surfaces. Copyright © 2005 John Wiley & Sons, Ltd.
Some anomalies of the SER spectra of symmetrical molecules adsorbed on transition metal substrates: consideration by the dipole–quadrupole SERS theory
Authors: Polubotko A. M. 
Published: July 2005

AbstractThe SERS spectra of pyridine, pyrazine and benzene, adsorbed on Ni and some other transition metals are analyzed from the point of view of the dipole–quadrupole SERS theory. There are some anomalies in the regularities of these spectra with respect to those with the molecules adsorbed on Ag, Cu and Au. The breathing mode may not be the most enhanced. Although quadrupole interactions were not found precisely in the spectra of pyridine and pyrazine, they manifest in the SERS spectra of benzene adsorbed on silica‐supported Ni. From the analysis of the enhancement coefficient and some another points, we consider that these anomalies may be associated with the larger influence of the dipole interaction or a smaller role of the quadrupole interactions for the considered molecules and metals and also with some features of the complex dielectric constants in the above systems. Further, a strong influence of the minor quadrupole moments on the enhancement factor of strongly distorted benzene was found. This result is very important, because it indicates the complexity of the SERS dipole–quadrupole theory. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced Raman spectroscopy of bacteria: the effect of excitation wavelength and chemical modification of the colloidal milieu
Authors: Zeiri L. , Efrima S. 
Published: July 2005

AbstractSurface‐enhanced Raman spectroscopy (SERS) of bacteria infused with silver or gold colloids reflects the different biochemicals in the immediate, nano‐vicinity of the particles. In particular we demonstrate high sensitivity to flavins, important coenzymes usually associated with the cell wall. Varying the type of colloid, the laser excitation wavelength and the chemical protocols enables us to observe spectral signatures from different biochemical components of the microorganism. Specifically, we can switch between normal Raman, resonance Raman, and SERS, altering the relative spectral contributions of the various cell constituents. Copyright © 2005 John Wiley & Sons, Ltd.
Density functional theory study of surface‐enhanced Raman scattering spectra of pyridine adsorbed on noble and transition metal surfaces
Authors: Wu De‐Yin , Duan Sai , Ren Bin , Tian Zhong‐Qun 
Published: July 2005

AbstractPyridine as a typical model molecule has been most extensively studied by SERS for three decades. However, there has not been a satisfactory explanation for the dramatic change of the relative intensity of the two main bands assigned to the total symmetric modes at 1008 and 1035 cm−1 when pyridine was adsorbed on silver and many other metals. We performed a hybrid density functional theory study of the binding interaction and the Raman spectral properties of pyridine–metal clusters. The calculated Raman scattering factors of total symmetric modes for the pyridine–metal complexes indicate that the bonding interaction influences the relative Raman intensity of the vibrational modes. The surface bonding mechanism was analyzed on the basis of molecular orbital theory. It was also shown that the binding interaction will lead to geometric changes and result in metal‐dependent surface Raman spectra. This theoretical approach could be helpful in using SERS to study surface chemistry, in particular to extract valuable information about surface bonding and adsorption through calculating and analyzing the relative intensity of SERS spectra. Copyright © 2005 John Wiley & Sons, Ltd.