Self-assembled structures of alkanethiols on gold-coated cantilever tips and substrates for atomic force microscopy: Molecular organisation and conditions for reproducible deposition

Measurements of surface-liquid interactions (contact-angle goniometry) and tip-surface adhesion forces (atomic force microscopy) combined with infrared spectroscopic studies have been used to investigate surface-preparation and solution-deposition conditions for the reproducible formation of self-assembled molecular structures on gold-coated tips and substrates for atomic force microscopy. Preliminary data show that surface-saturated self-assembled monolayers form reproducibly on prolonged (>20 h) exposure of gold-coated glass substrates to ethanolic solutions of ω-functionalised alkanethiols in the concentration range 80-160 mmol dm⁻³. The data also show that exposure for 16 h to alkanethiol concentrations in the range 160-240 mmol dm⁻³ promote bilayer formation whereas concentrations of 240-320 mmol dm⁻³ result in the deposition of multilayers, the average orientation of which is parallel to that of the first molecular layer; the use of parent 1-undecanethiol solutions at concentrations of 1-80 mmol dm⁻³ results in incomplete monolayer coverage.     

Organoselenium chemistry : A study of intermediates in the fragmentation of aliphatic ketoselenoxides. Characterization of selenoxides,selenenamides and selenolseleninates by 1H-,13C-and 77Se-NMR

A series of β-ketoselenenic acids was generated at low temperature ( - 20° to - 50°) by selenoxide syn elimination of appropriate selenoxides (13-ox, 16-ox, 35-ox, 38-ox, and 39-ox). No evidence for the buildup of significant concentrations of selenenic acid was obtained. A selenolseleninate (15, 2,2' - diseleno - bis(1 - phenyl - 2 - methyl -1 - propanone) - Se - oxide) was detected as an intermediate in the decomposition of 13-ox and 16-ox. This compound, which is stable in solution below - 50° was charaeterized by NMR spectroscopy (¹H, ¹³C,⁷⁷Se) and by its thermal decomposition and reactions with phosphite (reduction to diselenide 6) and dialkylamines (formation of selenenamide 11). Decomposition of 15 in the presence of dibenzylamine resulted in trapping of a selenenic acid-like species (RSeSeOH) to give RSeSeN(CH₂Ph)₂ (R = PhC(O)C(CH₃)₂). Although 15 could not be prepared by oxidation of diselenide 6, it was possible to prepare a cyclic selenolseleninate (4,4-dimethyl-1,2-diselenolane monoxide, 20) by oxidation of the related diselenide (19). Attempts to prepare more stable aliphatic selenenic acids by blocking the principal decomposition pathway of 15 were not successful. Thus 1 - benzoyl -1 - cyclopropaneselenenic acid was generated from 35-ox and 38-ox and 1 - benzoyl - 2,2 - dimethylcyclopropaneselenenic from 39-ox. The former underwent normal disproportionation (to 36 and 37) even when prepared at -49°. The latter gave what appeared to be a selenolseleninate (40) which again disproportionated at -17°.     

Phosphine-stabilized arsenium salts: water-stable,labile, coordination complexes

A series of air- and water-stable tertiary phosphine-stabilized arsenium salts of the type R(3)P-->AsR(2)(+)PF(6)(-) has been isolated. In the crystal structures of two chiral triarylphosphine complexes of prochiral methylphenylarsenium hexafluorophosphate, the stereochemistry around arsenic is trigonal pyramidal with the phosphorus atom occupying the apical position, the As-P bond being orthogonal to the plane of the trigonal (lone-pair included) arsenium ion: Ph(3)P-->AsMePh(+) PF(6)(-), P2(1)/c, a = 10.7775(2) A, b = 17.7987(3) A, c = 13.3797(2) A, beta = 109.066(1) degrees, V = 2425.78(7) A(3), T = 200 K, Z = 4; Ph(2)(2-MeOC(6)H(4))P-->AsMePh(+) PF(6)(-), P1, a = 10.8077(2) A, b = 10.9741(2) A, c = 13.5648(2) A, alpha = 99.0162(9) degrees, beta = 105.2121(9) degrees, gamma = 116.4717(9) degrees, V = 1318.11(5) A(3), T = 200 K, Z = 2. The arsenium ion in each case appears to be further stabilized by conjugation of the lone pair with the phenyl group, with which the arsenic and methyl-carbon atoms are almost coplanar. In the crystal structure of the 2-(methoxymethylphenyl)diphenylphosphine adduct of methylphenylarsenium hexafluorophosphate, there operates a counteractive chelate effect in which anchimeric oxygen coordination to arsenic destabilizes the arsenic-phosphorus bond in the six-membered chelate ring. Although they are stable, phosphine-stabilized arsenium salts undergo rapid phosphine exchange and attack at arsenic by anionic carbon and oxygen nucleophiles to give tertiary arsines and arsinous acid esters, respectively, with liberation of the phosphine.     

Polymeric Structure and Solid ~(31)P, ~(113)Cd NMR Spectra of Cadmium(II) Dialkyldithiophosphate (Alkyl = n-butyl and i-butyl)

Owing to the particular affinity toward metal ion and changeable dentation, dialkyldithiophosphates are commonly used ligands in preparation of the transition metal complexes with variable structures1. For instances relevant to this work, the IIB metal complexes defined crystallographically so far have provided the examples as follows2-12 (Scheme 1). Scheme 1 The frameworks of IIB metal complexes of dialkyldithiophosphate MMMMMMMMMM a b c d …     

Multicomponent coupling approach to (+/-)-frondosin B and a ring-expanded analogue

[reaction: see text] A recently discovered multicomponent coupling reaction is used to give direct access to a late intermediate in the synthesis of frondosin B. This intermediate can also be efficiently converted to a ring-expanded analogue of frondosin B by sustained heating of the reaction mixture. An unprecedented tandem 1,7-hydrogen shift, 8pi-electrocyclization is proposed to explain the formation of this ring-expanded species.     

PHOTOCHEMISTRY OF TYPE I ACID-SOLUBLE CALF SKIN COLLAGEN: DEPENDENCE ON EXCITATION WAVELENGTH

Although previous studies have demonstrated that the predominant photochemistry of type I collagen under 254 nm irradiation may be attributed either to direct absorption by tyrosine/phenylalanine or to peptide bonds, direct collagen photochemistry via solar UV wavelengths is much more likely to involve several age- and tissue-related photolabile collagen fluorophores that absorb in the latter region. In this study, we compare and contrast results obtained from irradiation of a commercial preparation of acid-soluble calf skin type I collagen in solution with UVC (primarily 254 nm), UVA (335–400nm) and broad-band solar-simulating radiation (SSR; 290^1–00nm). Excitation spectroscopy and analysis of photochemically induced disappearance of fluorescence (fluorescence fading) indicates that this preparation has at least four photolabile fluorescent chromophores. In addition to tyrosine and L-3,4-dihydroxyphenylalanine, our sample contains two other fluorophores. Chromophore I, with emission maximum at 360 nm, appears to be derived from interacting aromatic moieties in close mutual proximity. Chromophore II, with broad emission at430–435 nm, may be composed of one or more age-related molecules. Collagen fluorescence fading kinetics are sensitive to excitation wavelength and to conformation. Under UVC, chromophore I fluorescence disappears with second-order kinetics, indicating a reaction between two proximal like molecules. Adherence to second-order kinetics is abrogated by prior denaturation of the collagen sample. A new broad, weak fluorescence band at400–420 nm, attributable to dityrosine, forms under UVC, but not under solar radiation. This band is photolabile to UVA and UVB wavelengths. Amino acid analysis indicates significant destruction of aromatic amino acids under UVC, but not under UVA or SSR. When properly understood, collagen fluorescence fading phenomena may act as a sensitive molecular probe of structure, conformation and reactivity.     

Factors affecting propagating fronts of addition polymerization: Velocity,front curvature,temperatue profile,conversion, and molecular weight distribution

Several properties of propagating fronts of addition polymerization were studied. A power function could be fit to the velocity dependence on initiator concentration, but not with the exponents predicted by current models or in agreement with other published work. Bubbles from the volatile by-products of initiator decomposition were found to affect the front velocity and curvature. The front velocity for triethylene glycol dimethacrylate polymerization was found to depend linearly on temperature over a moderate range. The conversion of methacrylic acid in fronts varied greatly with initiator type and concentration. Benzoyl peroxide produced much lower conversion than t-butyl peroxide, but fronts with tBPO propagated slower. A dual initiator system of BPO and tBPO produced rapidly propagating fronts with good conversion but the contribution of each initiator to the velocity was not additive. The possibility of chain branching was considered. The apparent molecular weight distributions were very broad, often trimodal, and found to depend on initiator type and concentration as well as the tube diameter. The temperature profiles were measured and found to be very sharp for BPO and broader for tBPO but both had front temperatures in excess of 200°C, indicating a high ceiling temperature. © 1995 John Wiley & Sons, Inc.     

Syntheses and Structures of Two New Cu/Nb/pyrazine Complexes: Three dimensional CuNb(pyz)2OF5 · (pyz)(H2O) and two dimensional [Cu(pyz)2.5]+ [NbF6]− · (pyz)

Crystals of CuNb(pyz)₂OF₅ · (pyz)(H₂O) ( 1 ) and [Cu(pyz)_2.5]⁺ [NbF₆]^? · (pyz) ( 2 ) were grown (150°C and autogeneous pressures) from CuO, 1/2(Nb₂O₅), (HF)_x · pyridine, and H₂O in excess pyrazine. Light blue single crystals of ( 1 ) are orthorhombic, crystallizing in space group Cccm (No. 66), with a = 14.547(1) Å, b = 16.135(2) Å, c = 13.803(2) Å, and Z = 8. The structure of ( 1 ) contains corner shared [Cu(pyz)_4/2F_2/2]⁺, [Cu(pyz)_4/2O_2/2], and [NbF₄O_1/2F_1/2]^?0.5 octahedra. Orange crystals of ( 2 ) are monoclinic, crystallizing in space group C2/c (No. 15), with a = 11.792(8) Å, b = 17.123(3) Å, c = 17.051(5) Å, β = 90.04(4)°, and Z = 8. The structure of ( 2 ) contains puckered rings of corner shared [Cu(pyz)(pyz)_3/2]⁺ tetrahedra and isolated [NbF₆]^? anions within the rings.     

Efficient synthesis of an imidazole-substituted delta-amino acid by the integration of chiral technologies

Two methods to produce (2S)-5-amino-2-(1-n-propyl-1H-imidazol-4-ylmethyl)-pentanoic acid were investigated. Diastereoisomeric salt resolution, using the quinidine salt, gave the desired intermediate in 98% ee and 33% yield. Asymmetric hydrogenation of various substrates gave high conversions, with up to 83% ee. Integration of these two approaches via asymmetric hydrogenation of a quinidine salt substrate followed by crystallization provided the desired intermediate in 94% ee and 76% yield.