Showing papers in "Journal of the American Chemical Society in 1989"

Formation of monolayer films by the spontaneous assembly of organic thiols from solution onto gold

Abstract: : Long-chain alkanethiols, HS(CH2)nX, adsorb from solution onto gold surfaces and form ordered, oriented monolayer films. The properties of the interfaces between the films and liquids are largely independent of chain length when n > 10; in particular, wetting is not directly influenced by the proximity of the underlying gold substrate. The specific interaction of gold with sulfur and other soft nucleophiles and its low reactivity toward most hard acids and bases make it possible to vary the structure of the terminal group, X, widely and thus permit the introduction of a great range of functional groups into a surface. Studies of wettability of these monolayers, and of their composition using X-ray photoelectron spectroscopy (XPS), indicate that the monolayers are oriented with the tail group, X, exposed at the monolayer-air or monolayer- liquid interface. The adsorption of simple n-alkanethiols generates hydrophobic surfaces whose free energy (19 mJ/sq. m) is the lowest of any hydrocarbon surface studied to date. Measurement of contact angles is a useful tool for studying the structure and chemistry of the outermost few angstroms of a surface. This work used contact angles and optical ellipsometry to study the kinetics of adsorption of monolayer films and to examine the experimental conditions necessary for the formation of high-quality films.

Voltammetric studies of the interaction of metal chelates with DNA. 2. Tris-chelated complexes of cobalt(III) and iron(II) with 1,10-phenanthroline and 2,2'-bipyridine

Abstract: Voltammetric methods were used to probe the interaction (electrostatic or intercalative) of metal complexes, ML 3 3+/2+ , with calf thymus DNA. Binding constants (K n+ ) and binding site sizes (s) were determined from voltammetric data. The exact magnitude for the parameters depends on whether the ML 3 3+/2+ /DNA reaction is assumed to be static (S) or mobile (M) within the characteristic time of a voltammetric experiment

An internal-coordinate Monte Carlo method for searching conformational space

Abstract: An internal coordinate, random-search method for finding the low-energy conformations of organic molecules is described. The search is biased toward the low-energy regions of conformational space by choosing starting geometries for each step in the conformational search from among previously stored low-energy conformers. Tendencies of the search method to concentrate in certain regions of conformational space at the expense of others are reduced by uniform usage of stored structures as starting geometries and by using varying numbers or torsional rotations in each step. Tests of the procedure's ability to find all low-energy conformations of several acrylic, medium- and large-ring molecules are described. Direct comparisons of the method with Cartesian coordinate random methods and systematic internal coordinate grid searches are carried out with symmetrical and unsymmetrical molecules. The method makes it possible to carry out global conformational searches on symmetrical and unsymmetrical molecules having up to a dozen variable torsion angles.

Formation of monolayers by the coadsorption of thiols on gold: variation in the head group, tail group, and solvent

Abstract: Long-chain alkanethiols, HS(CH2)nX, adsorb from solution onto gold and form oriented. ordered monolayers. Monolayers exposing more than one functional group at the surface can be generated by coadsorption of t*'o or more thiols from solution. In general, the ratio of the concentration of the t\r'o components in a mixed monolayer is not the same as in solution but reflects the relative solubilities of the components in solution and interactions between the tail groups, X, in the monolayers. Multicomponent monolayers do not phase-segregate into single-component domains large enough to influence the contact angle (a few tens of angstroms across) and also do not act as ideal two.dimensional solutions. In the two-component system HS(CH2)J/HS(CH2)"CH3 in ethanol, where X is a polar tail group such as CH2OH or CN, adsorption of the polar component is particularly disfavored at low concentrations of the polar component in the monolayer. These isotherms may arise from poor solvation of the polar tail groups in the quasi-two-dimensional alkane solution provided by the methyl tail groups. From dilute solutions in alkanes, adsorption of HS(CH2)r9CH2OH is strongly preferred over HS(CHz)roCHr, probably due to the stabilization afforded by intramonolayer hydrogen bonds between the hydroxyl tail groups. The wettability of mixed monolayers is not linear in the composition of the surface. (n a surface comprised of a polar and a nonpolar component, the polar component is more hydrophilic when its concentration in the monolayer is low than when the monolayer is composed largely of the polar component.

Molecular mechanics. The MM3 force field for hydrocarbons. 3. The van der Waals' potentials and crystal data for aliphatic and aromatic hydrocarbons

Abstract: The van der Waals' potentials used for interactions between carbon and hydrogen in both aliphatic and aromatic systems have been improved from those available in MM2, and the new values are used in MM3. The atoms are slightly larger and somewhat softer than they were with MM2. These values were optimized by fitting to the crystal parameters (six cell constants) and the heats of sublimation for the normal alkanes from C6 to C,,, plus C,2, and also diamond, graphite, benzene, biphenyl, and hexamethylbenzene, in addition to fitting structural and energy data on congested molecules as reported earlier. The parameters developed give good crystal structures and heats of sublimation for these molecules. Biphenyl is calculated to be twisted about 40' in the gas phase, but lattice forces cause it to flatten into a planar conformation in the crystal.

The nature of halogen.cntdot..cntdot..cntdot.halogen interactions: are short halogen contacts due to specific attractive forces or due to close packing of nonspherical atoms?

Abstract: Revue des differentes interpretations des interactions intermoleculaires courtes et application au cas des interactions halogene-halogene dans un cristal. Pour determiner leur nature, utilisation d'une approche statistique. Les resultats demontrent la nature attractive et anisotropique des interactions Cl--Cl, Br--Br, et I--I dans des cristaux moleculaires

Two-dimensional infrared spectroscopy

Abstract: Two-dimensional infrared (2D IR) spectroscopy, a novel technique based on time-resolved IR spectroscopy, is introduced In 2D IR, a system is excited by an external perturbation, which induces a dynamic fluctuation of the IR spectrum A correlation analysis is applied to the time-dependent IR signals to yield a spectrum defined by two independent wavenumbers By spreading IR peaks over the second dimension, a complex spectrum consisting of overlapped peaks can be substantially simplified, and spectral resolution is enhanced Peaks located on a 2D spectral plane provide information on connectivity and interactions among functional groups associated with the IR bands 2D IR spectra are presented for a system consisting of a mixture of atactic polystyrene (PS) and low-density polyethylene (PE) to illustrate these features The spectroscopic evidence clearly shows PS and PE in a blend are segregated at the molecular level, allowing the components to respond to an applied external perturbation independently of each other

Formation of Monolayers by the Coadsorption of Thiols on Gold: Variation in the Length of the Alkyl Chain

Abstract: when X = Y = oH' the pure monolayers a." *eti"d by water, but the mixed monolayers are less hydrophilic because nonpolar polymethylenecha insareexposedatt hesurface. whinX =cHr, Y = oH (n=2L,m= ll),averysharptr ansitionoccurs from a monolayer composed largely of the longer, methyl-terminated component to the shorter, hydroxyl-terminated component as the mole fraction of HS(CHz) r roH in-the adsorption solution is increased. From solutions .ontuining two thiols, adsorption of the thiol with the longer chain is preferred. Tiris preference is greater when the monolayers are adsorbed from ethanol than from isooctane' The mixed monolayers do not uit ", ideal two-dimensional solutions. T-he adsorption isotherms suggest a positive excess free energy of mixing of the two components in the monolayer. The compositions of the monolayers appear to be determined largely by thermodynamics, although in some cases there is also a kinetic contribution. The two components in the mixed monolayers do not phase-segregate into macroscopic islands (greater than a few tens of angstroms across) but are probably not randomly dispersed within the monolayer, ThL wettability of mixed, methyl-terminatea can be partially rationalized by the geometric m€an approximition, but a full description probably requires inclusion of the entropy of mixing at the monolayer-liquid interface. The hysteresis in the contact angle on these monolayers cannot be explained by theories of wetting based on macroscopic heterogeneity. Contact angles ur? -o.. sensitive than optical ellipsometry or X-ray photoelectron spectroscopy to certain types of changes in the coriposition and structure of these monolayers.

A new population analysis based on atomic polar tensors

Abstract: A new population analysis which provides values of atomic charges in molecules is proposed. The atomic charge is related to trace of the corresponding atomic polar tensor (tensor of the derivatives of dipole moment with respect to atomic Cartesian coordinates). The resulting electron populations do not require any direct reference to the basis set used, and they can be calculated for both HF and correlated wave functions. The charges are computed for several molecules and compared with the results of Mulliken population analysis.

Electronegativity is the average one-electron energy of the valence-shell electrons in ground-state free atoms

Abstract: It is argued that electronegativity is the third dimension of the Periodic Table, and that x{sub spec} = (m{epsilon}{sub p} + n{epsilon}{sub s})/(m + n), for representative elements where {epsilon}{sub p}, {epsilon}{sub s} are the p, s ionization energies and m, n the number of p, s electrons. Values of spectroscopic x are obtained to high accuracy from the National Bureau of Standards atomic energy level tables and closely match the widely accepted Pauling and Allred Rochow scales. x{sub spec} rationalizes the diagonal separation between metals and non-metals in the Periodic Table, the formation of noble gas molecules, metallization of the elements as one descends groups l-V, and the force definition used by Allred Rochow. {Delta}x{sub spec} = x{sub spec}{sup A} - x{sub spec}{sup B}, the energy difference of an average electron in atom A and in atom B, is able to systematize properties of the vast array of known materials: ionic solids, covalent molecules, metals, minerals, inorganic and organic polymers, semiconductors, etc. Transition-metal electronegativity cannot be simply determined because of the nature of d-orbital radial distributions and this is reflected in its paucity of use among transition-metal chemists. Estimates for first transition series x{sub spec} are obtained and amore » computational method to address this problem is given. It also proves possible to translate free atom, ground-state x{sub spec} into the in situ molecular orbital definition of average one-electron energy for orbitals localized on an atomic center. This leads to an improved definition of group (or substituent) electronagetivity, extension and refinements in the use of electronegativity perturbations in qualitative and semiquantitative molecular orbital theory, and understanding of hybrid orbital electronegativity ordering rules such as sp > sp{sup 2} > sp{sup 3}.« less

Three-dimensional heteronuclear NMR of nitrogen-15 labeled proteins

Abstract: The use of a 3D NMR technique has been shown to remove the problems of degenerate chemical shifts. A very sensitive 3D experiment for unraveling the regular protein NOESY spectrum is reported here. The method involves the {sup 15}N labelling of the protein and is described in some detail. The sensitivity of this heteronuclear 3D technique is excellent, and resonance overlap in the S. Nase 3D spectrum is minimal, despite the relatively coarse digitization. 14 references, 2 figures.

Silsesquioxanes as models for silica surfaces

Abstract: The hydrolytic condensation of cyclohexyltrichlorosilane (CySiCl/sub 3/) affords (Cy/sub 7/Si/sub 7/O/sub 9/(OH)/sub 3/) (1), (Cy/sub 6/Si/sub 6/O/sub 9/) (2), and (Cy/sub 8/Si/sub 8/O/sub 11/(OH)/sub 2/) (3a). Trisilanol 1 and 3b, the bis(triphenyltin) derivative of 3a, have been structurally characterized by single-crystal X-ray diffraction studies. Trisilanol 1 undergoes corner-capping reactions with trifunctional monomers (e.g., R'SiCl/sub 3/, MeGeCl/sub 3/, MeSnCl/sub 3/), is selectively monosilylated to (Cy/sub 7/Si/sub 7/O/sub 9/(OH)/sub 2/OTMS) (6a) with chlorotrimethylsilane, and can be dehydrated to (Cy/sub 7/Si/sub 7/O/sub 10/(OH)) (7a). Comparison of the molecular structure of 1 with (111) /beta/-cristobalite and (0001) /beta/-tridymite reveals many structural similarities. Silsesquioxanes 1, 6a, and 7a are discussed as models for silica surfaces.

New approach to mesophase stabilization through hydrogen-bonding molecular interactions in binary mixtures

Abstract: The authors report a new approach to novel liquid crystalline moieties having a greatly enhanced mesomorphic range through the formation of intermolecular hydrogen bonds between two dissimilar mesogens. In liquid crystals, mesomorphicity results from a proper combination of the shape of a molecule and the magnitude and direction of molecular interactions between molecules. While the importance of dipole-dipole interactions in the formation of mesophases has long been established, we have hypothesized that the occurrence of intermolecular hydrogen bonding should have great potential for ordering thermotropic liquid crystals because H-bonding is much stronger than dipole-dipole interactions.

Synthesis, x-ray crystal structures, and cation-binding properties of alkyl calixaryl esters and ketones, a new family of macrocyclic molecular receptors

Abstract: Des calix[n]arenes (n=4, 6, 8) portant des cetoesters sont etudies; pour n=4, ils presentent a l'etat solide une conformation en cone, leurs cryptates avec des metaux alcalins sont egalement etudies dans le but d'evaluer leurs proprietes ionophores

Reactions of carbonyl compounds with Grignard reagents in the presence of cerium chloride

Abstract: The addition of Grignard reagents to ketones is significantly enhanced by cerium chloride with remarkable suppression of side reactions, particularly enolization. Some esters, which are prone to side reactions, also react readily with Grignard reagents in the presence of cerium chloride to give normal reaction products in reasonable to high yields.

Starburst dendrimers. 5. Molecular shape control

Abstract: The recently reported starburst dendrimer and arborol systems provide unimolecular prototypes that allow for both endo-receptor and exo-supramolecular controlled molecular morphogenesis. This present communication describes the use of molecular simulation and reiterative generation development as a means for predicting and controlling molecular size and topology.

Glycosylation of unreactive substrates

Abstract: Synthese de glucosteroide et d'arylglucoside a partir d'ethoxycarbonyloxy-3 hydroxy-7 cholanoate-24 acide et de phenyl tetra-O-benzyl-2,3,4,6 glucopyranosyl sulfoxyde

The structure of self-assembled monolayers of alkylsiloxanes on silicon: a comparison of results from ellipsometry and low-angle x-ray reflectivity

Abstract: The thicknesses of C10-C18 alkylsiloxane monolayers on silicon-silicon dioxide substrates have been measured with ellipsometry and low-angle X-ray reflection. Although, for any given sample, thicknesses measured by the two methods agree to within experimental error, ellipsometric measurements are systematically larger by approximately 2 {angstrom}. This difference may result from variations in the sensitivity of the two techniques to the structure of the interface between silicon dioxide and the alkylsiloxane monolayer. The x-ray reflectivity measurements provide evidence that these organic monolayers do not build up as island structures and demonstrate that the approximate area projected by each alkyl group in the plane of the monolayer is {approximately} 21 {plus minus}3 {angstrom}{sup 2}. Preliminary studies indicate that this technique can be used to follow the changes in the structure of a monolayer which result from chemical transformations. The influence of damage that is induced by x-ray radiation on these measurements is discussed.