共查询到20条相似文献，搜索用时 314 毫秒
1.
Luntz AC 《Science (New York, N.Y.)》2003,302(5642):70-71
Vibrational excitations of specific bonds in molecules have been used to enhance the reactivity of the molecules in direct gas-phase reactions. In his Perspective, Luntz highlights a report by Beck et al., who show that such vibrational control may also be possible for catalytic reactions at a surface. The authors demonstrate that differently excited deuterated methane molecules have different dissociation probabilities on a nickel surface, even though the energies of the different molecules are similar. 相似文献
2.
Near-atomic resolution images of a two-dimensional heteroepitaxial crystal composed of the relatively "functionally rich" chiral liquid crystal mesogen MDW 74 on graphite have been obtained by scanning tunneling microscopy (STM). This work is aimed at developing an improved understanding of the commercially crucial phenomenon of liquid crystal alignment by studying well-characterized surfaces. Herein is reported molecular-level characterization of the surface underlying a ferroelectric liquid crystal in situ, a requisite starting point for understanding the liquid crystal-solid interface at the molecular level. The results are also important in the context of developing a model for the molecular. origins of the contrast observed in STM images of organic monolayers on conductor surfaces. The data and analysis provide strong evidence that neither frontier orbital alone (highest occupied or lowest unoccupied molecular orbital) is sufficient to describe the observed tunneling efficiency. 相似文献
3.
This article represents only a brief glimpse into one of the fascinating aspects of the chemistry of electronically excited molecules. More exhaustive coverage of synthetic applications of photochemical reactions may be found in texts and reviews. We summarize here by noting that classification of reliable photoreactions is possible, but that the most effective application of the classification scheme requires a familiarity with some of the special features of the chemistry of electronically excited states. Thus, knowledge of the properties of singlet and triplet states, the factors which determine photochemical efficiency and reactivity, and the special technical aspects of photochemical procedures such as selective direct and phtosensitized excitation and photoexcitation at low temperatures or solid matrices make photochemistry a useful tool for the construction of organic molecules. It is our hope that the material and flavor of this article will whet the intellectual and experimental appetites of researchers who are concerned with various aspects of the synthesis of oganic molecules. 相似文献
4.
Two-dimensional molecular patterns were obtained by the adsorption of long-chain alkanes, alcohols, fatty acids, and a dialkylbenzene from organic solutions onto the basal plane of graphite. In situ scanning tunneling microscopy (STM) studies revealed that these molecules organize in lamellae with the extended alkyl chains oriented parallel to a lattice axis within the basal plane of graphite. The planes of the carbon skeletons, however, can be oriented either predominantly perpendicular to or predominantly parallel with the substrate surface, causing the lamellar lattice to be either in or near registry with the substrate (alkanes and alcohols) or not in registry (fatty acids and dialkylbenzenes). In the case of the alcohols and the dialkylbenzene the molecular axes are tilted by +30 degrees or -30 degrees with respect to an axis normal to the lamella boundaries, giving rise to molecularly well-defined domain boundaries. Fast STM image recording allowed the spontaneous switch between the two tilt angles to be observed in the alcohol monolayers on a time scale of a few milliseconds. 相似文献
5.
S Chen RS Ingram MJ Hostetler JJ Pietron RW Murray TG Schaaff JT Khoury MM Alvarez RL Whetten 《Science (New York, N.Y.)》1998,280(5372):2098-2101
A transition from metal-like double-layer capacitive charging to redox-like charging was observed in electrochemical ensemble Coulomb staircase experiments on solutions of gold nanoparticles of varied core size. The monodisperse gold nanoparticles are stabilized by short-chain alkanethiolate monolayers and have 8 to 38 kilodaltons core mass (1.1 to 1.9 nanometers in diameter). Larger cores display Coulomb staircase responses consistent with double-layer charging of metal-electrolyte interfaces, whereas smaller core nanoparticles exhibit redox chemical character, including a large central gap. The change in behavior is consistent with new near-infrared spectroscopic data showing an emerging gap between the highest occupied and lowest unoccupied orbitals of 0.4 to 0.9 electron volt. 相似文献
6.
The scanning tunneling microscope (STM) can be used to measure current-voltage characteristics on an atomic scale. The attachment of copper phthalocyanine molecules, in contrast to a variety of other molecules, to graphite changes the electrical characteristics of the STM from relatively symmetric to highly asymmetric or rectifying. Evidence is presented to show that the asymmetry arises because of the electronic energy levels of the copper phthalocyanine. The organic molecules were bonded to the graphite by an acid-base reaction that may have wide applicability. 相似文献
7.
Wachowiak A Yamachika R Khoo KH Wang Y Grobis M Lee DH Louie SG Crommie MF 《Science (New York, N.Y.)》2005,310(5747):468-470
We present a low-temperature scanning tunneling microscopy (STM) study of K(x)C60 monolayers on Au(111) for 3 < or = x < or = 4. The STM spectrum evolves from one that is characteristic of a metal at x = 3 to one that is characteristic of an insulator at x = 4. This electronic transition is accompanied by a dramatic structural rearrangement of the C60 molecules. The Jahn-Teller effect, a charge-induced mechanical deformation of molecular structure, is directly visualized in the K4C60 monolayer at the single-molecule level. These results, along with theoretical analyses, provide strong evidence that the transition from metal to insulator in K(x)C60 monolayers is caused by the Jahn-Teller effect. 相似文献
8.
Otero R Lukas M Kelly RE Xu W Laegsgaard E Stensgaard I Kantorovich LN Besenbacher F 《Science (New York, N.Y.)》2008,319(5861):312-315
Nonsymmetrical organic molecules adsorbed on solid surfaces may assemble into random networks, thereby providing model systems for organic glasses that can be directly observed by scanning tunneling microscopy (STM). We investigated the structure of a disordered cytosine network on a gold(111) surface created by thermal quenching, to temperatures below 150 K, of the two-dimensional fluid present on the surface at room temperature. Comparison of STM images to density functional theory calculations allowed us to identify three elementary structural motifs (zigzag filaments and five- and six-membered rings) that underlie the whole supramolecular random network. The identification of elementary structural motifs may provide a new framework for understanding medium-range order in amorphous and glassy systems. 相似文献
9.
Near-ultraviolet sunlight is absorbed by sulfur dioxide to produce electronically excited molecules, which react with n-butane in the presence of wet or dry air to produce an aerosol and eventually a liquid precipitate. The product is a heavily oxidized, sulfur-containing, organic strong acid. The possible role of these reactions in photochemical air pollution is discussed. 相似文献
10.
Electronically excited molecules, being better electron donors and acceptors than their ground states, form charge-transfer complexes (exciplexes) which can lead to radical ions. Exciplex emission is widely used to probe polymers and organized media such as membranes and micelles. Exciplexes are also intermediates in photoreactions that lead to unique products. Photochemical electron-transfer processes, which are the basis of silver halide photography and electrophotography, are involved in many reactions of wide scope. Recent studies have led to the discovery of several electron-transfer photooxygenations with a diversity that will probably rival that of singlet oxygen. Both exciplex emission and photochemical electron transfer play important roles in organic photochemistry. 相似文献
11.
Hettich C Schmitt C Zitzmann J Kühn S Gerhardt I Sandoghdar V 《Science (New York, N.Y.)》2002,298(5592):385-389
By performing cryogenic laser spectroscopy under a scanning probe electrode that induces a local electric field, we have resolved two individual fluorescent molecules separated by 12 nanometers in an organic crystal. The two molecules undergo a strong coherent dipole-dipole coupling that produces entangled sub- and superradiant states. Under intense laser illumination, both molecules are excited via a two-photon transition, and the fluorescence from this doubly excited system displays photon bunching. Our experimental scheme can be used to optically resolve molecules at the nanometer scale and to manipulate the degree of entanglement among them. 相似文献
12.
Kraack H Ocko BM Pershan PS Sloutskin E Deutsch M 《Science (New York, N.Y.)》2002,298(5597):1404-1407
The structure of organic monolayers on liquid surfaces depends sensitively on the details of the molecular interactions. The structure of a stearic acid film on a mercury surface was measured as a function of coverage with angstrom resolution. Unlike monolayers on water, the molecules were found here to undergo a transition from surface-parallel to surface-normal orientation with increasing coverage. At high coverage, two condensed hexatic phases of standing-up molecules were found. At low coverage, a two-dimensional (2D) gas phase and condensed single- and double-layered phases of flat-lying molecular dimers were revealed, exhibiting a 1D longitudinal positional order. This system should provide a broader tunability range for nanostructure construction than solid-supported self-assembled monolayers. 相似文献
13.
Heathcock CH 《Science (New York, N.Y.)》1981,214(4519):395-400
For the scientist who wishes to synthesize complex organic compounds, the most difficult problem is often establishing the correct configuration at the various chiral centers as the synthesis is being carried out. In the past decade, there has been an increasing effort to find direct solutions to this problem, which is particularly acute in the synthesis of acyclic and other conformationally flexible molecules. One of the oldest organic reactions, the aldol condensation, is emerging as a powerful tool for use in achieving such stereocontrol. 相似文献
14.
Waite JH Young DT Cravens TE Coates AJ Crary FJ Magee B Westlake J 《Science (New York, N.Y.)》2007,316(5826):870-875
Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH4 and N2). Up to now, it has been assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (approximately 1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH4 and N2) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (approximately 8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected. 相似文献
15.
Tunneling electrons from a scanning tunneling microscope (STM) were used to excite photon emission from individual porphyrin molecules adsorbed on an ultrathin alumina film grown on a NiAl(110) surface. Vibrational features were observed in the light-emission spectra that depended sensitively on the different molecular conformations and corresponding electronic states obtained by scanning tunneling spectroscopy. The high spatial resolution of the STM enabled the demonstration of variations in light-emission spectra from different parts of the molecule. These experiments realize the feasibility of fluorescence spectroscopy with the STM and enable the integration of optical spectroscopy with a nanoprobe for the investigation of single molecules. 相似文献
16.
The results have shown that selective excitation obtained with a tunable monochromatic laser is a useful technique for studying photochemical and energy transfer processes. A new phenomenon in the photochemistry of bromine was observed, in which bound excited molecules, and not atoms, were formed in the primary process. The mechanism of the subsequent reaction consists of collisional dissociation of the excited molecules into atoms, which then initiated free-radical chains. A quantitative estimate of the collisional electronic relaxation rate for excited bromine molecules was obtained, and a new upper limit to the continuous absorption strength at 14,400 cm(-1) was determined. 相似文献
17.
Schierbaum KD Weiss T van Veizen EU Engbersen JF Reinhoudt DN Göpel W 《Science (New York, N.Y.)》1994,265(5177):1413-1415
It is shown by angle-resolved x-ray photoelectron spectroscopy that cavitands derived from resorcin[4]arenes provided with four dialkylsulfide chains form stable monolayers on gold surfaces that are well organized by self-assembly. The cavitand headgroups at the surface of the resorcin[4]arene monolayer act as molecular recognition sites for small organic molecules with remarkable selectivity for perchloroethylene (C(2)Cl(4)). Comparative thermal desorption experiments indicate binding sites with high interaction energies of C(2)Cl(4) at the surface of the resorcin[4]arene monolayers. Fast and reversible "host-guest" interactions were found by the monitoring of extremely small mass changes (in the nanogram range) with a quartz microbalance oscillator provided with gold electrodes coated by resorcin[4]arene monolayers. 相似文献
18.
Molecular structure of DNA by scanning tunneling microscopy 总被引：5，自引：0，他引：5
A Cricenti S Selci A C Felici R Generosi E Gori W Djaczenko G Chiarotti 《Science (New York, N.Y.)》1989,245(4923):1226-1227
Uncoated DNA molecules marked with an activated tris(l-aziridinyl) phosphine oxide (TAPO) solution were deposited on gold substrates and imaged in air with the use of a high-resolution scanning tunneling microscope (STM). Constant-current and gap-modulated STM images show clear evidence of the helicity of the DNA structure: pitch periodicity ranges from 25 to 35 angstroms, whereas the average diameter is 20 angstroms. Molecular structure within a single helix turn was also observed. 相似文献
19.
Graphite: a mimic for DNA and other biomolecules in scanning tunneling microscope studies 总被引：11，自引：0，他引：11
Highly ordered pyrolytic graphite (HOPG) is the substrate often used in scanning tunneling microscope (STM) studies of biomolecules such as DNA. All of the images presented in this article are of freshly cleaved HOPG surfaces upon which no deposition has occurred. These images illustrate features previously thought to be due to biological molecules, such as periodicity and meandering of "molecules" over steps. These features can no longer be used to distinguish real molecules from features of the native substrate. The feasibility of the continued use of HOPG as a substrate for biological STM studies is discussed. 相似文献
20.
Formaldehyde could have been produced by photochemical reactions in Earth's primitive atmosphere, at a time when it consisted mainly of molecular nitrogen, water vapor, carbon dioxide, and trace amounts of molecular hydrogen and carbon monoxide. Removal of formaldehyde from the atmosphere by precipitation can provide a source of organic carbon to the oceans at the rate of 10(11) moles per year. Subsequent reactions of formaldehyde in primeval aquatic environments would have implications for the abiotic synthesis of complex organic molecules and the origin of life. 相似文献