Vibrationally quantum-state-specific reaction dynamics of H atom abstraction by CN radical in solution

Solvent collisions can often mask initial disposition of energy to the products of solution-phase chemical reactions. Here, we show with transient infrared absorption spectra obtained with picosecond time resolution that the nascent HCN products of reaction of CN radicals with cyclohexane in chlorinated organic solvents exhibit preferential excitation of one quantum of the C-H stretching mode and up to two quanta of the bending mode. On time scales of approximately 100 to 300 picoseconds, the HCN products undergo relaxation to the vibrational ground state by coupling to the solvent bath. Comparison with reactions of CN radicals with alkanes in the gas phase, known to produce HCN with greater C-H stretch and bending mode excitation (up to two and approximately six quanta, respectively), indicates partial damping of the nascent product vibrational motion by the solvent. The transient infrared spectra therefore probe solvent-induced modifications to the reaction free energy surface and chemical dynamics.     

Probing the threshold to H atom transfer along a hydrogen-bonded ammonia wire

We characterized the entrance channel, reaction threshold, and mechanism of an excited-state H atom transfer reaction along a unidirectionally hydrogen-bonded "wire" -O-H...NH3...NH3...NH3...N. Excitation of supersonically cooled 7-hydroxyquinoline.(NH3)3 to its vibrationless S1 state produces no reaction, whereas excitation of ammonia-wire vibrations induces H atom transfer with a reaction threshold approximately 200 wave numbers. Further translocation steps along the wire produce the S1 state 7-ketoquinoline.(NH3)3 tautomer. Ab initio calculations show that proton and electron movement along the wire are closely coupled. The rate-controlling S1 state barriers arise from crossings of a pipi* with a Rydberg-type pisigma* state.     

Structural basis for methyl transfer by a radical SAM enzyme

The radical S-adenosyl-L-methionine (SAM) enzymes RlmN and Cfr methylate 23S ribosomal RNA, modifying the C2 or C8 position of adenosine 2503. The methyl groups are installed by a two-step sequence involving initial methylation of a conserved Cys residue (RlmN Cys(355)) by SAM. Methyl transfer to the substrate requires reductive cleavage of a second equivalent of SAM. Crystal structures of RlmN and RlmN with SAM show that a single molecule of SAM coordinates the [4Fe-4S] cluster. Residue Cys(355) is S-methylated and located proximal to the SAM methyl group, suggesting the SAM that is involved in the initial methyl transfer binds at the same site. Thus, RlmN accomplishes its complex reaction with structural economy, harnessing the two most important reactivities of SAM within a single site.     

Energy transfer across a metal film mediated by surface plasmon polaritons

Coupled surface plasmon polaritons (SPPs) are shown to provide effective transfer of excitation energy from donor molecules to acceptor molecules on opposite sides of metal films up to 120 nanometers thick. This variant of radiative transfer should allow directional control over the flow of excitation energy with the use of suitably designed metallic nanostructures, with SPPs mediating transfer over length scales of 10(-7) to 10(-4) meters. In the emerging field of nanophotonics, such a prospect could allow subwavelength-scale manipulation of light and provide an interface to the outside world.     

Realizing all-spin-based logic operations atom by atom

An ultimate goal of spintronic research is the realization of concepts for atomic-scale all-spin-based devices. We combined bottom-up atomic fabrication with spin-resolved scanning tunneling microscopy to construct and read out atomic-scale model systems performing logic operations. Our concept uses substrate-mediated indirect exchange coupling to achieve logical interconnection between individual atomic spins. Combined with spin frustration, this concept enables various logical operations between inputs, such as NOT and OR.     

RAFT技术合成含醛基的嵌段共聚物

选择末端含有双硫酯结构的聚苯乙烯(分子量Mn=6 500,分散度Mw/Mn=1. 05)活性链为可逆加成-断裂链转移(Reversible addition-fragmentation chain transfer, RAFT)试剂,以α,α′-偶氮二异丁腈(α,α′azodiisobutyronitrile, AIBN)为引发剂、在60 ℃下四氢呋喃(Tetrahydrofuran,THF)中成功实现了苯乙烯(Styrene, St)与1,2:3,4-di-O-异亚丙基-6-O-(2-甲醛-4-苯乙烯基)-D-半乳糖(1,2:3,4-di-O-isopropylidene-6-O-(2-formyl-4-vinylphenyl)-D-galactopyranose, IVDG)的"活性"/可控RAFT自由基嵌段共聚合.聚合物分子量(Mn)随单体转化率几乎线性增加,而且整个反应过程中分子量分布 (Mw/Mn < 1.2)保持在较窄的范围.核磁共振(1H-NMR)分析进一步证实了聚合物链的结构.     

Image of a sulfur atom

Two-dimensional signal averaging has been applied to dark field electron micrographs of molecules of 2,3,4,5-tetraacetoxymercurithiophene. Only the mercury atom images are seen in single micrographs. However, in the composite image, resulting from photographic superposition of 64 individual images, the sulfur atom in the molecule is clearly revealed.     

Low-energy atom scattering from surfaces

Experiments in which low-energy atoms are scattered from surfaces represent an important new method for structural analysis of the topmost atomic layers of a surface. The method and its application in a number of areas in surface science, such as detecting surface defects and studying lattice dynamics, are discussed.     

Fast atom bombardment mass spectrometry

Fast atom bombardment mass spectrometry has become a powerful structural tool since the first reports of its use in 1981. Samples are ionized in the condensed state, usually in a glycerol matrix, by bombarding the matrix with xenon or argon atoms with energies of 5000 to 10,000 electron volts. This yields both positive and negative secondary ions, which are sputtered from the surface. The technique has been used to detect inorganic ion clusters to mass 25,800 and biologically active peptides to mass 5700, and it gives molecular ions of such highly polar or labile organic compounds as glycosphingolipids and polyene antibiotics. It can be especially valuable in determining the sequences of amino acids in polypeptides.     

A well-collimated quasi-continuous atom laser

Extraction of sodium atoms from a trapped Bose-Einstein condensate (BEC) by a coherent, stimulated Raman process is demonstrated. Optical Raman pulses drive transitions between trapped and untrapped magnetic sublevels, giving the output-coupled BEC fraction a well-defined momentum. The pulsed output coupling can be run at such a rate that the extracted atomic wave packets strongly overlap, forming a highly directional, quasi-continuous matter wave.     

Living polymerization methods

Living polymerization techniques can be used to achieve a high degree of control over polymer chain architecture. Examples of the type of polymers that can be synthesized include block copolymers, comb-shaped polymers, multiarmed polymers, ladder polymers, and cyclic polymers. This control of structure, in turn, results in polymers with widely diverse physical properties, even though they are made from readily available low-cost monomers.     

Scientists implicated in atom test deception

Described here are the circumstances leading to a recent court ruling that Atomic Energy Commission officials suppressed data indicating a link between two 1953 atomic bomb tests in Nevada and sheep deaths in neighboring Utah, and then pressured scientific investigators to concur with the AEC position. In a 1956 trial, a federal judge denied compensation to the sheep owners. Evidence of the deception came to light in 1979 and 1980 and, on 24 August 1982, the same judge ordered a new trial, ruling that several AEC attorneys and scientists had indeed misrepresented the facts. His ruling has been appealed by the Department of Justice.     

Probing two-electron dynamics of an atom

Coherent short-pulse laser excitation has been used to control the approximate energy and relative proximity of two valence electrons within the same alkaline-earth atom, thereby providing insight into the dynamical evolution of a three-body Coulomb system. Our time-domain experiments enable direct experimental study of the electron dynamics at the classical limit of a two-electron atom. As an example, we look at the mechanism of autoionization for one two-electron configuration class and find that the doubly excited atom decays through a single violent electron-electron collision rather than a gradual exchange of energy between the electrons.