Thermal stability of intermediate sized acetylenic compounds and the heats of formation of propargyl radicals

4-Methylhexyne-1, 5-methylhexyne-1, hexyne-1, and 6-methylheptyne-2 have been decomposed in comparative-rate single-pulse shock-tube experiments. Rate expressions for the initial decomposition reactions at 1100°K and from 2 to 6 atm pressure are In combination with previous results, rate expressions for propargyl C? C bond cleavage are related to that for the alkanes by the expression These results yield a propargyl resonance energy of D(nC₃H₇-H) – D(C₃H₃-H) = 36 ± 2 kJ, in excellent agreement with a previous shock-tube study. They also lead to D(CH₃C≡CCH₂-H) – D(C₃H₃-H) = 0.6 ± 3 kJ, D(sC₄H₉-H) – D(iC₃H₇-H) = 0 ± 3 kJ, D(iC₄H₉-H) – D(nC₃H₇-H) = 2 ± 3 kJ, and D(nC₃H₇-H) – D(iC₃H₇-H) = 13.9 ± 3 kJ (all values are for 300°K). The systematics of the molecular decomposition process are explored.     

A convolution approximation for Van Hove correlation function in liquids

The relative motions of particles in a simple liquid may be described as one-dimensional radial diffusion in an effective potential of the mean force. This effective potential may be obtained by an iteration procedure with the Vineyard approximation as the initial step. Using a harmonic approximation for the potential, the “distinct” part of the Van Hove time-dependent correlation function is then the convolution of a modified radial distribution function with a modified self-correlation function. The former describes the average positions of particles whereas the latter describes the density profile around the average position. For liquid argon, this modified convolution procedure appears to give results in satisfactory agreement with molecular dynamics.     

The dependence of inelastic electron tunneling on junction roughness

We have made a series of Al-Al₂O₃-benzoic acid-Pb inelastic electron tunneling spectroscopy (IETS) junctions on substrates roughened by varying thicknesses of CaF₂. The vibrational peaks due to the molecular monolayer of benzoic acid disappear as the substrates are made rougher. We speculate on the cause of this disappearance.     

XPS studies of ion-bombardment damage of transition metal sulfides

Radiation damage to the surface of transition metal sulfides under ～1-keV ion bombardment has been investigated by photoelectron spectroscopy. Under     

Efficient calculation of electron diffraction for the structural determination of nanomaterials

A critical advance in the technique of low-energy electron diffraction is presented and shown to enable determining detailed structures of nanomaterials, based on experimental methods that already exist or have been proposed. Our new cluster approach speeds up the computation to scale as n logn, rather than the current n3 or n2, with n the number of atoms, for example. Applications are illustrated for C60 molecules adsorbed on a Cu(111) surface, with and without coadsorbed metal atoms, exhibiting sensitivity to important structural features such as buckyball size and deformation.     

Electrochemically fabricated nanovolcano arrays for SERS applications

Nanovolcano arrays were fabricated by electrodepositing Ni/Cu alloys into a monolayer of self‐assembled nanospheres and then electrochemically etching the deposited alloy film. The fabricated nanovolcano arrays feature highly ordered hexagonally arranged concave nanobowls decorated with triangular nanopores at their interstices. After coated with Ag, the nanovolcano arrays serve as high‐performance substrates for surface enhancement Raman spectroscopy (SERS) measurements. The experimental study shows that the structural features of the nanovolcano arrays, including concave nanobowls, nanopores, and their long‐range orders, all contribute to the observed strong SERS enhancement in a synergetic manner, which is further confirmed by the simulation results obtained using the finite‐difference time‐domain method. Copyright © 2012 John Wiley & Sons, Ltd.     

Orbital-Overlap control in the solid-state reactivity of beta-azido-propiophenones: selective formation of cis-azo-dimers

Solid-state photolysis of 1a,b yields selectively cis-3a,b. X-ray analysis of 1a,b reveals the molecules adopt an extended structure and as such the crystal packing arrangement consists of planar, pi-stacked molecules. The shortest intermolecular distance between adjacent N-atoms is approximately 3.76 A and would lead to formation of trans-3a,b, whereas cis-3a,b is formed by dimerization between N-atoms that are approximately 3.9 A apart. We propose that the molecular orbital alignment of the adjacent nitrenes controls the solid-state reactivity.     

Ultra-Compact and Efficient Integrated Multichannel Mode Multiplexer in Silicon for Few-Mode Fibers

Space-division multiplexing (SDM) is one of the key enabling technologies to increase the capacity of fiber communication systems. However, implementing SDM-based systems using multimode fiber has been challenging with the need for compact, low-cost, and scalable mode de/multiplexer (DE/MUX). Here a novel integrated mode MUX for few-mode fibers (FMFs) is presented which can launch up to eight spatial and polarization channels. The new design is composed of a 2D multimode grating coupler (MMGC), highly compact spot size converters (SSCs), and adiabatic directional couplers (ADCs). Eight data lanes in FMFs can be selectively launched with integrated optical phase shifters. Experimental results reveal efficient chip-to-fiber coupling with peak efficiencies of −3.8, −5.5, −3.6, and −4.1 dB for LP₀₁, LP_11a, LP_11b, and LP_21b modes, respectively. Thanks to the use of an integrated subwavelength Mikaelian lens for mode-independent field size conversion with loss ≤−0.25 dB, the total footprint of the MMGC and SSCs is only 35×35 µm². The proposed design shows great potential for densely integrated photonic circuits in future SDM applications.     

Automerizations in benzenoid hydrocarbons. New mechanistic insights from the thermal rearrangement of benz[a]anthracene-5-13C.

Scrambling of the carbon atoms in benz[a]anthracene between positions 5 and 6 has been shown to occur at high temperatures; a mechanism involving thermal generation of a carbene is proposed.