Kinetics of the thermal decomposition of ferrocenyl azide: Character of ferrocenyl nitrene

The Arrhenius parameters for the thermal decomposition of ferrocenyl azide in isooctane are A = (5.1 ± 1.4) × 10¹² s^?1 and E_act = 113.1 ± 0.9 (kJ mol^?1) and the rate is relatively insensitive to solvent (isooctane, benzene, acetonitrile: 1:1.7:2.4). The results indicate a relatively nonpolar transition state which is considerably “tighter” than for a normal bond fission reaction. The Arrhenius parameters are comparable to those for aromatic azides and do not offer any support for anchimeric assistance by the iron atom. A kinetic scheme is presented which accounts for the observed products: Nitrogen, ferrocene, aminoferrocene, azoferrocene, and insoluble material. Rates of hydrogen abstraction by the intermediate ferrocenyl nitrene from cyclohexane, benzene, and acetonitrile are used to show that the nitrene is nucleophilic. © 1994 John Wiley & Sons, Inc.     

A Framework for Representing Reticulate Evolution

Acyclic directed graphs (ADGs) are increasingly being viewed as more appropriate for representing certain evolutionary relationships, particularly in biology, than rooted trees. In this paper, we develop a framework for the analysis of these graphs which we call hybrid phylogenies. We are particularly interested in the problem whereby one is given a set of phylogenetic trees and wishes to determine a hybrid phylogeny that embeds each of these trees and which requires the smallest number of hybridisation events. We show that this quantity can be greatly reduced if additional species are involved, and investigate other combinatorial aspects of this and related questions.Received November 12, 2003     

Leakage of the fundamental mode in photonic crystal fiber tapers

We report detailed measurements of the optical properties of tapered photonic crystal fibers (PCFs). We observe a striking long-wavelength loss as the fiber diameter is reduced, despite the minimal airhole collapse along the taper. We associate this loss with a transition of the fundamental core mode as the fiber dimensions contract: At wavelengths shorter than this transition wavelength, the core mode is strongly confined in the fiber microstructure, whereas at longer wavelengths the mode expands beyond the microstructure and couples out to higher-order modes. These experimental results are discussed in the context of the so-called fundamental mode cutoff described by Kuhlmey et al. [Opt. Express 10, 1285 (2002)], which apply to PCFs with a finite microstructure.     

Heterocyclic tautomerism. XI. Structures of 5,5′-bitetrazole and 1-methyl-5-(2′-pyridyl)tetrazole at 130 K

The X-ray crystal structures of 5,5-bitetrazole and 1-methyl-5-(2-pyridyl)tetrazole at 130 K are described. 5,5-Bitetrazole exists as the 1 H, 1'H-tautomer and packs in chains held together by pairs of intermolecular hydrogen bonds. It crystallizes in the monoclinic space group P2₁/n witha=4.945(1),b=6.367(1),c=8.491(1) Å and =99.233(7)^o. In 1-methyl-5-(2-pyridyl)tetrazole the two heterocyclic rings are nearly coplanar and the molecular packing is controlled by intermolecular – interactions. It crystallizes in the triclinic space group P1 witha=6.667(1),b=7.135(1),c=9.202(1)Å, =89.59(1), =69.08(1), and =66.62(1)^o.     

Convenient preparation of tert-butyl beta-(protected amino)esters

Refluxing an aldehyde 1 with benzotriazole and benzylcarbamate in the presence of a catalytic amount of p-TsOH gave the corresponding benzyloxycarbonylamino-1-(1-benzotriazolyl)alkane 2 in good yields. Compounds 2 treated with substituted tert-butyl acetates 3 using LDA as a base afford smoothly and under mild conditions the N-2-protected 3-aminoalkanoic esters 4.     

Near-field scanning optical microscopy of single molecules by femtosecond two-photon excitation

We present a demonstration of near-field scanning optical microscopy of single molecules based on ultrafast two-photon-induced fluorescence. Measurements were performed by use of 100-fs pulses at 800 nm from a Ti:sapphire laser to excite the two-photon transition in Rhodamine B molecules. Although near-field probes are normally metal coated to achieve superresolution, we used uncoated tips to achieve sufficiently high optical powers to generate acceptable fluorescence emission rates. Images of single molecules demonstrate a resolution of ~175nm(< lambda/4) on a topographically smooth surface, which surpasses the apparent lambda/2 resolution limit for uncoated tips operating in the linear response regime.     

Functionalised enaminones and their use in heterocyclic synthesis

A series of enaminones derived from 2,4-dioxocyclohexane carboxylic acid esters has been prepared. The use of some of the new derivatives in the synthesis of some quinazolones and imidazoloquinazolines is illustrated.     

Donor-mediated band gap reduction in a homologous series of conjugated polymers

A family of six donor-acceptor-donor monomers was synthesized using combinations of thiophene, 3,4-ethylenedioxythiophene and 3,4-ethylenedioxypyrrole as donor moieties, and cyanovinylene as the acceptor moiety, to understand the effects of modified donor ability on the optoelectronic and redox properties of the resulting electropolymerized materials. Spectroelectrochemistry, differential pulse voltammetry, and cyclic voltammetry results indicate band gaps ranging from 1.1 to 1.6 eV and suggest that these polymers can be both p-type and n-type doped at accessible potentials. In situ conductivity results indicate that the n-type conductivity magnitude is modest, and the conductivity profile indicates a redox conductivity mechanism as opposed to a delocalized electronic band mechanism as observed for p-type doping.