The six-membered cyclic allenes given in the title have been studied theoretically by means of an MR-CI approach. For all compounds, the allene structures were found to be the ground states in the gas phase. In the cases of cyclohexa-1,2-diene (1), the isobenzene 2, and the isonaphthalene 7, the most stable structures having a planar allene moiety are the diradicals 1b, 2b, and 7b, representing the transition states for the racemization of 1a, 2a, and 7a and being less stable than the latter by 14.1, 8.9, and 11.2 kcal/mol, respectively. At variance with this order, the 3delta(2)-pyran 4 and the chromene 5 have the zwitterions 4c and 5c as the most stable planar structures, which lie only 1.0 and 5.4 kcal/mol above 4a and 5a, respectively. According to the simulation of the solvent effect, 4c even becomes the ground state of 4 in THF solution. The frontier orbitals of the respective states of 2 and 4 suggest different rates and sites for the reaction with nucleophiles. For the first time, the pyran 4 has been generated and trapped. As a precursor for 4, 3-bromo-4H-pyran (9) was chosen, the synthesis of which was achieved on two routes from 4H-pyran. The treatment of 9 with potassium tert-butoxide (KOt-Bu)/18-crown-6 gave 4-tert-butoxy-4H-pyran as the only discernible product, whether styrene or furan was present, indicating the interception of 4 by KOt-Bu. Finally, the disagreement between the experiment and the theory concerning the heat of formation and the electronic nature of the isobenzene 2 is resolved by demonstrating that the experimental data can provide only an upper limit of the DeltaH(f) degrees value.

No abstract

We report various nonlinear phenomena observed in the oxidation of ascorbic acid by molecular (H 2 Asc) oxygen in the presence of copper(II) ions in a continuous Ñow stirred tank reactor (CSTR) as well as in a batch reactor (closed system). Transient irregular oscillations were observed in a batch reactor by varying the initial concentrations of and copper(II) ions. The behavior of the system in a CSTR was studied by varying the H 2 Asc Ñow rate and the inÑow concentrations of and copper(II) ions. At low inÑow concentrations of (k f ) H 2 Asc copper(II) ions the system exhibits bistability and small amplitude regular oscillations. In a two-dimensional parametric plot with coordinates and the regions of bistability and oscillations form a cross-shapedFor increasing values of oscillations emerge H 2 Asc k f via a Hopf bifurcation and disappear due to a homoclinic orbit. Simultaneously applied small amplitude periodic and stochastic perturbations imposed onto the Ñow rate close to the bistable region revealed the phenomenon of stochastic resonance, which is conÐrmed by an analysis of probability distribution functions, power spectra, and singular value decomposition.

No abstract

A variety of spatial patterns emerging in a system containing Methylene Blue, sulfide, and oxygen in a liquid layer and in a polyacrylamide gel are presented. We show the formation of long-lasting mosaic patterns arising from hydrodynamic instability and the transition from an irregular to a more regular, striped structure which occurs under the influence of an external electric field. We also demonstrate the formation of Turing patterns in a gel matrix. Here the gel plays an important role in the pattern formation process by slowing down the diffusion of the activator MB + . We present experiments showing that pattern formation can be influenced by visible light. An inhomogeneous illumination of the gel-sheet leads to selected regions of activity, which generates stripes instead of hexagons.