Evolutionary trajectories of codistributed taxa with comparable ecological preferences and dispersal abilities may be similarly impacted by historical landscape-level processes. Species' responses to changes in a shared biogeographic landscape may be purely concerted, completely independent, or classified as falling within an intermediate part of the continuum bounded by these two extremes. With sufficient molecular data, temporal contrasts of congruence among taxa with respect to these responses can be made. Such contrasts provide insights into the relative influence of ancient versus more recent climatic (and other) impacts on genetic structuring. Using phylogenetic, allele frequency, and genotypic data from two low-mobility, rotting-log-adapted (saproxylic) springtail species (Collembola) from an isolated 100-km-long section of the Great Dividing Range in southeastern Australia, we tested the concerted-response hypothesis over three timescales. Tests of phylogeographic, demographic, and contemporary population-genetic congruence were performed using an integrative approach that draws on both direct (pattern-based) and indirect (scenario-based) analyses. Our data revealed a general pattern of broad-scale similarities in species' responses to the interaction between Pleistocene climatic cycles and landscape setting, overlaid with some species-specific differences on local geographic and more recent temporal scales. This general pattern of phylogeographic congruence was accompanied by evidence for contemporaneous demographic incongruence indicating that, even at relatively small spatial scales, biogeographic context can exert an overarching influence on genetic structuring.

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Some problems associated with unrestricted wave functions for open-shell molecules are discussed in the contest of coupled-cluster calculation of molecular properties. Particular attention is given to a phenomenon akin to the "triplet instability" of closed-shell molecules, where the approximate spin pairing of a nominal pair of electrons in the unrestricted Hartree-Fock wave function begins to give way to significant spin polarization. This problem-which gives rise to pronounced spin contamination-is discussed from the point of view of orbital instability and occupation numbers of the charge density matrix. The onset, rather than the magnitude of the spin contamination is analyzed in detail for diatomics, especially heteronuclear cases where the transition to significant spin contamination does not occur discontinuously. It is shown that the qualitative description of this phenomenon satisfactorily explains anomalous results for NO and PO, although the magnitude of spin contamination in these molecules is significantly less than in other cases where anomalous results are not observed. It appears that calculations of equilibrium molecular properties using coupled-cluster methods based on unrestricted Hartree-Fock reference should be monitored carefully for any molecule containing a multiple bond, especially when the bonded pair of atoms appear in different rows of the periodic table.

This work is devoted to the investigation of propagating polymerization fronts converting a liquid monomer into a liquid polymer. We consider a simplified mathematical model which consists of the heat equation and equation for the depth of conversion for one-step chemical reaction and of the Navier-Stokes equations under the Boussinesq approximation. We fulfill the linear stability analysis of the stationary propagating front and find conditions of convective and thermal instabilities. We show that convection can occur not only for ascending fronts but also for descending fronts. Though in the latter case the exothermic chemical reaction heats the cold monomer from above, the instability appears and can be explained by the interaction of chemical reaction with hydrodynamics. Hydrodynamics changes also conditions of the thermal instability. The front propagating upwards becomes less stable than without convection, the front propagating downwards more stable. The theoretical results are compared with experiments. The experimentally measured stability boundary for polymerization of benzyl acrylate in dimethyl formamide is well approximated by the theoretical stability boundary. (c) 1998 American Institute of Physics.

Frontal polymerization is a promising approach for preparing polymeric materials in which a localized reaction zone propagates through an unstirred medium. Large thermal and compositional gradients occur in the front that can lead to convective instabilities that interfere with the front propagation. With monomers that produce thermoplastic polymers, the Rayleigh-Taylor instability can destroy a descending front as the more dense molten polymer "fingers" into the unreacted monomer. Ascending fronts are not possible because simple convection caused by the heat release of the reaction quenches the front. Using a saltwater dispersion, we have shown for the first time that stable descending fronts with monomers such as benzyl acrylate can propagate through a saltwater dispersion if the density of the aqueous phase is greater than the density of the polymer. However, hydrolysis followed by intermolecular anhydride formation occurs at the high front temperature, resulting in a lightly cross-linked copolymer.