At 25 "C and atmospheric pressure, the iodide-catalyzed decomposition of aqueous hydrogen peroxide in an undisturbed solution results in a maximum dissolved oxygen concentration of 0.12 M, or about 100 times the equilibrium solubility. At this concentration loss of gas occurs by homogeneous nucleation of bubbles. The supersaturation is readily destroyed by agitation or by sonication. As the external pressure is increased, the maximum attainable oxygen concentration increases linearly, reaching 0.18 M at 50 atm. In a plot of dissolved gas concentration versus external pressure, the supersaturation line is parallel to the Henry's law (saturation) line. The maximum supersaturation concentration at elevated pressure is independent of the nature of the pressurizing gas. While the pressure dependence of the supersaturation limit can be understood in terms of existing nucleation theory, the theory fails in attempts to calculate that limit quantitatively.

The solution structures of ion pairs formed by quarternary ammonium ions derived from quinine alkaloid with small hard anions (BH(4)(-) or Cl(-)) in CDCl(3) have been characterized by nuclear magnetic resonance methods. Structural observations have been correlated with the sense of asymmetric induction observed in the phase-transfer reduction of 9-anthryl trifluoromethyl ketone by borohydride (BH(4)(-)) when catalyzed by the quaternary N-benzylquinine ammonium ion. From interionic nuclear Overhauser effects (NOEs), it appears that the BH(4)(-) ion occupies two of the four trigonal pyramidal sites formed by substituents of the quarternary nitrogen of the catalyst cation. One of these sites is in close proximity to the cation's hydroxyl group that is strictly required for asymmetric induction in the model reaction, while the other site is near the vinyl group on the cation. The vinyl group does not appear to be important for determining the sense or extent of asymmetric induction. Using energy-minimized structures derived from NMR data, it was predicted that the N-(9-methyleneanthryl)quinine-quarternary ammonium catalyst would give improved asymmetric induction in the model reaction due to a preferred anion occupancy at the site near the hydroxyl group. An improvement in enantiomeric excess (ee) is observed using the anthryl-modified catalyst, and NMR studies on the modified catalyst confirm the predicted change in anion binding site occupancies. The changes in site occupancies determined by NMR can be fitted to a simple kinetic model that correctly predicts the extent of change in ee.

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The behavior of oscillatory phenomena has become an active area of research in the past two decades, and many complex systems have been studied and modeled. The subject of this paper, the soda bottle oscillator, is relatively simple and easily observed: shake a bottle of soda water at room temperature several times, let it stand a minute, then puncture the cap with a thumb tack or similar object. As pressure is released from the bottle, the dissolved gas is seen to bubble off in oscillatory bursts rather than in a steady single flow.In order for far-from-equilibrium systems to exhibit oscillatory behavior, the mechanism must contain a nonlinear step and a step in which feedback inhibits the nonlinear step. Oscillations in reactions that are of interest to chemists can have either a chemical or a physical mechanism. Reactions such as the Belousov-Zhabotinsky (B-Z) reaction (1) exhibit chemical oscillations, whereas the oscillations in gas evolution processes, such as the Morgan reaction (2), have a physical origin.Chemical oscillations are an effect of reaction kinetics, and it is the concentration of intermediate chemical species that oscillates. To exhibit oscillatory behavior, a reaction mechanism must have an autocatalytic step and a step that produces a species that inhibits the autocatalysis. The inhibitory step provides the feedback that controls the nonlinear (autocatalytic) step. The complete mechanism for an oscillating chemical reaction is complex, as are the models that illustrate their behavior.In a gas evolution oscillator (GEO), a dissolved gas (carbon monoxide from formic acid in the Morgan reaction) is produced by chemical reaction at atmospheric pressure. Once the solution becomes fully supersaturated, homogeneous nucleation of bubbles occurs. Diffusion of dissolved gas into growing bubbles depletes the solution sufficiently to shut off further bubble formation, and this is the feedback responsible for oscillation (3). The oscillations are sustained by continuous chemical production of the dissolved gas.The mechanism for oscillation in the soda bottle oscillator is also physical. However, it is very different from the mechanism in a GEO. We believe that the oscillations in this case occur as the system returns to equilibrium in response to the changing pressure of CO 2 above the solution. The process starts with dissolved gas in equilibrium with gas in the headspace, in accordance with Henry's law:where c eq is the concentration of dissolved gas, p is the external pressure, and κ is the Henry's law constant, which for CO 2 is 0.034 M atm ᎑1 at 25 °C. When a small hole is made in the cap, the initial decrease in pressure causes the onset of bubble nucleation. As the bubbles escape into the headspace, they tend to increase the pressure, and if the rate of increase is greater than the rate of decrease due to the pinhole, then the net result is that the headspace becomes partially repressurized. This is the feedback that "turns off" the nucleation process. The cycle repeats several times before reach...

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