Minimal Ice: Water clusters comprising fewer than 100 molecules have long been studied in gas phase to model the more complex structures of ice and liquid water. At some stage, as clusters grow larger, they effectively become tiny crystals of ice, but it has been hard to pinpoint precisely where in the range between 100 and 1000 molecules the formal transition takes place. Pradzynski et al. (p. 1529 ) used vibrational spectroscopy to show that the onset of an icelike structure, indicated by a characteristically distinct absorption band in the infrared, occurs at a cluster size of approximately 275 molecules.

A detailed reaction mechanism for n-heptane oxidation has been compiled and subsequently simplified. The model is based on a kinetic model for C1–C4 fuel oxidation of Hoyermann et al. [Phys. Chem. Chem. Phys., 2004, 6, 3824] and a detailed mechanism for n-heptane oxidation developed by Curran et al. [Combust. Flame, 1998, 114, 149]. The generated mechanism is kept compact by limiting the application of the low temperature oxidation pathways to the fuel molecule. The first reaction steps and the complex low temperature paths in the oxidation process have been simplified and reorganized by linear chemical lumping. The reported procedure allows a decrease in number of species and reactions with only a minor loss of model accuracy. The simplified model is of very compact size and gives an advantageous starting point for further model reduction. By this chemically lumped general mechanism without further adjustments the large set of experimental data for the high and low temperature oxidation (ignition delay times, species concentration profiles, heat release and engine pressure profiles, flame speeds and flame structure data) for conditions ranging from very low to high temperatures (550–2300 K), very lean to extremely fuel rich (0.22 < ϕ < 3) mixtures and pressures between 1 and 42 bar is consistently described providing a basis for reliable predictions for future applications, (i) building reaction mechanisms for similar but chemically more complex fuels (e.g. iso-octane, n-decane,…) and (ii) calculating complex flow fields (“fluid dynamics”) after further simplification with advanced reduction tools.

Size selected water clusters are generated by photoionizing sodium doped clusters close to the ionization threshold. This procedure is free of fragmentation. Upon infrared excitation, size- and isomer-specific OH-stretch spectra are obtained over a large range of cluster sizes. In one application of this method the infrared spectra of single water cluster sizes are investigated. A comparison with calculations, based on structures optimized by genetic algorithms, has been made to tentatively derive cluster structures which reproduce the experimental spectra. We identified a single all-surface structure for n = 25 and mixtures with one or two interior molecules for n = 24 and 32. In another application the sizes are determined at which the crystallization sets in. Surprisingly, this process strongly depends on the cluster temperature. The crystallization starts at sizes below n = 200 at higher temperatures and the onset is shifted to sizes above n = 400 at lower temperatures.

On the basis of existing detailed kinetic schemes a general and consistent mechanism of the oxidation of hydrocarbons and the formation of higher hydrocarbons was compiled for computational studies covering the characteristic properties of a wide range of combustion processes. Computed ignition delay times of hydrocarbon–oxygen mixtures (CH₄-, C₂H₆-, C₃H₈-, n-C₄H₁₀-, CH₄ + C₂H₆-, C₂H₄, C₃H₆-O₂) match the experimental values. The calculated absolute flame velocities of laminar premixed flames (CH₄-, C₂H₆-, C₃H₈-, n-C₄H₁₀-, C₂H₄-, C₃H₆-, and C₂H₂-air) and the dependence on mixture strength agree with the latest experimental investigations reported in the literature. With the same model concentration profiles for major and intermediate species in fuel-rich, non-sooting, premixed C₂H₂-, C₃H₆- air flames and a mixed C₂H₂/C₃H₆ (1:1)-air flame at 50 mbar are predicted in good agreement with experimental data. An analysis of reaction pathways shows for all three flames that benzene formation can be described by propargyl combination.

Significance: How small can a water cluster be and still stabilize ice I, the macroscopic crystal polymorph? What is the nature of coexistence between ice and liquid in small clusters? In this work, we show that the smallest water clusters that can sustain ice contain only around 90 water molecules. Our experimental and theoretical analysis suggests that the melting transition in these small droplets is very different from macroscopic water and our everyday experience. When cooled slightly below their freezing temperature, water clusters with less than ∼ 150 molecules can oscillate in time between the liquid and crystalline state. These results can be significant to understand the state of water confined in proteins and other materials.