The number of water molecules needed to form the smallest ice crystals has proven challenging to pinpoint experimentally. This information would help to better understand the hydrogen-bonding interactions that account for the macroscopic properties of water. Here, we report infrared (IR) spectra of precisely size-selected (H(2)O)(n) clusters, with n ranging from 85 to 475; sodium doping and associated IR excitation-modulated photoionization spectroscopy allowed the study of this previously intractable size domain. Spectral features indicating the onset of crystallization are first observed for n = 275 ± 25; for n = 475 ± 25, the well-known band of crystalline ice around 3200 cm(-1) dominates the OH-stretching region. The applied method has the potential to push size-resolved IR spectroscopy of neutral clusters more broadly to the 100- to 1000-molecule range, in which many solvents start to manifest condensed phase properties.

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.

Understanding the combustion chemistry of the butene isomers is a prerequisite for a comprehensive description of the chemistry of C1 to C4 hydrocarbon and oxygenated fuels such as butanol. For the development and validation of combustion models, it is thus crucial to improve the knowledge about the C4 combustion chemistry in detail. Premixed low-pressure (40 mbar) flat argon-diluted (25%) flames of the three butene isomers (1-butene, trans-2-butene and i-butene) were studied under fuel-rich (=1.7) conditions using a newly developed analytical combination of high-resolution in-situ molecular-beam mass 2 spectrometry (MBMS) and in-situ gas chromatography (GC). The time-of-flight MBMS with its high mass resolution enables the detection of both stable and reactive species, while the gas chromatograph permits the separation of isomers from the same sampling volume. The isomer-specific species information and the quantitative mole fraction profiles of more than 30 stable and radical species measured for each fuel were used to extend and validate the C4 subset of a comprehensive flame simulation model. The experimental data shows different destruction pathways for the butene isomers, as expected, and the model is well capable to predict the different combustion behavior of the isomeric flames. The detailed analysis of the reaction pathways in the flame and the respective model predictions are discussed.

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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 < phi < 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.