Two families of halopyridnium hexacyanometallate salts, (3-XpyMe) 3 [M(CN) 6 ] and (3,5-X 2 pyMe) 3 [M(CN) 6 ] (X = I, Br; 3-XpyMe = N-methyl-3-halopyridinium; 3,5-XpyMe = N-methyl-3,5-dihalopyridinium; M = Cr, Fe, Co), have been synthesized and characterized by single crystal X-ray diffraction. Five of the six members of each family are characterized as isostructural compounds, two structures are reported as solvates, (3-IpyMe) 3 [Fe(CN) 6 ]•2MeCN (2•2MeCN) and (3,5-Br 2 pyMe) 3 [Cr(CN) 6 ]•4H 2 O (10•4H 2 O), and the solvate (3-IpyMe) 3 [Co(CN) 6 ]•2MeCN (3•2MeCN) has been characterized in addition to the unsolvated 3. All halogens participate in halogen bonding, forming C−X•••NC(M) halogen bonds and in one case a C−Br•••O halogen bond (in 10•4H 2 O). The halogen bond distances are shorter than the corresponding sum of van der Waals radii, and stronger interactions are formed by iodine than bromine (I•••N 2.789(7)−3.116(7), R IN 0.790− 0.883; Br•••N 2.884(3)−3.166(2), R BrN 0.848−0.931). Longer halogen bonds are formed in 10•4H 2 O (Br•••N 3.041(6)− 3.380(6), R BrN 0.894−0.994) due to competition from O−H•••N hydrogen bonding. All halogen bonds have interaction geometries at the halogen close to linearity (most have C−X•••N > 165°; smallest angle is 154.1(3)°). The geometry of interaction of the halogen bond donor (C−X) with the cyanide ligand either suggests interaction predominantly with the exo lone pair of the nitrogen atom (CN•••X > 145°) or predominant involvement of the CN π-bond in the halogen bond (CN•••X < 105°) acceptor role. The former are shorter interactions than the latter. Halogen bonds become shorter across each isostructural series for Cr > Fe > Co, and this is discussed in the context of metal-to-cyanide π-back-donation.

Abstract-New model organic microparticles are used to assess the thermal ablation that occurs during aerogel capture at speeds from 1 to 6 km s −1 . Commercial polystyrene particles (20 µm diameter) were coated with an ultrathin 20 nm overlayer of an organic conducting polymer, polypyrrole. This overlayer comprises only 0.8% by mass of the projectile but has a very strong Raman signature, hence its survival or destruction is a sensitive measure of the extent of chemical degradation suffered. After aerogel capture, microparticles were located via optical microscopy and their composition was analyzed in situ using Raman microscopy. The ultrathin polypyrrole overlayer survived essentially intact for impacts at ~1 km s −1 , but significant surface carbonization was found at 2 km s −1 , and major particle mass loss at ≥3 km s −1 . Particles impacting at ~6.1 km s −1 (the speed at which cometary dust was collected in the NASA Stardust mission) were reduced to approximately half their original diameter during aerogel capture (i.e., a mass loss of 84%). Thus significant thermal ablation occurs at speeds above a few km s −1 . This suggests that during the Stardust mission the thermal history of the terminal dust grains during capture in aerogel may be sufficient to cause significant processing or loss of organic materials. Further, while Raman D and G bands of carbon can be obtained from captured grains, they may well reflect the thermal processing during capture rather than the pre-impact particle's thermal history.

The study of hyper-velocity impacts of micrometeoroids is important for the calibration of dust sensors in space applications. For this purpose, submicron-sized synthetic dust grains comprising either polystyrene or poly[bis(4-vinylthiophenyl)sulfide] were coated with an ultrathin overlayer of an electrically conductive organic polymer (either polypyrrole or polyaniline) and were accelerated to speeds between 3 and 35 km s(-1) using the Heidelberg Dust Accelerator facility. Time-of-flight mass spectrometry was applied to analyse the resulting ionic impact plasma using a newly developed Large Area Mass Analyser (LAMA). Depending on the projectile type and the impact speed, both aliphatic and aromatic molecular ions and cluster species were identified in the mass spectra with masses up to 400 u. Clusters resulting from the target material (silver) and mixed clusters of target and projectile species were also observed. Impact velocities of between 10 and 35 km s(-1) are suitable for a principal identification of organic materials in micrometeoroids, whereas impact speeds below approximately 10 km s(-1) allow for an even more detailed analysis. Molecular ions and fragments reflect components of the parent molecule, providing determination of even complex organic molecules embedded in a dust grain. In contrast to previous measurements with the Cosmic Dust Analyser instrument, the employed LAMA instrument has a seven times higher mass resolution--approximately 200--which allowed for a detailed analysis of the complex mass spectra. These fundamental studies are expected to enhance our understanding of cometary, interplanetary and interstellar dust grains, which travel at similar hyper-velocities and are known to contain both aliphatic and aromatic organic compounds.

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