Hydrophobic hydration of the hydrocarbon adamantane in amorphous ice

Hydrophobic molecules are by definition difficult to hydrate. Previous studies in the area of hydrophobic hydration have therefore often relied on using amphiphilic molecules where the hydrophilic part of a molecule enabled the solubility in liquid water. Here, we show that the hydrophobic adamantane (C10H16) molecule can be fully hydrated through vapour codeposition with water onto a cryogenic substrate at 80 K resulting in the matrix isolation of adamantane in amorphous ice. Using neutron diffraction in combination with the isotopic substitution method and the empirical potential structure refinement technique, we find that the first hydration shell of adamantane is well structured consisting of a hydrogen-bonded cage of 28 water molecules that is also found in cubic structure II clathrate hydrates. The four hexagonal faces of the 51264 cage are situated above the four methine (CH) groups of adamantane whereas the methylene (CH2) groups are positioned below the edges of two adjoining pentagonal faces. The oxygen atoms of the 28 water molecules can be categorised on the basis of symmetry equivalences as twelve A, twelve B and four C oxygens. The water molecules of the first hydration shell display orientations consistent with those expected for a clathrate-hydrate-type cage, but also unfavourable ones with respect to the hydrogen bonding between the water molecules. Annealing the samples at 140 K, which is just below the crystallisation temperature of the matrix, removes the unfavourable orientations and leads to a slight increase in the structural order of the first hydration shell. The very closest water molecules display a tendency for their dipole moments to point towards the adamantane which is attributed to steric effects. Other than this, no significant polarisation effects are observed which is consistent with weak interactions between adamantane and the amorphous ice matrix. FT-IR spectroscopy shows that the incorporation of adamantane into amorphous ice leads to a weakening of the hydrogen bonds. In summary, the matrix-isolation of the highly symmetric adamantane in amorphous ice provides an interesting test case for hydrophobic hydration. Studying the structure and spectroscopic properties of water at the interface with hydrophobic hydrocarbons is also relevant for astrophysical environments, such as comets or the interstellar medium, where amorphous ice and hydrocarbons have been shown to coexist in large quantities.

Infrared photodesorption of CO from astrophysically relevant ices studied with a free-electron laser

The infrared excitation and photodesorption of carbon monoxide (CO) and water-containing ices have been investigated using the FEL-2 free-electron laser light source at the FELIX laboratory, Radboud University, The Netherlands. CO-water mixed ices grown on a gold-coated copper substrate at 18 K were investigated. No CO photodesorption was observed, within our detection limits, following irradiation with light resonant with the C-O vibration (4.67 μm). CO photodesorption was seen as a result of irradiation with infrared light resonant with water vibrational modes at 2.9 μm and 12 μm. Changes to the structure of the water ice, which modifies the environment of the CO in the mixed ice, were also seen subsequent to irradiation at these wavelengths. No water desorption was observed at any wavelength of irradiation. Photodesorption at both wavelengths is due to a single-photon process. Photodesorption arises due to a combination of fast and slow processes of indirect resonant photodesorption (fast), and photon-induced desorption resulting from energy accumulation in the librational heat bath of the solid water (slow) and metal-substrate-mediated laser-induced thermal desorption (slow). Estimated cross-sections for the slow processes at 2.9 μm and 12 μm were found to be ∼7.5 × 10-18 cm2 and ∼4.5 × 10-19 cm2, respectively.

Quantum Chemical Cluster Studies of Cation-Ice Reactions for Astrochemical Applications: Seeking Experimental Confirmation

ConspectusInterstellar clouds and the outer reaches of protostellar and protoplanetary systems are very cold environments where chemistry is limited to processes that have little or no reaction barrier (in the absence of external energy input). This account reviews what is known about cation-ice reactions, which are not currently incorporated in astrochemical network models. Quantum chemical cluster calculations using density functional theory have shown that barrierless reactions can occur when gas phase cations such as HCO⁺, OH⁺, CH₃⁺, and C⁺ are deposited on an icy grain mantle with energies commensurate with other gas phase species. When cations react with molecules on ice surfaces, the pathways and products often differ significantly from gas phase chemistry due to the involvement of water and other molecules in the ice. The reactions studied to date have found pathways to abundant and important astromolecules such as methanol, formic acid, and carbon dioxide that are very favorable and may be more efficient pathways than gas phase processes. Other products that can be produced include glycolonitrile, its precursors, and related isocyanide compounds. This account describes for the first time ice surface reactions between the carbon cation, C⁺, and two common astromolecules, methanol (CH₃OH) and formic acid (HCOOH), which can yield precursors to glyoxal, hydroxyketene, vinyl alcohol, and acetaldehyde. The quantum chemical methodology used to explore reaction surfaces is also used to predict both vibrational and electronic spectra of reactant and product ices, which offers guidance for possible experimental studies of these reactions. While theoretical calculations indicate that cation-ice reactions are efficient and offer novel pathways to important astrochemical compounds, experimental confirmation would be very welcome. Cations and ice-covered grain mantles are certainly present in cold astrophysical environments. The account concludes with a discussion of how cation-ice reactions could be incorporated into reaction network models of the formation and destruction of molecules in interstellar clouds and protoplanetary systems. Further studies will involve characterizing additional rcactions and more extensive treatment of the most important cation-ice reactions to better ascertain reaction branching outcomes.

Sign flipping of spontaneous polarization in vapour-deposited films of small polar organic molecules

Films of polar molecules vapour-deposited on sufficiently cold substrates are not only amorphous, but also exhibit charge polarization across their thickness. This is an effect known for 50 years, but it is very poorly understood and no mechanism exists in the literature that can explain and predict it. We investigated this bulk effect for 18 small organic molecules as a function of substrate temperature (30-130 K). We found that, as a rule, alcohol films have the negative end on the vacuum side at all temperatures. Alkyl acetates and toluene showed positive voltages which reached a maximum around the middle of the temperature range investigated. Tetrahydrofuran showed positive voltages which dropped with increasing deposition temperature. Diethyl ether, acetone, propanal, and butanal showed positive film voltages at low temperatures, negative at intermediate temperatures and again positive voltages at higher temperatures. In all cases, film voltages were monitored during heating leading to film evaporation. Film voltages were irreversibly eliminated before film elimination, but voltage profiles during temperature ramps differed vastly depending on compound and deposition temperature. In general, there was a gradual voltage reduction, but propanal, butanal, and diethyl ether showed a change in voltage sign during temperature ramp in films deposited at low temperatures. All these data expand substantially the experimental information regarding spontaneous polarization in vapour-deposited films, but still require complementary measurements as well as numerical simulations for a detailed explanation of the phenomenon.

Quantitative Anisotropic Analysis of Molecular Orientation in Amorphous N2O at 6 K by Infrared Multiple-Angle Incidence Resolution Spectrometry

The existence of molecular orientational order in nanometer-thick films of molecules has long been implied by surface potential measurements. However, direct quantitative determination of the molecular orientation is challenging, especially for metastable amorphous thin films at low temperatures. This study quantifies molecular orientation in amorphous N₂O at 6 K using infrared multiple-angle incidence resolution spectrometry (IR-MAIRS). The intensity ratio of the weak antisymmetric stretching vibration band of the ¹⁴N¹⁵NO isotopomer between the in-plane and out-of-plane IR-MAIRS spectra provides an average molecular orientation angle of 65° from the surface normal. No discernible change is observed in the orientation angle when a different substrate material is used (Si and Ar) at 6 K or the Si substrate temperature is changed in the range of 6-14 K. This suggests that the transient mobility of N₂O during physisorption is key in governing the molecular orientation in amorphous N₂O.

Non-linear and non-local behaviour in spontaneously electrical solids

We show that solids displaying spontaneous dipole orientation possess quite general non-local and non-linear characteristics expressed through their internal electric fields.