Formation of a new benzene-ethane co-crystalline structure under cryogenic conditions.

We report the first experimental finding of a solid molecular complex between benzene and ethane, two small apolar hydrocarbons, at atmospheric pressure and cryogenic temperatures. Considerable amounts of ethane are found to be incorporated inside the benzene lattice upon the addition of liquid ethane onto solid benzene at 90-150 K, resulting in formation of a distinctive co-crystalline structure that can be detected via micro-Raman spectroscopy. Two new features characteristic of these co-crystals are observed in the Raman spectra at 2873 and 1455 cm(-1), which are red-shifted by 12 cm(-1) from the υ1 (a1g) and υ11 (eg) stretching modes of liquid ethane, respectively. Analysis of benzene and ethane vibrational bands combined with quantum mechanical modeling of isolated molecular dimers reveal an interaction between the aromatic ring of benzene and the hydrogen atoms of ethane in a C-H···π fashion. The most favored configuration for the benzene-ethane dimer is the monodentate-contact structure, with a calculated interaction energy of 9.33 kJ/mol and an equilibrium bonding distance of 2.66 Å. These parameters are comparable to those for a T-shaped co-crystalline complex between benzene and acetylene that has been previously reported in the literature. These results are relevant for understanding the hydrocarbon cycle of Titan, where benzene and similar organics may act as potential hydrocarbon reservoirs due to this incorporation mechanism.

Insights into hydrogen bonding via ice interfaces and isolated water.

Water in a confined environment has a combination of fewer available configurations and restricted mobility. Both affect the spectroscopic signature. In this work, the spectroscopic signature of water in confined environments is discussed in the context of competing models for condensed water: (1) as a system of intramolecular coupled molecules or (2) as a network with intermolecular dipole-dipole coupled O-H stretches. Two distinct environments are used: the confined asymmetric environment at the ice surface and the near-isolated environment of water in an infrared transparent matrix. Both the spectroscopy and the environment are described followed by a perspective discussion of implications for the two competing models. Despite being a small molecule, water is relatively complex; perhaps not surprisingly the results support a model that blends inter- and intramolecular coupling. The frequency, and therefore the hydrogen-bond strength, appears to be a function of donor-acceptor interaction and of longer-range dipole-dipole alignment in the hydrogen-bonded network. The O-H dipole direction depends on the local environment and reflects intramolecular O-H stretch coupling.

Water: a responsive small molecule.

Unique among small molecules, water forms a nearly tetrahedral yet flexible hydrogen-bond network. In addition to its flexibility, this network is dynamic: bonds are formed or broken on a picosecond time scale. These unique features make probing the local structure of water challenging. Despite the challenges, there is intense interest in developing a picture of the local water structure due to water's fundamental importance in many fields of chemistry. Understanding changes in the local network structure of water near solutes likely holds the key to unlock problems from analyzing parameters that determine the three dimensional structure of proteins to modeling the fate of volatile materials released into the atmosphere. Pictures of the local structure of water are heavily influenced by what is known about the structure of ice. In hexagonal I(h) ice, the most stable form of solid water under ordinary conditions, water has an equal number of donor and acceptor bonds; a kind of symmetry. This symmetric tetrahedral coordination is only approximately preserved in the liquid. The most obvious manifestation of this altered tetrahedral bonding is the greater density in the liquid compared with the solid. Formation of an interface or addition of solutes further modifies the local bonding in water. Because the O-H stretching frequency is sensitive to the environment, vibrational spectroscopy provides an excellent probe for the hydrogen-bond structure in water. In this Account, we examine both local interactions between water and small solutes and longer range interactions at the aqueous surface. Locally, the results suggest that water is not a symmetric donor or acceptor, but rather has a propensity to act as an acceptor. In interactions with hydrocarbons, action is centered at the water oxygen. For soluble inorganic salts, interaction is greater with the cation than the anion. The vibrational spectrum of the surface of salt solutions is altered compared with that of neat water. Studies of local salt-water interactions suggest that the picture of the local water structure and the ion distribution at the surface deduced from the surface vibrational spectrum should encompass both ions of the salt.

Competitive binding of methanol and propane for water via matrix-isolation spectroscopy: implications for inhibition of clathrate nucleation.

Methanol is a well-known thermodynamic inhibitor of clathrate hydrate formation. The interactions responsible for the inhibition, however, are not well-identified. Propane is a relatively simple hydrocarbon that forms a clathrate hydrate under mild conditions. This paper reports data about the interaction of methanol with water-propane complex. Methanol, water, and propane are isolated in carbon tetrachloride, and the interaction is probed with infrared spectroscopy. Water is known to interact with propane via the oxygen lone pairs and the propane methylene hydrogens. Experimental evidence indicates that methanol hydrogen bonds to water via donation of the hydroxyl hydrogen (K = 4.4 × 10(2)). Methanol does not have a direct interaction with propane. These results are consistent with an inhibitory mechanism in which methanol competes with propane for the oxygen atom of water.

Spectroscopic identification of water-propane interaction: implications for clathrate nucleation.

Propane is one of several hydrocarbons known to form a clathrate hydrate. To probe interactions leading to clathrate nucleation, the water-propane interaction is investigated in carbon tetrachloride with infrared spectroscopy. Isotopic substitution provides compelling evidence that the water-propane interaction occurs between the propane methylene hydrogen atoms and the water lone pair. In addition, interaction between propane and water results in clustering of water molecules, a clustering that is identified by the appearance of a peak between the symmetric and asymmetric stretches of water.

A co-crystal between benzene and ethane: a potential evaporite material for Saturn's moon Titan.

Using synchrotron X-ray powder diffraction, the structure of a co-crystal between benzene and ethane formed in situ at cryogenic conditions has been determined, and validated using dispersion-corrected density functional theory calculations. The structure comprises a lattice of benzene molecules hosting ethane molecules within channels. Similarity between the intermolecular interactions found in the co-crystal and in pure benzene indicate that the C-H⋯π network of benzene is maintained in the co-crystal, however, this expands to accommodate the guest ethane molecules. The co-crystal has a 3:1 benzene:ethane stoichiometry and is described in the space group [Formula: see text] with a = 15.977 (1) Šand c = 5.581 (1) Šat 90 K, with a density of 1.067 g cm(-3). The conditions under which this co-crystal forms identify it is a potential that forms from evaporation of Saturn's moon Titan's lakes, an evaporite material.

Hydrogen Bonding between Water and Tetrahydrofuran Relevant to Clathrate Formation.

Tetrahydrofuran (THF) is well-known as a clathrate former as well as a promoter for gas hydrate formation. This work examines interactions between water and tetrahydrofuran via the effect on water's vibrational spectrum. Due to water's large oscillator strength in the hydrogen-bonded region, interactions are diagnosed by isolating small clusters in a transparent medium (carbon tetrachloride in this study). A weak THF/water hydrogen bond is reflected by a 3450 cm(-1) OH-donor vibration (blue shifted from the water/water hydrogen bond) and a 3685 cm(-1) nonbonded OH stretch (blue shifted 22 cm(-1) from the decoupled OH stretch in this medium). Increasing the THF concentration results in another 20 cm(-1) blue shift of the OH-donor stretch. Additional THF does not complex with free water but rather joins with existing THF/water structures to form a cluster enriched in THF. These results complement previous work examining THF vibrations in clathrate hydrates. Together, they generate a picture in which water mediates between THF pairs--mediation that affects vibrational frequencies of both species. In addition to a frequency shift, water's hydrogen-bonded resonance gains oscillator strength due to its mediating configuration.

Experimental study on the effect of ammonia on the phase behavior of tetrahydrofuran clathrates.

Clathrate hydrates, ice-like crystalline compounds in which small guest molecules are enclosed inside cages formed by tetrahedrally hydrogen-bonded water molecules, are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. A prototypical example is Saturn's moon Titan, where dissociation of methane clathrates, a major crustal component, could contribute significantly to the replenishment of atmospheric methane. Ammonia is an important clathrate inhibiting agent that may be present (potentially at high concentrations) in Titan's interior. In this study, low-temperature Raman experiments are conducted to examine the dissociation point of tetrahydrofuran clathrates, an ambient-pressure analogue of methane clathrates, over a wide range of ammonia concentrations from 0 to 25 wt %. A phase diagram for the H2O-THF-NH3 system is generated, showing two main results: (i) ammonia lowers the dissociation point of clathrate hydrates to a similar extent compared to the melting of water ice and (ii) THF clathrate exhibits a "liquidus-like" behavior in the presence of ammonia, with a eutectic temperature of about 203.6 K. As temperatures higher than this estimated eutectic are anticipated within Titan's icy crust, these results imply that partial dissociation of clathrates can occur readily and may contribute to outgassing from the interior.