On the mechanism of solid-state phase transitions in molecular crystals - the role of cooperative motion in (quasi)racemic linear amino acids.

During single-crystal-to-single-crystal (SCSC) phase transitions, a polymorph of a compound can transform to a more stable form while remaining in the solid state. By understanding the mechanism of these transitions, strategies can be developed to control this phenomenon. This is particularly important in the pharmaceutical industry, but also relevant for other industries such as the food and agrochemical industries. Although extensive literature exists on SCSC phase transitions in inorganic crystals, it is unclear whether their classications and mechanisms translate to molecular crystals, with weaker interactions and more steric hindrance. A comparitive study of SCSC phase transitions in aliphatic linear-chain amino acid crystals, both racemates and quasi-racemates, is presented. A total of 34 transitions are considered and most are classified according to their structural change during the transition. Transitions without torsional changes show very different characteristics, such as transition temperature, enthalpy and free energy, compared with transitions that involve torsional changes. These differences can be rationalized using classical nucleation theory and in terms of a difference in mechanism; torsional changes occur in a molecule-by-molecule fashion, whereas transitions without torsional changes involve cooperative motion with multiple molecules at the same time.

Molecular dynamics simulations of energy dissipation and non-thermal diffusion on amorphous solid water.

Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. Especially, saturated, hydrogen-rich molecules are formed through surface chemistry where the interstellar grains act as a meeting place and absorbing energy. Here we present the results of thousands of molecular dynamics simulations to quantify the outcome of an energy dissipation process. Admolecules on top of an amorphous solid water surface have been given translational energy between 0.5 and 5 eV. Three different surface species are considered, CO2, H2O and CH4, spanning a range in binding energies, number of internal degrees of freedom and molecular weight. The results are compared against a previous study using a crystalline water ice surface. Possible outcomes of a dissipation process are adsorption - possibly after long-range diffusion-, desorption and desorption of a surface molecule. The three admolecules were found to bind at different locations on the surface, particularly in terms of height. Water preferably binds on top of the surface, whereas methane fills the nanopores on the surface. This has direct consequences for desorption, travelled distance, and kick-out probabilities. The admolecules are found to frequently travel several tens of angstroms before stabilizing on a binding site, allowing follow-up reactions en route. We present kick-out probabilities and we have been able to quantify the desorption probability which depends on the binding energy of the species, the translational excitation, and a factor that accounts for difference in binding site height. We provide expressions that can be incorporated in astrochemical models to predict grain surface formation and return into the gas phase of these products.

Inhibition of the Vapor-Mediated Phase Transition of the High Temperature Form of Pyrazinamide.

Tailor-made additives can prove an effective method to prolong the lifetime of metastable forms of pharmaceutical compounds by surface stabilization. Pyrazinamide (PZA) is a pharmaceutical compound with four polymorphic forms. The high temperature γ form, which can be produced by spray drying or sublimation growth, is metastable at room temperature and transforms within days when produced by spray drying, and within several months up to years for single crystals produced by sublimation. However, when PZA is cospray dried with 1,3-dimethylurea (DMU), it has been reported to remain in its γ form for several years. Scanning electron microscopy (SEM) images reveal that the phase transition from γ-PZA to the low temperature forms involves a vapor-mediated recrystallization, while the reverse phase transition upon heating is a nucleation-and-growth solid-solid phase transition. The lifetime-extending effect of DMU on spray-dried PZA has been investigated in more detail and compared with high-energy ball milling of sublimation-grown γ-PZA crystals. Co-ball milling of PZA and DMU is found to extend the lifetime of the high temperature form of PZA to a few months, while separate ball milling leads to an extension of merely a few weeks. DMU acts as an additive that most likely stabilizes the surface of γ-PZA, which would reduce the vapor pressure of PZA, thereby reducing the transition rate. Alternatively, DMU could prevent nucleation of low temperature forms of PZA.

Spectroscopic constraints on CH3OH formation: CO mixed with CH3OH ices towards young stellar objects.

The prominent infrared absorption band of solid CO - commonly observed towards young stellar objects (YSOs) - consists of three empirically determined components. The broad 'red component' (2136 cm-1, 4.681 µm) is generally attributed to solid CO mixed in a hydrogen-bonded environment. Usually, CO embedded in the abundantly present water is considered. However, CO:H2O mixtures cannot reproduce the width and position of the observed red component without producing a shoulder at 2152 cm-1, which is not observed in astronomical spectra. Cuppen et al. showed that CO:CH3OH mixtures do not suffer from this problem. Here, this proposition is expanded by comparing literature laboratory spectra of different CO-containing ice mixtures to high-resolution (R = λ/Δλ = 25000) spectra of the massive YSO AFGL 7009S and of the low-mass YSOL1489 IRS. The previously unpublished spectrum of AFGL 7009S shows a wide band of solid 13CO, the first detection of 13CO ice in the polar phase. In this source, both the 12CO and 13CO ice bands are well fitted with CO:CH3OH mixtures, while respecting the profiles and depths of the methanol bands at other wavelengths, whereas mixtures with H2O cannot. The presence of a gradient in the CO:CH3OH mixing ratio in the grain mantles is also suggested. Towards L1489 IRS, the profile of the 12CO band is also better fitted with CH3OH-containing ices, although the CH3OH abundance needed is a factor of 2.4 above previous measurements. Overall, however, the results are reasonably consistent with models and experiments about formation of CH3OH by the hydrogenation of CO ices.

The formation of ice mantles on interstellar grains revisited--the effect of exothermicity.

Modelling of grain surface chemistry generally deals with the simulation of rare events. Usually deterministic methods or statistical approaches such as the kinetic Monte Carlo technique are applied for these simulations. All assume that the surface processes are memoryless, the Markov chain assumption, and usually also that their rates are time independent. In this paper we investigate surface reactions for which these assumptions are not valid, and discuss what the effect is on the formation of water on interstellar grains. We will particularly focus on the formation of two OH radicals by the reaction H + HO2. Two reaction products are formed in this exothermic reaction and the resulting momentum gained causes them to move away from each other. What makes this reaction special is that the two products can undergo a follow-up reaction to form H2O2. Experimentally, OH has been observed, which means that the follow-up reaction does not proceed with 100% efficiency, even though the two OH radicals are formed in each other's vicinity in the same reaction. This can be explained by a combined effect of the directionality of the OH radical movement together with energy dissipation. Both effects are constrained by comparison with experiments, and the resulting parametrised mechanism is applied to simulations of the formation of water ice under interstellar conditions.

Interactions of adsorbed CO2 on water ice at low temperatures.

We present a computational study into the adsorption properties of CO2 on amorphous and crystalline water surfaces under astrophysically relevant conditions. Water and carbon dioxide are two of the most dominant species in the icy mantles of interstellar dust grains and a thorough understanding of their solid phase interactions at low temperatures is crucial for understanding the structural evolution of the ices due to thermal segregation. In this paper, a new H2O-CO2 interaction potential is proposed and used to model the ballistic deposition of CO2 layers on water ice surfaces, and to study the individual binding sites at low coverages. Contrary to recent experimental results, we do not observe CO2 island formation on any type of water substrate. Additionally, density functional theory calculations are performed to assess the importance of induced electrostatic interactions.

Efficient surface formation route of interstellar hydroxylamine through NO hydrogenation. II. The multilayer regime in interstellar relevant ices.

Hydroxylamine (NH(2)OH) is one of the potential precursors of complex pre-biotic species in space. Here, we present a detailed experimental study of hydroxylamine formation through nitric oxide (NO) surface hydrogenation for astronomically relevant conditions. The aim of this work is to investigate hydroxylamine formation efficiencies in polar (water-rich) and non-polar (carbon monoxide-rich) interstellar ice analogues. A complex reaction network involving both final (N(2)O, NH(2)OH) and intermediate (HNO, NH(2)O·, etc.) products is discussed. The main conclusion is that hydroxyl-amine formation takes place via a fast and barrierless mechanism and it is found to be even more abundantly formed in a water-rich environment at lower temperatures. In parallel, we experimentally verify the non-formation of hydroxylamine upon UV photolysis of NO ice at cryogenic temperatures as well as the non-detection of NC- and NCO-bond bearing species after UV processing of NO in carbon monoxide-rich ices. Our results are implemented into an astrochemical reaction model, which shows that NH(2)OH is abundant in the solid phase under dark molecular cloud conditions. Once NH(2)OH desorbs from the ice grains, it becomes available to form more complex species (e.g., glycine and ß-alanine) in gas phase reaction schemes.

Long-timescale simulations of diffusion in molecular solids.

Kinetic processes play a crucial role in the formation and evolution of molecular layers. In this perspective we argue that adaptive kinetic Monte Carlo is a powerful simulation technique for determining key kinetic processes in molecular solids. The applicability of the method is demonstrated by simulating the diffusion of a CO admolecule on a water ice surface, which is an important process for the formation of organic compounds on interstellar dust grains. CO diffusion is found to follow Arrhenius behavior and the corresponding effective activation energy for diffusion is determined to be 50 ± 1 meV. A coarse graining algorithm is applied which greatly enhances the efficiency of the simulations at low temperatures, down to 10 K, without altering the underlying physical processes. Eventually, we argue that a combination of both on- and off-lattice kinetic Monte Carlo techniques is a good way for simulating large-scale processes in molecular solids over long time spans.

Molecular dynamics simulations of D2O ice photodesorption.

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D(2)O in an amorphous D(2)O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H(2)O, the main processes after UV photodissociation are trapping and desorption of either fragments or D(2)O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D(2)O molecule photodesorption. D(2)O desorption takes places either by direct desorption of a recombined D(2)O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D(2)O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D(2)O in the top four monolayers and are compared quantitatively with those for H(2)O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5-2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D(2)O photodesorption probability is larger than that of H(2)O (by about a factor of 1.4-2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D(2)O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D(2)O ice, because the D atom formed after D(2)O photodissociation has a larger momentum than photogenerated H atoms from H(2)O, and D transfers momentum more easily to D(2)O than H to H(2)O. The total (OD + D(2)O) yield has been compared with experiments and the total (OH + H(2)O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D(2)O ice than when we compare with calculated yields for H(2)O ice.

Water formation by surface O3 hydrogenation.

Three solid state formation routes have been proposed in the past to explain the observed abundance of water in space: the hydrogenation reaction channels of atomic oxygen (O + H), molecular oxygen (O(2) + H), and ozone (O(3) + H). New data are presented here for the third scheme with a focus on the reactions O(3) + H, OH + H and OH + H(2), which were difficult to quantify in previous studies. A comprehensive set of H/D-atom addition experiments is presented for astronomically relevant temperatures. Starting from the hydrogenation/deuteration of solid O(3) ice, we find experimental evidence for H(2)O/D(2)O (and H(2)O(2)/D(2)O(2)) ice formation using reflection absorption infrared spectroscopy. The temperature and H/D-atom flux dependence are studied and this provides information on the mobility of ozone within the ice and possible isotope effects in the reaction scheme. The experiments show that the O(3) + H channel takes place through stages that interact with the O and O(2) hydrogenation reaction schemes. It is also found that the reaction OH + H(2) (OH + H), as an intermediate step, plays a prominent (less efficient) role. The main conclusion is that solid O(3) hydrogenation offers a potential reaction channel for the formation of water in space. Moreover, the nondetection of solid ozone in dense molecular clouds is consistent with the astrophysical picture in which O(3) + H is an efficient process under interstellar conditions.

Water formation at low temperatures by surface O2 hydrogenation I: Characterization of ice penetration.

Water is the main component of interstellar ice mantles, is abundant in the solar system and is a crucial ingredient for life. The formation of this molecule in the interstellar medium cannot be explained by gas-phase chemistry only and its surface hydrogenation formation routes at low temperatures (O, O(2), O(3) channels) are still unclear and most likely incomplete. In a previous paper we discussed an unexpected zeroth-order H(2)O production behavior in O(2) ice hydrogenation experiments compared to the first-order H(2)CO and CH(3)OH production behavior found in former studies on hydrogenation of CO ice. In this paper we experimentally investigate in detail how the structure of O(2) ice leads to this rare behavior in reaction order and production yield. In our experiments H atoms are added to a thick O(2) ice under fully controlled conditions, while the changes are followed by means of reflection absorption infrared spectroscopy (RAIRS). The H-atom penetration mechanism is systematically studied by varying the temperature, thickness and structure of the O(2) ice. We conclude that the competition between reaction and diffusion of the H atoms into the O(2) ice explains the unexpected H(2)O and H(2)O(2) formation behavior. In addition, we show that the proposed O(2) hydrogenation scheme is incomplete, suggesting that additional surface reactions should be considered. Indeed, the detection of newly formed O(3) in the ice upon H-atom exposure proves that the O(2) channel is not an isolated route. Furthermore, the addition of H(2) molecules is found not to have a measurable effect on the O(2) reaction channel.

Water formation at low temperatures by surface O2 hydrogenation II: The reaction network.

Water is abundantly present in the Universe. It is the main component of interstellar ice mantles and a key ingredient for life. Water in space is mainly formed through surface reactions. Three formation routes have been proposed in the past: hydrogenation of surface O, O(2), and O(3). In a previous paper [Ioppolo et al., Astrophys. J., 2008, 686, 1474] we discussed an unexpected non-standard zeroth-order H(2)O(2) production behaviour in O(2) hydrogenation experiments, which suggests that the proposed reaction network is not complete, and that the reaction channels are probably more interconnected than previously thought. In this paper we aim to derive the full reaction scheme for O(2) surface hydrogenation and to constrain the rates of the individual reactions. This is achieved through simultaneous H-atom and O(2) deposition under ultra-high vacuum conditions for astronomically relevant temperatures. Different H/O(2) ratios are used to trace different stages in the hydrogenation network. The chemical changes in the forming ice are followed by means of reflection absorption infrared spectroscopy (RAIRS). New reaction paths are revealed as compared to previous experiments. Several reaction steps prove to be much more efficient (H + O(2)) or less efficient (H + OH and H(2) + OH) than originally thought. These are the main conclusions of this work and the extended network concluded here will have profound implications for models that describe the formation of water in space.

Kinetic Monte Carlo studies of hydrogen abstraction from graphite.

We present Monte Carlo simulations on Eley-Rideal abstraction reactions of atomic hydrogen chemisorbed on graphite. The results are obtained via a hybrid approach where energy barriers derived from density functional theory calculations are used as input to Monte Carlo simulations. By comparing with experimental data, we discriminate between contributions from different Eley-Rideal mechanisms. A combination of two different mechanisms yields good quantitative and qualitative agreement between the experimentally derived and the simulated Eley-Rideal abstraction cross sections and surface configurations. These two mechanisms include a direct Eley-Rideal reaction with fast diffusing H atoms and a dimer mediated Eley-Rideal mechanism with increased cross section at low coverage. Such a dimer mediated Eley-Rideal mechanism has not previously been proposed and serves as an alternative explanation to the steering behavior often given as the cause of the coverage dependence observed in Eley-Rideal reaction cross sections.

Thickness-dependent ordering of water layers at the NaCl(100) surface.

We have determined the ordering properties of water adsorbed at room temperature on the rock salt (100) surface under four different conditions: ultrahigh vacuum, dry nitrogen atmosphere, 45% and 75% relative humidity. Details of the atomic structure are determined for both sides of the solid-liquid interface. The top most layer of NaCl shows a small relaxation that changes from an expansion to a contraction with increasing humidity. Under all measured conditions water monolayers with different ordering properties are present at the interface. Surprisingly, we find that the amount of ordering in the first layer is increasing with increasing thickness of the water film. At a solid-liquid interface, the ordering appears to be correlated with the solubility.