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The structural evolution with pressure of icy mixtures of simple molecules is a poorly explored field despite the fundamental role they play in setting the properties of the crustal icy layer of the outer planets and of their satellites. Water and ammonia are the two major components of these mixtures, and the crystal properties of the two pure systems and of their compounds have been studied at high pressures in a certain detail. On the contrary, the study of their heterogeneous crystalline mixtures whose properties, due to the strong N–H⋯O and O–H⋯N hydrogen bonds, can be substantially altered with respect to the individual species has so far been overlooked. In this work, we performed a comparative Raman study with a high spatial resolution of the lattice phonon spectrum of both pure ammonia and water–ammonia mixtures in a pressure range of great interest for modeling the properties of icy planets’ interiors. Lattice phonon spectra represent the spectroscopic signature of the molecular crystals’ structure. The activation of a phonon mode in plastic NH3-III attests to a progressive reduction in the orientational disorder, which corresponds to a site symmetry reduction. This spectroscopic hallmark allowed us to solve the pressure evolution of H2O–NH3–AHH (ammonia hemihydrate) solid mixtures, which present a remarkably different behavior from the pure crystals likely to be ascribed to the role of the strong H-bonds between water and ammonia molecules characterizing the crystallites’ surface.

The formation of a clathrate hydrate crystal is characterized by several steps, each of them distinguished by a different structural arrangement and temporal duration. A precise definition of these different forms is a challenging task, because the entirety of the formation dynamics spans over a time interval ranging from few nanoseconds to several days. Computational methods are powerful and essential to define the nucleation step, but they fail in providing a reliable picture of the long-range order establishment. On the other side, the experimental methods employed in the study of the growth dynamics usually monitor the hydrate growth at the interface with the fluid and thus are limited by the diffusion of the guest molecules through the newly formed hydrate phase. This problem is overcome here by the confinement of an argon hydrate sample in a sapphire anvil cell, allowing monitoring of the melting and crystallization of hydrates under moderate pressures by FTIR and Raman spectroscopies. This approach, besides providing a spectroscopic characterization of this hydrate, allowed the time windows characteristic of the formation of a macroscopic amorphous phase to be identified, possibly coincident with the so-called blob, and its rapid evolution toward the achievement of the local structure. Long-range ordering takes place on a longer time scale, most of it is realized in few hours but still evolving for weeks. No hints for supporting the so-called memory effect are gained through this study.

Carbon nanothreads are the most exciting carbon based nanomaterials recently discovered. Obtained by compressing aromatics around 20 GPa, they are characterized by potentially exceptional mechanical properties. The reaction mechanisms have been partly elucidated through computational studies and x-ray diffraction experiments. However, in all these studies, the electronic modifications to which the molecule is subjected with increasing pressure are neglected as also if, and to which extent, the electronic excited states are involved in the high-pressure reactivity. In fact, the pressure increase induces remarkable changes in the electronic properties of molecular crystals, which are often directly related to the reaction’s onset and path. We report the pressure evolution of the two-photon induced emission spectrum of crystalline stilbene, the archetype of a class of molecules from which double-core nanothreads are obtained, with the twofold purpose of gaining insight into the reaction mechanism and monitoring if the structural changes observed in x-ray diffraction studies have a detectable counterpart in the electronic properties of the system. The freezing of the spectral diffusion observed on rising pressure is ascribed to a hampered conformational rearrangement because of the larger stiffness of the local environment. The transition to the high pressure phase where the nanothreads form is revealed by the slope change of the pressure shift of all spectral components, while the progressive intensification with pressure of the 0-0 transition suggests a strengthening of the ethylenic bond favoring the charge delocalization on the benzene moieties, which is likely the trigger of the chemical instability.

Poly(ionic liquid)s are an interesting class of compounds because of their unique chemical and physical properties gathering the characteristics of ionic liquids and polymers. Pressure and temperature have been demonstrated to be alternative parameters to obtain polymers from monomeric species using only physical tools. In this work, we investigate the reaction under high pressure and room temperature of the ionic liquid 1-allyl-3-methylimidazolium iodide by using the diamond anvil cell technique in combination with synchrotron X-ray diffraction and electronic and vibrational spectroscopies. The results indicate a chemical reaction happening through the terminal double bond of the allyl group both in crystalline and glassy phases with the onset of the reaction around ∼7 GPa. Vibrational spectra present evidence for an oligomerization reaction in both the phases. The reaction occurring both in glassy and crystal phases indicates a mechanism not driven by collective motions and likely related to local topological arrangements. The results presented herein extend our understanding of ionic liquid instability boundaries under high pressure and contribute to the development of alternative synthetic routes to achieve poly(ionic liquids).

The presence of ions induces perturbations in the water network, these structural and dynamic modifications can extent over space scales overcoming the local solvation shell: aqueous solutions of sodium perchlorate (NaClO4) are characterized by extended phenomena of structure breaking of the solvent network. The aim of the present work is the experimental investigation of the interplay between the local structural modifications induced by the perchlorate ions and the collective dynamical properties of the solvent. Ultrafast Optical Kerr Effect (OKE) and time resolved infrared absorption are the experimental techniques adopted: OKE is mostly sensitive to the collective properties of the sample, while transient IR provides access to local properties of the solvent. Classical Molecular Dynamics (MD) simulations support the analysis of the experimental results. All experiments and simulations are performed at room temperature, varying the concentrations (0–6 M) and varying the applied pressure (10−4–1.3 GPa). Experiments and computer simulations confirm that pressure and concentration have convergent effects on the water dynamics, due to the analogous modification of the short-range liquid structure that cancel some dynamical anomalies typical of pure water. Both local and collective dynamic observables point to structural properties as responsible for their peculiar pressure and concentration dependence.

The pressure-induced reactivity of aromatic molecules in the crystal phase has been recently demonstrated to be a practicable route for the synthesis of crystalline nanothreads. The formation of th...

Self-assembly of organic macrocycles has been exploited as a preliminary step in the synthesis of soluble and tailorable carbon-based nanostructures. Functionalized nanotubes have been prepared using, as core building blocks, nearly planar ring structures containing several alkyne units, exploiting the geometry achieved in the spontaneous preassembling step driven by π interaction. Covalent cross-linking between these units was achieved by thermal or photochemical activation with UV light. Here, we apply a moderate pressure in a sapphire anvil cell (1.0 GPa) to facilitate the preassembling and induce the cross-linking under pressure either with visible light, absorbed by two-photon absorption, or thermally. We observe a high yield of enhanced quality cross-linked nanotubes in a sample, showing, at ambient pressure, only side-chain decomposition. These results show that moderate pressures, easily achievable in large volume cells, are able to selectively favor topochemical reactions in such complex organic ...

The high-pressure chemical reactivity of nitromethane, under irradiation with visible and near-UV laser light, was investigated by in situ FTIR spectroscopy in a diamond anvil cell. The reactivity was probed at different pressures (0.2, 1.2, 5.0, 15.3, and 28.0 GPa) with different excitation wavelengths (514, 458, and 350 nm), with all absorbed through a two-photon process. Insight into the reaction mechanism was gained by measuring the near-UV absorption spectrum of nitromethane as a function of pressure to 32 GPa, the threshold pressure above which it reacts spontaneously in the absence of electronic excitation. We were thus able to determine the pressure evolution of the two lowest-energy transitions (σ → π* and a singlet–triplet transition). The information obtained from the absorption spectra together with the reactivity data allowed us to locate the red absorption edge of the higher-energy π → π* transition and its pressure shift. The excitation of the σ → π* transition was not able to induce any ph...

Despite water being the most common and most widely studied substance in the world, it still presents unknown aspects. In particular, water shows several thermodynamic and dynamical anomalies in the liquid and supercooled metastable phases, and the natures of these phases are still hotly debated. Here, we report measurements of water using the optical Kerr effect as a function of pressure along two isotherms, at 273 K from 0.1 to 750 MPa and at 297 K from 0.1 to 1350 MPa, reaching the supercooled metastable phase. The structural relaxation and the low frequency vibrational dynamics of water show a peculiar pressure dependence similar to that of other dynamical properties. The data analysis suggests the presence in the water phase diagram of a crossover area that divides two regions characterized by different dynamic regimes, which appear to be related to two liquid forms, one dominated by the high density water and the other by the low density water.

The n-paraphenylene family comprises chains of phenylene units linked together by C-C bonds that are between single- and double-bonded, and where n corresponds to the number of phenylene units. In this work, we compare the response of the optical properties of different phenylene arrangements. We study linear chains (LPP), cyclic systems (CPPs), and non-conjugated cyclic systems with two hydrogenated phenylenes (H4[n]CPP). Particularly, the systems of interest in this work are [6]LPP, [12]- and [6]CPP and H4[6]CPP. This work combines Raman and infrared spectroscopies with absorption and fluorescence (one- and two-photon excitations) measured as a function of pressure up to maximum of about 25 GPa. Unprecedented crystallographic pressure-dependent results are shown on H4[n]CPP, revealing intramolecular &pi, &pi, interactions upon compression. These intramolecular interactions justify the H4[n]CPP singular optical properties with increasing fluorescence lifetime as a function of pressure.

Homogeneous melting of crystals is a complex multistep process involving the formation of transient states at temperatures considerably higher than the melting point. The nature and persistence of these metastable structures are intimately connected to the melting process, and a precise definition of the temporal boundaries of these phenomena is not yet available. We set up a specifically designed experiment to probe by transient infrared absorption spectroscopy the entire dynamics, ranging from tens of picoseconds to microseconds, of superheating and melting of an ice crystal. In spite of a large excess of energy provided, only about 30% of the micrometric crystal liquefies in the first 20-25 ns because of the long persistence of the superheated metastable phase that extends for more than 100 ns. This behavior is ascribed to the population of low-energy states that trap a large amount of energy, favoring the formation of a metastable, likely plastic, ice phase.

The pressure-induced transformation of molecular crystals can give rise to new materials characterized by intriguing hardness or energetic properties. Mechanisms regulating these reactions at the molecular level result from a complex interplay among crystal structure, lattice dynamics, and electronic properties. Here, we show that the formation of a three-dimensional amorphous carbon nitride by compressing phase II s-triazine is controlled by the competition between two different mechanisms, one entirely structural and the other electronic, representing the first example where such occurrence is demonstrated. Temperature drives the reactivity below 8 GPa by ruling the lattice dynamics, whereas above 8 GPa the electronic modifications, uniquely governed by pressure, trigger the chemical transformation. The amorphous material synthesized has a bonding structure characterized by a bulk typical of a strongly conjugated three-dimensional carbon nitride with hydrogen atoms migrated to saturate C and N terminations.

One-step synthesis using only physical tools is an appealing “green” method for the realization of technological materials. High-pressure conditions are particularly suitable in the attainment of extended supramolecular networks and in combination with high-temperature are effective in selecting specific reaction pathways to increase product quality. Here we show how pressures below 3 GPa and temperatures on the order of 420 K are effective for the synthesis, from urea crystal phase IV, of 2D graphitic carbon nitride oxide with enhanced crystalline quality. Angle-dispersive X-ray diffraction and Fourier transform IR spectroscopy show that the reaction becomes less selective at higher pressure with the formation of melamine and ammonium carbamate as byproducts. An anomalous positive slope is shown by the P–T instability boundary, likely arising from unfavorable hydrogen bonding with respect to the topochemical path, making urea an interesting case study for gaining insight into the role of hydrogen bonding in solid-state reactivity.

The effect of pressure on the electronic properties of crystalline s-triazine has been studied up to 14 GPa by using two-photon induced fluorescence. Emission and excitation spectra have been measured as a function of pressure on samples compressed in a diamond anvil cell. The different two-photon absorption cross sections to the nπ* and ππ* excited states account for the selectivity in the excitation wavelength observed in the high pressure photoinduced reactivity. The comparison between excitation and emission spectra highlights a remarkable red shift with rising pressure of the higher electronic excited states having ππ* character, which contrasts with the pressure insensitivity of the lowest nπ* states. Pressure is therefore extremely efficient at progressively destabilizing the π bonding orbitals, causing a reduction of the ring aromaticity, and driving the high pressure reactivity.

The high-pressure transformation of s-triazine to an extended amorphous carbon incorporating a large amount of nitrogen has been studied by Fourier transform infrared spectroscopy. The reaction, oc...

Dissociation of ethanol by two-photon absorption of UVML laser emission centered at 350 nm was employed to trigger a chemical reaction at ambient temperature with red phosphorus for pressures ranging between 0.2 and 1.5 GPa. The reaction products, identified by infrared and Raman spectroscopy, indicate a quite selective reactivity ascribable to the two main dissociation channels involving the splitting of the O-H and C-O bonds of ethanol. The ethoxy radical, obtained through the splitting of the O-H bond, has been identified as the main responsible for the phosphorus reactivity, giving rise to triethylphosphate. The same dissociation channel is also responsible for the formation of a consistent amount of molecular hydrogen, phosphine, and diethyl ether, whereas ethane and ethylene, the latter observed only in traces, likely derive from the C-O dissociation. The reaction is accelerated by increasing pressure from 0.2 to 0.6 GPa but is not favored, as also observed in pure ethanol, by a further pressure increase. The reaction proceeds until ethanol is completely consumed, and further irradiation determines the decomposition of the products, especially of diethyl ether, leading to the formation of CO2, methane, and ethane.

Picene is a polycyclic aromatic hydrocarbon belonging to the class of phenacenes which have been recently found to behave as high-temperature superconductors upon alkali metal doping. The electronic properties of organic crystals can be finely and largely modified by the density changes obtained by the application of an external pressure. In this work, the role of pressure in tuning the optical properties of crystalline picene has been investigated from room conditions up to 15 GPa through the measurement of UV-visible absorption spectra, two-photon excitation profiles, and one- and two-photon excited fluorescence spectra in a diamond anvil cell. The pressure dependence of the optical band gap was determined, and the frequencies of several vibronic bands belonging to electronic transitions from the ground state (S-0) to the four lowest-energy excited singlet states (S-1 to S-4) were determined as a function of pressure. We evidence a very different density dependence of the transition energy of S-0 -> S-1, which undergoes a remarkable red shift of similar to 400 cm(-1)/GPa, and of the transitions from S-0 to the higher excited states, which remain constant in the whole investigated range. This is consistent with a S-1 state of L-1(a) character in solid picene. The high-pressure chemical stability of solid picene was investigated through visible absorption and Fourier transform infrared spectroscopy (FTIR). A chemical transformation involving the bulk picene crystal occurs above similar to 23 GPa, giving rise to a disordered material similar to the amorphous hydrogenated carbon obtained in the pressure-induced reactivity of benzene. The combination of electronic and vibrational data allows us to identify the presence of reaction intermediates at similar to 10 GPa, preferentially forming at crystal defects.

Angle-dispersive X-ray diffraction and Fourier transform infrared spectroscopy have been employed to study the phase diagram of urea crystal beyond 15 GPa and at temperatures in excess of 400 K. Previously reported Bridgman phase II was structurally characterized for the first time, and it is discovered that it coincides with room-temperature phase IV. Large metastability P-T regions were identified for all phases in the sequence I-III-IV-V, ascribed to the difficulty to disrupt the H- bonding network, a prerequisite to accomplish the molecular rearrangement necessary for the structural transformation. High-temperature studies and use of a hydrostatic compression medium allows the thermodynamic boundaries of phase III, and partly of phase IV, to be identified therefore making a considerable step forward in the knowledge of the phase diagram of urea. (Graph Presented).

Two-photon excitation profiles and fluorescence spectra have been measured as a function of pressure in a diamond anvil cell up to 15.5 GPa in crystal phases I and II and in the glassy form of pyridine. The fluorescence emission intensity increases by about 6 orders of magnitude in going from the liquid to the crystalline phases at 3 GPa and further increases with pressure. This is explained by an energy inversion of the lowest (1)B(1) (nπ*) and (1)B(2) (ππ*) excited states likely due to the involvement of the lone pair of the N atom in intermolecular CH···N bonds. These interactions characterize the crystal phases and are stabilized by pressure. The glassy form, accordingly, is characterized by a much weaker fluorescence. Excimer emission is also observed. Comparison of the emission of several samples with different compression and annealing histories, the lack of reversibility in the excimer emission with decompression, and the larger relative intensity of the excimer band in the glassy form suggest that excimer formation occurs at crystal defects. This results support the conclusions of a previous investigation proposing that pressure-induced reactivity of pyridine is limited to crystal defects and agrees with the present knowledge of the solid-state chemistry of aromatic crystals.

Clarifying the structure/dynamics relation of water hydrogen-bond network has been the aim of extensive research over many decades. By joining anvil cell high-pressure technology, femtosecond 2D infrared spectroscopy, and molecular dynamics simulations, we studied, for the first time, the spectral diffusion of the stretching frequency of an HOD impurity in liquid water as a function of pressure. Our experimental and simulation results concordantly demonstrate that the rate of spectral diffusion is almost insensitive to the applied pressure. This behavior is in contrast with the previously reported pressure-induced speed up of the orientational dynamics, which can be rationalized in terms of large angular jumps involving sudden switching between two hydrogen-bonded configurations. The different trend of the spectral diffusion can be, instead, inferred considering that the first solvation shell preserves the tetrahedral structure with pressure and the OD stretching frequency is only slight perturbed.

Pressure is extremely efficient to tune intermolecular interactions, allowing the study of the mechanisms regulating, at the molecular level, the structure and dynamics of condensed phases. Among the simplest molecules, water represents in many respects a mystery despite its primary role in ruling most of the biological, physical, and chemical processes occurring in nature. Here we report a careful characterization of the dynamic regime change associated with low-density and high-density forms of liquid water by measuring the line shape of the OD stretching mode of HOD in liquid water along different isotherms as a function of pressure. Remarkably, the high-pressure studies have been here extended down to 240 K, well inside the supercooled regime. Supported by molecular dynamics simulations, a correlation among amorphous and crystalline solids and the two different liquid water forms is attempted to provide a unified picture of the metastable and thermodynamic regimes of water.

Liquid water has a primary role in ruling life on Earth in a wide temperature and pressure range as well as a plethora of chemical, physical, geological, and environmental processes. Nevertheless, a full understanding of its dynamical and structural properties is still lacking. Water molecules are associated through hydrogen bonds, with the resulting extended network characterized by a local tetrahedral arrangement. Two different local structures of the liquid, called low-density (LDW) and high-density (HDW) water, have been identified to potentially affect many different chemical, biological, and physical processes. By combining diamond anvil cell technology, ultrafast pump-probe infrared spectroscopy, and classical molecular dynamics simulations, we show that the liquid structure and orientational dynamics are intimately connected, identifying the P-T range of the LDW and HDW regimes. The latter are defined in terms of the speeding up of the orientational dynamics, caused by the increasing probability of breaking and reforming the hydrogen bonds.

The optical properties of picene at ambient conditions have been investigated through the measurement of UV/Vis absorption and fluorescence spectra and of excitation profiles, using one-and two-photon excitation, in solution and in the crystal phase. For solvated picene an assignment of the vibronic structure of the transitions to the four lowest-energy excited singlet states (S-1-S-4) has been obtained from the absorption data, and the vibronic structure of the fluorescence spectra has been assigned. The absorption and fluorescence spectra of the solid phase can be interpreted according to the single molecule analysis. Nevertheless, the strong increase of the optical density in the spectral region of the lowest HOMO-LUMO transitions and the frequency shift of absorption and fluorescence bands may be explained by a mixing of the states of adjacent molecules in the crystal. Moreover, peculiar emission features depending on the crystal dimensions (10(-1) to 10(2) mu m) are observed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770265]

The room-temperature reactivity of liquid methanol induced by two- photon absorption of near UV photons (350 nm) was studied as a function of pressure. Different chemical reactions were triggered by the radical species produced through the population of the lowest electronic excited singlet state because of its dissociative character. Experiments were performed at room temperature between 0.1 and 1.8 GPa on CH3OH and between 0.2 and 1.5 GPa on CD3OH. Different irradiation cycles were performed at constant pressure conditions, and FTIR and Raman spectra were measured to monitor the reaction evolution. Methoxymethanol and methylformate were the main products and the only ones detected in all the experiments. Ethylene glycol formed only at low pressure (0.2-0.3 GPa), whereas small amounts of methane, water, and unsaturated (C=C) species were also detected independently of the reaction pressure. Only dissociation along the O-H and C-O coordinates was relevant in the investigated pressure range. Ethylene glycol, methoxymethanol, and methylformate derive from the dissociation channel involving the O-H bond cleavage, whereas methane and unsaturated species come from the dissociation along the C-O bond. The comparison of the results obtained for the two isotopomers at the different investigated pressures allowed the identification of three different reactive paths that, starting from the methoxy radical, lead to the formation of the main products. The important effect of pressure on the reaction evolution could suggest a modification of the potential energy surface of the lowest electronic excited state along the O-H coordinate on increasing pressure.

Syntheses based on physical methods, such as pressure and light, are extremely attractive to prepare novel materials from pure molecular systems in condensed phases. The structural and electronic modifications induced by selective optical excitation can trigger unexpected chemical reactions by exploiting the high density conditions realized at high pressure. The identification of the microscopic mechanisms regulating this reactivity, mandatory to design synthetic environments appealing for practical applications, requires a careful characterization of both structural and electronic properties as a function of pressure. Here, we report a spectroscopic study, by FTIR and Raman techniques, of the ambient temperature photoinduced reactivity of liquid C(2)H(5)OD up to 1 GPa. The results have been interpreted by comparison with those relative to the fully hydrogenated isotopomer. The dissociation along the O-H (D) coordinate is the primary reactive channel, but the different reactivity of the two isotopomers with rising pressure highlights a dramatic pressure effect on the energy surface of the first electronic excited state. Dissociation along the O-H (D) coordinate becomes the reaction rate-limiting step due to an increase with pressure of the binding character along this coordinate.

In this work we have performed an extensive high pressure study of the condensed phases of pyridine by Raman and IR spectroscopy. We have evidenced three different polymorphs, two crystalline, and one glassy and established the pressure conditions in which they exist as stable or metastable phases by several compression/decompression experiments both on annealed and not annealed samples. Crystallization and phase transitions are found to be kinetically driven. The vibrational spectra are extremely complex due to the low symmetry of the crystals, which implies a large number of crystal components. This complexity required a careful analysis of both IR and Raman data that led to the identification of 20 out of 21 external modes expected for phase II. We did not find any conclusive indication of phase transitions on compressing phase II thus indicating that phase II is likely the stable phase at the onset pressure of the chemical transformation of pyridine. The latter starts at 18 GPa and relevant differences from the well characterized benzene reaction suggest that it is likely driven by crystal defects.