Reply to the 'Comment on Cage occupancy of methane clathrate hydrates in the ternary H2O-NH3-CH4 system' by S. Alavi and J. Ripmeester, Chem. Commun., 2022, , DOI: 10.1039/D1CC06526B

Our recent Communication suggested that ammonia in aqueous solution may preferentially destabilize large cages in methane clathrate hydrates. A Comment favored ammonia incorporation instead, but it did not accurately describe our proposed mechanism and relied primarily on studies conducted in different chemical systems and/or which used other preparation methods.

Cage occupancy of methane clathrate hydrates in the ternary H2O-NH3-CH4 system

The incorporation of ammonia inside methane clathrate hydrate is of great interest to the hydrate chemistry community. We investigated the phase behavior of methane clathrate formed from aqueous ammonia solution. Ammonia's presence decreases methane occupancy in the large cages, without definitive Raman spectroscopic evidence for its incorporation inside the structure.

Promoting the Insertion of Molecular Hydrogen in Tetrahydrofuran Hydrate With the Help of Acidic Additives

Among hydrogen storage materials, hydrogen hydrates have received a particular attention over the last decades. The pure hydrogen hydrate is generated only at extremely high-pressure (few thousands of bars) and the formation conditions are known to be softened by co-including guest molecules such as tetrahydrofuran (THF). Since this discovery, there have been considerable efforts to optimize the storage capacities in hydrates through the variability of the formation condition, of the cage occupancy, of the chemical composition or of the hydrate structure (ranging from clathrate to semi-clathrate). In addition to this issue, the hydrogen insertion mechanism plays also a crucial role not only at a fundamental level, but also in view of potential applications. This paper aims at studying the molecular hydrogen diffusion in the THF hydrate by in-situ confocal Raman microspectroscopy and imaging, and at investigating the impact of strong acid onto this diffusive process. This study represents the first report to shed light on hydrogen diffusion in acidic THF-H₂ hydrate. Integrating the present result with those from previous experimental investigations, it is shown that the hydrogen insertion in the THF hydrate is optimum for a pressure of ca. 55 bar at 270 K. Moreover, the co-inclusion of perchloric acid (with concentration as low as 1 acidic molecules per 136 water molecules) lead to promote the molecular hydrogen insertion within the hydrate structure. The hydrogen diffusion coefficient—measured at 270 K and 200 bar—is improved by a factor of 2 thanks to the acidic additive.

Gas Hydrate Crystallization in Thin Glass Capillaries: Roles of Supercooling and Wettability

We designed and implemented an experimental methodology to investigate gas hydrate formation and growth around a water-guest meniscus in a thin glass capillary, thus mimicking pore-scale processes in sediments. The glass capillary acts as a high-pressure optical cell in a range of supercooling conditions from 0.1 °C, i.e., very close to hydrate dissociation conditions, to ∼35 °C, very near the metastability limit. Liquid or gaseous CO₂ is the guest phase in most of the experiments reported in this paper, and N₂ in a few of them. The setup affords detailed microscopic observation of the roles of the key parameters on hydrate growth and interaction with the substrate: supercooling and substrate wettability. At low supercooling (less than 0.5 °C), a novel hydrate growth process is discovered, which consists of a hollow crystal originating from the meniscus and advancing on the guest side along the glass, fed by a thick water layer sandwiched between the glass and this crystal.

Unraveling the metastability of the SI and SII carbon monoxide hydrate with a combined DFT-neutron diffraction investigation

Clathrate hydrates are crystalline compounds consisting of water molecules forming cages (so-called "host") inside of which "guest" molecules are encapsulated depending on the thermodynamic conditions of formation (systems stable at low temperature and high pressure). These icelike systems are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. Carbon monoxide hydrate might be considered an important component of the carbon cycle in the solar system since CO gas is one of the predominant forms of carbon. Intriguing fundamental properties have also been reported: the CO hydrate initially forms in the sI structure (kinetically favored) and transforms into the sII structure (thermodynamically stable). Understanding and predicting the gas hydrate structural stability then become essential. The aim of this work is, thereby, to study the structural and energetic properties of the CO hydrate using density functional theory (DFT) calculations together with neutron diffraction measurements. In addition to the comparison of DFT-derived structural properties with those from experimental neutron diffraction, the originality of this work lies in the DFT-derived energy calculations performed on a complete unit cell (sI and sII) and not only by considering guest molecules confined in an isolated water cage (as usually performed for extracting the binding energies). Interestingly, an excellent agreement (within less than 1% error) is found between the measured and DFT-derived unit cell parameters by considering the Perdew-Burke-Ernzerhof (denoted PBE) functional. Moreover, a strategy is proposed for evaluating the hydrate structural stability on the basis of potential energy analysis of the total nonbonding energies (i.e., binding energy and water substructure nonbonding energy). It is found that the sII structure is the thermodynamically stable hydrate phase. In addition, increasing the CO content in the large cages has a stabilizing effect on the sII structure, while it destabilizes the sI structure. Such findings are in agreement with the recent experimental results evidencing the structural metastability of the CO hydrate.

Selective trapping of CO2 gas and cage occupancy in CO2-N2 and CO2-CO mixed gas hydrates

Hydrate-based CO2 trapping from CO2-N2 and CO2-CO gas mixtures is shown by Raman spectroscopy - the results are of interest for new separation and capture technology. A better trapping efficiency is measured for low CO2 concentrations and N2-based gas mixtures. Moreover, it is observed that CO molecules would impede hydrate formation from ice when a CO-enriched gas mixture is considered.

BDNF-TrkB signaling through Erk1/2 MAPK phosphorylation mediates the enhancement of fear memory induced by glucocorticoids

Activation of glucocorticoid receptors (GR) by glucocorticoid hormones (GC) enhances contextual fear memories through the activation of the Erk1/2(MAPK) signaling pathway. However, the molecular mechanism mediating this effect of GC remains unknown. Here we used complementary molecular and behavioral approaches in mice and rats and in genetically modified mice in which the GR was conditionally deleted (GR(NesCre)). We identified the tPA-BDNF-TrkB signaling pathway as the upstream molecular effectors of GR-mediated phosphorylation of Erk1/2(MAPK) responsible for the enhancement of contextual fear memory. These findings complete our knowledge of the molecular cascade through which GC enhance contextual fear memory and highlight the role of tPA-BDNF-TrkB-Erk1/2(MAPK) signaling pathways as one of the core effectors of stress-related effects of GC.

The enhancement of stress-related memory by glucocorticoids depends on synapsin-Ia/Ib

The activation of glucocorticoid receptors (GR) by glucocorticoids increases stress-related memory through the activation of the MAPK signaling pathway and the downstream transcription factor Egr-1. Here, using converging in vitro and in vivo approaches, respectively, GR-expressing cell lines, culture of hippocampal neurons, and GR genetically modified mice (GR(NesCre)), we identified synapsin-Ia/Ib as one of the effectors of the glucocorticoid signaling cascade. Stress and glucocorticoid-induced activation of the GR modulate synapsin-Ia/Ib through two complementary mechanisms. First, glucocorticoids driving Egr-1 expression increase the expression of synapsin-Ia/Ib, and second, glucocorticoids driving MAPK activation increase its phosphorylation. Finally, we showed that blocking fucosylation of synapsin-Ia/Ib in the hippocampus inhibits its expression and prevents the glucocorticoid-mediated increase in stress-related memory. In conclusion, our data provide a complete molecular pathway (GR/Egr-1/MAPK/Syn-Ia/Ib) through which stress and glucocorticoids enhance the memory of stress-related events and highlight the function of synapsin-Ia/Ib as molecular effector of the behavioral effects of stress.

Kinetics of molecular transport in a nanoporous crystal studied by confocal Raman microspectrometry: single-file diffusion in a densely filled tunnel

Confocal Raman microspectrometry has been used as an in situ probe of the transport of guest molecules along the one-dimensional tunnels in a crystalline urea inclusion compound, under conditions of guest exchange in which "new" guest molecules (pentadecane) are introduced at one end of the tunnel and displace the "original" guest molecules (1,8-dibromooctane). The Raman spectra, recorded as a function of position along the tunnel direction and as a function of time, have been used to establish details of the kinetics of the guest transport process. In particular, the transport of the new pentadecane guest molecules along the tunnel is found to exhibit a linear dependence on time, with the rate of the process in the region of 70-100 nm s-1. Mechanistic aspects relating to the guest transport process are discussed.

Significant conformational changes associated with molecular transport in a crystalline solid

Confocal Raman microspectrometry has been applied as an in situ probe of the transport of guest molecules along the one-dimensional tunnels in a crystalline urea inclusion compound, under conditions of guest exchange in which "new" guest molecules (pentadecane) are introduced at one end of the tunnel and displace the "original" guest molecules (1,8-dibromooctane). The Raman spectra, recorded as a function of position along the tunnel direction and as a function of time, demonstrate that the transport process is associated with a significant change in the conformational properties of the original (1,8-dibromooctane) guest molecules. In particular, in the boundary region between the original and new guest molecules, there is a substantial increase in the proportion of 1,8-dibromooctane guest molecules that have the gauche end-group conformation. The wider implications of this observation are discussed in relation to fundamental aspects of the molecular transport process in this material.

Enhancement of the Raman scattering signal due to a nanolens effect

The Raman scattering signal of a substrate is investigated using a polystyrene nanolens of a few hundred nanometers inserted within the light path of a confocal microspectrometer. As observed in solid immersion microscopy, the nanolens is at the origin of the improvement of the spatial resolution. Furthermore, enhancement of the Raman scattering signal of the substrate is observed when measuring through the polystyrene bead. The enhancement factors have been measured for silicon, highly ordered pyrolytic graphite, and gallium arsenide substrates. This setup provides a new way of enhancing the Raman signal by means of a nanolens.

Mechanistic Aspects of the Solid-State Transformation of Ammonium Cyanate to Urea at High Pressure

The chemical transformation of ammonium cyanate into urea has been of interest to many generations of scientists since its discovery by Friedrich Wöhler in 1828. Although widely studied both experimentally and theoretically, several mechanistic aspects of this reaction remain to be understood. In this paper, we apply computational methods to investigate the behavior of ammonium cyanate in the solid state under high pressure, employing a theoretical approach based on the self-consistent-charges density-functional tight-binding method (SCC-DFTB). The ammonium cyanate crystal structure was relaxed under external pressure ranging from 0 to 700 GPa, leading to the identification of five structural phases. Significantly, the phase at highest pressure (above 535 GPa) corresponds to the formation of urea molecules. At ca. 25 GPa, there is a phase transition of ammonium cyanate (from tetragonal P4/nmm to monoclinic P21/m) involving a rearrangement of the ammonium cyanate molecules. This transformation is critical for the subsequent transformation to urea. The crystalline phase of urea obtained above 535 GPa also has P21/m symmetry (Z = 2). This polymorph of urea has never been reported previously. Comparisons to the known (tetragonal) polymorph of urea found experimentally at ambient pressure suggests that the new polymorph is more stable above ca. 8 GPa. Our computational studies show that the transformation of ammonium cyanate into urea is strongly exothermic (enthalpy change -170 kJ mol-1 per formula unit between 530 and 535 GPa). The proposed mechanism for this transformation involves the transfer of two hydrogen atoms of the ammonium cation toward nitrogen atoms of neighboring cyanate anions, and the remaining NH2 group creates a C-NH2 bond with the cyanate unit.

The dynamical properties of the aromatic hydrogen bond in NH4(C6H5)4B from quasielastic neutron scattering

NH(4)(C(6)H(5))(4)B represents a prototypical system for understanding aromatic H bonds. In NH(4)(C(6)H(5))(4)B an ammonium cation is trapped in an aromatic cage of four phenyl rings and each phenyl ring serves as a hydrogen bond acceptor for the ammonium ion as donor. Here the dynamical properties of the aromatic hydrogen bond in NH(4)(C(6)H(5))(4)B were studied by quasielastic incoherent neutron scattering in a broad temperature range (20< or =T< or =350 K). We show that in the temperature range from 67 to 350 K the ammonium ions perform rotational jumps around C(3) axes. The correlation time for this motion is the lifetime of the "transient" H bonds. It varies from 1.5 ps at T=350 K to 150 ps at T=67 K. The activation energy was found to be 3.14 kJ mol, which means only 1.05 kJ mol per single H bond for reorientations around the C(3) symmetry axis of the ammonium group. This result shows that the ammonium ions have to overcome an exceptionally low barrier to rotate and thereby break their H bonds. In addition, at temperatures above 200 K local diffusive reorientational motions of the phenyl rings, probably caused by interaction with ammonium-group reorientations, were found within the experimental observation time window. At room temperature a reorientation angle of 8.4 degrees +/-2 degrees and a correlation time of 22+/-8 ps were determined for the latter. The aromatic H bonds are extremely short lived due to the low potential barriers allowing for molecular motions with a reorientational character of the donors. The alternating rupture and formation of H bonds causes very strong damping of the librational motion of the acceptors, making the transient H bond appear rather flexible.

Water Dynamics in Hardened Ordinary Portland Cement Paste or Concrete: From Quasielastic Neutron Scattering

Portland cement reacts with water to form an amorphous paste through a chemical reaction called hydration. In concrete the formation of pastes causes the mix to harden and gain strength to form a rock-like mass. Within this process lies the key to a remarkable peculiarity of concrete: it is plastic and soft when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses, and dams. The character of the concrete is determined by the quality of the paste. Creep and shrinkage of concrete specimens occur during the loss and gain of water from cement paste. To better understand the role of water in mature concrete, a series of quasielastic neutron scattering (QENS) experiments were carried out on cement pastes with water/cement ratio varying between 0.32 and 0.6. The samples were cured for about 28 days in sealed containers so that the initial water content would not change. These experiments were carried out with an actual sample of Portland cement rather than with the components of cement studied by other workers. The QENS spectra differentiated between three different water interactions: water that was chemically bound into the cement paste, the physically bound or "glassy water" that interacted with the surface of the gel pores in the paste, and unbound water molecules that are confined within the larger capillary pores of cement paste. The dynamics of the "glassy" and "unboud" water in an extended time scale, from a hundred picoseconds to a few nanoseconds, could be clearly differentiated from the data. While the observed motions on the picosecond time scale are mainly stochastic reorientations of the water molecules, the dynamics observed on the nanosecond range can be attributed to long-range diffusion. Diffusive motion was characterized by diffusion constants in the range of (0.6-2) 10(-9) m(2)/s, with significant reduction compared to the rate of diffusion for bulk water. This reduction of the water diffusion is discussed in terms of the interaction of the water with the calcium silicate gel and the ions present in the pore water.

Direct time-resolved and spatially resolved monitoring of molecular transport in a crystalline nanochannel system

Confocal Raman microspectrometry has been applied successfully as an in situ probe of the transport of guest molecules through the one-dimensional channel system in a crystalline inclusion compound, yielding insights into the spatial distribution of guest molecules and, in particular, the variation in the spatial distribution of the guest molecules as a function of time during the transport process.

Inelastic Neutron Scattering Study of Electron Reduction in Mn12 Derivatives

We report inelastic neutron scattering (INS) studies on a series of Mn(12) derivatives, [Mn(12)O(12)(O2CC6F5)16(H2O)4]z, in which the number of unpaired electrons in the cluster is varied. We investigated three oxidation levels: z = 0 for the neutral complex, z = -1 for the one-electron reduced species and z = -2 for the two-electron reduced complex. For z = 0, the ground state is S = 10 as in the prototypical Mn12-acetate. For z = -1, we have S = 19/2, and for z = - 2, an S = 10 ground state is retrieved. INS studies show that the axial zero-field splitting parameter D is strongly suppressed upon successive electron reduction: D = -0.45 cm(-1) (z = 0), D = -0.35 cm(-1) (z = -1), and D approximately -0.26 cm(-1) (z = -2). Each electron reduction step is directly correlated to the conversion of one anisotropic (Jahn-Teller distorted) Mn3+ (S = 2) to one nearly isotropic Mn2+ (S = 5/2).

Rotational dynamics of methyl groups in m-xylene

Methyl group dynamics of m-xylene was investigated by using incoherent inelastic and quasi-elastic neutron scattering. Inelastic measurements were carried out at the high flux backscattering spectrometer HFBS at the National Institute of Standards, quasi-elastic measurements at the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institute. Rotational potentials are derived which describe the tunnel splittings, first librational, and activation energies of the two inequivalent CH(3) groups. Indications for coupling of the methyl rotation to low-energy phonons have been found. The finite width of one tunneling transition at He temperature is described by direct methyl-methyl coupling. The combined results of the experiments and the calculations allow a unique assignment of rotor excitations to crystallographic sites.

Proton dynamics in the perchloric acid clathrate hydrate HClO4⋅5.5H2O

In the perchloric acid clathrate hydrate HClO4.5.5H2O, the perchlorate anions are contained inside an aqueous host crystalline matrix, positively charged because of the presence of delocalized acidic protons. Our experimental results demonstrate that the microscopic mechanisms of proton conductivity in this system are effective on a time scale ranging from nanosecond to picosecond. In the present paper, we discuss more specifically on the relaxation processes occurring on a nanosecond time scale by combining high-resolution quasielastic neutron scattering and 1H pulse-field-gradient nuclear magnetic resonance experiments. The combination of these two techniques allows us to probe proton dynamics in both space and time domains. The existence of two types of proton dynamical processes has been identified. The slowest one is associated to long-range translational diffusion of protons between crystallographic oxygen sites and has been precisely characterized with a self-diffusion coefficient of 3.5 x 10(-8) cm2/s at 220 K and an activation energy of 29.2+/-1.4 kJ/mol. The fastest dynamical process is due to water molecules' reorientations occurring every 0.7 ns at 220 K with an activation energy of 17.4+/-1.5 kJ/mol. This powerful multitechnique approach provides important information required to understand the microscopic origin of proton transport in an ionic clathrate hydrate.

[An animal model of human declarative (relational) memory and of its dysfunction]

The present work was aimed at determining, both at the psychological and at the neurobiological levels, aspects of rodent memory that fall into line with human declarative memory which is known to be selectively impaired in amnesic subjects and during the course of ageing. The ability to compare and to contrast items in memory, and to support inferential use of memories in novel situations (flexibility), were considered to be the two key psychological features of human declarative memory that were altered by both hippocampal lesions and hippocampal dysfunction. Adult and aged mice were trained on learning tasks using two-stage paradigms, the aim of which was to assess memory performance through these two psychological aspects in the same subjects. Results suggest that ageing specifically impairs the ability to both compare and contrast items in memory (declarative/relational memory based on complex associations), without altering memory based on simple S-R associations (procedural memory). Hippocampal lesions in adult mice produced the same dissociation between relational memory (impaired) and procedural memory (spared). Pharmacological experiments showed that, depending on the drug used, the relational memory deficit of aged mice may be selectively reversed (i.e. without changes in procedural memory) and that the behavioural efficacy of certain treatments was shown to parallel their potency in re-establishing normal (i.e. adult) levels of hippocampal plasticity-related mechanisms. Together with previous findings, these results suggest that the storage and use of relational representations would critically depend on the plasticity of hippocampal synapses, which via their connections with cortical areas, would support the storage of associations between perceptual, behavioral and cognitive events.

Vasopressin in the lateral septum promotes elemental conditioning to the detriment of contextual fear conditioning in mice

Previous experiments using a classical fear conditioning paradigm have provided evidence that the processing of contextual conditional stimuli (CSs) by the hippocampus would be controlled by the amygdala through a modulation of hippocampal-lateral septal (H-LS) excitability. More specifically, our suggestion was that vasopressin release into the LS would occur in an elemental conditioning case [pairing CS-US (unconditional stimulus) procedure] and would result in less hippocampal-dependent contextual stimuli processing (i.e. overshadowing of CSs by the simple CS). Conversely, when an unpairing CS-US procedure is used, this would result in more contextual stimuli processing through a decrease in vasopressin release into the LS. The aim of the present experiment was to test this hypothesis using intraseptal injection of vasopressin or its V1/V2 antagonist. In agreement with this hypothesis, results suggest that vasopressin release into the LS would constitute a device by which priority is given to the more salient simple stimulus to the detriment of contextual information.

Differential modulation of changes in hippocampal-septal synaptic excitability by the amygdala as a function of either elemental or contextual fear conditioning in mice

Recent data obtained using a classic fear conditioning paradigm showed a dissociation between the retention of associations relative to contextual information (dependent on the hippocampal formation) and the retention of elemental associations (dependent on the amygdala). Furthermore, it was reported that conditioned emotional responses (CERs) could be dissociated from the recollection of the learning experience (declarative memory) in humans and from modifications of the hippocampal-septal excitability in animals. Our aim was to determine whether these two systems ("behavioral expression" system and "factual memory" system) interact by examining the consequences of amygdalar lesions (1) on the modifications of hippocampal-septal excitability and (2) on the behavioral expression of fear (freezing) resulting from an aversive conditioning during reexposure to conditional stimuli (CSs). During conditioning, to modulate the predictive nature of the context and of a discrete stimulus (tone) on the unconditional stimulus (US) occurrence, the phasic discrete CS was paired with the US or randomly distributed with regard to the US. After the lesion, the CER was dramatically reduced during reexposure to the CSs, whatever the type of acquisition. However, the changes in hippocampal-septal excitability persisted but were altered. For controls, a decrease in septal excitability was observed during reexposure to the conditioning context only for the "unpaired group" (predictive context case). Conversely, among lesioned subjects this decrease was observed in the "paired group" (predictive discrete CS case), whereas this decrease was significantly reduced in the unpaired group with respect to the matched control group. The amplitude and the direction of these modifications suggest a differential modulation of hippocampal-septal excitability by the amygdala to amplify the contribution of the more predictive association signaling the occurrence of the aversive event.