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

Exploiting solid-state dynamic nuclear polarization NMR spectroscopy to establish the spatial distribution of polymorphic phases in a solid material

Solid-state DNP NMR can enhance the ability to detect minor amounts of solid phases within heterogenous materials. Here we demonstrate that NMR contrast based on the transport of DNP-enhanced polarization can be exploited in the challenging case of early detection of a small amount of a minor polymorphic phase within a major polymorph, and we show that this approach can yield quantitative information on the spatial distribution of the two polymorphs. We focus on the detection of a minor amount (<4%) of polymorph III of m-aminobenzoic acid within a powder sample of polymorph I at natural isotopic abundance. Based on proposed models of the spatial distribution of the two polymorphs, simulations of 1H spin diffusion allow NMR data to be calculated for each model as a function of particle size and the relative amounts of the polymorphs. A comparison between simulated and experimental NMR data allows the model(s) best representing the spatial distribution of the polymorphs in the system to be established.

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.

Solvent-polymer guest exchange in a carbamazepine inclusion complex: structure, kinetics and implication for guest selection

Solvent–polymer guest exchange in a carbamazepine inclusion complex in a stirred solution was studied and a mechanism was proposed.

A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds

n-Alkane/urea inclusion compounds are crystalline materials in which n-alkane 'guest' molecules are located within parallel one-dimensional 'host' tunnels formed by a helical hydrogen-bonded arrangement of urea molecules. The periodic repeat distance of the guest molecules along the host tunnels is incommensurate with the periodic repeat distance of the host substructure. The structural properties of the high-temperature phase of these materials (phase I), which exist at ambient temperature, are described by a (3 + 1)-dimensional superspace. Recent publications have suggested that, in the prototypical incommensurate composite systems, n-nonadecane/urea and n-hexadecane/urea, two low-temperature phases II and 'III' exist and that one or both of these phases are described by a (3 + 2)-dimensional superspace. We present a phenomenological model based on symmetry considerations and developed in the frame of a pseudo-spin-phonon coupling mechanism, which accounts for the mechanisms responsible for the I ↔ II ↔ 'III' phase sequence. With reference to published experimental data, we demonstrate that, in all phases of these incommensurate materials, the structural properties are described by (3 + 1)-dimensional superspace groups. Around the temperature of the II ↔ 'III' transition, the macroscopic properties of the material are not actually associated with a phase transition, but instead represent a 'crossover' between two regimes involving different couplings between relevant order parameters.

The creation of modulated monoclinic aperiodic composites in n-alkane/urea compounds

n-Dodecane/urea is a member of the prototype series of n-alkane/urea inclusion compounds. At room temperature, it presents a quasi-one dimensional liquid-like state for the confined guest molecules within the rigid, hexagonal framework of the urea host. At lower temperatures, we report the existence of two other phases. Below T_c=248 K there appears a phase with rank four superspace group P6₁22(00γ), the one typically observed at room temperature in n-alkane/urea compounds with longer guest molecules. A misfit parameter, defined by the ratio γ=c_h /c_g (c_host/c_guest), is found to be 0.632±0.005. Below T_c1=123 K, a monoclinic modulated phase is created with a constant shift along c of the guest molecules in adjacent channels. The maximal monoclinic space group for this structure is P12₁1(α0γ). Analogies and differences with n-heptane/urea, which also presents a monoclinic, modulated low-temperature phase, are discussed.

X-ray Birefringence Imaging of Materials with Anisotropic Molecular Dynamics

The X-ray birefringence imaging (XBI) technique, reported very recently, is a sensitive tool for spatially resolved mapping of the local orientational properties of anisotropic materials. In this paper, we report the first XBI measurements on materials that undergo anisotropic molecular dynamics. Using incident linearly polarized X-rays with energy close to the Br K-edge, the X-ray birefringence is dictated by the orientational properties of the C-Br bonds in the material. We focus on two materials (urea inclusion compounds containing 1,8-dibromooctane and 1,10-dibromodecane guest molecules) for which the reorientational dynamics of the brominated guest molecules (and hence the reorientational dynamics of the C-Br bonds) are already well characterized by other experimental techniques. The XBI results demonstrate clearly that, for the anisotropic molecular dynamics in these materials, the effective X-ray optic axis for the X-ray birefringence phenomenon is the time-averaged resultant of the orientational distribution of the C-Br bonds.

Tunable porous organic crystals: structural scope and adsorption properties of nanoporous steroidal ureas

Previous work has shown that certain steroidal bis-(N-phenyl)ureas, derived from cholic acid, form crystals in the P6(1) space group with unusually wide unidimensional pores. A key feature of the nanoporous steroidal urea (NPSU) structure is that groups at either end of the steroid are directed into the channels and may in principle be altered without disturbing the crystal packing. Herein we report an expanded study of this system, which increases the structural variety of NPSUs and also examines their inclusion properties. Nineteen new NPSU crystal structures are described, to add to the six which were previously reported. The materials show wide variations in channel size, shape, and chemical nature. Minimum pore diameters vary from ~0 up to 13.1 Å, while some of the interior surfaces are markedly corrugated. Several variants possess functional groups positioned in the channels with potential to interact with guest molecules. Inclusion studies were performed using a relatively accessible tris-(N-phenyl)urea. Solvent removal was possible without crystal degradation, and gas adsorption could be demonstrated. Organic molecules ranging from simple aromatics (e.g., aniline and chlorobenzene) to the much larger squalene (M(w) = 411) could be adsorbed from the liquid state, while several dyes were taken up from solutions in ether. Some dyes gave dichroic complexes, implying alignment of the chromophores in the NPSU channels. Notably, these complexes were formed by direct adsorption rather than cocrystallization, emphasizing the unusually robust nature of these organic molecular hosts.

Controlling spatial distributions of molecules in multicomponent organic crystals, with quantitative mapping by confocal Raman microspectrometry

We report four experimental strategies for controlling the three-dimensional arrangement of molecules in multicomponent organic crystals, exploiting confocal Raman microspectrometry to quantify the three-dimensional spatial distributions. Specifically, we focus on controlling the distribution of two types of guest molecule in solid organic inclusion compounds to produce composite core-shell crystals, crystals with a homogeneous distribution of the components, crystals with continuous compositional variation from the core to the surface, and crystals with alternating shells of the components. In this context, confocal Raman microspectrometry is particularly advantageous over optical microscopy as it is nondestructive, offers micrometric spatial resolution, and relies only on the component molecules having different vibrational properties.

Confined linear molecules inside an aperiodic supramolecular crystal: the sequence of superspace phases in n-hexadecane/urea

High-resolution studies of the host-guest inclusion compound n-hexadecane/urea are reported at atmospheric pressure, using both cold neutrons and x-ray diffraction. This intergrowth crystal presents a misfit parameter, defined by the ratio c(h)/c(g) (c(host)/c(guest)), which is temperature independent and irrational (γ = 0.486 ± 0.002) from 300 to 30 K. Three different structural phases are reported for this aperiodic crystal over this temperature range. The crystallographic superspaces are of rank 4 in phases I and II, whereas phase III is associated with an increase in rank to 5, with a supplementary misfit parameter (δ = 0.058 ± 0.002) that is constant throughout this phase. The superspace group of phase I is hexagonal P6(1)22(00γ) down to T(c1) = 149.5 ± 0.5 K; phase II, which persists down to T(c2) = 127.8 ± 0.5 K is orthorhombic P2(1)2(1)2(1)(00γ), and phase III is orthorhombic P2(1)2(1)2(1)(00γ)(00δ).

MOLECULAR TRANSPORT AND FLUIDICS IN CARBON NANOTUBE

In this article we review recent developments in molecular transport and fluidics in carbon nanotube (CNT)-based nanochannels. Atomic molecular dynamics simulations and theoretical studies based on Fokker–Planck diffusion equation on the transport of large and long polymer molecules in CNTs are the focus of the article. Fast translocation and diffusion processes of large molecules in CNTs are reviewed and discussed, considering the effects of interfacial interactions and molecular conformations and structures at interface. The transport features for multiple molecules diffusing through CNTs are also discussed.

Anhydrous proton conduction at 150 °C in a crystalline metal-organic framework

Metal organic frameworks (MOFs) are particularly exciting materials that couple porosity, diversity and crystallinity. But although they have been investigated for a wide range of applications, MOF chemistry focuses almost exclusively on properties intrinsic to the empty frameworks; the use of guest molecules to control functions has been essentially unexamined. Here we report Na(3)(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) (named β-PCMOF2), a MOF that conducts protons in regular one-dimensional pores lined with sulfonate groups. Proton conduction in β-PCMOF2 was modulated by the controlled loading of 1H-1,2,4-triazole (Tz) guests within the pores and reached 5 × 10(-4) S cm(-1) at 150 °C in anhydrous H(2), as confirmed by electrical measurements in H(2) and D(2), and by solid-state NMR spectroscopy. To confirm its potential as a gas separator membrane, the partially loaded MOF (β-PCMOF2(Tz)(0.45)) was also incorporated into a H(2)/air membrane electrode assembly. The resulting membrane proved to be gas tight, and gave an open circuit voltage of 1.18 V at 100 °C.

Nitrogen and hydrogen adsorption by an organic microporous crystal

Quick on the uptake: Following its identification during a targeted search, the intriguing crystal structure of 3,3',4,4'-tetra(trimethylsilylethynyl)biphenyl was investigated. Simple removal of the included solvent provides an organic crystal with an open microporous structure that has a striking similarity to that of zeolite A (see picture). Reversible adsorption of nitrogen and hydrogen gases at 77 K confirms that the microporosity is permanent.

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.

Pressure-induced lock-in in an aperiodic nanoporous crystal

This Letter reports on the first observation of a commensurate lock-in inside an aperiodic composite. This result is obtained by neutron diffraction, under hydrostatic pressure, in the prototype self-assembled crystal of hexadecane urea. A selective compressibility of the sublattices is a required condition together with the existence of a lock-in energy term in these supramolecular materials. This measurement, under one-dimensional confinement, opens the way for the use of continuously controlled forces to manipulate molecular properties or functions.