Single-crystal deuterium NMR study of the symmetry breaking in an incommensurate organic inclusion compound (original) (raw)
Related papers
Royal Society Open Science
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 r...
Comments on the interpretation of dynamic deuterium NMR spectra from solid inclusion compounds
The Journal of Physical Chemistry, 1987
Guest molecules trapped within the regular voids of crystalline adducts, called inclusion compounds, are likely to experience mobility. Recent solid-state *H NMR studies indicated these motions to be primarily discrete jumps related to the molecular symmetry, to that of the environment, or to some other geometric feature of the system investigated. In this work we offer a more consistent picture based primarily on the symmetry of the voids, but also including the molecular symmetry. We show that the 2H NMR spectra from a whole class of guest molecules can be interpreted in terms of planar threefold jumps between unequally populated sites, such that the populations follow a simple Boltzmann law. The enthalpy difference between these sites is of the order of 1-2 kcal reflecting a rather weak but nonuniform guest-host interaction.
Royal Society Open Science
In a recent paper (Couzi et al. 2018 R. Soc. open sci. 5 , 180058. ( doi:10.1098/rsos.180058 )), we proposed a new phenomenological model to account for the I↔II↔“III” phase sequence in incommensurate n -alkane/urea inclusion compounds, which represents an alternative interpretation to that proposed in work of Toudic et al. In a Comment (Toudic et al. 2019 R. Soc. open sci. 6 , 182073. ( doi:10.1098/rsos.182073 )), Toudic et al. have questioned our assignment of the superspace group of phase II of n -nonadecane/urea, which they have previously assigned, based on a (3 + 2)-dimensional superspace, as C222 1 (00 γ )(10 δ ). In this Reply, we present new results from a comprehensive synchrotron single-crystal X-ray diffraction study of n -nonadecane/urea, involving measurements as a detailed function of temperature across the I↔II↔“III” phase transition sequence. Our results demonstrate conclusively that “main reflections” ( h, k, l , 0) with h+k odd are observed in phase II of n -nonad...
Phase transitions and ordering in urea inclusion compounds with n-paraffins
Journal of Physics and Chemistry of Solids, 1986
Urea inclusion compounds consist of a pseudo-hexagonal framework of urea with "infinite" channels parallel to c, into which paraffins are embedded. The compound with hexadecane exhibits phase transitions at -365, -148 and -120 K (phases I-IV), which have been investigated by X-ray and neutron scattering. Phase I is hexagonal with a (lon~tudinal and o~entational) random dist~bution of the included molecules. In phase II, which uniquely occurs in the adduct with C,,H,, a doubling of the lattice constant c,, of the host structure indicates partial lon~tudinal ordering. This is assigned to the approximately equal length of the hexadecane mote&e and 2c, leading to mutual deformations of both constituents. The transition II+111 is connected with a lateral orientational ordering of the guest molecules in adjacent channels and a simultaneous orthorhombic deformation of the host. Correlated rotational motions of the included molecules around their long axes are deduced from the increasing critical scattering above Tc. In phase IV a crystal-like ordering tendency of the paraffins plays the dominant role, while the host breaks into a heavily disordered domain structure. Experimental results indicate a transformation process as no thermal equilibrium was reached during the measurements. The different ordering mechanisms are discussed in terms of interactions between host and guest, guest and guest along each channel, and guest and guest in adjacent channels.
Structures and Energies of Mono-, Di-, and Triprotonated Urea: NMR/ab Initio/IGLO Study
The Journal of Organic Chemistry, 1994
The structures and NMR chemical shifts (13C and 15N) of urea and its mono-, di-, and triprotonated forms were calculated using high-level ab initio and IGLO methods, respectively. The results were compared with the experimental data obtained on mono-and diprotonated urea. Triprotonated urea was not observed experimentally.
Magnetic Resonance in Chemistry, 2004
The intergrowth of 1,10-decanedicarboxylic acid and urea give infinite hydrogen-bonded chains of the guest included in the hexagonal urea host. A deuterium high-resolution solid-state NMR study of the selectively deuterated intergrowth compound 1,10-decanedicarboxylic acid/hydrogenated urea at variable temperature in the range 90 ≤ T ≤ 300 K was performed on a single crystal. The analysis of the second moment as a function of temperature is shown to be compatible with the known phase transition occurring near T c = 203 K. Moreover, the spectra indicate that the orientational disorder is strong, and is compared to an axial uniform disorder. For this purpose, the general equation for the second moment of a system with uniform two-dimensional axial orientational disorder is given, and a method to take into account the non-uniform excitation of the pulse sequence is proposed.
Journal of the American Chemical Society, 2008
Flexible chiral molecules undergoing fast interconversion (on the NMR time scale) between different conformational enantiomers may yield "average" axial species with enantiotopically related sites. Contrary to the situation observed for rigid axial molecules, signals from these enantiotopic sites in NMR spectra recorded in chiral liquid-crystalline solvents can be resolved. In the present work, we studied the deuterium NMR spectra of tridioxyethylenetriphenylene (compound 4) statistically deuterated to 10% in the flexible side chains and dissolved in chiral and achiral lyotropic liquid crystals based on poly(γbenzylglutamate). The fast chair-chair flipping of the side chains in 4 on average renders the molecule axially symmetric (D 3h) with pairs of enantiotopic ethylene deuterons. These deuterons exhibit unusually large enantiodiscrimination. To explain this observation, we first describe how the average symmetry of flexible molecules can be derived from the symmetry of the "frozen" conformers and the nature of the averaging process. The procedure is then applied to 4 and used to analyze the NMR results. It is shown that the large enantiodiscrimination in the present case reflects a large difference in the orientational ordering of the conformational enantiomers participating in the interconversion processes as well as a large geometrical factor due to the special shape of the dioxyethylene side groups. 1 H and 13 C NMR spectra of 4 in the same lyotropic liquid crystalline solvent are analyzed to determine its ordering characteristics. Several related cases are also discussed.
Journal of Molecular Structure, 2007
Proton NMR spin-lattice relaxation times T 1 were measured for urea as a function of temperature. An activation energy of 46.3 ± 4.7 kJ/mol was extracted and compared with the range of 38-65 kJ/mol previously reported in the literature as measured by different magnetic resonance techniques. In addition, proton NMR spin-lattice relaxation times in the rotating frame T 1ρ were measured as a function of temperature. These measurements provide acquisition conditions for the 13 C and 15 N CP/MAS spectra of pure urea in the crystalline phase.
The Journal of Chemical Physics, 2012
n-Heptane/urea is an aperiodic inclusion compound in which the ratio of host and guest repeats along the channel axis is very close to unity and is found to have a constant value (0.981) from 280 K to 90 K. Below 280 K, two phase transitions are observed. The first (T c1 = 145 K) is a ferroelastic phase transition that generates superstructure reflections for the host while leaving the guest with 1D order. The second (T c2 = 130 K) is a "phase ordering" transition to a fourdimensional structure (P2 1 11(0βγ)) with pronounced host-guest intermodulation and a temperature dependent phase shift between guests in adjacent channels.