Molecular motion in crown ethers. Application of 13C and 2H NMR to the study of 4-carboxybenzo-24-crown-8 ether and its KNCS complex in solution and in the solid phase

Large-amplitude solid phase molecular motion has been detected in the macrocyclic ring of the title crown either via 13C CPMAS NMR. To study the details of the dynamic processes, two selectively deuterated d4 derivatives have been prepared and examined via 2H NMR as a function of temperature. A phase change occurring around 277 K has been verified by differential scanning calorimetry (DSC) and a model for the motional processes has been developed involving equivalent two-site flips of the CD2 groups. The amplitude of the CD2 motions apparently decreases the closer the group is to the aromatic ring. The influence of KNCS complexation on the 13C CPMAS spectrum and on 13C spin lattice relaxation times in solution has been explored

Symmetry, disorder, and dynamics in solid crown ether complexes: 2H NMR studies of 15-crown-5·NaI, 15-crown-5·NaCIO4, and 21-crown-7·KI

2H NMR line shapes have been obtained as a function of temperature for partially deuteriated 15-crown-5·Nal, 15-crown-5·NaCI04 and 21-crown-7·KI. Sudden changes in the line shapes above 330 K correspond to phase transitions: DSC shows transitions at 338, 347 K for 15-crown-5.NaI, and 356, 374, 383 K for 15-crown-5·NaCI04. The 2H NMR line shapes for the room temperature (RT) phases show the onset of a motion of the macrocycles above 200K, which is rapid a little above room temperature. Through detailed analysis of the complicated line shapes it has been shown that the most consistent interpretation of the dynamics in these RT phases is a 'merry-go-round' type of motion similar to that found for 18-crown-6 and 12-crown-4 complexes in which O-CH2-CH2 units exchange sites around the ring, simultaneously adjusting their conformation to fit the site. In the high-temperature phases of all three complexes all the ring atoms are dynamically equivalent. This can only happen through a combination of increased symmetry and disorder for which possible models have been devised

Solid phase stereochemical dynamics of 18-crown-6 ether and its KNCS complex as studied by low temperature carbon-13 nuclear magnetic resonance

We report resolution of individual carbon environments of 18-crown-6 ether and its KNCS complex using 13C CPMAS nuclear magnetic resonance spectroscopy at low temperature. The importance of torsional angle effects in determining chemical shifts in these solids is discussed

12-Crown-4 ethers: Solid-state stereochemical features of dibenzo-12-crown-4, derived dicyclohexano-12-crown-4 isomers, and a lithium thiocyanate complex as determined via 13C CPMAS nuclear magnetic resonance and X-ray crystallographic methods

Solid-phase 13C NMR spectra are presented for the title systems. For dibenzo-12-crown-4, cis-anti-cis-di-cyclohexano-12-crown-4, and the LiNCS complex of the cis-syn-cis isomer, asymmetric units derived from the NMR data are consistent with single-crystal X-ray data. In the uncomplexed cis-syn-cis isomer, intermolecular crystal packing effects are shown to render intramolecularly equivalent carbons nonequivalent. Some factors contributing to the 13C steric shifts in these molecules are discussed

A molecular merry-go-round: Motion of the large macrocyclic molecule 18-crown-6 in its solid complexes studied by2H NMR

2H NMR line shape measurements were used to confirm and refine a model to account for large-amplitude motions in solid 18-crown-6 complexes previously identified from 13C and 1H NMR results. Reminiscent of the motion of a merry-go-round, the motion is a combined rotation and conformational adjustment of the macrocycle, in which individual -OCH2CH2- units jump to adjacent sites in the crystal. Specifically, for the malononitrile complex, 18-crown-6·2CH2(CN)2, jump rates were obtained by modeling the intermediate-rate line shapes, yielding an activation energy of 47.6 ±0.8 kJ/mol. Because the motion does not introduce disorder, it is diffraction invisible

SOME STRUCTURAL STUDIES OF CLATHRATE HYDRATES

Nous avons préparé les hydrates d'oxygène. d'azote, de monoxyde de carbone et d'air dans leur domaine de stabilité à haute pression. Excepté pour l'hydrate de monoxyde de carbone, dont la structure n'est pas cependant confirmée, les diagrammes de diffraction X et de neutron montrent que leur structure est du type Stackelberg II. Les clathrates hydrates de méthylcyclohexane, pinacolore et t-butyl-methylether avec soit H2S soit Xe comme gaz support ont été identifiés à partir des sectres 2H de RMN des molécules (hôtes) deutérées ou partiellement deutérées et à partir des écrantages chimiques du 129Xe comme étant distincts des hydrates de type II. A partir des diagrammes des diffraction des rayons X et de neutrons il apparait que les nouveaux clathrates hydrates sont isostructuraux avec le clathrasil dodecasil-1H.We have prepared the hydrates of oxygen, nitrogen, carbon monoxide and air in their regions of stability at high pressure. Except for carbon monoxide hydrate, whose structure is not yet confirmed, the X-ray and/or neutron powder diffraction patterns show their structures to be von Stackelberg's type II. Clathrate hydrates of methylcyclohexane, pinacolone and t-butyl methyl ether with H2S or Xe as help gas, were identified from the 2H NMR spectra of deuterated or partially deuterated guest molecules and from 129Xe chemical shieldings as being distinct from type II hydrates. On the basis of neutron and X-ray powder diffraction patterns, it is suggested that the new clathrate hydrates are isostructural with the clathrasil dodecasil-1H

Methanol incorporation in clathrate hydrates and the implications for oil and gas pipeline flow assurance and icy planetary bodies

One of the best-known uses of methanol is as antifreeze. Methanol is used in large quantities in industrial applications to prevent methane clathrate hydrate blockages from forming in oil and gas pipelines. Methanol is also assigned a major role as antifreeze in giving icy planetary bodies (e.g., Titan) a liquid subsurface ocean and/or an atmosphere containing significant quantities of methane. In this work, we reveal a previously unverified role for methanol as a guest in clathrate hydrate cages. X-ray diffraction (XRD) and NMR experiments showed that at temperatures near 273 K, methanol is incorporated in the hydrate lattice along with other guest molecules. The amount of included methanol depends on the preparative method used. For instance, single-crystal XRD shows that at low temperatures, the methanol molecules are hydrogen-bonded in 4.4% of the small cages of tetrahydrofuran cubic structure II hydrate. At higher temperatures, NMR spectroscopy reveals a number of methanol species incorporated in hydrocarbon hydrate lattices. At temperatures characteristic of icy planetary bodies, vapor deposits of methanol, water, and methane or xenon show that the presence of methanol accelerates hydrate formation on annealing and that there is unusually complex phase behavior as revealed by powder XRD and NMR spectroscopy. The presence of cubic structure I hydrate was confirmed and a unique hydrate phase was postulated to account for the data. Molecular dynamics calculations confirmed the possibility of methanol incorporation into the hydrate lattice and show that methanol can favorably replace a number of methane guests.Peer reviewed: YesNRC publication: Ye