Structures and properties of solvated and unsolvated isopropyl functionalised calix[4]arenes

The tetra-iso-propyl ethers of calix[4]arene and p-t-butylcalix[4]arene have been isolated in the cone conformation, and structurally characterized as chloroform solvates. Thermogravimetric analysis demonstrated that the parent iso-propylcalix[4]arene solvate is significantly more stable than the p-t-butylcalix[4]arene analogue, retaining the solvent up to a temperature of of 125 °C. It was found that the calix[4]arene ether sublimes at atmospheric pressure, and solvent-free crystals appropriate for structure determination were produced at reduced pressure. The p-t-butylcalix[4]arene ether was also isolated without solvent in the lattice, but in this case the calixarene was crystallized from acetone, as sublimation did not produce crystals of sufficient quality

Non-equilibrium adiabatic molecular dynamics simulations of methane clathrate hydrate decomposition

Nonequilibrium, constant energy, constant volume (NVE) molecular dynamics simulations are used to study the decomposition of methane clathrate hydrate in contact with water. Under adiabatic conditions, the rate of methane clathrate decomposition is affected by heat and mass transfer arising from the breakup of the clathrate hydrate framework and release of the methane gas at the solid-liquid interface and diffusion of methane through water.We observe that temperature gradients are established between the clathrate and solution phases as a result of the endothermic clathrate decomposition process and this factor must be considered when modeling the decomposition process. Additionally we observe that clathrate decomposition does not occur gradually with breakup of individual cages, but rather in a concerted fashion with rows of structure I cages parallel to the interface decomposing simultaneously. Due to the concerted breakup of layers of the hydrate, large amounts of methane gas are released near the surface which can form bubbles that will greatly affect the rate of mass transfer near the surface of the clathrate phase. The effects of these phenomena on the rate of methane hydrate decomposition are determined and implications on hydrate dissociation in natural methane hydrate reservoirs are discussed.Peer reviewed: YesNRC publication: Ye

NMR investigations of solid (CH3)3CCOOH (trimethylacetic acid) and (CH3)3CCOOD

(CH3)3CCOOH (trimethylacetic or pivalic acid) and (CH3)3CCOOD have been investigated in the plastic and brittle modifications by pulsed and continuous wave protonmagnetic resonance methods between 77\u2009\ub0K and the melting point (310\u2009\ub0K). For the low-temperature phase of (CH3)3CCOOD, the second moment and spin\u2013lattice relaxation time (T 1) of the protons are in agreement with a combination of methyl group (C 3) and t-butyl group (C 3\u2032) reorientations having activation energies (E a ) of 2.35\ub10.15 and 4.00\ub10.25 kcal/mole, respectively. In the high-temperature plastic phase above the transition at 280\u2009\ub0K, overall molecular tumbling with an E a of 6.0\ub10.6 kcal/mole governs T 1, and self-diffusion with an E a of 12\ub12 kcal/mole is evident from the spin\u2013lattice relaxation time in the rotating frame (T 1\u3c1). Also, it is found that T 1\u3c1 falls significantly below T 1 in the 30\ub0 just below the transition. The deviation increases to as much as an order of magnitude as the transition temperature is approached, being about threefold larger for the protonated form of the acid than for the deuterated. This behavior is consistent with the slow onset of molecular tumbling. We suggest that the faster rate in the protonated compound may be attributed to the importance of quantum mechanical tunneling in the breaking and reforming of hydrogen bonds during the tumbling process.Peer reviewed: YesNRC publication: Ye

Dielectric evidence for the formation of a clathrate hydrate by sulfuryl chloride

A hydrate formed by SO2Cl2 at 1220 \ub0C is shown by its low temperature dielectric absorption to be a structure II clathrate hydrate. The activation energy for reorientation of encaged SO2Cl2 molecules is 1.1\u2002kcal mol 121 between 19 and 41\u2002K.Peer reviewed: YesNRC publication: Ye

Raman spectroscopic study of CO2 in hydrate cages

The Raman spectra of CO2 molecules in hydrate cages was studied for structure I CO2 hydrate and structure II tetrahydrofuran (THF)+CO2 binary hydrate. The results obtained for sII hydrate indicated that the Fermi dyad peaks of CO2 in the small cage of THF-CO2 hydrate are located at 1274cm-1 and 1380cm-1. Numerical fitting of the sI CO2 hydrate Raman spectrum shows CO2 Fermi dyad peaks in the small cage are located at 1275cm-1 and 1382cm-1. The unperturbed frequencies of the symmetric stretching v1 and overtone bending v2 modes were calculated for CO2 in various states. It was found that the v1 frequency does not follow the loose cage-tight cage model for CO2 in hydrate cages. Explanations are given as to why such a relationship is not expected in terms of the incomplete sampling of the void space of the non-spherical cages by the linear guest molecule and the contribution of H-bonding to the guest-host interactions.Peer reviewed: YesNRC publication: Ye

Solid phase carbon-13 NMR studies of dicyclohexyl-18-crown-6 ethers and some alkali metal phenoxide complexes

From C-13 CPMAS spectra, the asymmetric units in the crystalline state of a number of title species have been determined. Results are compared with available x-ray data and liquid phase C-13 spectra

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

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

Isoxazole hydrate

X-Ray, n.m.r., and dielectric studies show that the hydrate of isoxazole is a typical type II clathrate hydrate in structure, in reorientation rates of host and guest molecules, and in composition.Peer reviewed: YesNRC publication: Ye

Enhanced rate of gas hydrate formation in a fixed bed column filled with sand compared to a stirred vessel

10.1016/j.ces.2011.10.030Chemical Engineering Science681617-623CESC