Capsule formation, carboxylate exchange, and DFT exploration of cadmium cluster metallocavitands: highly dynamic supramolecules.

A family of molecular heptacadmium carboxylate clusters templated inside [3 + 3] Schiff base macrocycles has been isolated and studied by variable temperature solution and solid-state NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and density functional theory (DFT) calculations. These metallocavitand cluster complexes adopt bowl-shaped structures, induced by metal coordination, giving rise to interesting host-guest and supramolecular phenomena. Specifically, dimerization of these metallocavitands yields capsules with vacant coordination and hydrogen-bonding sites accessible to encapsulated guests.

Solid-state NMR and computational studies of 4-methyl-2-nitroacetanilide.

Studies on the solid-state structure of two polymorphs of 4-methyl-2-nitroacetanilide (MNA) were conducted using magic-angle spinning (13)C, (15)N and (1)H NMR spectroscopy, together with first-principles computations of NMR shielding (including use of a program that takes explicit account of the translational symmetry inherent in crystalline structures). The effects on (13)C chemical shifts of side-chain rotations have been explored. Information derived from these studies was then incorporated within a systematic space-search methodology for elucidation of trial crystallographic structures from powder XRD.

A combined first principles computational and solid-state NMR study of a molecular crystal: flurbiprofen.

The 1H, 13C and 19F magic-angle spinning NMR spectra have been recorded for Form 1 of flurbiprofen. In the case of 19F, spinning sideband analysis has produced data for the components of the shielding tensor. The chemical shift of the hydrogen-bonded proton was found to be 14.

[R,S]-Ethambutol dihydrochloride: variable-temperature studies of a dimorphic system with very similar packing.

The two polymorphs (Forms I and II) of [R,S]-ethambutol dihydrochloride transform enantiotropically and reversibly in a single-crystal-to-single-crystal phase transformation mode. These structurally very similar forms have been characterized and their thermodynamic relationship has been investigated by variable-temperature solid-state carbon-13 nuclear magnetic resonance, variable-temperature powder X-ray diffraction, differential scanning calorimetry, and optical microscopy. The nuclear magnetic resonance results are compared with those for the two polymorphs of the [S,S] diastereomer with known structures.

Refinement of hydrogen atomic position in a hydrogen bond using a combination of solid-state NMR and computation.

DFT computations of the proton chemical shift for the intermolecular hydrogen bond in the white form of methylnitroacetanilide, together with the experimental value obtained by high-speed magic-angle spinning NMR, enable the N-H distance to be determined as 1.03 +/- 0.02 A.

(1)H NMR study of the states of water in equilibrium poly(HEMA-co-THFMA) hydrogels.

The spin-spin relaxation times, T(2), of hydrated samples of poly(hydroxymethyl methacrylate), PHEMA, poly(tetrahydrofurfuryl methacrylate), PTHFMA, and the corresponding HEMA-THFMA copolymers have been examined to probe the states of the imbibed water in these polymers. The decay in the transverse magnetization of water in fully hydrated samples of PHEMA, PTHFMA, and copolymers of HEMA and THFMA was described by a multiexponential function. The short component of T(2) was interpreted as water molecules that were strongly interacting with the polymer chains.

PFG-NMR measurements of the self-diffusion coefficients of water in equilibrium poly(HEMA-co-THFMA) hydrogels.

The self-diffusion coefficients for water in a series of copolymers of 2-hydroxyethyl methacrylate, HEMA, and tetrahydrofurfuryl methacrylate, THFMA, swollen with water to their equilibrium states have been studied at 310 K using PFG-NMR. The self-diffusion coefficients calculated from the Stejskal-Tanner equation, D(obs), for all of the hydrated polymers were found to be dependent on the NMR storage time, as a result of spin exchange between the proton reservoirs of the water and the polymers, reaching an equilibrium plateau value at long storage times. The true values of the diffusion coefficients were calculated from the values of D(obs) in the plateau regions by applying a correction for the fraction of water protons present, obtained from the equilibrium water contents of the gels.