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We present an efficient route to tetramethylalumoxane by the controlled hydrolysis of AlMe3 in the presence of pyridine. The AlMe3(pyr) hydrolysis by 0.5 and 1 equiv. of H2O has been followed with real-time 1H NMR. Based on high-level quantum-chemical calculations, we conclude that hypervalent, pentacoordinate aluminium species are critical in the first steps of hydrolysis. [ABSTRACT FROM AUTHOR]

According to the damped quantum rotation (DQR) theory, hindered rotation of methyl groups, evidenced in nuclear magnetic resonance (NMR) line shapes, is a nonclassical process. It comprises a number of quantum-rate processes measured by two different quantum-rate constants. The classical jump model employing only one rate constant is reproduced if these quantum constants happen to be equal. The values of their ratio, or the nonclassicallity coefficient, determined hitherto from NMR spectra of single crystals and solutions range from about 1.20 to 1.30 in the latter case to above 5.0 in the former, with the value of 1 corresponding to the jump model. Presently, first systematic investigations of the DQR effects in wide-line NMR spectra of a powder sample are reported. For 1,1,1-triphenylethane deuterated in the aromatic positions, the relevant line-shape effects were monitored in the range 99-121 K. The values of the nonclassicality coefficient dropping from 2.7 to 1.7 were evaluated in line shape fits to the experimental powder spectra from the range 99-108 K. At these temperatures, the fits with the conventional line-shape model are visibly inferior to the DQR fits. Using a theoretical model reported earlier, a semiquantitative interpretation of the DQR parameters evaluated from the spectra is given. It is shown that the DQR effects as such can be detected in wide-line NMR spectra of powdered samples, which are relatively facile to measure. However, a fully quantitative picture of these effects can only be obtained from the much more demanding experiments on single crystals. [ABSTRACT FROM AUTHOR]

The longitudinal nuclear spin relaxation rates for the 199Hg(spin-1/2) and 14N (spin-1) nuclei, and quadrupolar cross-correlation coefficient for the 14N nuclei are estimated for a toluene solution of bis(hexamethyldisilylamido)mercury(II) in a broad temperature range. Study provides the evidence of the utility of quadrupolar cross-correlation effects for structural studies in isotropic liquids.

A supramolecular complex, formed by encapsulation of C60 fullerene in a molecular container built from two resorcin[4]arene rims zipped together by peptidic arms hydrogen bonded into a cylindrical β‐sheet, was studied by X‐ray crystallography, solid‐state and solution NMR, EPR spectroscopy and differential scanning calorimetry (DSC). The crystal structure, determined at 100 K, reveals that the complex occupies 422 site symmetry, which is compatible with the molecular symmetry of the container but not of the fullerene molecule, which has only 222 symmetry. The additional crystallographic symmetry leads to a complicated but discrete disorder, which could be resolved and modelled using advanced features of the existing refinement software. Solid‐state NMR measurements at 184–333 K indicate that the thermal motion of C60 in this temperature range is fast but has different activation energies at different temperatures, which was attributed to a phase transition, which was confirmed by DSC. Intriguingly, the activation energy for reorientations of C60 in the solid state is very similar for the free and encaged molecules. Also, the rotational diffusion coefficients seem to be very similar or even slightly higher for the encaged fullerene compared to the free molecule. We also found that chemical shift anisotropy (CSA) is not the main relaxation mechanism for the 13C spins of C60 in the studied complex. [ABSTRACT FROM AUTHOR]

Nuclear-spin relaxation is considered in a molecular system undergoing two types of dynamic processes: asymmetric-top small-step rotational diffusion and discrete multisite local jumps. The two processes are assumed to be uncorrelated. Time correlation functions for relevant rank-two interactions and corresponding spectral density functions are derived for a general relation between the characteristic rate constants. In addition, limiting cases of fast and slow local motions and of some specific jump conditions are also investigated. [ABSTRACT FROM AUTHOR]

A dimethylformamide-[d.sub.7] solution was used to study the carbon-13 nuclear spin relaxation in 1,8-bis(dimethylamino)naphthalene (DMAN). The results showed that changes in reorientation of the DMAN molecule under protonation are caused by formation of ion pairs rather than by charge-amplified interaction with solvent.

Subtle features of (super 13)C NMR spectra of perdeuterated methyl groupings are discussed. The study revealed that the (super 13)C line shape is dependent upon both the scalar coupling between magnetically equivalent deuterons and deuteron autocorrelated, and cross-correlated quadrupolar spectral densities.