Monday, July 14, 2008

Joel's Journal Updates

NMR crystallography of p-tert-butylcalix[4]arene host-guest complexes using 1H complexation-induced chemical shifts
Darren H. Brouwer, Saman Alavi, and John A. Ripmeester

1H magic-angle spinning (MAS) NMR spectra of p-tert-butylcalix[4]arene inclusion compounds with toluene and pyridine show large complexation-induced shifts of the guest proton resonances arising from additional magnetic shielding caused by the aromatic rings of the cavities of the host calixarene lattice. In combination with ab initio calculations, these observations can be employed for NMR crystallography of host–guest complexes, providing important spatial information about the location of the guest molecules in the host cavities.

Deuteron quadrupole coupling in benzene: librational corrections using a temperature-dependent Einstein model, and summary. The symmetries of electric field gradients and conditions for eta = 1
Pekka Pyykko and Fatemeh Elmi

Librational corrections are added to previous single-crystal and polycrystalline measurements of the deuteron quadrupole coupling constant in benzene. The results are related to gas-phase and liquid-crystal measurements and to theoretical values. A temperature-dependent Einstein model is introduced for the purpose. The group-theoretical relations of the electric field-gradient (EFG) tensor to the nuclear site symmetry are discussed. The conditions for h = 1 for reorientational processes are identified.

Revealing complex formation in acetone–n-alkane mixtures by MAS PFG NMR diffusion measurement in nanoporous hosts
Moises Fernandez, André Pampel, Ryoji Takahashi, Satoshi Sato, Dieter Freude and Jörg Kärger

Magic-angle spinning pulsed field gradient nuclear magnetic resonance (MAS PFG NMR) was applied for selective self-diffusion measurements of acetone–n-alkane (C6 up to C9) mixtures in nanoporous silica gel. Two specimens of silica gel with mean pore sizes of about 4 and 10 nm are considered. In the smaller pores, the n-alkane diffusivities are by about one and the acetone diffusivities by about two orders of magnitude smaller than in the larger pores. In addition, the acetone diffusivities in the narrow-pore specimen exhibit a pronounced oscillation with increasing chain length of the solvent n-alkanes: the diffusivities of acetone dissolved in odd-carbon number n-alkanes exceed those of acetone dissolved in even-carbon number n-alkanes by about 50%! These findings reproduce the tendencies observed in previous macroscopic release studies (Phys. Chem. Chem. Phys. 2003, 5, 2476) and suggest the formation of acetone–n-alkane complex-like assemblages in the narrow-pore silica gel.

Direct DIVAM Experiment: A Spin Dynamics Analysis
Paul hazendonk, Philip Wormald, and Tony Montina

Domain selection in polymer NMR is limited to experiments specifically suited to each structural domain owing to its particular spin dynamics and relaxation properties. The DIVAM experiment can be tuned to select for signal from the domain of interest, making it possible to obtain signals specific to different domains using only one experiment. An early description of this sequence explains this tunability using a simple one-spin-relaxation model, thereby limiting the selection mechanism to incoherent processes and thus ignoring the coherent terms such as chemical shift anisotropy (CSA), dipolar coupling and offset terms. Experiments have shown that when the DIVAM sequence is applied directly to the nucleus of interest, referred to as direct DIVAM (DD), transient behavior is observed in the signal intensity on the sample spinning time scale. This indicates that the coherent terms are involved in the selection process; the exact role of these terms is explored in this work. SIMPSON simulations illustrate that the CSA and offset terms can play a dominant role in domain selection; however, the dipole term was relatively ineffective and required large values before substantial selection was predicted. Using a one-spin-relaxation model, which now includes a chemical shift evolution term, an analytical expression for the signal intensity was provided as a function of interpulse delay (?), excitation angle (?), relaxation time (T2), and offset frequency (??). These indicate that the selection behavior varies substantially with differing time scales and excitation angles. For small angles and long delay times DD behaves primarily as a relaxation filter, whereas for larger angles and short delay times the coherent terms take over dominated by the CSA interactions. The DD sequence can therefore be set to select on the basis of the transverse relaxation rate or the strength of the CSA interaction, depending on the excitation angle used.

A Technique for In Situ Monitoring of Crystallization from Solution by Solid-State 13C CPMAS NMR Spectroscopy
Hughes, C. E.; Harris, K. D. M.
JPCA(2008) ASAP.

We report a technique for carrying out in situ solid-state NMR studies of crystallization from solution, allowing the evolution of different solid state structures (polymorphs) produced during the crystallization process to be identified. The technique exploits selectivity in NMR properties (specifically, the efficiency of cross-polarization from 1H to 13C) between molecules in the solid and solution states, such that the first solid particles produced during the crystallization process are observed selectively, without detecting any signal from dissolved solute (or solvent) molecules. The application of the technique is demonstrated to reveal new insights concerning an isotope effect on the polymorphic outcome of crystallization of glycine from water. As revealed by this example, the in situ solid-state NMR approach reported here creates significant new opportunities for probing and understanding details of the evolution of solid state structures produced during crystallization from solution.

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