Saturday, March 03, 2007

PCCP Update from Jan. 07 - Present

NMR measurements of scalar-coupling distributions in disordered solids
Sylvian Cadars, Anne Lesage, Michel Trierweiler, Laurent Heux and Lyndon Emsley

The measurement of scalar (J) couplings by solid-state NMR is a field of great interest, since this interaction is a rich source of local structural information, complementary to dipolar and chemical shift interactions. Here, we first demonstrate that J-coupling distributions exist and can be observed in disordered solids, as illustrated with the observation of a pair-specific distribution of 2J(31P–N–31P) couplings in a bis-phosphino amine, and we investigate the potential effects of such distributions on the measurement of average J-coupling constants. Second, we show that the measurement of two-dimensional (2D) distributions of J-couplings provides a much richer probe of local structural disorder than one-dimensional distributions, and we introduce new methods that provide different (selective or non-selective) ways of measuring 2D J distributions in a wide range of disordered systems. These methods are finally applied to a slightly disordered polymorphic sample of fully 13C-enriched cellulose, and then to the bis-phosphino amine sample, from which 2D 2JPP-coupling distributions are clearly identified and interpreted

NMR crystallography of oxybuprocaine hydrochloride, Modification II°
Robin K. Harris, Sylvian Cadars, Lyndon Emsley, Jonathan R. Yates, Chris J. Pickard, Ram K. R. Jetti and Ulrich J. Griesser

The 13C CPMAS spectrum is presented for the polymorph of oxybuprocaine hydrochloride which is stable at room temperature, i.e. Mod. II°. It shows crystallographic splittings arising from the fact that there are two molecules, with substantially different conformations, in the asymmetric unit. An INADEQUATE two-dimensional experiment was used to link signals for the same independent molecule. The chemical shifts are discussed in relation to the crystal structure. Of the four ethyl groups attached to NH+ nitrogens, one gives rise to unusually low chemical shifts, very different from those of the other three ethyl groups. This is attributed empirically to -gauche conformational effects, as is confirmed by shielding computations. These considerations allow 13C signals to be assigned to specific carbons in the two crystallographically inequivalent molecules in the crystal structure. Indeed, information about the conformations is inherent in the NMR spectrum, which thus provides data of crystallographic significance. A 13C/1H HETCOR experiment enabled resolution to be obtained in the 1H dimension and allowed 1H and 13C signals for the same independent molecule to be linked.

Experimental and quantum-chemical determination of the 2H quadrupole coupling tensor in deuterated benzenes
Anu M. Kantola, Susanna Ahola, Juha Vaara, Jani Saunavaara and Jukka Jokisaari

Deuterium Quadrupole Coupling Constant (DQCC) in benzene was determined both experimentally by Nuclear Magnetic Resonance spectroscopy in Liquid Crystalline solutions (LC NMR) and theoretically by ab initio electronic structure calculations. DQCCs were measured for benzene-d1 and 1,3,5-benzene-d3 using several different liquid crystalline solvents and taking vibrational and deformational corrections into account in the analysis of experimental dipolar couplings, used to determine the orientational order parameter of the dissolved benzene. The experimental DQCC results for the isotopomers benzene-d1 and 1,3,5-benzene-d3 are found to be 187.7 kHz and 187.3 kHz, respectively, which are essentially equal within the experimental accuracy (±0.4 kHz). Theoretical results were obtained at different C–D bond lenghts, and by applying corrections for electron correlation and rovibrational motion on top of large-basis-set Hartree–Fock results. The computations give a consistent DQCC of ca. 189 kHz for three different isotopomers; benzene-d1, 1,3,5-benzene-d3, and benzene-d6, revealing that isotope effects are not detectable within the present experimental accuracy. Calculations carried out using a continuum solvation model to account for intermolecular interaction effects result in very small changes as compared to the data obtained in vacuo. The comparison of theoretical and experimental results points out the selection of the underlying molecular geometry as the most likely source of the remaining discrepancy of less than 2 kHz. Such an agreement between the calculated and the experimental DQCC results can only be achieved if rovibrational effects are considered on one hand in the experimental direct dipolar coupling data, and on the other hand in the theoretical property calculation, as is done presently.

A double quantum 129Xe NMR experiment for probing xenon in multiply-occupied cavities of solid-state inclusion compounds
Darren H. Brouwer, Saman Alavi and John A. Ripmeester

A method is presented for detecting multiple xenon atoms in cavities of solid-state inclusion compounds using 129Xe double quantum NMR spectroscopy. Double quantum filtered 129Xe NMR spectra, performed on the xenon clathrate of Dianins compound were obtained under high-resolution Magic-Angle Spinning (MAS) conditions, by recoupling the weak 129Xe–129Xe dipole–dipole couplings that exist between xenon atoms in close spatial proximity. Because the 129Xe–129Xe dipole–dipole couplings are generally weak due to dynamics of the atoms and to large internuclear separations, and since the 129Xe Chemical Shift Anisotropy (CSA) tends to be relatively large, a very robust dipolar recoupling sequence was necessary, with the symmetry-based SR26114 dipolar recoupling sequence proving appropriate. We have also attempted to measure the 129Xe–129Xe dipole–dipole coupling constant between xenon atoms in the cavities of the xenon–Dianins compound clathrate and have found that the dynamics of the xenon atoms (as investigated with molecular dynamics simulations) as well as 129Xe multiple spin effects complicate the analysis. The double quantum NMR method is useful for peak assignment in 129Xe NMR spectra because peaks arising from different types of absorption/inclusion sites or from different levels of occupancy of single sites can be distinguished. The method can also help resolve ambiguities in diffraction experiments concerning the order/disorder in a material.

A solid-state 55Mn NMR spectroscopy and DFT investigation of manganese pentacarbonyl compounds
Kirk W. Feindel, Kristopher J. Ooms and Roderick E. Wasylishen
PCCP(2007)9, 1226.

Central transition 55Mn NMR spectra of several solid manganese pentacarbonyls acquired at magnetic field strengths of 11.75, 17.63, and 21.1 T are presented. The variety of distinct powder sample lineshapes obtained demonstrates the sensitivity of solid-state 55Mn NMR to the local bonding environment, including the presence of crystallographically unique Mn sites, and facilitates the extraction of the Mn chemical shift anisotropies, CSAs, and the nuclear quadrupolar parameters. The compounds investigated include molecules with approximate C4v symmetry, LMn(CO)5(L = Cl, Br, I, HgMn(CO)5, CH3) and several molecules of lower symmetry (L = PhCH2, Ph3–nClnSn (n= 1, 2, 3)). For these compounds, the Mn CSA values range from <100>

Ultraslow Li diffusion in spinel-type structured Li4Ti5O12—A comparison of results from solid state NMR and impedance spectroscopy
Martin Wilkening, Roger Amade, Wojciech Iwaniak and Paul Heitjans

The cubic spinel oxides Li1+xTi2–xO4 (0 x 1/3) are promising anode materials for lithium-ion rechargeable batteries. The end member of the Li–Ti–O series, Li4Ti5O12, can accommodate Li ions up to the composition Li7Ti5O12. Whereas a number of studies focus on the electrochemical behaviour of Li insertion into and Li diffusion in the Li intercalated material, only few investigations about low-temperature Li dynamics in the non-intercalated host material Li4Ti5O12 have been reported so far. In the present paper, Li diffusion in pure-phase microcrystalline Li4Ti5O12 with an average particle size in the ┬Ám range was probed by 7Li solid state NMR spectroscopy using spin-alignment echo (SAE) and spin–lattice relaxation (SLR) measurements. Between T = 295 K and 400 K extremely slow Li jump rates –1 ranging from 1 s–1 to about 2200 s–1 were directly measured by recording the decay of spin-alignment echoes as a function of mixing time and constant evolution time. The results point out the slow Li diffusion in non-intercalated Li4Ti5O12·–1 (1/T) follows Arrhenius behaviour with an activation energy EASAE of about 0.86 eV. Interestingly, EASAE is comparable to activation energies deduced from conductivity measurements (0.94(1) eV) and from SLR measurements in the rotating frame (0.74(2) eV) rather than from those performed in the laboratory frame, EAlow-T = 0.26(1) eV at low T.

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