PCCP, vol. 11 issues, Issues 45 - 48 and vol. 12, Issues 1 - 9

Phys. Chem. Chem. Phys., 2009, 11, 10331 - 10339, DOI: 10.1039/b822560e

NMR shielding as a probe of intermolecular interactions: ab initio and density functional theory studies

James A. Platts and Konstantinos Gkionis
Ab initio and density functional theory (DFT) calculations of nuclear magnetic resonance shielding tensors in benzene–methane and two isomers of the benzene dimer are reported, with the aim of probing the changes in shielding induced by the formation of supramolecular complexes from isolated molecules. It is shown that the changes in shielding (and hence of chemical shift) for hydrogen nuclei are broadly in line with expectations from shielding cones based on aromatic ring current, but that changes for carbon nuclei are rather more subtle. More detailed analysis indicates that the change in isotropic shielding results from much larger changes in individual components of the shielding tensor and in diamagnetic/paramagnetic shielding contributions. Benchmark data were obtained using Møller–Plesset 2nd order perturbation theory with a medium-sized basis set, but it is shown that Hartree–Fock and most density functional theory methods reproduce all essential changes in shielding, and do so in a reasonably basis set independent fashion. The chosen method is then applied to a DNA–intercalator complex


Phys. Chem. Chem. Phys., 2009, 11, 10391 - 10395, DOI: 10.1039/b914468d

Double-quantum 19F–19F dipolar recoupling at ultra-fast magic angle spinning NMR: application to the assignment of 19F NMR spectra of inorganic fluorides

Qiang Wang, Bingwen Hu, Franck Fayon, Julien Trébosc, Christophe Legein, Olivier Lafon, Feng Deng and Jean-Paul Amoureux
A broadband dipolar recoupling method robust to chemical shift is introduced to observe 19F–19F proximities in fluoroaluminates in high magnetic field and at ultra-fast magic angle spinning (>60 kHz).



Phys. Chem. Chem. Phys., 2009, 11, 11404 - 11414, DOI: 10.1039/b919860a

NMR tensors in planar hydrocarbons of increasing size

Suvi Ikäläinen, Perttu Lantto, Pekka Manninen and Juha Vaara

13C nuclear shielding and 13C–13C spin–spin coupling tensors were calculated using density functional theory linear response methods for a series of planar hydrocarbons. As calculation of the spin–spin coupling is computationally demanding for large molecules due to demands placed on basis-set quality, novel, compact completeness-optimized (co) basis sets of high quality were employed. To maximize the predictive value of the data, the convergence of the co basis sets was compared to well-known basis-set families. The selection of the exchange–correlation functional was performed based on the available experimental data and coupled-cluster calculations for ethene and benzene. The series of hydrocarbons, benzene, coronene, circumcoronene and circumcircumcoronene, was chosen to simulate increasingly large fragments of carbon nanosheets. It was found that the nuclear shielding and the one-, two-, and three-bond spin–spin coupling constants, as well as the corresponding anisotropies with respect to the direction normal to the plane, approach convergence as the number of carbon atoms in the fragment is increased. Predictions of the investigated properties can then be done for the limit of large planar hydrocarbons or carbon nanosheets. From the results obtained with a judicious choice of the functional, PBE, and co basis close to convergence, limiting values are estimated as follows: = 54 ± 1 ppm [corresponding to the chemical shift of 134 ppm with methane (CH4) as a reference], = 207 ± 4 ppm, 1J = 59.0 ± 0.5 Hz, 1J = -1.5 ± 0.5 Hz, 2J = 0.2 ± 0.4 Hz, 2J = -4.6 ± 0.2 Hz, 3J = 6 ± 1 Hz, and 3J = 3 ± 1 Hz



Phys. Chem. Chem. Phys., 2009, 11, 11487 - 11500, DOI: 10.1039/b916076k

Mg-25 ultra-high field solid state NMR spectroscopy and first principles calculations of magnesium compounds

Peter J. Pallister, Igor L. Moudrakovski and John A. Ripmeester

Due to sensitivity problems, 25Mg remains a largely under-explored nucleus in solid state NMR spectroscopy. In this work at an ultrahigh magnetic field of 21.1 T, we have studied at natural abundance the 25Mg solid state (SS) NMR spectra for a number of previously unreported magnesium compounds with known crystal structures. Some previously reported compounds have been revisited to clarify the spectra that were obtained at lower fields and were either not sufficiently resolved, or misinterpreted. First principles calculations of the 25Mg SS NMR parameters have been carried out using plane wave basis sets and periodic boundary conditions (CASTEP) and the results are compared with experimental data. The calculations produce the 25Mg absolute shielding scale and give us insight into the relationship between the NMR and structural parameters. At 21.1 T the effects of the quadrupolar interactions are reduced significantly and the sensitivity and accuracy in determining chemicals shifts and quadrupole coupling parameters improve dramatically. Although T1 measurements were not performed explicitly, these proved to be longer than assumed in much of the previously reported work. We demonstrate that the chemical shift range of magnesium in diamagnetic compounds may approach 200 ppm. Most commonly, however, the observed shifts are between -15 and +25 ppm. Quadrupolar effects dominate the 25Mg spectra of magnesium cations in non-cubic environments. The chemical shift anisotropy appears to be rather small and only in a few cases could the contribution of the CSA be detected reliably. A good correspondence between the calculated shielding constants and experimental chemical shifts was obtained, demonstrating the good potential of computational methods in spectroscopic assignments of solid state 25Mg NMR spectroscopy



Phys. Chem. Chem. Phys., 2010, 12, 583 - 603, DOI: 10.1039/b909870d

Calculation of NMR parameters in ionic solids by an improved self-consistent embedded cluster method

Johannes Weber and Jörn Schmedt auf der Günne

A new embedded cluster method (extended embedded ion method = EEIM) for the calculation of NMR properties in non-conducting crystals is presented. It is similar to the Embedded Ion Method (EIM) (ref. 1) in the way of embedding the quantum chemically treated part in an exact, self-consistent Madelung potential, but requires no empirical parameters. The method is put in relation to already existing cluster models which are classified in a brief review. The influence of the cluster boundary and the cluster charge is investigated, which leads to a better understanding of deficiencies in EIM. A recipe for an improved semi-automated cluster setup is proposed which allows the treatment of crystals composed of highly charged ions and covalent networks. EIM and EEIM results for 19F and 31P shielding tensors in NaF and in four different magnesium phosphates are compared with experimental values from solid state MAS NMR, some of which are measured here for the first time. The quantum part of the clusters is treated at hybrid DFT level (mPW1PW) with atomic basis sets up to 6-311G(3df,3pd). The improved agreement of EEIM allows new signal assignments for the different P-sites in Mg2P4O12, -Mg2P2O7 and MgP4O11. Conversion equations of the type = A + B between calculated absolute magnetic shieldings and the corresponding experimental chemical shifts are obtained independently from linear regressions of plots of isotropically averaged versus values on 19 31P signals of small molecules



Phys. Chem. Chem. Phys., 2010, 12, 1535 - 1542, DOI: 10.1039/b919118f

31P solid-state NMR studies of the short-range order in phosphorus–selenium glasses

Aleksei Bytchkov, Franck Fayon, Dominique Massiot, Louis Hennet and David L. Price

The local structure of P-rich and Se-rich phosphorus–selenium glasses was studied using high-resolution 31P solid-state MAS NMR. Two-dimensional 31P homonuclear through-bond correlation MAS experiments and 2D homonuclear J-resolved MAS measurements were performed at high spinning frequency to probe P–P and P–Se–P connectivities between the different P sites for the compounds in two glass-forming regions, P2.5Se97.5–P50Se50 and P67Se33–P84Se16. Amorphous phosphorus and crystalline -P4Se3 and -P4Se3 were also studied as reference materials. Glasses from the Se-rich region contain mainly three- and four-coordinated P sites linked together by Sen chains, whereas P-rich glasses contain a mixture of P4Se3 molecular units and possibly other structural units embedded in a red-phosphorus-like polymeric network

Journal of Magnetic Resonance

Mechanism of 1H-14N cross-relaxation in immobilized proteins

from ScienceDirect Publication: Journal of Magnetic Resonance

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 25 January 2010
Erik P., Sunde , Bertil, Halle

A resonant enhancement of the water-¹H relaxation rate at three distinct frequencies in 0.5 – 3 MHz range has been observed in a wide range of aqueous biological systems. These so-called quadrupole (Q) peaks have been linked to a dipolar flip-flop polarization transfer from ¹H nuclei to rapidly relaxing amide ¹⁴N nuclei in rotationally immobilized proteins. While the Q-peak frequencies conform to the known amide ¹⁴N quadrupole coupling parameters, a molecular model that accounts for the intensity and shape of the Q peaks has not been available. Here, we present such a model and test it against an extensive set of Q-peak data from two fully hydrated crosslinked proteins under conditions of variable temperature, pH and H/D isotope composition. We propose that polarization transfer from bulk water to amide ¹⁴N occurs in three steps: from bulk water to a so-called intermediary proton via material diffusion/exchange, from intermediary to amide proton by cross-relaxation driven by exchange-mediated orientational randomization of their mutual dipole coupling, and from amide proton to ¹⁴N by resonant dipolar relaxation ’of the second kind’, driven by ¹⁴N spin fluctuations, which, in turn, are induced by restricted rigid-body motions of the protein. An essentially equivalent description of the last step can be formulated in terms of coherent ¹H→¹⁴N polarization transfer followed by fast ¹⁴N relaxation. Using independent structural and kinetic information, we show that the Q peaks from these two proteins involve 7 intermediary protons in internal water molecules and side-chain hydroxyl groups with residence times of order 10^–5 s. The model not only accounts quantitatively for the extensive data set, but also explains why Q peaks are not observed from gelatin gels.

PGSE-NMR Measurement of the non-local dispersion tensor for flow in porous media

from ScienceDirect Publication: Journal of Magnetic Resonance

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 28 January 2010
M.W., Hunter , A.N., Jackson , P.T., Callaghan

The purpose of this work was to design and implement constant adiabadicity gradient modulated pulses that have improved slice profiles and reduced artifacts for spectroscopic imaging on 3T clinical scanners equipped with standard hardware. The newly proposed pulses were designed using the gradient offset independent adiabaticity (GOIA, Tannus and Garwood, 1997) method using WURST modulation for RF and gradient waveforms. The GOIA-WURST pulses were compared with GOIA-HSn (GOIA based on nth-order hyperbolic secant) and FOCI (Frequency Offset Corrected Inversion) pulses of the same bandwidth and duration. Numerical simulations and experimental measurements in phantoms and healthy volunteers are presented. GOIA-WURST pulses provide improved slice profile that have less slice smearing for off-resonance frequencies compared to GOIA-HSn pulses. The peak RF amplitude of GOIA-WURST is much lower (40% less) than FOCI but slightly higher (14.9% more) to GOIA-HSn. The quality of spectra as shown by the analysis of line-shapes, eddy currents artifacts, subcutaneous lipid contamination and SNR is improved for GOIA-WURST. GOIA-WURST pulse tested in this work shows that reliable spectroscopic imaging could be obtained in routine clinical setup and might facilitate the use of clinical spectroscopy.

Spectroscopic Imaging with Improved Gradient Modulated Constant Adiabadicity Pulses on High-Field Clinical Scanners

from ScienceDirect Publication: Journal of Magnetic Resonance

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 28 January 2010 Ovidiu C. Andronesi^{a, ,} , Saadallah Ramadan^b, Eva-Maria Ratai^a, Dominique Jennings^a, Carolyn E. Mountford^b and A. Gregory Sorensen^a

The purpose of this work was to design and implement constant adiabadicity gradient modulated pulses that have improved slice profiles and reduced artifacts for spectroscopic imaging on 3T clinical scanners equipped with standard hardware. The newly proposed pulses were designed using the gradient offset independent adiabaticity (GOIA, Tannus and Garwood, 1997) method using WURST modulation for RF and gradient waveforms. The GOIA-WURST pulses were compared with GOIA-HSn (GOIA based on nth-order hyperbolic secant) and FOCI (Frequency Offset Corrected Inversion) pulses of the same bandwidth and duration. Numerical simulations and experimental measurements in phantoms and healthy volunteers are presented. GOIA-WURST pulses provide improved slice profile that have less slice smearing for off-resonance frequencies compared to GOIA-HSn pulses. The peak RF amplitude of GOIA-WURST is much lower (40% less) than FOCI but slightly higher (14.9% more) to GOIA-HSn. The quality of spectra as shown by the analysis of line-shapes, eddy currents artifacts, subcutaneous lipid contamination and SNR is improved for GOIA-WURST. GOIA-WURST pulse tested in this work shows that reliable spectroscopic imaging could be obtained in routine clinical setup and might facilitate the use of clinical spectroscopy.

Measurement of Vorticity Diffusion by NMR Microscopy

from ScienceDirect Publication: Journal of Magnetic Resonance

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 1 February 2010
Jennifer R., Brown , Paul T., Callaghan

In a Newtonian fluid, vorticity diffuses at a rate determined by the kinematic viscosity. Here we use rapid NMR velocimetry, based on a RARE sequence, to image the time-dependent velocity field on start-up of a fluid-filled cylinder and therefore measure the diffusion of vorticity. The results are consistent with the solution to the vorticity diffusion equation where the angular velocity on the outside surface of the fluid, at the cylinder’s rotating wall, is fixed. This method is a means of measuring kinematic viscosity for low viscosity fluids without the need to measure stress.

Solid State Nuclear Magnetic Resonance

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