Wednesday, February 24, 2010

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

No comments: