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Angelika Sebald

Publications and source records attributed to Angelika Sebald.

8 recordsLinked to original sources
PubMed2005

Magnitudes and orientations of 31P chemical shielding tensors in Pt(II)-phosphine complexes and other four-fold coordinated phosphorus sites.

31P MAS and double-quantum filtered 31P MAS NMR experiments at and near the n = 0 rotational resonance condition, as well as off-magic angle spinning 31P NMR experiments on two polycrystalline samples of Pt(II)-phosphine thiolate complexes are reported. Numerical simulations yield complete descriptions of the two 31P spin pairs. 195Pt MAS NMR spectra are straightforward to obtain but sensitively reflect only some parameters of the 195Pt(31P)2 three-spin system. Based on the 31P NMR results obtained and in conjunction with a large body of literature data and irrespective of the chemical nature of the specimen, a unified picture of the dominating motif of 31P chemical shielding tensor orientations of phosphorus sites with 4-fold coordination is identified as a local (pseudo)plane rather than the directions of P element bond directions.

Journal Article↗
PubMed2005

MAS NMR with and without double-quantum filtration at and near the n=0 rotational resonance condition.

Spectral lineshapes of MAS NMR spectra of dipolar (re)coupled spin pairs exhibiting considerable chemical shielding anisotropies at and near the so-called n=0 rotational resonance (R2) condition are considered. The n=0 R2 condition is found to be not extremely sharp. Anisotropic interaction parameters such as chemical shielding tensor orientations and the magnitude of the dipolar coupling constant remain sensitively encoded in such lineshapes even when differences in isotropic chemical shielding values of up to 400 Hz (corresponding to ca. half the size of the dipolar coupling constant) are present. Additional double-quantum filtration (DQF) may enhance the sensitivity of spectral lineshapes to anisotropic interaction parameters for even larger differences in isotropic chemical shielding values. The dependence of the DQF efficiency on spin-system parameters as well as on external parameters (Larmor and MAS frequencies) is investigated. Away from R2 conditions a trend to lower DQF efficiencies is found whereas some spin-system parameters are more sensitively encoded in the corresponding spectral lineshapes. Our study is based on numerical simulations, with the known parameters of the 31P spin pair in Na4P2O7.10H2O representing our model case.

Journal Article↗
PubMed2005

Double-quantum filtered 1H MAS NMR spectra.

It is shown that straightforward double-quantum filtered (1)H MAS NMR experiments yield spectral lineshapes that permit to estimate the minimum number of (1)H spins in a cluster. The approach may offer an alternative to multiple-quantum experiments for the characterisation of (1)H spin clusters of moderate size. The duration of the double-quantum excitation period has to be chosen suitably, it is necessary to find a practical compromise between optimum double-quantum filtration efficiency and optimum information content of the spectral lineshapes. Some (1)H MAS NMR experiments on partially deuterated maleic acid are reported as well as numerical simulations.

Algorithms↗
PubMed2003

29Si and 19F MAS NMR spectra of isolated 29Si(19F)2 and 29Si(19F)3 spin systems: experiments and simulations.

We consider 29Si and 19F MAS NMR spectra of isolated 29Si(19F)2 and 29Si(19F)3 spin systems in two organosilicon compounds of the type RR'SiF2 and RSiF3(R,R' = organic ligands). Experimental spectra are analysed by means of numerical simulations. It is found that the SiF3 group in RSiF3 is reorienting rapidly around the molecular Si-C bond direction in the solid state. The two 19F shielding tensors in RR'SiF2 have strongly differing orientations relative to the two Si-F bond directions in the molecule. Possibilities and limitations of straightforward MAS NMR approaches for the full characterisation of 29Si(19F)2 and 29Si(19F)3 spin systems and other dipolar coupled two and three-spin systems are discussed.

Journal Article↗
PubMed2001

Structure of compounds E(SnMe3)4 (E = Si, Ge) as seen by high-resolution X-ray powder diffraction and solid-state NMR.

The compounds tetrakis(trimethylstannyl)germane, Ge(SnMe3)4 (1), and tetrakis(trimethylstannyl)silane, Si(SnMe3)4 (2), have crystal structures with the quasispherical molecules in a closed-packed stacking. At room temperature both structures have the space group P1 (Z = 2) with a = 9.94457 (5), b = 14.52927 (8), c = 9.16021 (5) A, alpha = 90.53390 (30), beta = 111.73080 (30), gamma = 90.0049 (4) degrees, and V = 1229.414 (12) A3 for (1) and a = 9.92009 (7), b = 14.51029 (11), c = 9.13585 (7) A, alpha = 90.4769 (4), beta = 111.6724 (4), gamma = 89.9877 (6) degrees, and V = 1222.037 (16) A3 for (2). The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as shown by analyzing the distances between the atoms. Upon heating, both compounds undergo a first-order phase transition at temperatures T(c) = 348 +/- 5 K, as characterized by a relative jump of the lattice parameter of approximately 16%. At 353 K, both structures have the space group P1 (Z = 4), with a = 14.2037 (2) A, and V = 2865.52 (7) A3 for (1) and a = 14.1346 (2) A, and V = 2823.90 (7) A3 for (2). Rietveld refinements were performed for the low-temperature phases measured at T = 295 K [R(wp) = 0.0844 for (1), R(wp) = 0.0940 for (2)] and for the high-temperature phases measured at T = 353 K [R(wp) = 0.0891 for (1), R(wp) = 0.0542 for (2)]. The combination of high-resolution X-ray powder diffraction measurements and variable-temperature magic-angle-spinning 13C, 29Si and 119Sn NMR experiments demonstrates low crystallographic and molecular (C1) symmetries for the low-temperature phases of (1) and (2) at temperatures T < 348 +/- 5 K and high crystallographic symmetry due to rotational disorder for the high-temperature phases at temperatures T > 348 +/- 5 K.

Journal Article↗
PubMedDate not supplied

Double-quantum filtered MAS NMR in the presence of chemical shielding anisotropies and direct dipolar and J couplings.

Double-quantum filtered MAS NMR spectra of an isolated homonuclear spin-1/2 pair are considered, at and away from rotational resonance conditions. The pulse sequence used is the solid-state NMR equivalent of double-quantum filtered COSY, known from solution-state NMR. The 119Sn spin pair in [(chex3Sn)2S] is characterized by a difference in isotropic chemical shielding smaller than the two chemical shielding anisotropies and by direct dipolar and isotropic J-coupling constants of similar magnitudes. At rotational resonance, one-dimensional double-quantum filtered 119Sn lineshapes yield the relative orientation of the two 119Sn chemical shielding tensors. Good double-quantum filtration efficiencies are found at and away from rotational resonance conditions, despite the presence of large chemical shielding anisotropies. Numerical simulations illustrate the interplay of the direct dipolar and J-coupling pathways and identify the latter as the main pathway even at rotational resonance conditions.

Anisotropy↗
PubMedDate not supplied

X-[1H, 19F] triple resonance with a X-[1H] CP MAS probe and characterisation of a 29Si-19F spin pair.

An economic approach for implementing X-[1H,19F] double-decoupling MAS NMR experiments with a conventional X-[1H] dual-channel CP MAS probe is demonstrated. The parameters characterising the isolated 29Si-19F spin pair in an organosilicon compound R(3)SiF (R = 9-anthryl) are determined. In addition, we discuss the optimum choice of experimental parameters for determining all 29Si-19F spin-pair parameters from straightforward 29Si MAS NMR spectra with only 1H decoupling applied during acquisition.

Crystallography↗