Ab initio calculations of 17O and nT NMR parameters (nT=31P, 29Si) in H3
TOTH3 dimers and T
3O9 trimeric rings |
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| Authors: | C G Lindsay J A Tossell |
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| Institution: | 1. Department of Chemistry and Biochemistry, University of Maryland at College Park, 20742, College Park, MD, USA
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| Abstract: | NMR shieldings (σ) and electric field gradients (eq) are calculated using ab initio methods at the O and T nuclei (where T=P, Si) in two different types of molecules-TH3 dimers, i.e. H3SiOSiH3 and H3POPH 3 2+ , and TO4 trimeric rings, i.e., Si3O 9 6- and P3O 9 3- , which serve as models for assessing the effects of polymerization, bond length and bond angle variation on the NMR properties of polymerized silicates and phosphates. In agreement with earlier ab initio studies on H3SiOSiH3 we confirm that σ(29Si), σ(31P), σ(17O) and eq(17O) all decrease as θ(SiOSi) decreases in the range from 180° to 100°. However, correction for artifacts due to distant core electrons leads to a considerably reduced value for the anisotropy in σ O, bringing it into better agreement with estimated experimental values. The qualitative change in σ(29Si) with θ(SiOSi) can be understood on the basis of changes in the energies of the highest energy occupied MO's and consequent variations in their contributions to the paramagnetic part of the shielding. For H3POPH 3 2+ we calculate a larger value of eqO than for the analog Si compound but the same type of variation of σ(17O) with θ(TOT). The change in σ(31P) with θ(POP) is, however, calculated to be much smaller than in the Si case and a maximum is predicted for intermediate angles. For the trimeric rings we obtain energy optimized geometries in good agreement with x-ray structural data, with T-O terminal distances systematically shorter than the T-O bridging distances. Calculated σ(T) anisotropies are also in good agreement with experiment and can be simply related to the calculated structure. After correction for distant core effects we obtain a change in σ(31P) between PO 4 3- and P3O 9 3- in reasonable agreement with experiment. |
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