Correlations between 29Si, 17O, and 1H NMR properties and local structures in silicates:: an ab initio calculation

In order to gain insight into the correlations between Si-29, O-17 and H-1 NMR properties (chemical shift and quadrupolar coupling parameters) and local structures in silicates, ab initio self-consistent field Hartree-Fock molecular orbital calculations have been carried out on silicate clusters of various polymerizations and intertetrahedral (Si-O-Si) angles. These include Si(OH)(4) monomers (isolated as well as interacting), Si2O(OH)(6) dimers (Cz symmetry) with the Si-O-Si angle fixed at 5 degrees intervals from 120 degrees to 180 degrees, Si3O2(OH)(8) linear trimers (C-2 symmetry) with varying Si-O-Si angles, Si3O3(OH)(6) three-membered rings (D-3 and C-1 symmetries), Si4O4(OH)(8) four-membered ring (C-4 symmetry) and Si8O12(OH)(8) octamer (D-4 symmetry). The calculated Si-29, O-17 and H-1 isotropic chemical shifts (delta(i)(Si), delta(i)(O) and delta(i)(H)) for these clusters are all close to experimental NMR data for similar local structures in crystalline silicates. The calculated O-17 quadrupolar coupling constants (QCC) of the bridging oxygens (Si-O-Si) are also in good agreement with experimental data. The calculated O-17 QCC of silanols (Si-O-H) are much larger than those of the bridging oxygens, but unfortunately there are no experimental data for similar groups in well-characterized crystalline phases for comparison. There is a good correlation between delta(i)(Si) and the mean Si-O-Si angle for both Q(1) and Q(2), where Q(n) denotes Si with n other tetrahedral Si next-nearest neighbors. Both the delta(i)(O) and the O-17 electric field gradient asymmetry parameter, eta of the bridging oxygens have been found to depend strongly on the O site symmetry, in addition to the Si-O-Si angle. On the other hand, the O-17 QCC seems to be influenced little by structural parameters other than the Si-O-Si angle, and is thus expected to be the most reliable O-17 NMR parameter that can be used to decipher Si-O-Si angle distribution information. Both the O-17 QCC and the H-2 QCC of silanols decrease with decreasing length of hydrogen bond to a second O atom (Si-O-H ... O), and the delta(i)(H) increase with the same parameter.