Electric-field gradients (EFG), qalpha beta , and nuclear magnetic resonance (NMR) shielding tensors, sigma alpha beta , have been obtained from ab initio near-Hartree-Fock wavefunctions for SiF4 and BF3. Using the recently estimated value of the nuclear quadrupole moment, Q(19F*)=0.072+or-0.004*10-24 cm2, we obtained values of the NQR coupling constant, e2qQ/h. The result for SiF4 is in good agreement with the experimental value. For BF3 the calculated 19F coupling constant is very different from experiment, suggesting difficulties with the analysis of the experimental data. By contrast, e2qQ/h at the B nucleus in BF3 is calculated to be 3.04 MHz, in reasonable agreement with the experimental value of 2.64+or-0.04 in solid BF3. B-F pi bonding in the 1a2" orbital of BF3 reduces the total coupling constant at B by around 40%. Calculated average NMR shieldings at F (using the central atom as gauge origin) differ from experimental values only by about 1.5% and the difference in shielding between SiF4 and BF3 is reproduced to within 2 p.p.m. The NMR shielding anisotropy in SiF3 is 367 p.p.m. while the difference of largest and smallest shielding components in BF3 is only 205 p.p.m. The paramagnetic shielding at F shows significant contributions from a number of valence MO, in contrast to the central-atom shielding which is dominated by sigma to pi excitations in BF3 and sigma to sigma * excitations in SiF4.A corrigendum for this article has been published in 1987 J. Phys. B: At. Mol. Phys. 20 5257
|Anno di pubblicazione:||1986|
|Titolo:||Ab initio calculation of nuclear quadrupole coupling constants and NMR shieldings in SiF_4 and BF_3|
|Autori:||J. A. TOSSELL; P. LAZZERETTI|
|Appare nelle tipologie:||Articolo su rivista|
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