NMR chemical shifts in the fluorocarbonate ion FCO2-: a comparative study of experimental and theoretical IGLO NMR results

Based on the MP2/SCF optimized geometry of the FCO2- anion, the nuclear mag netic shielding both at the carbon and fluorine nuclei are calculated by th e IGLO-method. The different contributions to the shielding, stemming from the respective bonds, lone pairs and inner electron shells are reported. Th e calculated chemical shifts are compared to the experimentally observed C- 13 and F-19-MAS chemical shifts. The shielding depends on the C-F bond leng th and the corresponding O-C-O bond angle. C-F bond lengthening is accompan ied with a decrease of the magnetic shielding of the F-19 nucleus, mainly c aused by the fluorine lone pairs, and an increase of the magnetic shielding of the C-13 nucleus which is mainly affected by the C-F bond electrons. Finally, this study confirms the expected geometric particularity of the io n: that is a weak C-F bond and open O-C-O bond angle of a planar entity. Th e interpolated C-F bond lengths derived from the experimental chemical shif ts by this methodology is 1.55 Angstrom (C-13 NMR) and 1.601 Angstrom (F-19 NMR) and are thus close to that of 1.505 Angstrom resulting from an ab ini tio geometric optimization calculation. (C) 1999 Elsevier Science S.A. All rights reserved.