Relative Abilities of Fluorine and Chlorine to Stabilize Carbenium Ions. Crystal Structures of Two Fluoro-Substituted Carbocations and of As2F11
Abstract
The first crystal structures of fluoro-substituted carbocations without heteroatom stabilization and of the As2F11(-) anion are reported. The experimental geometries of the carbenium ions in (CH3)2CF(+)asF6(-), (M-CF3C6H4)(C6H5)CF(+)asF6(-) and (M-CF3C6H4)(C6H5)CF(+)as2F11(-) and their comparison with that of the (0-ClC6H4)(C6H5)CCl(+) cation show that, in accord with previous theoretical calculations, chlorine stabilizes carbenium ions more efficiently than fluorine. The apparent discrepancy between these findings and a previously reached conclusion, based on an analysis of sup 13 C NMR chemical shift difference data, are reconciled by using the direct sup 13 C chemical shifts for judging the donor strength of a ligand. The sup 13 C and sup 19 F NMR spectra of the (M-CF3C6H4)(C6H5)CF(+) cation were recorded and analyzed with the help of RHF/6-31G(d,p) calculations using the GIAO method. In each of the three fluoro-substituted carbocation crystal structures studied, the carbenium centers are further stabilized by forming two fluorine bridges with the anions, resulting in pseudo-trigonal bipyramidal environments around the carbenium centers. The F5As-F-AsF5(-) anion in (M-CF3C6H4)(C6H5)CF(+)as2F11(-) possesses a symmetric fluorine bridge with an As-F-As angle of 156.5(13) deg and staggered AsF4 groups.
Document Details
- Document Type
- Technical Report
- Publication Date
- Nov 01, 1999
- Accession Number
- ADA408564
Entities
People
- George A. Olah
- Joachim Hegge
- Karl O. Christe
- Robert Bau
- Xiongzhi Zhang
Organizations
- Air Force Research Laboratory