On the Existence of the Halocarbonyl and Trifluoromethyl Cations in the Condensed Phase
Abstract
Vibrational and multinuclear NMR spectroscopy were used to reexamine previous claims, which were based solely on C-13-NMR spectra, for the existence of the FCO(+) and ClCO(+) cations in the condensed phase. The previously reported C-13-NMR signal, observed in the protolytic ionization of tert-butyl fluoro-formate and attributed to FCO(+), could not be reproduced. Furthermore, there was no evidence for the formation of FCO(+), when F2CO was reacted either with solid SbF5 matrices or with AsF5 or SbF5 in SO2ClF solutions at low-temperatures. The only observable products were the known oxygen-bridged donor-acceptor adducts between F2CO and the Lewis acids. Similarly, for Cl2CO or ClFCO and stoichiometric amounts of Lewis acids, only the corresponding oxygen-bridged donor-acceptor adducts were observed. However, in the presence of a two-fold excess of SbF5, the thermally unstable, ionic salt, ClCO(+) Sb3F(16), could be isolated. The ClCO(+) cation was characterized by vibrational spectroscopy and theoretical calculations. Frenking's natural bond orbital (NBO) analysis for CF3(+) and related species, which contain only one type of p(pi) back-donating ligands, was extended to systems containing two types of competing, p(pi) back-donating ligands. It is shown that p(pi) back-donation increases in the order F<Cl< O for the XCO(+) (X=F or Cl) cations, resulting in C-O triple bonds, while sigma-donation suppresses generation of a positive charge on oxygen. Born-Haber cylces were calculated for the SbF6(-) salts of CF3(+), FCO(+), and ClCO(+), as well as ClCO(+)Sb3F16(-). These cycles demonstrate that unfavorable overall thermodynamics, rather than p(pi) back-donation, are the main reasons for the elusiveness of CF3(+) and FCO(+) salts in the condensed phase. For the F2CO/SbF5 system, the overall thermodynamics are also responsible for the preferential formation of an oxygen-bridged donor-acceptor adduct.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 06, 1999
- Accession Number
- ADA408582
Entities
People
- B. Hoge
- G. A. Olah
- G. K. Prakash
- J. A. Boatz
- K. O. Christe
Organizations
- Air Force Research Laboratory