EMPIRICAL AND ABINITIO ESTIMATES OF THE STABILITIES OF FLUORINE-CONTAINING CATIONS

Heats of formation of two classes of monofluorinated cations, fluoronium ions and α-fluoro carbocations, are estimated ab initio and empirically. C3H6F+ structures that are isoelectronic to stable C3H6O isomers are examined on the SCF potential energy surface (6-31G** basis set). The cations isoelectronic to alcohols all correspond to ion-molecule complexes between C3H5+ and hydrogen fluoride where the equilibrium C-F bond distances are calculated to be >2 Å. The cation isoelectronic to propylene oxide is predicted to be unstable with respect to 1-fluoroisopropyl structures with planar skeletons. The remaining cations resemble their isoelectronic neutrals. Empirical estimates are based on core-ionization energies by using the method of equivalent cores. The two methods provide estimates within 15 kJ mol-1 (= 3.6 Kcal mol-1 =0.16 eV) of each other in a number of cases so long as appropriate isodesmic reactions are used. For nonisodesmic reactions equivalent-cores approximations can be substantially in error. The agreement between equivalent cores and ab initio estimates are evaluated for the two isodesmic reactions R1OR2 + (CH3)2F+ → R1FR2+ + (CH3)2O for fluoronium ions and O=CXY + CH3CHF+ → FCXY+ + CH3CHO for α-fluoro carbocations. The SCF electronic energy changes, ∆E1 when the ions are constrained to the geometries of their isoelectronic neutrals are found, in most cases, to agree with experimental values. By using correlations based on ∆E1, core-ionization energies for oxygen-containing molecules are predicted: 539.4 eV for acetone enol, >539.5 eV for the lowest triplet state of acetone, and 538.9 eV for formamide with the nitrogen lone pair twisted out of conjugation with the carbonyl. ∆Hf values computed for ions in their relaxed geometries agree, for the most part, with equivalent cores estimates. Substituent effects are evaluated ab initio by computing the energy barrier for rotation about the C=N bond of RCH=NH2+: SCF values of ∆H#x002A; for R = H, F, and vinyl at 6-31G** are 261, 190, and 166 kJ mol-1, respectively. For cyclic fluoronium ions, the four-member ring ion fluoretanium is estimated to have ∆Hf = 660 kJ mol-1, within 10 kJ mol-1 of two other C3H6F+ isomers, FCH2CHCH3+ (14) and CH3CH2CHF+ (2). The five-member ring fluorolanium and the six-member ring fluoranium are estimated to have heats of formation of 555 and 515 kJ mol-1, respectively. © 1990, American Chemical Society. All rights reserved.