THE GAS-PHASE ACIDITIES OF VERY STRONG NEUTRAL BRONSTED ACIDS

An interlocking ladder of relative gas-phase acidities of a large numb er of very strong CH (substituted phenylmalononitriles, 2,4,6-(CF3SO2) (3)C6H2CH3, CF3SO2- and FSO2-substituted methanes, TNT, etc.), NH (bis (perfluoroalkyl)sulfonyl imides, 2,4,6-(CF3SO2)(3)C6H2NH2, etc.), OH ( triflic acid, picric acid, 2,4,6-(CF3SO2)(3)C6H2OH, CH3SO3H, etc.) and SH (CF3COSH) Bronsted acids has been obtained using the pulsed FT ion cyclotron resonance (ICR) equilibrium constant method. The new intrin sic acidity scale covers a wide range from (CF3)(2)NH (Delta G(acid) = 324.3 kcal/mol) to (C4F9SO2)(2)NH (Delta G(acid) = 284.1 kcal/mol) an d is anchored to the thermodynamic Delta G(acid) value (318.1 kcal/mol ) of HBr. In several cases, the gas-phase acidity of compounds which m ake up the scale exceeds the acidity of such traditionally strong mine ral acids as HCl, HBr, HI, or H2SO4 by more than 30 powers of 10. The roles of the acidity sites (CH, NH, OH, SH) and the structural factors (i) field/inductive effects (F), (ii) pi-electron-acceptor resonance effects (R), and (iii) substituent polarizability (P) effects on incre asing the gas-phase acidity of Bronsted acids are discussed. The effec ts of multiple substitution in families of Bronsted acids have been me asured and discussed. The strong and extensive chains of conjugation i n the resonance-stabilized planar conjugate anion of (p-nitrophenyl)ma lononitrile lead to the same gas-phase CH acidity (Delta G(acid) = 299 .5 kcal/mol) as for OH superacid CF3SO3H. A single para substituent in aniline has been found that exerts such a powerful ''electron-withdra wing'' effect that this aniline has a stronger gas-phase acidity than CH3SO3H. The substituent is S(O)(=NSO2CF3)CF3, one of the family of Ya gupolskii superacceptor substituents that is generated by replacing =O by =NSO2CF3 at a bonded S, P, or I (including in addition to the abov e -S(=NSO2CF3)(2)CF3, -P(=NSO2CF3)(C3F7)(2) and -I=NSO2CF3). Perfluoro aromatic acids have been identified as well-behaved compounds for gas- phase acidity determinations. Their moderately strong inherent acidify ing effects are illustrated by the fact that a Delta G(acid) value of 302 +/- 1 kcal/mol applies to all of the following: (4-C5F4N)(2)CHCN, (p-CF3C6F4)(2)CHCN, beta-C10F7CH(CN)(2), and (CF3SO2)(2)CH2. The intro duction of more bulky strong electron-acceptor substituents in CH4 or NH3 has been found to be accompanied by an especially strong nonadditi ve increase in gas-phase acidity. The nonadditivity is least in CN-sub stituted compounds, such as for the above compounds. These results are discussed as being associated with saturation and particularly with s teric repulsion in the anions. In spite of the nonadditivity of the ac idities for disubstituted NH3 and CH4 (as well as X(3)CH), the acidify ing R, F, and P effects tend still to be quite significant for strongl y electron-withdrawing substituents. It will be noted that the acid (C 4F9SO2)(2)NH presently holds the record as the strongest measured gas- phase superacid. Extensive ab initio calculations are reported for sev eral basis sets which have been found useful for the study of : struct ures and electronic and vibrational energies, as well as entropy effec ts. A major tool is provided for the prediction of new extremely acidi c compounds. The present experimental and theoretical results set the grounds for preparation and measurement of new even stronger gas-phase superacids. Such results hold promise not only for extending basic kn owledge but also in providing many new practical applications for cond ensed-phase chemistry, including those that utilize the acidic substru ctures.