The free energy changes (Delta G, boron cation basicity; BCB) for the reaction [(MeO)(2)B]L+ = (MeO)(2)B+ + L (L = acetophenones) were determined in the gas phase by measuring ligand exchange equilibria using an FT-ICR mass spectrometer. On the basis of the correlation analysis by the Yukawa-Tsuno equation, Delta G = rho(sigma degrees + r(+)Delta(sigma) over bar (+)(R)), the substituent effect oil Delta BCB of acetophertone was characterized by a rho value (in kJ mol(-1) (sigma) over bar (-1) unit) of -43.2 and an r(+) value of 0.89. Both the rho and r(+) values were found to be similar to the corresponding values for protonation, indicating that the bond between (MeO)(2)B+ and the oxygen atom of the carbonyl group has a high covalent character similar to the H+-O=C bond. This conclusion was consistent with the geometrical features and the charge distribution calculated at DF-F-B3LYP/6-311+G(d,p) level of theory. A comparison with the results for a series of Lewis cation basicity of the acetophenone system showed that the r(+) value decreases in the order of H+ = (MeO)(2)B+ > Me3Si+ > Me3Ge+ > Cu+ > Li+. This decreasing order is related to increasing ionic (ion-dipole interaction) nature of the bonding interaction between Lewis cations and the carbonyl oxygen atom. This was also supported by the theoretical calculations.