Solution-based EB and CB parameters are applied to fit gas-phase ion-molecule enthalpy changes, greatly extending the range of systems covered and consequently the utility of the E and C model. Several significant insights result from the data fit. The use of EB and CB parameters to fit enthalpy changes for cations coordinating to Lewis bases in the gas phase supports earlier proposals that the solution enthalpies used to obtain E and C values have minimal solvation contributions. The reported analysis provides a direct link between conventional Lewis acid-base interactions and gas-phase ion-molecule reactions. The trends in the fraction of covalent and electrostatic contributions (CACB/EAEB) to most reactions M+(g) + B(g) ⇌ MB+(g) are seen to parallel base HOMO and acid LUMO energies. The trends in the covalent, electrostatic, and transfer terms for the amines reacting with the proton are in good agreement with similar quantities from literature ab initio calculations on these systems. Comparison of gas-phase interaction energies of a series of bases with a new ion to those for the proton, or to other ions, as a reference is found to be a questionable procedure because different donor orders result for acids that are significantly more electrostatic or covalent in their interaction than the reference. On the other hand, an ECT analysis with the parameters reported here provides a quantitative estimate of σ donor strength, and when deviations do occur they signal the existence of unusual effects in the gas-phase chemistry, e.g. repulsive effects, π-back bonding, or adduct geometry variation from multiple donor coordination sites. Insights relative to the proper design of a gas-phase experiment are also provided by this analysis. If the C/E ratio of all the bases studied is the same, the covalent and electrostatic nature of the cation cannot be determined no matter how large the data set. These data will plot up linearly with the proton affinity, and the ion will be incorrectly interpreted as being similar. The CB and EB values reported here should be employed to select different bases for study in a properly designed experiment. This analysis demonstrates that care must be exercised in using gas-phase cation enthalpies to analyze solution data (e.g. proton affinities and pKB). In the case of gas-phase interactions, the enthalpy is dominated by a transfer term that is largely cancelled out in the solution phase where displacement reactions usually occur. The transfer contribution is very large for the proton and makes proton affinities poor reference acids to employ for the interpretation of solution chemistry. © 1990, American Chemical Society. All rights reserved.
