Alkali Metal Cations Bonding to Carboxylate Anions: Studies using Mass Spectrometry and Quantum Chemical Calculations

Abstract

Data on the gas-phase energetics of anion/cation interactions are relatively scarce. In this work, gas-phase alkali metal cation basicity (AMCB) scales were established for a series of 15 benzoate ions XC6H4COO– with Li+, Na+, K+, Rb+, and Cs+ on the basis of mass spectrometry experiments and high-level calculations. A wide range of electron-donating and electron-withdrawing substituents were included in the study. The thermochemical values were calculated by ab initio methodologies and extrapolated to the complete basis set limit. For each metal cation, the experimental relative cation basicity values of the anions were established quantitatively by applying the Cooks’ kinetic method to the cation-bound heterodimers [(XC6H4COO–)­M+(YC6H4COO–)]−, generated by electrospray ionization. The self-consistency of these AMCB scales was ascertained by multiple overlap of the individual relative basicities. In parallel, the proton gas-phase basicities (GBs) of the benzoate anions (gas-phase acidities of the respective benzoic acids) were calculated in order to compare the results of the theoretical method with known experimental GB values. The experimental and calculated GB values agree quite accurately (average absolute deviation = 3.2 kJ mol–1). The relative experimental AMCB scales and the absolute calculated AMCB scales are highly correlated, and the two sets agree by better than 4 kJ mol–1. It is also demonstrated that the five series of calculated AMCBs are highly correlated with the calculated GB.

Gas‐Phase Alkali‐Metal Cation Affinities of Stabilized Enolates

“…This error cannot be derived from the experiments, which only determine relative quantities. A tentative estimate of this error can be obtained from a comparison of the independent theoretical calculations of the Δ H M+ ( B j − ) energies by Mayeux et al [12] . and us.…”

“…Mayeux et al. had recently determined the alkali‐metal ion affinities of these benzoates for Li + , Na + , and K + in a joined experimental and theoretical study [12] . The reported ΔΔ H M+ values span energy ranges between 40 (K + , Na + ) and 50 kJ mol −1 (Li + ).…”

Electrospray-Ionization Mass Spectrometry for the Analysis and Quantification of Carbanions

“…The same erroneous order in alkali metal cation affinities had previously been suggested by calculations conducted by Gal as well. 61 This exemplifies the improvement in certainty of the thermodynamic parameters by combining the calculated reference values with experiments, as individual inaccuracies are mitigated by interpolation.…”

“…They had recently been shown to be amenable to calculation via quantum chemical methods by confirming their relative affinities experimentally. 61 By using calculated affinity values of a total of six reference molecules in a single plot, slight deviations in the relative affinity within the series of reference compounds are mitigated through interpolation. This combination of experimental and calculated data should provide a collection of anions with improved thermodynamic accuracy that can be used as reference compounds themselves in the future.…”

“…Systematic deviations of the absolute values cannot be precluded entirely, though two separate calculations using different methods deviated by less than 1 kcal/mol. 61,215 Furthermore, the interpretation of the mass spectrometric data using the kinetic method can easily be updated with more accurate reference values once they are available, either from improved calculations or from novel experimental methods. The ratio of benzoic acid to carbon acid was chosen due to a generally higher responsiveness of benzoate anions towards ESI-MS. A total of six carbon acids were selected from methylenes featuring a mix of two cyano, keto, ester or amide groups.…”