Gas-Phase Proton Affinity and Protonation Entropy Scale between Acetaldehyde and Methyl Acetate. An Experimental Evaluation and Comparative Analysis

Abstract

The thermochemistry associated with the gas-phase proton affinity scale between acetaldehyde and methyl acetate has been reinvestigated by variable-temperature equilibrium measurements using pulsed electron beam high-pressure mass spectrometry. This experimentally difficult and sometimes kinetically inaccessible subset is particularly important since it provides the only bridge between the expansive lower and upper scales. Data were taken from 497 to 665 K, with an average temperature for all base pair combinations of 584 K. Special care was taken to ensure that the measurements accurately reflected both the ΔH° and ΔS° values within this temperature range. The present data indicate that earlier values from this laboratory within the same subset were expanded by ∼13% in both ΔG° and ΔH°, although no evidence was found for a similar expansion in our original ladder for bases with proton affinities above isobutene, which was used as the reference base in that study. A detailed analysis and comparison betwee...