Intramolecular Solvation of Carboxylate Anions in the Gas Phase

Abstract

Proton exchange reactions between acetate, n-butanoate, 2-ethylhexanoate, and n-decanoate were studied experimentally by the use of PHPMS and theoretically by the use of ab initio methods. The occurrence of a curvature in some of the van't Hoff plots suggests isomerization of at least one of the participants in the equilibrium. This isomerization is suggested to be an intramolecular solvation of the carboxylate anions via unconventional hydrogen bonding. These interactions are discussed in terms of charge distributions in the unfolded and folded conformers of the carboxylates. Thermochemical values for the intramolecular solvation were deconvoluted from the curved van't Hoff plots by a fitting procedure. The thermochemical data for the intramolecular solvation was used to calculate the conformer composition of the carboxylate anions. Various properties related to the intramolecular solvation of the carboxylate anions are discussed.

Observation of Weak CH⋅⋅⋅O Hydrogen Bonding to Unactivated Alkanes

“…[30,31] The second spectral band was vibrationally resolved for n = 0-2, with a vibrational frequency of about 640 cm À1 that is characteristic of the CO 2 bending motion. [32] The vibrational structure is smeared out in the larger systems in the room temperature spectra. Significant changes were observed in the PES spectra at a trapping temperature of 70 K. For n = 0-4 ( Figure 1, left column), the low-temperature spectra are much sharper and the CO 2 bending vibrational progression in the second band is much better resolved in all the spectra.…”

“…[32,33] Because of the anticipated weakness of this CÀH···O hydrogen bonding, the folded conformation can only be observed at lower temperatures owing to the large entropy contributions at higher temperatures for the linear structure. To obtain further insight into the folding transition, we performed temperature-dependent studies for n = 6 and 8 at trapping temperatures from 300 to 18 K (Figure 2).…”

Isomerization of the protonated acetone dimer in the gas phase

“…On close inspection of the van't Hoff plot for the association reaction between protonated acetone and acetone (Fig. 2), a slight curvature is observed, which is normally taken to indicate the presence of isomeric adducts 19, 23, 37. The curvature is more evident by sequentially disregarding the low‐ or high‐temperature points on each side of the bend in the derivation of the reaction enthalpy; the curvature is real if the derived reaction enthalpy changes systematically as a function of temperature.…”

Observation of Weak CH⋅⋅⋅O Hydrogen Bonding to Unactivated Alkanes

“…The two observed bands in all the spectra are due to detachment from three oxygen lone‐pair orbitals on the carboxylate, and the first broad band actually contains two detachment channels 30. 31 The second spectral band was vibrationally resolved for n =0–2, with a vibrational frequency of about 640 cm −1 that is characteristic of the CO 2 bending motion 32. The vibrational structure is smeared out in the larger systems in the room temperature spectra.…”

“…The blue shift observed for n =5–8 in the low‐temperature spectra suggests a conformational change in which the negative charge on the carboxylate is stabilized, most likely resulting from the formation of CH⋅⋅⋅O hydrogen bonds between the terminal CH 3 group and the ‐CO 2 − group to form a folded structure 32. 33 Because of the anticipated weakness of this CH⋅⋅⋅O hydrogen bonding, the folded conformation can only be observed at lower temperatures owing to the large entropy contributions at higher temperatures for the linear structure.…”