Gas Phase Acidity Measurement of Local Acidic Groups in Multifunctional Species: Controlling the Binding Sites in Hydroxycinnamic Acids

Abstract

The applicability of the extended kinetic method (EKM) to determine the gas phase acidities (GA) of different deprotonable groups within the same molecule was tested by measuring the acidities of cinnamic, coumaric, and caffeic acids. These molecules differ not only in the number of acidic groups but in their nature, intramolecular distances, and calculated GAs. In order to determine independently the GA of groups within the same molecule using the EKM, it is necessary to selectively prepare pure forms of the hydrogen-bound heterodimer. In this work, the selectivity was achieved by the use of solvents of different vapor pressure (water and acetonitrile), as well as by variation of the drying temperature in the ESI source, which affected the production of heterodimers with different solvation energies and gas-phase dissociation energies. A particularly surprising finding is that the calculated solvation enthalpies of water and the aprotic acetonitrile are essentially identical, and that the different gas-phase products generated are apparently the result of their different vapor pressures, which affects the drying mechanism. This approach for the selective preparation of heterodimers, which is based on the energetics, appears to be quite general and should prove useful for other studies that require the selective production of heterodimers in ESI sources. The experimental results were supported by density functional theory (DFT) calculations of both gas-phase and solvated species. The experimental thermochemical parameters (deprotonation ΔG, ΔH, and ΔS) are in good agreement with the calculated values for the monofunctional cinnamic acid, as well as the multifunctional coumaric and caffeic acids. The measured GA for cinnamic acid is 334.5 ± 2.0 kcal/mol. The measured acidities for the COOH and OH groups of coumaric and caffeic acids are 332.7 ± 2.0, 318.7 ± 2.1, 332.2 ± 2.0, and 317.3 ± 2.2 kcal/mol, respectively.

A Statistically Supported Antioxidant Activity DFT Benchmark—The Effects of Hartree–Fock Exchange and Basis Set Selection on Accuracy and Resources Uptake

“…We were able to find experimental evidence for BDE: 78.7 kcal/mol [ 71 ] and 81.2 kcal/mol [ 22 ] (averaged to 80.0 kcal/mol); similarly, for PA: 323.3 (±2.2) kcal/mol [ 72 ] through studying the literature. Since these values were obtained for the gaseous state, the computations were done in vacuum.…”

Gas-phase basicities of polyfunctional molecules. Part 4: Carbonyl groups as basic sites

“…These two types of methods generally lead to GB values within few kJ/mol (Bouchoux, ,). However, difficulties in the application of the kinetic methods when there are several sites of protonation in competition were recently documented (Bouchoux, ; Bourgoin‐Voillard et al, ; Guerrero et al, ). These experimental limitations are counterbalanced by the expanded use of increasingly accurate quantum chemical computations.…”

Static and Dynamic Properties of 1,1′‐Bi‐2‐naphthol and Its Conjugated Acids and Bases

“…Δ acid H 0 , GA, and Δ acid S 0 of BINOL were experimentally determined by means of the EKM43, 44, 45, 46 by using a triple‐quadrupole mass spectrometer (Agilent/Varian MS‐320) with an electrospray ionization source (ESI).…”

“…ð9Þ D acid H 0 , GA, and D acid S 0 of BINOL were experimentally determined by means of the EKM [43,44,45,46] À ) via the two competitive dissociation channels with rate constants k and k i , respectively ( Figure 10). If secondary fragmentation is negligible, it is assumed that the abundance ratio of these fragment ions is equal to the ratio of the two dissociation rate constants k and k i .…”