Gas-Phase Acidities and Basicities of Alanines and <i>N</i>‑Benzylalanines by the Extended Kinetic Method

This paper reports an experimental determination of the gas-phase acidities and basicities of <i>N</i>-benzylalanines, in both their α and β forms, by means of the extended kinetic method (EKM). The experimental gas-phase acidity of β-alanine was also determined. Standard ab initio molecular orbital calculations at the G3 level were performed for alanines, and at the G3­(MP2)//B3LYP level for <i>N</i>-benzylalanines. There is a very good agreement between the experimental and the calculated values. The more branched α-amino acids are more acidic and less basic than the linear β-amino acids

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

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 Δ<i>G</i>, Δ<i>H</i>, and Δ<i>S</i>) 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

Phase Transition Thermodynamics of Bisphenols

Herein we have studied, presented, and analyzed the phase equilibria thermodynamics of a bisphenols (BP-A, BP-E, BP-F, BP-AP, and BP-S) series. In particular, the heat capacities, melting temperatures, and vapor pressures at different temperatures as well as the standard enthalpies, entropies, and Gibbs energies of phase transition (fusion and sublimation) were experimentally determined. Also, we have presented the phase diagrams of each bisphenol derivative and investigated the key parameters related to the thermodynamic stability of the condensed phases. When all the bisphenol derivatives are compared at the same conditions, solids BP-AP and BP-S present lower volatilities (higher Gibbs energy of sublimation) and high melting temperatures due to the higher stability of their solid phases. Solids BP-A and BP-F present similar stabilities, whereas BP-E is more volatile. The introduction of −CH₃ groups in BP-F (giving BP-E and BP-A) leads an entropic differentiation in the solid phase, whereas in the isotropic liquids the enthalpic and entropic differentiations are negligible

Energetics and Structural Properties, in the Gas Phase, of <i>trans</i>-Hydroxycinnamic Acids

We have studied the energetics and structural properties of <i>trans</i>-cinnamic acid (CA), <i>o</i>-, <i>m</i>-, and <i>p</i>-coumaric acids (2-, 3-, and 4-hydroxycinnamic acids), caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4-hydroxy-3-methoxycinnamic acid), <i>iso</i>-ferulic acid (3-hydroxy-4-methoxycinnamic acid), and sinapic acid (3,5-dimethoxy-4-hydroxycinnamic acid). The experimental values of Δ_f<i>H</i>_m°(g), determined (in kJ·mol^–1) for CA (−229.8 ± 1.9), <i>p</i>-coumaric acid (−408.0 ± 4.4), caffeic acid (−580.0 ± 5.9), and ferulic acid (−566.4 ± 5.7), allowed us to derive Δ_f<i>H</i>_m°(g) of <i>o</i>-coumaric acid (−405.6 ± 4.4), <i>m</i>-coumaric acid (−406.4 ± 4.4), <i>iso</i>-ferulic acid (−565.2 ± 5.7), and sinapic acid (−698.8 ± 4.1). From these values and by use of isodesmic/homodesmotic reactions, we studied the energetic effects of π-donor substituents (-OH and -OCH₃) in cinnamic acid derivatives and in the respective benzene analogues. Our results indicate that the interaction between -OCH₃ and/or -OH groups in hydroxycinnamic acids takes place without significant influence of the propenoic fragment

Energetic and Structural Properties of Two Phenolic Antioxidants: Tyrosol and Hydroxytyrosol

Theoretical and experimental studies on the energetic, structural and some other relevant physicochemical properties of the antioxidant tyrosol (<b>1</b>), hydroxytyrosol (<b>1</b>OH) molecules and the corresponding radicals <b>1</b>_rad^• and <b>1</b>O_rad^• are reported in this work. The experimental values of the gas-phase enthalpy of formation, Δ_f<i>H</i>_m⁰(g), in kJ·mol^–1, of <b>1</b> (−302.4 ± 3.4) and <b>1</b>OH (−486.3 ± 4.1) have been determined. Quantum chemical calculations, at DFT (M05-2X) and composite <i>ab initio</i> G3 and G4 levels of theory, provided results that served to (i) confirm the excellent consistency of the experimental measurements performed, (ii) establish that the stabilizing effect of H-bond of hydroxyethyl chain and aromatic ring (OH···π interaction) is smaller in radicals than in parent molecules, (iii) deducecombining experimental data in isodesmic reactionsΔ_f<i>H</i>_m⁰(g) of radicals <b>1</b>_rad^• (−152.3 ± 4.4 kJ·mol^–1) and <b>1</b>O_rad^• (−370.6 ± 3.8 kJ·mol^–1), (iv) estimate a reliable O–H bond dissociation enthalpy, BDE of <b>1</b> (368.1 ± 5.6 kJ·mol^–1) and of <b>1</b>OH (333.7 ± 5.6 kJ·mol^–1), and (v) corroborateusing “BDE criteria”than <b>1</b>OH is a more effective antioxidant than <b>1</b>

Energetic and Structural Study of Bisphenols

We have studied thermochemical, thermophysical and structural properties of bisphenols A, E, F, and AP. In particular, the standard enthalpies of sublimation and the standard enthalpies of formation in the gas phase at 298.15 K for all these species were experimentally determined. A computational study, through M05-2X density functional theory, of the various species shed light on structural effects and further confirmed, by means of the isodesmic reaction scheme, the excellent consistency of the experimental results. Our results reflect also the fact that energetic substituent effects are transferable from diphenylalkanes to bisphenols

Acidities of <i>closo</i>-1-COOH-1,7‑C₂B₁₀H₁₁ and Amino Acids Based on Icosahedral Carbaboranes

Carborane clusters are not found in Nature and are exclusively man-made. In this work we study, both experimentally and computationally, the gas-phase acidity (measured GA = 1325 kJ·mol^–1, computed GA = 1321 kJ·mol^–1) and liquid-phase acidity (measured p<i>K</i>_a = 2.00, computed p<i>K</i>_a = 1.88) of the carborane acid <i>closo</i>-1-COOH-1,7-C₂B₁₀H₁₁. The experimental gas-phase acidity was determined with electrospray tandem mass spectrometry (ESI/MS), by using the extended Cooks kinetic method (EKM). Given the similar spatial requirements of the title icosahedral cage and benzene and the known importance of aminoacids as a whole, such a study is extended, within an acid–base context, to corresponding <i>ortho</i>, <i>meta</i>, and <i>para</i> amino acids derived from icosahedral carborane cages, 1-COOH-<i>n</i>-NH₂-1, <i>n</i>-R with {R = C₂B₁₀H₁₀, <i>n</i> = 2, 7, 12}, and from benzene {R = C₆H₄, <i>n</i> = 2, 3, 4}. A remarkable difference is found between the proportion of neutral versus zwitterion structures in water for glycine and the carborane derived amino acids