José-Luis M. Abboud

Linear solvation energy relations

Solvents have been parameterized by scales of dipolarity/polarizability π*, hydrogen-bond donor (HBD) strength α, and hydrogen-bond acceptor strength β. Linear dependence (LSERs) on these solvent parameters are used to correlate and predict a wide variety of solvent effects, as well as to provide an analysis in terms of knowledge and theoretical concepts of molecular structural effects. Some recent applications utilizing this approach are presented. Included are analyses of solvent effects on (a) the free energies of transfer of tetraalkylammonium halide ion pairs and dissociated ions, (b) rates of nucleophilic substitution reactions, (c) the contrast in solvent effects of water (HBD) and dimethyl sulfoxide (non-HBD) on the acidities of m- and p-substituted phenols, (d) partition coefficients of non-HBD solutes between solvent bilayers, and (e) family relationships between proton transfer (and non-protonic Lewis acid) basicities and corresponding β values for monomer HBA. A comprehensive summary of LSER with references is given.

Basicity of N-H- and N-Methyl-1,2,3-triazoles in the Gas Phase, in Solution, and in the Solid State − An Experimental and Theoretical Study

The gas-phase and aqueous basicities of six 1,2,3-triazoles have been determined, the former by FT-ICR and the latter by spectrophotometry and 1H NMR. The gas-phase experiments agree very well with the Gibbs free energies calculated at the B3LYP/6-31G* level. In contrast, only semiquantitative ascertainments are possible when basicities in the gas phase and in solution are compared. It is possible, with the aid of calculations, to obtain a complete picture of the complex equilibria involved in C-substituted N-H-1,2,3-triazoles. The crystal structures of 4(5)-phenyl-1,2,3-triazole (4) and 4(5)-nitro-1,2,3-triazole (15) have been determined. In the gas phase, 2H tautomers b always predominate, while in aqueous solution, both 1H and 2H tautomers − a and b − are present. Finally, in the solid state, 1 exists as a 1:1 mixture of 1a and 1b, while 4 is in the 4b tautomeric form and 15 is a 1H tautomer 15a. These conclusions − a in the gas phase, a + b in solution, and equal probabilities of finding either a or b in the crystal − are probably general for all 1,2,3-triazoles.

Bond activation by protonation in the gas phase

Abstract The structures of different first-row bases and their protonated species are compared using ab initio quantum-mechanical techniques at the 6-31G* and 6-31+G(d, p) levels of accuracy. A topological analysis of the Laplacian of the electronic charge density offers quantitative information on bond activation upon protonation. We have shown that protonation can be accompanied by bond breaking in cases where the basic center is highly electronegative as in alcohols or fluoroalkanes. In bidentate bases, protonation produces opposite effects depending on the center which undergoes protonation.

The ever-surprising chemistry of boron: enhanced acidity of phosphine.boranes.

The acidity-enhancing effect of BH(3) in gas-phase phosphineboranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas-phase acidities (GA) of MePH(2) and MePH(2)BH(3) differ by about 118 kJ mol(-1) (see picture).The gas-phase acidity of a series of phosphines and their corresponding phosphineborane derivatives was measured by FT-ICR techniques. BH(3) attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110 kJ mol(-1)). This acidity-enhancing effect of BH(3) is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High-level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH(3) association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH(3) group is present. Our study also shows that deprotonation of ClCH(2)PH(2) and ClCH(2)PH(2)BH(3) is followed by chloride departure. For the latter compound deprotonation at the BH(3) group is found to be more favorable than PH(2) deprotonation, and the subsequent loss of Cl(-) is kinetically favored with respect to loss of Cl(-) in a typical S(N)2 process. Hence, ClCH(2)PH(2)BH(3) is the only phosphineborane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.

Dihydrogen Generation from Amine/Boranes: Synthesis, FT-ICR, and Computational Studies†

A Fourier transform ion cyclotron resonance spectrometry (FT-ICR) study of the gas-phase protonation of ammonia-borane and sixteen amine/boranes R(1)R(2)R(3)N-BH(3) (including six compounds synthesized for the first time) has shown that, without exception, the protonation of amine/boranes leads to the formation of dihydrogen. The structural effects on the experimental energetic thresholds of this reaction were determined experimentally. The most likely intermediate and the observed final species (besides H(2)) are R(1)R(2)R(3)N-BH(4)(+) and R(1)R(2)R(3)N-BH(2)(+), respectively. Isotopic substitution allowed the reaction mechanism to be ascertained. Computational analyses ([MP2/6-311+G(d,p)] level) of the thermodynamic stabilities of the R(1)R(2)R(3)N-BH(3) adducts, the acidities of the proton sources required for dihydrogen formation, and the structural effects on these processes were performed. It was further found that the family of R(1)R(2)R(3)N-BH(4)(+) ions is characterized by a three-center, two-electron bond between B and a loosely bound H(2) molecule. Unexpected features of some R(1)R(2)R(3)N-BH(4)(+) ions were found. This information allowed the properties of amine/boranes most suitable for dihydrogen generation and storage to be determined.

Linear solvation energy relationships. Part 2. Correlations of electronic spectral data for aniline indicators with solvent π* and β values

The solvatochromic comparison method is employed with electronic spectral data to unravel solvent polarity and hydrogen bonding effects on the p→π* transitions of ethyl 4-aminobenzoate, 4-aminobenzophenone, 3,5-dinitro-aniline, 3-nitroaniline, and N-ethyl-3-nitroaniline. Values of b in the solvatochromic equation, ν(i)max=v0+sπ*+bβ are rationalized in terms of indicator HBD (hydrogen bond donor) acidities and solvent-induced rehybridization effects.

Structural effects on the thermochemical properties of carbonyl compounds I. Enthalpies of combustion, vapour pressures and enthalpies of sublimation, and enthalpies of formation of 2-methylpropanamide, 2,2-dimethylpropanamide, and 1-adamantyl carboxamide

Abstract The energies of combustion of 2-methylpropanamide, 2,2-dimethylpropanamide, and 1-adamantyl carboxamide have been determined using a static bomb calorimeter. The vapour pressures have been measured over a temperature range of about 15 K by the Knudsen-effusion technique. From the experimental results the following quantities for the three compounds, at 298.15 K, have been derived: Compound Δ f H m o (cr) Δ sub H m o Δ f H m o (g) kJ·mol −1 kJ·mol −1 kJ·mol −1 2-Methylpropanamide −368.6 ± 0.9 86.0 ± 0.2 −282.6 ± 0.9 2,2-Dimethylpropanamide −399.7 ± 1.3 86.6 ± 0.4 −313.1 ± 1.4 1-Adamantyl carboxamide −427.0 ± 2.4 108.0 ± 0.5 −319.0 ± 2.5 The structural effects on the thermochemical properties have been studied.

Thermodynamic Stabilities of Carbocations

Publisher Summary This chapter addresses the quantification of the concept of “stability”. Thermodynamic criteria have been applied to rank carbenium ions on the basis of their intrinsic stabilities—that is, stabilities in the absence of solvent. Both experimental and computational data have been used. It discusses the relationship between the intrinsic thermodynamic stability of these species and their reactivity in solution, from both the thermodynamic and the kinetic points of view. Relevant structural features of these species, as revealed by experimental or computational methods data, have been reviewed. Quantum mechanical calculations at the highest possible levels (within our computational capabilities) have been performed on most of the species. The scope of this chapter is limited to a relatively small number of ions that illustrate some representative examples of structural effects. The “low resolution” overview of the gas-phase stabilities of a variety of carbenium ions shows that they are by no means exotic data unrelated to solution reactivity. On the contrary, they are the fundamental factor determining the relative stabilities of the same species in solution.

Gas-Phase Basicities Around and Below Water Revisited

This work employs Fourier transform ion cyclotron resonance (FT-ICR) and the Gaussian quantum chemistry composite methods W1 and G2 to experimentally and computationally analyze gas-phase basicities (GB) for a series of weak bases in the basicity region around and below water. The study aims to clarify the long-standing discrepancy between reported GB values for weak bases obtained via high-pressure mass spectrometry (HPMS) and ICR; the ICR scale is observed to be more than 2 times contracted compared to the HPMS scale. The computational results of this work support published HPMS data. This agreement improves with increasing sophistication of the computational method and is excellent at the W1 level. Several equilibria were also re-examined experimentally using FT-ICR. In the experiments with some polyfluorinated weak bases (hexafluoro-2-propanol and nonafluoro-2-methyl-2-propanol), it was found that two protonation processes compete in the gas phase: protonation on oxygen and protonation on fluorine. In these species, protonation on fluorine proceeds faster and is statistically favored over protonation on oxygen but leads to cations that are thermodynamically less stable than oxygen-protonated cations. The process may also lead to the irreversible loss of HF. The rearrangement of fluorine-protonated cations to oxygen-protonated cations is very slow and is further suppressed by the process of HF abstraction. These results at least partially explain the discrepancy between published HPMS data and earlier FT-ICR findings and call for the utmost care in using FT-ICR for gas-phase basicity measurements of heavily fluorinated compounds. The narrower dynamic range of ICR necessitates the measurement of several problematic bases and produces some differences between the ICR results in the present work and the published HPMS data; the wider dynamic range allows HPMS to overcome these difficulties in connecting the ladder.

Empirical treatment of solvent-solute interactions: medium effects on the electronic absorption spectrum of ?-carotene

Solvent effects on the wavenumber of the maximum of the longest wavelength electronic absorption band of all-trans-β-carotene were determined in 34 solvents. Together with results from previous studies, a data set for 51 solvents, mostly non-hydrogen bond donors, was constructed. This information was analyzed in terms of reaction field models and also showed its value for correlation purposes when used either alone or in combination with standard empirical solvent polarity–polarizability scales.