Josefredo R. Pliego

Gibbs energy of solvation of organic ions in aqueous and dimethyl sulfoxide solutions

The Gibbs energy of solvation of several ions in water and dimethyl sulfoxide (DMSO) solutions was obtained through the use of thermodynamic equations relating ΔGsolv* of the ion with gas phase basicity, pKa, ΔGsolv* of neutral species and the Gibbs energy of solvation of the proton. We have used the most accurate and recent values for these properties, and this report provides 56 Gibbs energy of solvation values in aqueous solution and 30 in DMSO solution. Our results support the general view that anions are much better solvated in aqueous solution than in DMSO. An important example is the hydroxide ion for which the Gibbs energy of transfer from water to DMSO is 26 kcal mol−1. The majority of anions have a Gibbs energy of transfer in the range 10 to 15 kcal mol−1. In the case of cations, DMSO has a larger solvation ability but the difference in the Gibbs nenergy of solvation between water and DMSO is not greater than 5 kcal mol−1. The present data can be very useful for the development of continuum solvation models.

New values for the absolute solvation free energy of univalent ions in aqueous solution

Abstract The absolute solvation free energy of 30 univalent ions, mainly organic species, has been calculated from experimental and theoretical data on proton affinities, aqueous acidity constants, solvation free energy of neutral species, and the new value for the absolute solvation free energy of the proton determined by Tissandier et al. [J. Phys. Chem. A 102 (1998) 7787]. Our new values reveal considerable differences with previous compilations, and should be taken into consideration for comparison with liquid simulation results and in the development of implicit solvation models.

Parametrization of the PCM model for calculating solvation free energy of anions in dimethyl sulfoxide solutions

Abstract We report the first parametrization of a continuum model for the solvation of anions in DMSO solution. The present parameters used in conjunction with the PCM method predict the solvation free energy of 21 anions in DMSO solution with an average error of −1.2 kcal mol −1 , and a S.D. for the average error of only 2.2 kcal mol −1 . This low value of the S.D. shows that the present parametrization is capable of predicting accurate differences of the solvation free energies in DMSO solution and is reliable for modeling liquid phase chemical reactions.

A Theoretical Analysis of the Free-Energy Profile of the Different Pathways in the Alkaline Hydrolysis of Methyl Formate In Aqueous Solution

The free-energy profile for the different reaction pathways available to the hydroxide ion and methyl formate in aqueous solution is reported for the first time. The theoretical analysis was carried out by using the cluster-continuum method recently proposed by us for calculating the free energy of solvation of ions. Unlike the gas-phase reaction, our results are consistent with the fact that the reaction occurs mainly by nucleophilic attack of the hydroxide on the carbonyl carbon to yield a tetrahedral intermediate (B(AC)2 mechanism). However, an additional pathway, in which the hydroxide ion acts as a general base and a water molecule coordinated to this ion acts as the nucleophile, is also predicted to be important. The relative importance of these pathways is calculated to be 87 % and 13 %, respectively. The tetrahedral intermediate of the hydrolysis reaction has an estimated lifetime of 10 nanoseconds, and its conjugate acid has a pK(a) of 8.8. This tetrahedral intermediate is predicted to proceed to products by two pathways: elimination of methoxide ion (84 %) and by water catalyzed elimination of methanol (16 %). The less common reaction pathway, which involves attack of the hydroxide ion on the formyl hydrogen (decarbonylation mechanism) and leads to water, carbon monoxide, and methanol, is calculated to be only 3 kcal mol(-1) less favorable than the B(AC)2 mechanism. By comparison, direct attack of the hydroxide ion on the methyl group (B(AL)2 or S(N)2 mechanism) leading to an acyl-oxygen bond cleavage has a very high free energy of activation and is not expected to be important. The theoretically observed activation free energy at 298.15 K is calculated to be 15.5 kcal mol(-1), in excellent agreement with the experimentally measured value of 15.3 kcal mol(-1). This present model allows for a clear distinction between contributions due to solvation and those due to intrinsic (gas-phase) effects and proves to yield results in very good agreement with available experimental data.

The Gas-Phase Reaction Between Hydroxide Ion and Methyl Formate: A Theoretical Analysis of the Energy Surface and Product Distribution

The potential energy surface for the prototype solvent-free ester hydrolysis reaction: OH- +HCOOCH3 --> products has been characterized by high level ab initio calculations of MP4/6-311 + G(2df,2p)//MP2/6-31 + G(d) quality. These calculations reveal that the approach of an OH- ion leads to the formation of two distinct ion-molecule complexes: 1) the MS1 species with the hydroxide ion hydrogen bonded to the methyl group of the ester, and 2) the MS4 moiety resulting from proton abstraction of the formyl hydrogen by the hydroxide ion and formation of a three-body complex of water, methoxide ion and carbon monoxide. The first complex reacts to generate formate anion and methanol products through the well known B(AC)2 and S(N)2 mechanisms. RRKM calculations predict that these pathways will occur with a relative contribution of 85% and 15% at 298.15 K, in excellent agreement with experimentally measured values of 87% and 13%, respectively. The second complex reacts by loss of carbon monoxide to yield the water-methoxide complex through a single minimum potential surface and is the preferred pathway in the gas-phase. This water-methoxide adduct can further dissociate if the reactants have excess energy. These results provide clear evidence that the preferred pathways for ester hydrolysis in solution are dictated by solvation of the hydroxide ion.

Revisiting the reactions of nucleophiles with arenediazonium ions: dediazoniation of arenediazonium salts in aqueous and micellar solutions containing alkyl sulfates and alkanesulfonates and an ab initio analysis of the reaction pathway

Dediazoniation of 2,4,6-trimethylbenzenediazonium tetrafluoroborate, 1-ArN2BF4 (for the z-Ar compounds described in this paper, z refers to the length of the carbon chain of the substituent at C4 of the benzene ring), in aqueous solutions containing sodium methyl sulfate, NaMeSO4, or sodium methanesulfonate, NaMeSO3, yields 2,4,6-trimethylphenol, 1-ArOH, 2,4,6-trimethylphenyl methyl sulfate, 1-ArOSO3Me and 2,4,6-trimethylphenyl methanesulfonate, 1-ArO3SMe, respectively. The relative yields of 1-ArO3SMe or 1-ArOSO3Me and 1-ArOH depend on the NaMeSO4 or NaMeSO3 concentrations. 4-n-Hexadecyl-2,6-dimethylbenzenediazonium tetrafluoroborate, 16-ArN2BF4, was used to determine the local head group concentration in sodium dodecyl sulfate and sodium dodecanesulfonate micelles by chemical trapping comparing the relative product yields with those obtained in water using the short chain analogs. n Ab initio calculations of the spontaneous dediazoniation of phenyldiazonium ion in the gas phase, as well as in aqueous solution with, or without, added MeSO3−, yield potential energy surfaces for the reaction. For this model the calculated and experimental values of the spontaneous dediazoniation rate constants in aqueous solution, as well as the product composition, were similar to those obtained with 1-ArN2+. These results suggest that in aqueous solution nucleophiles can only compete with water if a diazonium ion·nucleophile complex is formed prior to N2 loss. Calculations show that the addition of nucleophiles to the arenediazonium ion occurs without a saddle point in the potential energy surface, suggesting that the free phenyl cation is not an obligatory intermediate in aqueous solutions. np

A theoretical abinitio and Monte Carlo simulation study of the pyridine+CCl2 reaction kinetics in the gas phase and in carbon tetrachloride solution using canonical flexible transition state theory

The potential energy surface for the pyridine+CCl2 reaction was studied at the abinitio MP4/6-311G(2df,p)//MP2/6-31G(*) level of theory. The MP4/6-311G(2df,p) energies were evaluated by the additivity approximation E[MP4/6-311G(2df,p)]≈E[MP4/6-31G(*)]+E[MP2/6-311G(2df,p)]-E[MP2/6-31G(*)]. The first step proceeds by the addition of CCl2 to pyridine forming a dipolar ylide structure without an activation barrier. Then this species rearranges to a more stable biradical like ylide on a picosecond time scale. The generalized transition state for dipolar ylide formation occurs at a large center of mass distance between the species, and to calculate the reaction rate constant we have used canonical flexible transition state theory. The configurational integral was solved by Monte Carlo simulation and statistical perturbation theory, and the potential of mean force in the gas phase was obtained. This procedure was extended to the liquid phase by including the solvent coordinates in the configurational integral. The activation free energy in the gas phase and in carbon tetrachloride solution was calculated as 1.44 and 2.62 kcal mol-1, respectively. The corresponding rate constants are 5.5×1011 and 7.5×1010 l mol-1 s-1. The last value is in reasonable agreement with the experimental result of 7×109 l mol-1 s-1 determined in isooctane solution.

Reaction of CCl2 with CH2NH and the formation of dipolar and biradical ylide structures

The potential energy surface for the reaction between CH2NH and CCl2 has been investigated using ab initio methods. We have performed geometry optimizations at the MP2/6-31G* level of theory and single point calculations at the MP4(SDQ)/6-311++G** level. The reaction step for ylide formation has a free energy of activation predicted to be 5.0 kcal mol–1. The parallel 1,2-cycloaddition reaction has a calculated free energy barrier of 16.5 kcal mol–1, indicating that this second pathway is not competitive with ylide formation. The structure of the azomethine ylide formed in the first reaction step is similar to that found for the ylide resulting from the reaction of methylene with ammonia and corresponds to a dipolar species. This is highly unstable and rearranges to its more stable isomer, the biradical azomethine ylide, which has a structure similar to the corresponding carbonyl ylide. This species has a free energy barrier to ring closure calculated to be 21.2 kcal mol–1, so it has reasonable kinetic stability. The resulting aziridine has a free energy of 24.1 kcal mol–1 lower than the biradical azomethine ylide, and the activation free energy of ring opening is calculated to be 45.3 kcal mol–1.