The effect of properties of water-organic solvent mixtures on the solvation enthalpy of 12-crown-4, 15-crown-5, 18-crown-6 and benzo-15-crown-5 ethers at 298.15 K

Crown ethers are preferential solvated by organic solvents in the mixtures of water with formamide, N-methylformamide, acetonitrile, acetone and propan-1-ol. In these mixed solvents the energetic effect of the preferential solvation depends quantitatively on the structural and energetic properties of mixtures. The energetic properties of the mixtures of water with hydrophobic solvents (N,N-dimethylformamide, dimethylsulfoxide, N,N-dimethylacetamide, hexamethylphosphortriamide) counteract the preferential solvation of the crown ether molecules. The effect of the hydrophobic and acid-base properties of the mixture of water with organic solvent on the solvation of 12-crown-4, 15-crown-5, 18-crown-6 and benzo-15-crown-5 ethers was discussed. The solvation enthalpy of one -CH₂CH₂O- group in water, N,N-dimethylformamide and hexamethylphosphortriamide is equal to −24.21, −16.04 and −15.91 kJ/mol, respectively. The condensed benzene ring with 15-crown-5 ether molecule brings about an increase in the exothermic effect of solvation of the crown ether in the mixtures of water with organic solvent.     

Theoretical study on redox potentials of organic radicals in different solvents

The BMK density functional theory method has been used to examine the redox potentials of organic radicals in different solvents (DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide; MeCN, acetonitrile). The polarizable continuum solvation model (PCM) was used to describe the solvation-free energies. The one-electron electrochemical standard potentials (E ⁰) of ca. 100 organic radicals in three solvents were calculated using a single, unified theoretical method whose reliability has been tested against almost all the available experimental data. It was found that the mean absolute deviation (MAD) between the theory and experiment was about 0.08 V. With the newly developed theoretical method in hand, more redox potentials of organic radicals in these three solvents were predicted by this single, unified method. The results showed that the redox potentials of organic radicals in different organic solvents including DMF and DMSO had good correlations with their redox potentials in MeCN.     

A novel quantum mechanical/molecular mechanical approach to the free energy calculation for isomerization of glycine in aqueous solution

The free energy change associated with the isomerization reaction of glycine in water solution has been studied by a hybrid quantum mechanical/molecular mechanical (QM/MM) approach combined with the theory of energy representation (QM/MM-ER) recently developed. The solvation free energies for both neutral and zwitterionic form of glycine have been determined by means of the QM/MM-ER simulation. The contributions of the electronic polarization and the fluctuation of the QM solute to the solvation free energy have been investigated. It has been found that the contribution of the density fluctuation of the zwitterionic solute is estimated as -4.2 kcal/mol in the total solvation free energy of -46.1 kcal/mol, while that of the neutral form is computed as -3.0 kcal/mol in the solvation free energy of -15.6 kcal/mol. The resultant free energy change associated with the isomerization of glycine in water has been obtained as -7.8 kcal/mol, in excellent agreement with the experimental data of -7.3 or -7.7 kcal/mol, implying the accuracy of the QM/MM-ER approach. The results have also been compared with those computed by other methodologies such as the polarizable continuum model and the classical molecular simulation. The efficiency and advantage of the QM/MM-ER method has been discussed.     

Aqueous solvation free energies of ions and ion-water clusters based on an accurate value for the absolute aqueous solvation free energy of the proton

Thermochemical cycles that involve pKa, gas-phase acidities, aqueous solvation free energies of neutral species, and gas-phase clustering free energies have been used with the cluster pair approximation to determine the absolute aqueous solvation free energy of the proton. The best value obtained in this work is in good agreement with the value reported by Tissandier et al. (Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. J.; Earhart, A. D.; Coe, J. V. J. Phys. Chem. A 1998, 102, 7787), who applied the cluster pair approximation to a less diverse and smaller data set of ions. We agree with previous workers who advocated the value of -265.9 kcal/mol for the absolute aqueous solvation free energy of the proton. Considering the uncertainties associated with the experimental gas-phase free energies of ions that are required to use the cluster pair approximation as well as analyses of various subsets of data, we estimate an uncertainty for the absolute aqueous solvation free energy of the proton of no less than 2 kcal/mol. Using a value of -265.9 kcal/mol for the absolute aqueous solvation free energy of the proton, we expand and update our previous compilation of absolute aqueous solvation free energies; this new data set contains conventional and absolute aqueous solvation free energies for 121 unclustered ions (not including the proton) and 147 conventional and absolute aqueous solvation free energies for 51 clustered ions containing from 1 to 6 water molecules. When tested against the same set of ions that was recently used to develop the SM6 continuum solvation model, SM6 retains its previously determined high accuracy; indeed, in most cases the mean unsigned error improves when it is tested against the more accurate reference data.     

Electrochemical oxidation of several oxocarbon salts in N,N-dimethylformamide

The oxocarbon salts of croconic acid and its dicyanomethylene derivatives have been shown to undergo two consecutive reversible one-electron transfers in N,N-dimethylformamide to produce stable radical anions and the neutral croconates. Disproportion equilibrium constants were found to be quite small for all the crononate radical anions investigated. Following chemical reactions accompanied the second oxidation process of dicyanomethylene-substituted crononates. Substituent effects were shown to be ring position-independent and are discussed with respect to the unique resonance structure of the crononates.     

Effects of synthesis-solvent on swelling and elastic properties of poly(N-isopropylacrylamide) hydrogels

Thermosensitive N-isopropylacrylamide (NIPA) hydrogels were synthesized by a free radical copolymerization with N,N′-methylenebisacrylamide (MBAA) in four solvents: water, ethanol, acetone and N,N-dimethylformamide. The swelling and elastic properties of the hydrogels were affected by the synthesis-solvents; the hydrogels (e.g. NIPA/MBAA = 1000/50 mol/m³-pre-gel solution) synthesized in water have smaller swelling volume and larger shear modulus at 10 °C than those synthesized in amphiphilic solvents. The network structure of hydrogels was estimated in terms of the conversion and two sorts of effective crosslinking density based on the Flory theory and the concentration of crosslinker. The hydrogels synthesized in water can have the microscopic inhomogeneous network arising from the entanglement of polymer chains, while the hydrogels synthesized in amphiphilic solvents can have the homogeneous network arising from the polymer concentration lower than the pre-gel solution and can be similar in network structure to the lightly crosslinked hydrogel synthesized in water.     

Formaldehyde oxime <--> nitrosomethane tautomerism

Formaldehyde oxime <--> nitrosomethane tautomerism, isomeric nitrone, and their common cations and anions are studied with Gaussian-2 theory using MP2(full)/6-31G geometries and with density functional theory using B3LYP/6-311+G**. Geometrical parameters, harmonic vibrational frequencies, relative stabilities, conformational stabilities, and ionization energies are compared with experimental gas-phase data when available. The formaldehyde oxime <--> nitrosomethane tautomerism is compared with the amide <--> imidol, imine <--> enamine, keto <--> enol, and nitro <--> aci-nitro tautomeric processes. Solvent effects are estimated by the self-consistent isodensity polarizable continuum model (SCIPCM). The influence of hydrogen bonding interactions with the solvent is addressed by including two water molecules. In the final evaluation, formaldehyde oxime is 15.8 kcal/mol more stable than nitrosomethane when the aqueous solvation correction of 3.8 kcal/mol is applied to the G2 energies. Unsolvated formaldehyde oxime is estimated to be 11.1 kcal/mol more stable than nitrone. The estimated gas-phase ionization energies (G2) are 362.5 kcal/mol for formaldehyde oxime, 350.6 kcal/mol for nitrosomethane, and 351.4 kcal/mol for nitrone.     

Experimentation with different thermodynamic cycles used for pKa calculations on carboxylic acids using complete basis set and Gaussian‐n models combined with CPCM continuum solvation methods

Complete basis set and Gaussian‐n methods were combined with Barone and Cossi's implementation of the polarizable conductor model (CPCM) continuum solvation methods to calculate pK_a values for six carboxylic acids. Four different thermodynamic cycles were considered in this work. An experimental value of ?264.61 kcal/mol for the free energy of solvation of H⁺, ΔG_s(H⁺), was combined with a value for G_gas(H⁺) of ?6.28 kcal/mol, to calculate pK_a values with cycle 1. The complete basis set gas‐phase methods used to calculate gas‐phase free energies are very accurate, with mean unsigned errors of 0.3 kcal/mol and standard deviations of 0.4 kcal/mol. The CPCM solvation calculations used to calculate condensed‐phase free energies are slightly less accurate than the gas‐phase models, and the best method has a mean unsigned error and standard deviation of 0.4 and 0.5 kcal/mol, respectively. Thermodynamic cycles that include an explicit water in the cycle are not accurate when the free energy of solvation of a water molecule is used, but appear to become accurate when the experimental free energy of vaporization of water is used. This apparent improvement is an artifact of the standard state used in the calculation. Geometry relaxation in solution does not improve the results when using these later cycles. The use of cycle 1 and the complete basis set models combined with the CPCM solvation methods yielded pK_a values accurate to less than half a pK_a unit. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

(Vapour + liquid) equilibrium in (N,N-dimethylacetamide + methanol + water) at the temperature 313.15 K

Total vapour pressures, measured at the temperature 313.15 K, are reported for the ternary mixture (N,N-dimethylacetamide + methanol + water), and for binary constituents (N,N-dimethylacetamide + methanol) and (N,N-dimethylacetamide + water). The present results are compared with previously obtained data for binary mixtures (amide + water) and (amide + methanol), where amide=N-methylformamide, N,N-dimethylformamide, N-methyl-acetamide, 2-pyrrolidinone and N-methylpyrrolidinone. Moreover, it was found that excess Gibbs free energy of mixing for binary mixtures varies roughly linearly with the molar volume of amide.     

Single-ion solvation free energies and the normal hydrogen electrode potential in methanol, acetonitrile, and dimethyl sulfoxide

The division of thermodynamic solvation free energies of electrolytes into contributions from individual ionic constituents is conventionally accomplished by using the single-ion solvation free energy of one reference ion, conventionally the proton, to set the single-ion scales. Thus, the determination of the free energy of solvation of the proton in various solvents is a fundamental issue of central importance in solution chemistry. In the present article, relative solvation free energies of ions and ion-solvent clusters in methanol, acetonitrile, and dimethyl sulfoxide (DMSO) have been determined using a combination of experimental and theoretical gas-phase free energies of formation, solution-phase reduction potentials and acid dissociation constants, and gas-phase clustering free energies. Applying the cluster pair approximation to differences between these relative solvation free energies leads to values of -263.5, -260.2, and -273.3 kcal/mol for the absolute solvation free energy of the proton in methanol, acetonitrile, and DMSO, respectively. The final absolute proton solvation free energies are used to assign absolute values for the normal hydrogen electrode potential and the solvation free energies of other single ions in the solvents mentioned above.     

Continuum polarizable force field within the Poisson-Boltzmann framework

We have developed and tested a complete set of nonbonded parameters for a continuum polarizable force field. Our analysis shows that the new continuum polarizable model is consistent with B3LYP/cc-pVTZ in modeling electronic response upon variation of dielectric environment. Comparison with experiment also shows that the new continuum polarizable model is reasonable, with accuracy similar to that of B3LYP/cc-pVTZ in reproduction of dipole moments of selected organic molecules in the gas phase. We have further tested the validity to interchange the Amber van der Waals parameters between the explicit and continuum polarizable force fields with a series of dimers. It can be found that the continuum polarizable model agrees well with MP2/cc-pVTZ, with deviations in dimer binding energies less than 0.9 kcal/mol in the aqueous dielectric environment. Finally, we have optimized atomic cavity radii with respect to experimental solvation free energies of 177 training molecules. To validate the optimized cavity radii, we have tested these parameters against 176 test molecules. It is found that the optimized Poisson-Boltzmann atomic cavity radii transfer well from the training set to the test set, with an overall root-mean-square deviation of 1.30 kcal/mol, an unsigned average error of 1.07 kcal/mol, and a correlation coefficient of 92% for all 353 molecules in both the training and test sets. Given the development documented here, the next natural step is the construction of a full protein/nucleic acid force field within the new continuum polarization framework.     

A new equation to reproduce the enthalpies of transfer of formamide, N-methylformamide and N,N-dimethylformamide from water to aqueous methanol mixtures at 298 K

The enthalpies of transfer of formamide (Form) N-methylformamide (NMF) and N,N-dimethylformamide (DMF) from water to aqueous methanol mixtures are reported and analysed in terms of the new solvation theory. It was found that a previous equation could not reproduce these data over the whole range of solvent compositions. Using the new solvation theory to reproduce the enthalpies of transfer shows excellent agreement between the experimental and calculated data over the entire range of solvent compositions. The analyses show that the solvation of DMF is random in the aqueous methanol mixtures while Form and NMF are preferentially solvated by methanol. It is also found that the interaction of the solutes is stronger with methanol than with water.     

Calculation of solvation free energies of charged solutes using mixed cluster/continuum models

We derive a consistent approach for predicting the solvation free energies of charged solutes in the presence of implicit and explicit solvents. We find that some published methodologies make systematic errors in the computed free energies because of the incorrect accounting of the standard state corrections for water molecules or water clusters present in the thermodynamic cycle. This problem can be avoided by using the same standard state for each species involved in the reaction under consideration. We analyze two different thermodynamic cycles for calculating the solvation free energies of ionic solutes: (1) the cluster cycle with an n water cluster as a reagent and (2) the monomer cycle with n distinct water molecules as reagents. The use of the cluster cycle gives solvation free energies that are in excellent agreement with the experimental values obtained from studies of ion-water clusters. The mean absolute errors are 0.8 kcal/mol for H(+) and 2.0 kcal/mol for Cu(2+). Conversely, calculations using the monomer cycle lead to mean absolute errors that are >10 kcal/mol for H(+) and >30 kcal/mol for Cu(2+). The presence of hydrogen-bonded clusters of similar size on the left- and right-hand sides of the reaction cycle results in the cancellation of the systematic errors in the calculated free energies. Using the cluster cycle with 1 solvation shell leads to errors of 5 kcal/mol for H(+) (6 waters) and 27 kcal/mol for Cu(2+) (6 waters), whereas using 2 solvation shells leads to accuracies of 2 kcal/mol for Cu(2+) (18 waters) and 1 kcal/mol for H(+) (10 waters).     

Combining the polarizable Drude force field with a continuum electrostatic Poisson–Boltzmann implicit solvation model

In this work, we have combined the polarizable force field based on the classical Drude oscillator with a continuum Poisson–Boltzmann/solvent‐accessible surface area (PB/SASA) model. In practice, the positions of the Drude particles experiencing the solvent reaction field arising from the fixed charges and induced polarization of the solute must be optimized in a self‐consistent manner. Here, we parameterized the model to reproduce experimental solvation free energies of a set of small molecules. The model reproduces well‐experimental solvation free energies of 70 molecules, yielding a root mean square difference of 0.8 kcal/mol versus 2.5 kcal/mol for the CHARMM36 additive force field. The polarization work associated with the solute transfer from the gas‐phase to the polar solvent, a term neglected in the framework of additive force fields, was found to make a large contribution to the total solvation free energy, comparable to the polar solute–solvent solvation contribution. The Drude PB/SASA also reproduces well the electronic polarization from the explicit solvent simulations of a small protein, BPTI. Model validation was based on comparisons with the experimental relative binding free energies of 371 single alanine mutations. With the Drude PB/SASA model the root mean square deviation between the predicted and experimental relative binding free energies is 3.35 kcal/mol, lower than 5.11 kcal/mol computed with the CHARMM36 additive force field. Overall, the results indicate that the main limitation of the Drude PB/SASA model is the inability of the SASA term to accurately capture non‐polar solvation effects. © 2018 Wiley Periodicals, Inc.     

Calculation of acidic dissociation constants in water: solvation free energy terms. Their accuracy and impact

Three polarizable continuum models, DPCM, CPCM, and IEFPCM, have been applied to calculate free energy differences for nine neutral compounds and their anions. On the basis of solvation free energies, the pK_a values were obtained for the compounds in question by using three thermodynamic cycles: one, involving the combined experimental and calculated data, as well as two other cycles solely with calculated data. This paper deals with the influence of factors such as the SCRF model applied, choice of a particular thermodynamic cycle, atomic radii used to build a cavity in the solvent (water), optimization of geometry in water, inclusion of electron correlation, and the dimension of the basis set on the solvation free energies and on the calculated pK_a values. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Solvation free energies and partition coefficients with the coarse-grained and hybrid all-atom/coarse-grained MARTINI models

We present the estimation of solvation free energies of small solutes in water, n-octanol and hexane using molecular dynamics simulations with two MARTINI models at different resolutions, viz. the coarse-grained (CG) and the hybrid all-atom/coarse-grained (AA/CG) models. From these estimates, we also calculate the water/hexane and water/octanol partition coefficients. More than 150 small, organic molecules were selected from the Minnesota solvation database and parameterized in a semi-automatic fashion. Using either the CG or hybrid AA/CG models, we find considerable deviations between the estimated and experimental solvation free energies in all solvents with mean absolute deviations larger than 10 kJ/mol, although the correlation coefficient is between 0.55 and 0.75 and significant. There is also no difference between the results when using the non-polarizable and polarizable water model, although we identify some improvements when using the polarizable model with the AA/CG solutes. In contrast to the estimated solvation energies, the estimated partition coefficients are generally excellent with both the CG and hybrid AA/CG models, giving mean absolute deviations between 0.67 and 0.90 log units and correlation coefficients larger than 0.85. We analyze the error distribution further and suggest avenues for improvements.     

Calculation of solvation free energies of Li+ and O2 ? ions and neutral lithium–oxygen compounds in acetonitrile using mixed cluster/continuum models

Solvation effects play a major role in determining the cycling characteristics of the non-aqueous rechargeable Li-air battery. We use a mixed cluster/continuum solvent model with varying number of explicit solvent molecules (n?=?4–10) to calculate the solvation free energies ( $ \Updelta G_{\text{solv}}^{*} $ ) of Li⁺ and O₂ ^? ions and neutral LiO₂, Li₂O₂, LiO, and Li₂O species in acetonitrile solvent. Calculations for complexes with the full first solvation shell around Li⁺ (n?=?4) and O₂ ^? (n?=?8) show excellent agreement with the solvation free energies obtained using the cluster pair approximation (the error is below 2.0?kcal/mol). The use of the pure continuum model fitted to reproduce the experimental values of $ \Updelta G_{\text{solv}}^{*} $ (Li⁺) and $ \Updelta G_{\text{solv}}^{*} $ (O₂ ^?) gives the solvation free energies of various lithium–oxygen species (Li _x O _y ; x, y?=?1, 2) that are in excellent agreement with the results obtained using mixed cluster/continuum models (n?≥?8). This provides a theoretical framework for including solvent effects in the theoretical models of oxygen reduction and evolution reactions in the aprotic Li-air battery.     

A DFT study on pyridine-derived N-heterocyclic carbenes

To appreciate the chemistry of N-heterocyclic carbenes (NHCs), eight carbenic tautomers of pyridine (azacyclohexadienylidenes) are studied at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G^∗ and B3LYP/6-311++G^∗∗//B3LYP/6-31+G^∗ levels of theory. Various thermodynamic parameters are calculated for these minima, along with a kinetic focus on carbene-pyridine tautomerization. Appropriate isodesmic reactions show stabilization energies of 2-azacyclohexa-3,5-dienylidene (1) and 4-azacyclohexa-2,5-dienylidene (6) as 119.4 and 104.1 kcal/mol, rather close to that of the synthesized 1,3-dimethylimidazol-2-ylidene (129.2 kcal/mol). Three different mechanisms are suggested for the tautomerizations including: [1,2]-H shift, [1,4]-H shift, and three sequential [1,2]-H shifts. The calculated energy barrier for [1,2]-H shift of 1 is 44.6 kcal/mol, while the first [1,2]-H shift for the proposed sequential mechanism of 6 requires 65.1 kcal/mol. Three preliminary minimum templates are introduced, which may possess the potential of synthetic consideration: 2,6-di(X)-3,5-dichloro-4-azacyclohexa-2,5-dienylidene for X=Mes, t-Bu, and Ad.     

Radical polymerization behavior of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate in water

The homogeneous polymerization of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate (MDAPS) with potassium peroxydisulfate (KPS) was kinetically in situ investigated in water by means of FT-near IR spectroscopy. The overall activation energy of the polymerization was calculated to be 16.0 kcal/mol. The initial polymerization rate (R_p) at 40 °C was expressed by R_p=k[KPS]^0.65[MDAPS]^1.0. The presence of alkaline metal salts was observed to accelerate the polymerization. The order of acceleration at 40 °C was CsCl > KCl > NaCl > LiCl when the chloride salts were used. NaCl showed higher acceleration effect than NaF. NaBr and NaI exhibited retardation and inhibition effect, respectively, because of reduction of KPS and its primary radical with bromide and iodide ions. The polymerization of MDAPS with KPS in water in the presence of NaCl at 2.0 mol/l gave R_p=k[KPS]^0.70[MDAPS]^1.4 at 40 °C. The overall activation energy of the polymerization in the presence of NaCl was estimated to be 11.6 kcal/mol being considerably lower value compared with that in its absence. The syndiotacticity of poly(MDAPS) tended to increase with rising temperature and decrease in the presence of NaCl.