Stepwise hydration of ionized aromatics. Energies, structures of the hydrated benzene cation, and the mechanism of deprotonation reactions

The stepwise binding energies (DeltaHdegree(n-1,n)) of 1-8 water molecules to benzene(.+) [Bz(.+)(H2O)n] were determined by equilibrium measurements using an ion mobility cell. The stepwise hydration energies, DeltaHdegree(n-1,n), are nearly constant at 8.5 +/- 1 kcal mol-1 from n = 1-6. Calculations show that in the n = 1-4 clusters, the benzene(.+) ion retains over 90% of the charge, and it is extremely solvated, that is, hydrogen bonded to an (H2O)n cluster. The binding energies and entropies are larger in the n = 7 and 8 clusters, suggesting cyclic or cage-like water structures. The concentration of the n = 3 cluster is always small, suggesting that deprotonation depletes this ion, consistent with the thermochemistry since associative deprotonation Bz(.+)(H2O)(n-1) + H2O-->C6H5. + (H2O)nH+ is thermoneutral or exothermic for n > or = 4. Associative intracluster proton transfer Bz(.+)(H2O)(n+1) + H2O-->C6H5.(H2O)nH+ would also be exothermic for n > or = 4, but lack of H/D exchange with D2O shows that the proton remains on C6H6(.+) in the observed Bz(.+)(H2O)n clusters. This suggests a barrier to intracluster proton transfer, and as a result, the [Bz(.+)(H2O)n]* activated complexes either undergo dissociative proton transfer, resulting in deprotonation and generation of (H2O)nH+, or become stabilized. The rate constant for the deprotonation reaction shows a uniquely large negative temperature coefficient of K = cT(-67+/-4) (or activation energy of -34+/- 1 kcal mol-1), caused by a multibody mechanism in which five or more components need to be assembled for the reaction.     

Correlation between hydration and deprotonation in hexaaqua metal complexes

The hydration energy of metallic cations determined with density functional calculations using a double-numerical plus p-polarization basis set, related to the acidity constants of hexaaqua metal complexes, was investigated in the present study. From the results calculated by Vosko-Wilk-Nusair (VWN), Becke-Perdew (BP) and Becke-Lee-Yang-Parr (BLYP) density functionals, a global linear correlation with the observed acidity constants in both main group [Mg(II), Ca(II) and Al(III)] and (post-)transition group [Mn(II), Zn(II), Cd(II), Sc(III), Cr(III), Fe(III), Ga(III) and In(III)] hexaaqua metal complexes has been established:

VWN density functional: pK_a = 16.5760 + 0.0173E_hydr kcal mol⁻¹

BP density functional: pK_a = 15.7329 + 0.0182E_hydr kcal mol⁻¹

BLYP density functional: pK_a = 15.9448 + 0.0185E_hydr kcal mol⁻¹     

Gas phase hydration and deprotonation of the cyclic C3H3+ cation. Solvation by acetonitrile, and comparison with the benzene radical cation

The binding energies of the first 5 H2O molecules to c-C3H3+ were determined by equilibrium measurements. The measured binding energies of the hydrated clusters of 9-12 kcal/mol are typical of carbon-based CH+...X hydrogen bonds. The ion solvation with the more polar CH3CN molecules results in stronger bonds consistent with the increased ion-dipole interaction. Ab initio calculations show that the lowest energy isomer of the c-C3H3+(H2O)4 cluster consists of a cyclic water tetramer interacting with the c-C3H3+ ion, which suggests the presence of orientational restraint of the water molecules consistent with the observed large entropy loss. The c-C3H3+ ion is deprotonated by 3 or more H2O molecules, driven energetically by the association of the solvent molecules to form strongly hydrogen bonded (H2O)nH+ clusters. The kinetics of the associative proton transfer (APT) reaction C3H3+ + nH2O --> (H2O)nH+ + C3H2* exhibits an unusually steep negative temperature coefficient of k = cT(-63+/-4) (or activation energy of -37 +/- 1 kcal mol(-1)). The behavior of the C3H3+/water system is exactly analogous to the benzene+*/water system, suggesting that the mechanism, kinetics and large negative temperature coefficients may be general to multibody APT reactions. These reactions can become fast at low temperatures, allowing ionized polycyclic aromatics to initiate ice formation in cold astrochemical environments.     

Langevin model of the temperature and hydration dependence of protein vibrational dynamics

The modification of internal vibrational modes in a protein due to intraprotein anharmonicity and solvation effects is determined by performing molecular dynamics (MD) simulations of myoglobin, analyzing them using a Langevin model of the vibrational dynamics and comparing the Langevin results to a harmonic, normal mode model of the protein in vacuum. The diagonal and off-diagonal Langevin friction matrix elements, which model the roughness of the vibrational potential energy surfaces, are determined together with the vibrational potentials of mean force from the MD trajectories at 120 K and 300 K in vacuum and in solution. The frictional properties are found to be describable using simple phenomenological functions of the mode frequency, the accessible surface area, and the intraprotein interaction (the displacement vector overlap of any given mode with the other modes in the protein). The frictional damping of a vibrational mode in vacuum is found to be directly proportional to the intraprotein interaction of the mode, whereas in solution, the friction is proportional to the accessible surface area of the mode. In vacuum, the MD frequencies are lower than those of the normal modes, indicating intramolecular anharmonic broadening of the associated potential energy surfaces. Solvation has the opposite effect, increasing the large-amplitude vibrational frequencies relative to in vacuum and thus vibrationally confining the protein atoms. Frictional damping of the low-frequency modes is highly frequency dependent. In contrast to the damping effect of the solvent, the vibrational frequency increase due to solvation is relatively temperature independent, indicating that it is primarily a structural effect. The MD-derived vibrational dynamic structure factor and density of states are well reproduced by a model in which the Langevin friction and potential of mean force parameters are applied to the harmonic normal modes.     

Pressure dependence of the mobility of negative charge carriers injected into liquid benzene

The mobility of negative charge carriers injected into liquid benzene was measured from atmospheric pressure up to 1.7 kbar at several temperatures. The pressure dependence of the mobility excludes a hopping mechanism but can be explained by assuming ionic conduction.     

Pressure and temperature dependence of hydrophobic hydration: volumetric, compressibility, and thermodynamic signatures

The combined effect of pressure and temperature on hydrophobic hydration of a nonpolar methanelike solute is investigated by extensive simulations in the TIP4P model of water. Using test-particle insertion techniques, free energies of hydration under a range of pressures from 1 to 3000 atm are computed at eight temperatures ranging from 278.15 to 368.15 K. Corresponding enthalpy, entropy, and heat capacity accompanying the hydration process are estimated from the temperature dependence of the free energies. Partial molar and excess volumes calculated using pressure derivatives of the simulated free energies are consistent with those determined by direct volume simulations; but direct volume determination offers more reliable estimates for compressibility. At 298.15 K, partial molar and excess isothermal compressibilities of methane are negative at 1 atm. Partial molar and excess adiabatic (isentropic) compressibilities are estimated to be also negative under the same conditions. But partial molar and excess isothermal compressibilities are positive at high pressures, with a crossover from negative to positive compressibility at approximately 100-1000 atm. This trend is consistent with experiments on aliphatic amino acids and pressure-unfolded states of proteins. For the range of pressures simulated, hydration heat capacity exhibits little pressure dependence, also in apparent agreement with experiment. When pressure is raised at constant room temperature, hydration free energy increases while its entropic component remains essentially constant. Thus, the increasing unfavorability of hydration under raised pressure is seen as largely an enthalpic effect. Ramifications of the findings of the authors for biopolymer conformational transitions are discussed.     

A molecular dynamics computer simulation study of the temperature dependence of hydration of 1,4-dioxane and 1,3-dioxane

This paper describes a study of the hydration of 1,3-dioxane and 1,4-dioxane at two different temperatures using different molecular dynamics (MD) computer simulation techniques. Three major conclusions have been drawn. Firstly, the simulations of 1,4-dioxane—water and 1,3-dioxane—water at constant pressure lead essentially to the same conclusions as earlir MD studies at constant volume. Secondly, the numerical values of dynamic properties depend critically on the density of the system. Simulations at constant pressure provide densities which are dependent on the periodicity requirement imposed on the system by the periodic boundary conditions. The smaller the periodic box, the stronger this effect is. Thirdly, in 1,4-dioxane—water an increase in temperature results in an enhanced mobility of water molecules in the solvation shell, whereas in the case of 1,3-dioxane—water these water molecules become more strongly bound by the solute. This effect is entirely due to a reduction of the mobility of water molecules in the 1,3-dioxane oxygen hydration subshells. The contrasting behavior is explained in terms of a situation where solvent—solvent interactions dominate solute—solvent interactions in 1,4-dioxane—water at both temperatures and in 1,3-dioxane—water at the lower temperature, while the opposite situation holds for 1,3-dioxane—water at the higher temperature.     

Crystal structures of n-BuLi adducts with (R,R)-TMCDA and the consequences for the deprotonation of benzene

Combinations of organolithium compounds and diamine bases have become a powerful tool in synthetic chemistry. Because of the structure-reactivity relationship, the elucidation of reaction mechanisms of these reagents is strongly connected with the structural determination of intermediate species. In mixtures of the diamine TMCDA (N,N,N',N'-tetramethylcyclohexane-1,2-diamine) and n-butyllithium, two different structures, the dimeric [n-BuLi x (R,R)-TMCDA]2 and the aggregate [(n-BuLi)2 x (R,R)-TMCDA]2, can be isolated, depending on the n-BuLi/TMCDA ratio. Thereby, [(n-BuLi)2 x (R,R)-TMCDA]2 is a rare example of an organolithium compound with a ladder arrangement of the central four-membered Li-C-Li-C rings. Two isomers of the ladder structure are formed in the crystal by changing from the enantiomerically pure to racemic TMCDA. As n-BuLi/TMCDA mixtures are also able to deprotonate benzene, these structures give hint to possible mechanisms. Supported by theoretical studies, transition states based on the dimer, the ladder structure, and a hypothetical monomer are discussed.     

Thermal phenomena of alkali-activated metakaolin studied with a negative temperature coefficient system

Journal of Thermal Analysis and Calorimetry - The properties of alkali-activated materials (AAMs) depend on both the type of raw material used and their production procedure. This article presents...     

A negative temperature dependence of the electron self-exchange rates of zinc porphyrin pi radical cations

The one-electron oxidation of ZnT(t-Bu)PP (T(t-Bu)PP2- = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin dianion) by one equivalent of Ru(bpy)33+ (bpy = 2,2'-bipyridine) results in quantitative formation of ZnT(t-Bu)PP*+ which is detected by ESR. In the presence of excess ZnT(t-Bu)PP, the ESR line width becomes broader with increasing ZnT(t-Bu)PP concentration due to the electron self-exchange between ZnT(t-Bu)PP*+ and ZnT(t-Bu)PP. The line width of the ESR signal of ZnT(t-Bu)PP*+ becomes broader as the temperature is decreased from 313 to 233 K. This indicates that the electron self-exchange reaction becomes faster at a lower temperature. The substituent and solvent effects on such a negative temperature dependence of the electron self-exchange rates are reported.     

Radiolysis of benzene solutions in aqueous NaNO3: A temperature dependence study by HPLC and GC analytical techniques

The radiolysis of aqueous benzene solutions was carried out at 35 °C, 45 °C, 55 °C, and 65 °C. The results, obtained by coupling HPLC and GC separations, showed that the yields of all the radiolysis products increased with temperature. However, the comparison of the results must be limited to those compounds that are formed exclusively in aqueous solutions. It was found that the increase of the relative concentrations of the products with the temperature followed the order: nitrobenzene>phenol>o-nitrophenol >p-nitrophenol.     

Stepwise creep in polyethylenes of trademarked pipes in the premelting temperature mode

The stepwise creep in polyethylenes of the trademarked pipes Hostalen 4731b and Dowlex 2388 is compared to that of standard HDPE and LDPE. With the aim to study the deformation of the interlamellar macromolecules, a temperature of 100°C is chosen. At this temperature, the destruction of the interlamellar aggregates results in their easy deformation with the preservation of the integrity of the main crystalline framework. The process is performed under a light load, as a result of which there is deformation relaxation to some limiting value, and then the loading is repeated. The ordering of the lamellar structuring in the annealed samples leads to a sharp decrease in deformability of the interlamellar molecules, unlike what is observed in quenched samples. The shortening of samples is established after stepwise unloading similar to the loading. The Dowlex 2388 copolymer exhibits a sharp increase in tear resistance at 115°C accompanied by a local rise in the orientation stretching of the interlamellar molecules, whose subsequent breakage leads to a significant increase in the slot width after tearing at 115 ± 5°C, i.e., at exactly the maximum resistance.     

Micellization and catalytic activity of the cetyltrimethylammonium bromide-Brij 97-water mixed micellar system

Surface tension measurements and the kinetic study of the basic hydrolysis of ethyl p-nitrophenyl chloromethyl phosphonate were used to examine the structural behavior and catalytic activity of the cethyltrimethylammonium bromide (CTAB)-polyoxyethylene (10) oleyl ether, C(18)H(35)(OCH(2)CH(2))(10)OH (Brij 97)-water mixed micellar system. Application of the regular solution model to the experimental data yields the value of the interaction parameter beta as -4.6, which indicates an attractive interaction of the surfactants in the mixed micelle and reflects synergistic solution behavior of the mixture. The mixed micellar composition is found to be enriched in the surfactant with the lower critical micelle concentration (cmc). In the kinetic study a nonmonotonic change in the pseudo-first-order rate constant of basic hydrolysis of the substrate is observed with increasing mole fraction of nonionic surfactant. The pseudophase micellar model reveals that the concentration factor mainly contributes to the catalytic effect, while the microenvironmental factor plays a negative role.     

Numerical experiment simulating positive and negative hydration of ions in electrolyte solutions

Fortieth Anniversary of the October Revolution State University, Bashkir. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 2, pp. 174–177, March–April, 1988.