Homolytic dissociation in hydrogen-bonding liquids: energetics of the phenol O–H bond in methanol and the water O–H bond in water

The energetics of the phenol O–H bond in methanol and the water O–H bond in liquid water were investigated by microsolvation modelling and statistical mechanics Monte Carlo simulations. The microsolvation approach was based on density functional theory calculations. Optimised structures for clusters of phenol and the phenoxy radical with one and two methanol molecules are reported. By analysing the differential solvation of phenol and the phenoxy radical in methanol, we predict that the phenol O–H homolytic bond dissociation enthalpy in solution is 24.3±11 kJ/mol above the gas-phase value. The analysis of the water O–H bond dissociation by microsolvation was based on optimised structures of OH–(H₂O)_1–6 and –(H₂O)_1–7 clusters. Microsolvation modelling and statistical mechanics simulations predict that the HO–H bond dissociation enthalpies in the gas phase and in liquid water are very similar. Our results stress the importance of estimating the differences between the solvation enthalpies of the radical species and the parent molecule and the limitations of local models based on microsolvation.Proceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail     

Attachment of Hydrogen Molecules to Atomic Ions (Na+, Cl−): Examination of an Adiabatic Separation of the H2 Rotational Motion

Interactions between molecular hydrogen and ions are of interest in cluster science, astrochemistry and hydrogen storage. In dynamical simulations, H₂ molecules are usually modelled as point particles, an approximation that can fail for anisotropic interactions. Here, we apply an adiabatic separation of the H₂ rotational motion to build effective pseudoatom-ion potentials and in turn study the properties of (H₂)_nNa⁺/Cl⁻ clusters. These interaction potentials are based on high-level ab initio calculations and Improved Lennard-Jones parametrizations, while the subsequent dynamics has been performed by quantum Monte Carlo calculations. By comparisons with simulations explicitly describing the molecular rotations, it is concluded that the present adiabatic model is very adequate. Interestingly, we find differences in the cluster stabilities and coordination shells depending on the spin isomer considered (para- or ortho-H₂), especially for the anionic clusters.     

Can Water Clusters Ionize HCl under the Conditions of Arctic Stratosphere? 1. Intermolecular Interactions

Pseudopotential model was constructed to simulate the H₃O⁺(H₂O) _n Cl^– clusters at room and stratosphere temperatures using the Monte Carlo method. Numerical values of interaction parameters were restored from the experimental data on the free energy and entropy of vapor nucleation on ions in the combination with the data of quantum chemical calculations of the optimal configurations of HCl(H₂O) _n clusters. The stability of various cluster structures and the probability of the rupture of intramolecular HCl bond in these clusters were analyzed.     

Spatial delocalization in para-H2 clusters

We applied the quantum path integral Monte Carlo method for the study of (para-H)N (N = 5-33) clusters at T = 2 K, exploring static and dynamic order, which originates from the effects of zero-point energy, kinetic energy, and thermal fluctuations in quantum clusters. Information on dynamic structure was inferred from the asymptotic tails of the cage correlation function calculated from the centroid Monte Carlo trajectory. The centroid cage correlation function decays to zero for large clusters (N = 15-33), manifesting the interchange of molecules between different solvation shells, with statistically diminishing back interchange. Further evidence for the floppiness of para-hydrogen clusters emerges from the Monte Carlo evolution of the centroid of a tagged molecule, which exhibits significant changes in the list of its first and second solvation shells due to the interchange of molecules between these shells.     

Temperature dependence of the rate constant of ionic analogs of H + H2 → H2 + H reaction by quantum instanton method

Rate constant ratios k(T)/k(1,500K) for two symmetrical reactions H^? + H₂ → H₂ + H^? and H⁺ + H₂ → H₂ + H⁺ are reported. Direct method based on quantum instanton approximation for evaluation of the temperature dependence of the quantum‐mechanical reaction rate constant is used. Implementation of the theory involves thermodynamic integration and path integral Monte Carlo method. Results of anionic case shows resemblance to neutral case, whereas cationic case is significantly different and below 1,000K rate constant shows strong deviation form linearity of Arrhenius plot due to high activation barrier. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Structural and Infrared Spectroscopic Study on Solvation of Acetylene by Protonated Water Molecules

The effect of solvation on the conformation of acetylene has been studied by adding one water molecule at a time. Quantum chemical calculations of the H⁺(C₂H₂)(H₂O)_n (n=1-5) clusters indicate that the H₂O molecules prefer to form the OH…π interaction rather than the CH…O interaction. This solvation motif is different from that of neutral (C₂H₂)(H₂O)_n (n=1-4) clusters, in which the H₂O molecules prefer to form the CH…O and OH…C H-bonds. For the H⁺(C₂H₂)(H₂O)_n cationic clusters, the first solvation shell consists of one ring structure with two OH…π H-bonds and three water molecules, which is completed at n=4. Simulated infrared spectra reveal that vibrational frequencies of OH…π H-bonded O-H stretching afford a sensitive probe for exploring the solvation of acetylene by protonated water molecules. Infrared spectra of the H⁺(C₂H₂)(H₂O)_n(n=1-5) clusters could be readily measured by the infrared photodissociation technique and thus provide useful information for the understanding of solvation processes.     

Hydration of Fe+: A Monte Carlo simulation of water clusters and of a dilute aqueous solution

Monte Carlo statistical thermodynamic computer simulations are reported for several clusters Fe⁺ (H₂O)_n at different temperatures and for a dilute aqueous solution of Fe⁺ at 298 K. The energy of each configuration has been calculated in the pairwise additivity approximation using the MCY potential for the water–water interaction and an ab initio analytical potential built by us for the Fe⁺–H₂O interaction. Energy and structural analysis of the generated configurations lead to the prediction of a coordination number of six for the first hydration shell of the Fe⁺ ion, both in clusters and in dilute solution. Finally, the variation in the distance to the Fe⁺ ion of the energy and orientation of water molecules in the solution are discussed.     

Clustering and activation in reactions of CoCp with hydrogen and methane

Gas-phase clustering reactions of CoCp⁺ with H₂ and with CH₄ were investigated using temperature-dependent equilibrium experiments. In both systems, the CoCp⁺ ion was found to form strong interactions with two ligands. The first and second H₂ groups cluster to CoCp⁺ with bond energies of 16.2 and 16.8 kcal/mol, respectively, while the first and second CH₄ groups cluster to CoCp⁺ with bond energies of 24.1 and 12.1 kcal/mol, respectively. These bond energies are in good agreement with those determined by density functional theory (DFT). Molecular geometries for the four clusters determined with DFT are also presented. Weak experimental bond energies of 0.9 kcal/mol for the third H₂ and 2.2 kcal/mol for the third CH₄ clustering to CoCp⁺ suggest these ligands occupy the second solvation shell of the ion. In addition to clustering in the methane system, H₂-elimination from CoCp(CH₄)₂⁺ was observed. The mechanism for this reaction was investigated by collision-induced dissociation experiments and DFT, which suggest the predominate H₂-elimination product is (c-C₅H₆)Co⁺---C₂H₅. Theory indicates that dehydrogenation requires the active participation of the Cp ring in the mechanism. Transfer of H and CH₃ groups to the C₅-ring ligand allows the metal center to avoid the high-energy Co(IV) oxidation state required when it forms two covalent bonds in addition to its interaction with a C₅-ring ligand.     

Determination of the acid dissociation constants of p-benzohydroquinone by the INDO method

The stepwise acid dissociation constants for p-benzohydroquinone (QH₂) in aqueous media have been explicitly calculated for the first time, with the INDO parametrized SCF –MO method. We have optimized the geometries of QH₂, QH^?, and Q^2? and of the QH₂ · 6H₂O, QH^? · (H₃O⁺) · 5H₂O, and Q^2? · (H₃O⁺)₂ · 4H₂O systems that model the solvated species. The presence of the associated water molecules (and hydronium ions) account for the stabilization due to hydrogen bonding as well as for a part of the effect of interaction of these molecules with the respective reaction fields in an aqueous medium. To simulate the first solvation shell in a more complete manner, four more water molecules have been considered to be placed above and below the quinonoid ring and the optimized geometries of the resulting hydrated species, QH₂ · 10H₂O, QH^? · (H₃O⁺) · 9H₂O, and QH^? · (H₃O⁺) · 8H₂O, have been determined. The standard free-energy changes calculated for the dissociation of QH₂ into QH^? and H⁺ is 0.0251 Hartree (65.9 kJ mol^?1) and that of QH^? into Q^2? and H⁺ is 0.0285 Hartree (74.8 kJ mol^?1). Experimentally observed dissociation constants for these two steps correspond to free-energy changes of 0.0214 Hartree (56.2 kJ mol^?1) and 0.0248 Hartree (65.1 kJ mol^?1), respectively. © 1995 John Wiley & Sons, Inc.     

Rate constants of atomic hydrogen formation in H3O+(H2O) n + e → H + (H2O) n gas-phase processes

Using the Maxwellian electron velocity distribution and the Breit-Wigner approximation of the reaction cross section, the kinetic parameters of the hydrogen atom formation upon the electron capture by positively charged hydronium-water clusters are estimated. Calculations of the cross sections and rate constants are based on the data of quantum chemical studies of H₃O⁺(H₂O) _n and H₃O(H₂O) _n clusters, particularly on the detailed analysis of the spacing of high-lying states of the radicals and the character of the unpaired electron density distribution, as well as on the general trend in the electron affinity change of the cations depending on the number of water molecules. The lifetimes of the radicals before the dissociation are taken from the classical nonempirical molecular dynamics runs. The results are compared to available experimental data. The article is published in the original.     

Thermal Responsive Ion Selectivity of Uranyl Peroxide Nanocages: An Inorganic Mimic of K+ Ion Channels

An actinyl peroxide cage cluster, Li_48+mK₁₂(OH)_m[UO₂(O₂)(OH)]₆₀ (H₂O)_n (m≈20 and n≈310; U₆₀), discriminates precisely between Na⁺ and K⁺ ions when heated to certain temperatures, a most essential feature for K⁺ selective filters. The U₆₀ clusters demonstrate several other features in common with K⁺ ion channels, including passive transport of K⁺ ions, a high flux rate, and the dehydration of U₆₀ and K⁺ ions. These qualities make U₆₀ (a pure inorganic cluster) a promising ion channel mimic in an aqueous environment. Laser light scattering (LLS) and isothermal titration calorimetry (ITC) studies revealed that the tailorable ion selectivity of U₆₀ clusters is a result of the thermal responsiveness of the U₆₀ hydration shells.     

Structural peculiarities of particle solvation inN,N-dimethylformamide

Monte Carlo computer simulation of infinitely dilute solutions of Cs⁺ and I⁻ ions and Xe atoms inN,N-dimethylformamide (DMF) was performed. Peculiarities of intermolecular interactions and structural properties of the solutions were investigated. The solvation enthalpies of cations and anions are close, but the energies of the ion-solvent and solventsolvent interactions differ appreciably. The solute particles affect the energy and structural properties of DMF only in spatially localized domains. The second solvation shell of the solute particles was not structurally allocated. Two structurally and energetically differing states of DMF molecules (with normal and anomalous orientation of the dipole moments) exist in solution of Cs⁺ cations. The anomalous orientation of DMF molecules is due to the solvation steric effect, packing, and kinematic factors. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 584–596, April, 2000.     

Enthalpies of solvation for dopamine hydrochloride in water-ethanol solutions

The enthalpies of dissolution of dopamine hydrochloride (H₂Dop · HCl) in water-ethanol solvents containing from 0 to 0.8 mole fraction of ethanol are measured by calorimetry at 298.15 K. Standard enthalpies of transfer (??_tr H ^°) for the molecular (H₂Dop) and cationic (H₃Dop⁺) forms of dopamine from water into binary solvents are calculated from the obtained data. The enthalpies of transfer of H₃Dop⁺ cation are determined from the enthalpies of dissolution of H₂Dop · HCl using the familiar method of separating the molar quantities into ionic contributions (Ph₄P⁺ = BPh ₄ ^? ), and by an original alternative procedure. The effect of the composition of the binary solvent on the solvation of dopamine is considered.     

Dissociative Photoionization of m-Xylene

The photoionization and dissociative photoionization of m-xylene (C₈H₁₀) were researched by using synchrotron radiation vacuum ultraviolet (SR-VUV) and supersonic expanding molecular beam reflectron time-of-flight mass spectrometer (RFTOF-MS) system. The photoionization efficiency spectra (PIEs) of parent ion C₈H₁₀⁺ and main fragment ions C₈H₉⁺ and C₇H₇⁺ were observed, and the ionization energy (IE) of m-xylene and appearance energies (AEs) of main fragment ions C₈H₉⁺ and C₇H₇⁺ were determined to be 8.60 ± 0.03 eV, 11.76 ± 0.04 eV and 11.85 ± 0.05 eV, respectively. Structures of reactant, transition states (TSs), intermediates (INTs), and products involved in two dominant dissociation channels were optimized at the B3LYP/6-311++G(d,p) level, and the relative energies were calculated at the G3 level. Based on the results, two major dissociative photoionization channels, C₇H₇⁺+CH₃ and C₈H₉⁺+H were calculated at the B3LYP/6-311++G(d,p) level. On the basis of theoretical and experimental results, the dissociative photoionization mechanisms of m-xylene were proposed. The C–H or C–C bond dissociation and hydrogen migration are the main processes in the dissociation channels of m-xylene cation.

     

Solvent effects on the electronic absorption spectrum of formamide studied by a sequential Monte Carlo/quantum mechanical approach

Sequential Monte Carlo/quantum mechanical calculations are performed to study the solvent effects on the electronic absorption spectrum of formamide (FMA) in aqueous solution, varying from hydrogen bonds to the outer solvation shells. Full quantum-mechanical intermediate neglect of differential overlap/singly excited configuration interaction calculations are performed in the supermolecular structures generated by the Monte Carlo simulation. The largest calculation involves the ensemble average of 75 statistically uncorrelated quantum mechanical results obtained with the FMA solute surrounded by 150 water solvent molecules. We find that the n → π* transition suffers a blueshift of 1,600 cm⁻¹ upon solvation and the π → π* transition undergoes a redshift of 800 cm⁻¹. On average, 1.5 hydrogen bonds are formed between FMA and water and these contribute with about 20% and about 30% of the total solvation shifts of the n → π* and π → π* transitions, respectively. The autocorrelation function of the energy is used to sample configurations from the Monte Carlo simulation, and the solvation shifts are shown to be converged values. Received: 14 March 2002 / Accepted: 3 April 2002 / Published online: 24 June 2002     

The structure of the hydration shell of the ionized HCl molecule in water vapor

The H₃O⁺(H₂O) _n Cl^? clusters were simulated by the Monte Carlo method in a grand canonical ensemble in thermal and material contact with water vapor under the conditions close to the natural conditions in the stratosphere. A detailed model including nonpair polarization and covalent interactions was used. The correlation functions, density distributions, and free energy and entropy as functions of the interionic distance were calculated. The mechanism of ionized HCl state stabilization was determined by the formation of a special structure of the hydrate cluster component with low Gibbs energy and entropy.     

Tandem mass spectrometry studies of acetone and acetone/water cluster ions

Ion clusters were formed in a temperature-variable high-pressure ion source from neat acetone and acetone/water mixtures and subjected to tandem mass spectrometry studies-unimolecular and collisionally activated mass-analyzed ion kinetic energy spectroscopy. The predominance of water loss from H⁺(H₂0)(A) _l=3, where A = acetone, suggests that the solvation sphere around H₃O⁺ does not close at l = 3, contrary to the case of acetonitrile or dimethyl ether. The results may be interpreted in terms of suggested ion structures which involve isomerization enroute to dissociation. The virtual absence of H/D scrambling in the collisionally activated dissociation of H₃O⁺(DA)₃, DA =acetone-d ₆, and of D₃O⁺(A)₃ means that if enolization takes place, it is a rate-determining step in an irreversible isomerization. The stability of H⁺(H₂O)(A)₃ is a dominant factor in the observation of acetone loss from H⁺(H₂0)(A)₄.     

Monte Carlo bicanonical ensemble simulation for sodium cation hydration free energy in liquid water

A series of bicanonical ensemble Monte Carlo (BC MC) simulations has been performed to calculate Na⁺ hydration Gibbs energy in aqueous solution. The hydration Gibbs energy of Na⁺ ion in aqueous solution is the difference between formation free energies of Na⁺ (H₂O)_n and (H₂O)_n clusters at n → α. The convergence of the hydration free energy to bulk water value is fast, and the results at n = 60 turned out to be in good agreement with experimental ones and those calculated using free energy perturbation method [1]. The ion–water interaction has been described by Aqvist's pair potential [1] and SPC model [2] has been used for water–water interactions. The behaviour of the absolute Gibbs energy, the entropy, the internal energy of the clusters and the development of hydration shells’ structure with the increase of the number of water molecules are discussed.     

The σ‐Aromatic Clusters [Zn3]+ and [Zn2Cu]: Embryonic Brass

The triangular clusters [Zn₃Cp*₃]⁺ and [Zn₂CuCp*₃] were obtained by addition of the in situ generated, electrophilic, and isolobal species [ZnCp*]⁺ and [CuCp*] to Carmona’s compound, [Cp*Zn? ZnCp*], without splitting the Zn? Zn bond. The choice of non‐coordinating fluoroaromatic solvents was crucial. The bonding situations of the all‐hydrocarbon‐ligand‐protected clusters were investigated by quantum chemical calculations revealing a high degree of σ‐aromaticity similar to the triatomic hydrogen ion [H₃]⁺. The new species serve as molecular building units of Cu_nZn_m nanobrass clusters as indicated by LIFDI mass spectrometry.