Solvation of formic acid and proton transfer in hydrated clusters

Hydrated Formic Acid Clusters and their Interaction with Electrons

We investigate ion formation in hydrated formic acid (FA) clusters upon collision with electrons of variable energy, focusing on electron ionization at 70 eV (EI) and low-energy (1.5-15 eV) electron attachment (EA). To uncover details about the composition of neutral clusters, we aim to elucidate the ion formation processes in FAM ⋅ WN clusters initiated by interaction with electrons and determine the extent of cluster fragmentation. EI predominantly produces protonated [FAm+H]+ ions, and in FA-rich clusters, the stable ring structures surrounding H3O+ ions are formed. In contrast, EA leads to a competition between the formation of intact [FAm ⋅ Wn]- and dissociated [FAm ⋅ Wn-H]- fragment ions, influenced by the cluster size, level of hydration, and electron energy. Our findings reveal a predisposition of low-energy EA towards forming [FAm ⋅ Wn]-, while higher electron energies tend to favor the formation of [FAm ⋅ Wn-H]- due to intracluster ion-molecule reactions. The comparison of positive and negative ion spectra suggests that the mass spectra of FA-rich clusters may indicate their actual size and composition. On the other hand, the more weakly bound water evaporation from the clusters depends strongly on the ionization. Thus, for the hydrated clusters, the neutral cluster size can hardly be estimated from the mass spectra.

New Insights into the Pt-Catalyzed CH3OH Oxidation Mechanism: First-Principle Considerations on Thermodynamics, Kinetics, and Reversible Potentials

A systematic first-principle study of CH₃OH oxidation along indirect and direct pathways on Pt(111) has been carried out, and some new insights into CH₃OH oxidation pathways in direct CH₃OH fuel cells (DMFCs) are presented. The thermodynamics, kinetics, and reversible potentials for all possible elementary steps, initializing with C-H, O-H, and C-O bond cleavages and proceeding via sequential decomposition and oxidation from the reaction intermediates, are analyzed. Some key reactive intermediates are identified. By comparing the activation energies and reversible potentials of various possible elementary reaction steps, we can speculate that the initial CH₃OH oxidation step proceeds by the CH₃O intermediate under a nonelectrochemical environment, whereas it prefers to occur by the CH₂OH intermediate under electrochemical environment. Furthermore, CHO hydroxylation into HCOOH along a direct pathway is more facile to occur than CHO dehydrogenation into CO along an indirect pathway at the nonelectrochemical interface, whereas the indirect and direct pathways may be parallel pathways on Pt(111) under the present simulated electrochemical environment. Simultaneously, CH₃ can be easily formed through C-O bond cleavage in CH₃OH, which is a nonelectrochemical step. Thus, the CH _x (x = 0-3) species is possibly formed on Pt(111) during CH₃OH oxidation regardless of being under an electrochemical or nonelectrochemical environment. The adsorbed CH _x species will result in the blocking of the active sites and the prevention of further CH₃OH oxidation. Our present findings on the formation of carbonaceous deposits on Pt(111) are consistent with the experimentally observed C-O bond scission of CH₃OH into CH _x species. Thus, we propose that the adsorbed residues that poisoned the Pt surface and impeded the performance of DMFCs may be CH _x species, rather than CO species, since the direct pathway is more favorable on Pt(111) at the nonelectrochemical interface. However, the poisonous species that occupied the active sites of the Pt surface may be CH _x and CO species due to the simultaneous occurrence of oxidation pathways on Pt(111) under the present simulated electrochemical environment. Based on the present study, some new insights into CH₃OH oxidation mechanisms and designing strategies of Pt-based alloy catalysts for CH₃OH oxidation can be provided.

Quantum free energy landscapes from ab initio path integral metadynamics: Double proton transfer in the formic acid dimer is concerted but not correlated

With the goal of computing quantum free energy landscapes of reactive (bio)chemical systems in multi-dimensional space, we combine the metadynamics technique for sampling potential energy surfaces with the ab initio path integral approach to treating nuclear quantum motion. This unified method is applied to the double proton transfer process in the formic acid dimer (FAD), in order to study the nuclear quantum effects at finite temperatures without imposing a one-dimensional reaction coordinate or reducing the dimensionality. Importantly, the ab initio path integral metadynamics technique allows one to treat the hydrogen bonds and concomitant proton transfers in FAD strictly independently and thus provides direct access to the much discussed issue of whether the double proton transfer proceeds via a stepwise or concerted mechanism. The quantum free energy landscape we compute for this H-bonded molecular complex reveals that the two protons move in a concerted fashion from initial to product state, yet world-line analysis of the quantum correlations demonstrates that the protons are as quantum-uncorrelated at the transition state as they are when close to the equilibrium structure.

Calorimetric and spectroscopic studies on solvation energetics for H2storage in the CO2/HCOOH system

The role of solvent interactions in H₂-storage/delivery in the carbon dioxide–formic acid couple.

Influence of temperature on molecular interactions of imidazolium-based ionic liquids with acetophenone: thermodynamic properties and quantum chemical studies

The thermophysical properties of a series of alkyl imidazolium-based ionic liquids with acetophenone over the wide range of composition and at (293.15, 303.15, 313.15, 323.5 and 333.15) K under atmospheric pressure is reported in this study.

Microsolvation of the formic acid dimer — (HCOOH)2(H2O)n clusters with n = 1, . . ., 5

Density functional theory and spin-component-scaled Møller–Plesset perturbation theory calculations are used to examine the microsolvation of the formic acid dimer. The lowest energy structures with n water molecules consist of a n-water cluster, not necessarily of lowest energy, with two formic acid molecules attached to its surface by hydrogen bonds. The total number of hydrogen bonds does not correlate directly with relative stability.

A first principles theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions: D2O in hydration shells of Cl(-) ions

A theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions is presented from first principles without employing any empirical potential models. The present calculations are based on ab initio molecular dynamics for trajectory generation and wavelet analysis of the simulated trajectories for time dependent frequency calculations. Results are obtained for two different deuterated aqueous solutions: the first one is a relatively dilute solution of a single Cl(-) ion and the second one is a concentrated solution of NaCl ( approximately 3M) dissolved in liquid D(2)O. It is found that the frequencies of OD bonds in the anion hydration shell, i.e., those which are hydrogen bonded to the chloride ion, have a higher stretch frequency than those in the bulk water. Also, on average, the frequencies of hydration shell OD modes are found to increase with increase in the anion-water hydrogen bond distance. On the dynamical side, when the vibrational spectral diffusion is calculated exclusively for the hydration shell water molecules in the first solution, the dynamics reveals three time scales: a short-time relaxation ( approximately 200 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation ( approximately 3 ps) corresponding to the lifetimes of chloride ion-water hydrogen bonds, and another longer-time constant ( approximately 20 ps) corresponding to the escape dynamics of water from the anion hydration shell. Existence of such three time scales for hydration shell water molecules was also reported earlier for water containing a single iodide ion using classical molecular dynamics [B. Nigro et al., J. Phys. Chem. A 110, 11237 (2006)]. Hence, the present study confirms the basic results of this earlier work using a different methodology. However, when the vibrational spectral diffusion is calculated over all the OD modes, only two time scales of approximately 150 fs and approximately 2.7 ps are found without the slowest component of approximately 20 ps. This is likely because of the very small weight that the hydration shell water molecules carry to the overall spectral diffusion in the solution containing a single ion. For the concentrated solution also, the slowest component of approximately 20 ps is not found in the spectral diffusion of all water molecules because a distinct separation between the hydration shell and bulk water in terms of their stretch frequencies does not hold at this high concentration regime. The present first principles results are compared with those of the available experiments and classical simulations.

High-Resolution Microwave Studies of Ring-Structured Complexes between Trifluoroacetic Acid and Water

The rotational spectra of the complexes between one trifluoroacetic acid molecule and up to three water molecules have been recorded using a pulsed nozzle Fourier transform microwave spectrometer. The unambiguous assignments of them are made on the basis of the agreement between the experimentally determined rotational constants and the theoretical predictions from ab initio calculations using MP2/6-311++G(2df,2pd). All the complexes exhibit hydrogen-bonded ring structures. The fine splittings observed in some of the a-type transitions of the trifluoroacetic acid-H2O dimer were analyzed in terms of the likely tunneling motions of the hydrogens in the H2O molecule. Further calculations of the equilibrium constants for these three hydrated complexes of trifluoroacetic acid were also made to evaluate their fractions against the trifluoroacetic acid monomer in the atmosphere.

Deprotonation of solvated formic acid: Car-Parrinello and metadynamics simulations

The deprotonation of solvated formic acid was investigated theoretically with ab initio simulations. With the Car-Parrinello method, deprotonation and reprotonation by means of a proton wire were observed. The microscopics of these reactions were analyzed, and reveal the key role played by nearby water molecules in catalyzing the reactions. A constrained molecular dynamics calculation was carried out to estimate the dissociation free energy. Deprotonation of formic acid was further investigated with the recently developed metadynamics method using the formic acid oxygen coordination numbers as the collective variables. The determined free-energy landscape gives barriers similar to that obtained with the constrained free-energy calculation.

Does the most stable formic acid tetramer have π stacking or C–H⋯O interactions?

Density functional theory (DFT), Moller-Plesset (MP) perturbation theory, and coupled-cluster calculations are used to examine low-energy minima on the potential energy surface of the formic acid tetramer (HCOOH)(4). The potential energy surface is rather flat with respect to rotation of one of the dimers, relative to the other dimer in an aligned stack, about the axis passing through the inversion centers of the dimers. Our best calculations suggest that an aligned pi-pi stack of two dimers is very likely to be the global minimum but there are two other pi-pi stacks within 0.5 kcal /mol. Moreover, a fourth pi-pi stack, a planar association of two dimers held together by C-H...O interactions, and a bowl structure all lie within 1 kcal /mol of the lowest-energy structure.

New theoretical insight into the interactions and properties of formic acid: Development of a quantum-based pair potential for formic acid

We performed ab initio quantum-chemical studies for the development of intra- and intermolecular interaction potentials for formic acid for use in molecular-dynamics simulations of formic acid molecular crystal. The formic acid structures considered in the ab initio studies include both the cis and trans monomers which are the conformers that have been postulated as part of chains constituting liquid and crystal phases under extreme conditions. Although the cis to trans transformation is not energetically favored, the trans isomer was found as a component of stable gas-phase species. Our decomposition scheme for the interaction energy indicates that the hydrogen-bonded complexes are dominated by the Hartree-Fock forces while parallel clusters are stabilized by the electron correlation energy. The calculated three-body and higher interactions are found to be negligible, thus rationalizing the development of an atom-atom pair potential for formic acid based on high-level ab initio calculations of small formic acid clusters. Here we present an atom-atom pair potential that includes both intra- and inter molecular degrees of freedom for formic acid. The newly developed pair potential is used to examine formic acid in the condensed phase via molecular-dynamics simulations. The isothermal compression under hydrostatic pressure obtained from molecular-dynamics simulations is in good agreement with experiment. Further, the calculated equilibrium melting temperature is found to be in good agreement with experiment.

Interaction of formic acid with solid water

The interaction of formic acid (HCOOH) with solid water, deposited on tungsten at 80 K, was investigated. We have prepared and annealed formic acid (FA)/water interfaces (FA layers on thin films of solid water and H(2)O adlayers on thin FA films). Metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (He I and II) were utilized to study the electron emission from the 10a' to 6a' molecular orbitals (MOs) of FA, and the 1b(1), 3a(1), and 1b(2) MOs of H(2)O. These spectra were compared with results of density-functional theory calculations on FA-H(2)O complexes reported in Ref. 14 [A. Allouche, J. Chem. Phys. 122, 234703(2005), (preceding paper)]. Temperature programmed desorption was applied for information on the desorption kinetics. Initially, FA is adsorbed on top of the water film. The FA spectra are distorted with respect to those from FA monomers; it is concluded that a strong interaction exists between the adsorbates. Even though partial solvation of FA species takes place during annealing, FA remains in the top layer up to the desorption of the water film. When H(2)O molecules are offered to FA films at 80 K, no water network is formed during the initial stage of water exposure; H(2)O molecules interact individually via H bonds with the formic acid network. Experiment and theory agree that no water-induced deprotonation of the formic acid molecules takes place.

Quantum studies of hydrogen bonding in formic acid and water ice surface

The structure and spectroscopy (electronic and vibrational) of formic acid (HCOOH) dimers and trimers are investigated by means of the hybrid (B3LYP) density-functional theory. Adsorption of single and dimer HCOOH on amorphous water ice surface is modeled using two different water clusters. Particular attention has been given to spectroscopic consequences. Several hypotheses on formic acid film growing on ice and incorporation of a single water molecule in the formic acid film are proposed.

A comparative ab initio study of intramolecular proton transfer in model alpha-hydroxyalkoxides

A comparative ab initio study was performed on the intramolecular proton-transfer reaction that occurs in alpha-hydroxyethanoxy, alpha-hydroxyphenoxide, and alpha-hydroxyethenoxy anions. The intramolecular proton transfer occurs in a five-member atom arrangement, between two oxygen atoms separated by a carbon-carbon bond. The chosen systems serve as models for alpha-hydroxyalkoxide molecules where the carbon-carbon bond varies from a single bond (the glycolate anion or alpha-hydroxyethanoxide anion) to a part of an aromatic ring (the alpha-hydroxyphenoxide anion), and finally to a double bond (the alpha-hydroxyethenoxide anion). Particular attention was given to the evolution along the intrinsic reaction coordinate of such properties as energies, relevant structural parameters, Mulliken charges, dipole moments, and 1H-NMR chemical shifts to reveal the similarities and differences for the proton transfer in the model systems.

Matrix isolation infrared and ab initio study of the hydrogen bonding between formic acid and water

The infrared spectra of the formic acid-water complexes isolated in argon matrices are reported. Both supersonic jet expansion and a conventional effusive source followed by trapping in solid argon at 10K are used to obtain the matrices. The experimental IR spectra are compared to the data obtained from high level ab initio (MP2) and DFT (B3LYP) calculations with 6-311++G(d,p) and aug-cc-pVTZ basis sets. The complex formation results in red shifts in the C=O and O-H stretching vibrations and a blue shift in the C-O stretching vibration of formic acid. The O-H stretching modes of water also exhibit pronounced red shifts. Both the MP2 and B3LYP calculations located three minima corresponding to cyclic HCOOH...H2O complexes with two hydrogen bond interactions. The binding energies are -10.3, -5.1, and -3.5 kcal mol(-1), respectively, for the three complexes at the MP2/ aug-cc-pVTZ level, corrected for the basis set superposition error (BSSE) using the Boys-Bernardi counterpoise scheme. Comparison of the calculated frequencies of the three complexes with the matrix IR spectrum reveals that the lowest energy complex is formed. In addition, a complex of formic acid with two water molecules is observed.