Theoretical Studies on the Hydrogen Bonding Interaction of Complexes of Formic Acid with Water

Solubility of CO2 in Aqueous Formic Acid Solutions and the Effect of NaCl Addition: A Molecular Simulation Study

There is a growing interest in the development of routes to produce formic acid from CO₂, such as the electrochemical reduction of CO₂ to formic acid. The solubility of CO₂ in the electrolyte influences the production rate of formic acid. Here, the dependence of the CO₂ solubility in aqueous HCOOH solutions with electrolytes on the composition and the NaCl concentration was studied by Continuous Fractional Component Monte Carlo simulations at 298.15 K and 1 bar. The chemical potentials of CO₂, H₂O, and HCOOH were obtained directly from single simulations, enabling the calculation of Henry coefficients and subsequently considering salting in or salting out effects. As the force fields for HCOOH and H₂O may not be compatible due to the presence of strong hydrogen bonds, the Gibbs-Duhem integration test was used to test this compatibility. The combination of the OPLS/AA force field with a new set of parameters, in combination with the SPC/E force field for water, was selected. It was found that the solubility of CO₂ decreases with increasing NaCl concentration in the solution and increases with the increase of HCOOH concentration. This continues up to a certain concentration of HCOOH in the solution, after which the CO₂ solubility is high and the NaCl concentration has no significant effect.

Quantum vibration perturbation approach with polyatomic probe in simulating infrared spectra

The quantitative prediction of vibrational spectra of chromophore molecules in solution is challenging and numerous methods have been developed. In this work, we present a quantum vibration perturbation (QVP) approach, which is a procedure that combines molecular quantum vibration and molecular dynamics with perturbation theory. In this framework, an initial Newtonian molecular dynamics simulation is performed, followed by a substitution process to embed molecular quantum vibrational wave functions into the trajectory. The instantaneous vibrational frequency shift at each time step is calculated using the Rayleigh-Schrödinger perturbation theory, where the perturbation operator is the difference in the vibrational potential between the reference chromophore and the perturbed chromophore in the environment. Semi-classical statistical mechanics is employed to obtain the spectral lineshape function. We validated our method using HCOOH·nH₂O (n = 1-2) clusters and HCOOH aqueous solution as examples. The QVP method can be employed for rapid prediction of the vibrational spectrum of a specific mode in solution.

High level ab initio investigation of the catalytic effect of water on formic acid decomposition and isomerization

Formic acid (FA) is a ubiquitous molecule found in the atmosphere, and is relevant to many important processes. The FA molecule generally exists as the trans isomer, which can decompose into H2O and CO (dehydration). It can also exist in the less favorable cis isomer which can decompose into H2 and CO2 (decarboxylation). Our work examines the complexes formed between each isomer of FA with water. We present geometries and vibrational frequencies obtained at the reliable CCSD(T)/aug-cc-pVTZ level of theory for seven FAwater complexes. We utilize the focal point method to determine CCSDT(Q)/CBS plus corrections binding energies of 7.37, 3.36, and 2.02 kcal mol-1 plus 6.07, 3.79, 2.60, and 2.55 kcal mol-1 for the trans-FAwater and cis-FAwater complexes, respectively. Natural bond orbital analysis is used to further decompose the interactions in each complex and gain insight into their relative strengths. Furthermore, we examine the effect that a single water molecule has on the barrier heights to each decomposition pathway by optimizing the transition states and verifying their connectivity with intrinsic reaction coordinate computations as well as utilizing a kinetic model. Water lowers the barrier to dehydration by at most 15.78 kcal mol-1 and the barrier to decarboxylation by up to 15.90 kcal mol-1. Our research also examines for the first time the effect of one water molecule on the interconversion barrier and we find that the barrier from trans to cis is not catalyzed by water due to the strong FA and water interactions. Our results highlight some instances where different binary complexes result in different decomposition pathways and even a case where one binary complex can form the same decomposition products via two distinct mechanisms. Our results provide a reliable benchmark of the FAH2O system as well as provide insight into future studies of similar atmospheric systems.

Carboxylic acids in aqueous solutions: Hydrogen bonds, hydrophobic effects, concentration fluctuations, ionization, and catalysis

In the frequency range between 100 kHz and 2 GHz, ultrasonic absorption spectra have been measured for a series of carboxylic acids from formic to enanthic acid, including constitutional isomers. Also investigated have been the spectra for mixtures with water of short-chain formic, acetic, propionic, butyric, and isobutyric acid, in each case covering the complete composition range. The neat carboxylic acids feature two Debye-type relaxation terms with relaxation times between 5.6 and 260 ns as well as 0.14 and 1.4 ns, respectively, at room temperature. Depending on the composition, mixtures with water reveal an additional Debye relaxation term in the intermediate frequency range (acetic acid) or a term subject to a relaxation time distribution (propionic, butyric, and isobutyric acid). The relaxations of the neat acids are assigned to the equilibrium between monomers and single-hydrogen-bonded linear dimers and between linear and twofold-hydrogen-bonded cyclic dimers. The latter equilibrium is considerably catalyzed by hydronium and carboxylate ions. Several mixtures with water indicate one of the up to three Debye relaxations to reflect the protolysis of the organic acid. The term with underlying relaxation time distribution is due to noncritical fluctuations in the local concentrations. The Debye relaxations are evaluated to yield the parameters of the relevant elementary chemical reactions, such as the rate and equilibrium constants and the isentropic reaction volumes. A comparison of the correlation length of concentration fluctuations with data for other aqueous systems confirms the idea that the hydrophobic part of the organic constituent promotes the formation of a micro-heterogeneous liquid structure, whereas the hydrophilic moiety is of minor importance in this respect. The high-frequency limiting absorption suggests the equilibrium between conformers of linear dimers to contribute to the spectra well above the frequency range of measurements.

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.

Matrix-isolation studies of noncovalent interactions: more sophisticated approaches

Noncovalent interactions are crucial for many physical, chemical, and biological phenomena. Matrix isolation is a powerful method to study noncovalent interactions, including hydrogen-bonded species, and it has been extensively used in this field. However, there are difficult situations, such as in the case of species that are impossible to prepare in the gas phase. In this article, we describe some advanced approaches allowing studies of complexes that are problematic for the traditional methods. Photolysis of a suitable precursor in a matrix can lead to a large concentration of 1:1 complexes, which are otherwise very difficult to prepare (e.g., the H2O···O complex). Photolysis of species combined with annealing can lead to complexes of molecules with mobile atoms (e.g., the same H2O···O complex). Simultaneous photolysis of two species combined with annealing can produce complexes of radicals via reactions of the photogenerated complexes with mobile atoms (e.g., the H2O···HCO complex). Interaction of noble-gas (Ng) hydrides with other species is another topic (e.g., the N2···HArF complex) and very large blue shifts of the H-Ng stretching modes are normally observed for these systems. Complexes and dimers of the higher-energy conformer of formic acid have been prepared by using selective vibrational excitation of the ground-state conformer. The higher-energy conformer of formic acid can be efficiently stabilized in the complexes with strong hydrogen bonding. We also consider some problematic cases when the changes in the vibrational frequencies of the 1:1 complexes are very small (e.g., the phenol···Xe complex) and when the complex formation is prevented by strong solvation in the matrix (e.g., species in solid xenon).

Predicted organic compounds derived from rare gas atoms and formic acid

Organic insertion compounds of rare gas atoms into formic acid were investigated at the MP2(full)/aug-cc-pVTZ level. There are two configuration isomers for each molecule based on the location of H atoms: trans- and cis-HCOORgH (Rg = Ar, Kr, Xe). Their structures, harmonic frequencies, and decomposition energies have been calculated using the above ab initio method. Using trans-HCOOXeH as an example, natural bond orbital (NBO) and atom-in-molecules (AIM) analyses were also carried out to explore the binding nature of the rare gas atoms. The formation mechanism of molecular orbitals is also presented in this paper. The presented results indicate that HCOOXeH and HCOOKrH are potential candidates for experimental observation.

AN AB INITIO MOLECULAR DYNAMICS STUDY ON THE SOLVATION OF FORMATE ION AND FORMIC ACID IN WATER

Formate ion and formic acid are linked in water by the equilibrium for the acidic dissociation of formic acid, which as the simplest carboxylic acid is an important model system. In this study, the microscopic details of the solvation around a formate ion and around a formic acid molecule in aqueous solution are explored by ab initio molecular dynamics simulations, at 300, 500, 700, and 900 K. The formate ion exerts a strong influence on the surrounding solvent molecules by hydrogen bonding, which restricts the access of other water molecules. With rising temperature, the hydrogen bonds are disrupted, and the space around formic acid becomes more accessible. Solvation of the formic acid is marked by its partial dissociation to produce a proton, and the hydrogen bond interaction around a formic acid is not as strong as that around a formate ion. The acidic dissociation becomes less favorable as temperature rises, which indicates a lesser catalytic role for the water molecules in the thermal dissociation of formic acid.

Electronic Structures of Formic Acid (HCOOH) and Formate (HCOO(-)) in Aqueous Solutions

The electronic structures of formic acid (HCOOH) and formate (HCOO(-)) have been determined in aqueous solutions over a pH range of 1.88-8.87 using a combination of X-ray photoelectron spectroscopy (XPS), partial electron-yield X-ray absorption spectroscopy (PEY XAS), and density functional theory (DFT). The carbon 1s XPS measurements reveal a binding energy shift of -1.3 eV for deprotonated HCOO(-) compared with neutral HCOOH. Such distinction between neutral HCOOH and deprotonated HCOO(-) cannot be made based solely on the respective carbon K-edge PEY XA spectra. Independent of pH, the C1s → π* state excitations occur at 288.0 eV and may lead to the incorrect conclusion that the energy levels of the π* state are the same for both species. The DFT calculations are consistent with the experimental observations and show a shift to higher energy for both the occupied C1s (lower binding energy) and unoccupied π* orbitals of deprotonated HCOO(-) compared to neutral HCOOH in aqueous solutions.

Can hydridic-to-protonic hydrogen bonds catalyze hydride transfers in biological systems?

Catalysis of hydride transfer by hydridic-to-protonic hydrogen (HHH) bonding in α-hydroxy carbonyl isomerization reactions was examined computationally in the lithium salts of 7-substituted endo-3-hydroxybicyclo[2.2.1]hept-5-en-2-ones. The barrier for intramolecular hydride transfer in the parent system was calculated to be 17.2 kcal/mol. Traditional proton donors, such as OH and NH(3)(+), stabilized the metal cation-bridged transition state by 1.4 and 3.3 kcal/mol, respectively. Moreover, among the conformers of the OH systems, the one in which the proton donor is able to interact with the migrating hydride (H(m)) has an activation barrier lower by 1.3 and 1.7 kcal/mol than the other possible OH conformers. By contrast, the presence of an electronegative group such as F, which disfavors the migration electronically by opposing development of hydridic charge, destabilizes the hydride migration by 1.5 kcal/mol relative to the epimeric exo system. In both ground and transition states the H(m)···H distance decreased with increasing acidity of the proton donor, reaching a minimum of 1.58 Å at the transition state for NH(3)(+). Both Mulliken and NPA charges show enhancement of negative character of the migrating hydride in the cases in which HHH bonding is possible.

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.

Structure and stability of salicylic acid-water complexes and the effect of molecular hydration on the spectral properties of salicylic acid

Density functional theory with B3LYP parametrization and 6-311++G(d,p) basis set has been used to investigate the structure and stability of salicylic acid-water complexes. The vertical excitation energies for these complexes have been computed using time-dependent density functional theory (with B3LYP parametrization and a 6-311++G(d,p) basis set). It is shown that the hydrogen bond between the carboxylic hydrogen and the oxygen of water is the strongest among all possible hydrogen bonds in the system. The hydrogen bond strength in salicylic acid-water complexes seems to be nearly additive. The change in absorption maximum (lambda(max)) corresponding to the vertical excitation energy for the first three excited singlet and triplet states of the complex with 1-3 water molecules is nominal (approximately 1-3 nm). But with the addition of the fourth water molecule, the lambda(max) for S(1) and T(1) decreases by approximately 17 nm and it increases for S(2) and S(3) by about the same amount. The decrease in lambda(max) for transition to the T(2) state on the addition of the fourth water molecule is only approximately 9 nm. There seems to be an intersystem crossing between the S(1) and T(3) states that could account for the observed fluorescence quenching of salicylic acid in water.

Hydrogen-bonding interactions in acetic acid monohydrates and dihydrates by density-functional theory calculations

Equilibrium structures and the respective binding energies of acetic acid monohydrates and dihydrates have been determined by density-functional theory calculations with different basis sets, including 6-31+G(3d,p), 6-311++G(d,p), and 6-311++G(3df,3pd). Given that the C=O and OH groups in acetic acid provide the predominant hydrogen-bonding interactions with water, six stable conformer structures have been found each for the monohydrate and syn-dihydrate. Of the three syn- and three anti-conformers of acetic acid with water, the most stable monohydrate structure is found to be that of the syn-conformer bonding with water in a cyclic double H-bonded geometry. Similarly, the syn-conformer bonding with two water molecules in a cyclic double H-bonded geometry has also been determined to be the most stable among the six plausible structures for the syn-dihydrate. Frequency analysis of the stable conformers has been performed and the vibrational spectra of the most stable monohydrate and dihydrate structures are compared with the experimental gas-phase and matrix data. Furthermore, the calculated binding energies between an acetic acid and a water molecule for both monohydrate and dihydrate are larger than that between two water molecules, which supports our recent experimental observation of coevaporation of acetic acid with water upon annealing acetic acid on ice.

Vibrational spectroscopic properties of hydrogen bonded acetonitrile studied by DFT

Vibrational properties (band position, Infrared and Raman intensities) of the acetonitrile C[triple bond]N stretching mode were studied in 27 gas-phase medium intensity (length range: = 1.71-2.05 angstroms; -deltaE range = 13-48 kJ/mol) hydrogen-bonded 1:1 complexes of CH3CN with organic and inorganic acids using density functional theory (DFT) calculations [B3LYP-6-31++G(2d,2p)]. Furthermore, general characteristics of the hydrogen bonds and vibrational changes in the OH stretching band of the acids were also considered. Experimentally observed blue-shifts of the C[triple bond]N stretching band promoted by the hydrogen bonding, which shortens the triple bond length, are very well reproduced and quantitatively depend on the hydrogen bond length. Both predicted enhancement of the infrared and Raman nu(C[triple bond]N) band intensities are in good agreement with the experimental results. Infrared band intensity increase is a direct function of the hydrogen bond energy. However, the predicted increase in the Raman band intensity increase is a more complex function, depending simultaneously on the characteristics of both the hydrogen bond (C[triple bond]N bond length) and the H-donating acid polarizability. Accounting for these two parameters, the calculated nu(C[triple bond]N) Raman intensities of the complexes are explained with a mean error of +/- 2.4%.

An Experimental Raman and Theoretical DFT Study on the Self-Association of Acrylonitrile

The liquid structure of acrylonitrile (propenenitrile) has been investigated using Raman spectroscopy and density functional theory (DFT) ab initio calculations with the 6-311++G** basis set. Two different and complementary experimental approaches were undertaken: FT-Raman spectra of 13 acrylonitrile solutions in carbon tetrachloride (concentration range=0.25-12.0 mol.L-1) were studied in detail including principal component analysis (PCA) of the CN stretching band. Furthermore, dispersive Raman spectra of neat acrylonitrile were obtained at eight different temperatures from 238 up to 343 K. The complex and asymmetric acrylonitrile Raman CN stretching band can be decomposed into two components attributed to monomeric and self-associated forms. Ab initio results fully support this assignment and suggest that the self-associated complex is a nonplanar trimer held together by dipole-dipole interactions. At ambient temperature, the composition of acrylonitrile can be expressed as a mixture of 25% monomers and 75% trimers. Close to the boiling point, trimers still represent 65% of the liquid composition. The corresponding enthalpy of association was estimated to be -22+/-2 kJ.mol-1.

Furan−Formic Acid Dimers: An ab Initio and Matrix Isolation Study

The dimers formed by formic acid (FA) and furan are investigated by ab initio methods and matrix isolation spectroscopy. Nine complexes with binding energies between -3.91 and -0.82 kcal/mol (MP2/6-311++G(d,p) + ZPE + BSSE) are identified. Another five weaker bound complexes are localized at lower level of theory only. The binding in the furan-FA dimers can be described in terms of OH...O, C=O...H, HO...H, CH...O, OH...pi, and CH...pi interactions. Therefore, the furan-FA complexes are classified in two types: (1) the dimers where the OH hydrogen of formic acid interacts with the furan molecule and (2) the dimers where the main interactions of FA with the furan molecule are via the less acidic CH hydrogen. Duning's and Pople's triple and double basis sets were used to study the dependence of the geometries and energies of the complexes from the basis set. BSSE (basis set superposition error) counterpoise corrections (CP) were included during the geometry optimizations of all dimers at the MP2/6-31G(d,p) level of theory. Matrix isolation spectroscopy allowed us to record the IR spectrum of aggregates between FA and furan. By comparison of the experimental IR spectrum with calculated IR spectra of a variety of complexes, it was possible to identify the most stable furan-FA dimer as the major product of the aggregation.