Ab Initio Molecular Orbital Analysis of Dimers of cis-Formic Acid. Implications for Condensed Phases

Amidated Pectic Polysaccharides (Pectin) as Methane Hydrate Inhibitor at Constant Cooling and Isobaric Condition

This study aims to address the environmental impact of using common commercial hydrate inhibitors such as Methanol (MeOH) in extremely cold oil and gas environments. As a greener alternative, Pectic Polysaccharides (pectin) can act as a kinetic hydrate inhibitor (KHI) to delay hydrate formation. We evaluated the performance of amidated pectin (AMP), a type of pectin with higher electronegative functional groups, using a high-pressure micro-differential scanning calorimeter (HP µ-DSC) under isobaric conditions with constant cooling. We compared AMP to low-methoxylated pectin (LMP) and high-methoxylated pectin (HMP) and found that AMP was the best KHI among the tested pectin types. At a concentration of 1.0 wt.%, the AMP Relative Inhibitor Performance (RIP) was 0.10, and at 0.1 wt.%, it had an RIP of 0.07, which were the only positive RIPs obtained amongst the tested KHIs. The results suggest that AMP can be a sustainable KHI option in extremely cold environments where the KHI effectiveness typically declines.

Dissecting intermolecular interactions in the condensed phase of ibuprofen and related compounds: the specific role and quantification of hydrogen bonding and dispersion forces

Ibuprofen is a well-established non-steroidal anti-inflammatory drug, inhibiting the prostaglandin-endoperoxide synthase. One of the key features defining the ibuprofen structure is the doubly intermolecular O-HO[double bond, length as m-dash]C hydrogen bond in cyclic dimers as know from carboxylic acids and confirmed by X-ray analysis. Until now, there was neither information about the vaporization enthalpy of ibuprofen nor about how this thermal property is determined by the subtle balance between different types of intermolecular interaction. In this study we derive the vaporization enthalpy of ibuprofen from thermochemical experiments to be . We dissected the hydrogen bond energy, EHB = 45.0 kJ mol-1, exclusively from measured vaporization enthalpies of related aliphatic carboxylic acids, their homomorph methyl esters and alkyl acetates, respectively. This contribution from hydrogen bonding could be confirmed almost quantitatively from quantum chemical calculations of ibuprofen clusters, which also suggest dispersion interaction of similar order (Edisp = 47 kJ mol-1). Following the full analysis of the gas-vapor transition enthalpy, we studied the changing structural components from the solid to the liquid phase of ibuprofen by means of Attenuated Total Reflection Infrared (ATR-IR) spectroscopy. The cyclic dimers as observed in the X-ray patterns are essentially preserved in the liquid state just above the melting point. However, with increasing temperature the doubly hydrogen-bonded cyclic dimers are replaced by singly hydrogen-bonded linear dimers in the liquid ibuprofen. The transfer enthalpy from the temperature-dependent equilibria of both dimers as obtained from the IR intensity ratios of the vibrational bands quantifies for the first time the energy of the released, single hydrogen bond to be EHB = 21.0 kJ mol-1. Overall, we show that a combination of thermodynamics, infrared spectroscopy and quantum chemistry provides quantification and detailed understanding of structure and molecular interaction in ibuprofen and related compounds.

Bond Fragility Spectra for the Double Proton-Transfer Reaction in the Formic Acid-Type Dimers

The newly developed method of fragility spectra for observation of bond breaking and formation upon a reaction has been applied to the canonical reaction series of the double proton transfer (DPT). Formic acid and its thio-analogues HCXYH (X, Y = O, S) have been chosen for the analysis. Very accurate linear correlations have been determined between the nondiagonal elements of the connectivity matrix, essential for the method, and the Wiberg bond orders for the corresponding bonds. Relation of the slope of this correlation to the global softness and to the atomic numbers of the bonded atoms has been proved, thus corroborating the c-DFT formula describing the fragility spectra. The electron density changes in bonds, as observed by the fragility spectra, are in harmony with the curvature diagrams reported by other authors.

Theoretical study of the absolute inner-shell photoionization cross sections of the formic acid and some of its hydrogen-bonded clusters

Inner-shell absolute photoabsorption and photoionization cross sections of the formic acid, HCOOH, and its small hydrogen-bonded clusters, i.e., (HCOOH)₂, HCOOH₂ ⁺, HCOHOH⁺, and HCOOH·H₃O⁺, were calculated at the time-dependent density functional theory (TDDFT) level, and the results were used to analyze the effect of the formic acid clustering on the carbon and oxygen K-edge photoionization cross sections. The discrete electronic pseudospectra obtained with square-integrable (L²) basis set calculations were used in an analytic continuation procedure based on continued fraction functions to obtain the photoabsorption cross sections. Symmetry adapted cluster configuration interaction calculations on the small formic acid clusters have also been performed at the oxygen K-edge to assign the discrete transitions and ionization potentials in support to the TDDFT results.

Ab initio molecular dynamics studies of formic acid dimer colliding with liquid water

Ab initio molecular dynamics simulations of formic acid (FA) dimer colliding with liquid water at 300 K have been performed using density functional theory. The two energetically lowest FA dimer isomers were collided with a water slab at thermal and high kinetic energies up to 68kBT. Our simulations agree with recent experimental observations of nearly a complete uptake of gas-phase FA dimer: the calculated average kinetic energy of the dimers immediately after collision is 5 ± 4% of the incoming kinetic energy, which compares well with the experimental value of 10%. Simulations support the experimental observation of no delayed desorption of FA dimers following initial adsorption. Our analysis shows that the FA dimer forms hydrogen bonds with surface water molecules, where the hydrogen bond order depends on the dimer structure, such that the most stable isomer possesses fewer FA-water hydrogen bonds than the higher energy isomer. Nevertheless, even the most stable isomer can attach to the surface through one hydrogen bond despite its reduced hydrophilicity. Our simulations further show that the probability of FA dimer dissociation is increased by high collision energies, the dimer undergoes isomerization from the higher energy to the lowest energy isomer, and concerted double-proton transfer occurs between the FA monomers. Interestingly, proton transfer appears to be driven by the release of energy arising from such isomerization, which stimulates those internal vibrational degrees of freedom that overcome the barrier of a proton transfer.

Formic acid dimers in a nitrogen matrix

Formic acid (HCOOH) dimers are studied by infrared spectroscopy in a nitrogen matrix and by ab initio calculations. We benefit from the use of a nitrogen matrix where the lifetime of the higher-energy (cis) conformer is very long (∼11 h vs. 7 min in an argon matrix). As a result, in a nitrogen matrix, a large proportion of the cis conformer can be produced by vibrational excitation of the lower-energy (trans) conformer. Three trans-trans, four trans-cis, and three cis-cis dimers are found in the experiments. The spectroscopic information on most of these dimers is enriched compared to the previous studies in an argon matrix. The cis-cis dimers of ordinary formic acid (without deuteration) are reported here for the first time. Several conformational processes are obtained using selective excitation by infrared light, some of them also for the first time. In particular, we report on the formation of cis-cis dimers upon vibrational excitation of trans-cis dimers. Tunneling decays of several dimers have been detected in the dark. The tunneling decay of cis-cis dimers of formic acid as well as the stabilization of cis units in cis-cis dimers is also observed for the first time.

Ab initio study of hydrogen bonding in the H3PO2 dimer and H3PO2-DMF complex

The molecular structures and H-bonding interactions in the phosphinic acid dimer and the complex of phosphinic acid with N,N-dimethylformamide (DMF) were investigated by density functional theory calculations at the B3LYP level of theory. In order to better understand these phenomena, the individual molecules were also studied. The results were compared with previously obtained data for similar H-bonded complexes of phosphoric and phosphorous acids with DMF. Various correlations were found between geometric characteristics and parameters derived from Bader's theory and natural bond orbital analysis. Graphical abstract Irina V. Fedorova and Lyubov P. Safonova. Ab initio study of hydrogen bonding in the H₃PO₂ dimer and H₃PO₂-DMF complex. The acids of phosphorus are considered suitable candidates for ionomers due to their efficient proton transport properties. In order to better understand their molecular structures and their H-bond characteristics in real liquids, the most stable configurations of the H₃PO₂ dimer and the H₃PO₂-DMF complex were examined using computational methods in quantum chemistry. It was found that the strength of the H-bonding interactions in these systems depends strongly on the environment.

Competing intramolecular vs. intermolecular hydrogen bonds in solution

A hydrogen bond for a local-minimum-energy structure can be identified according to the definition of the International Union of Pure and Applied Chemistry (IUPAC recommendation 2011) or by finding a special bond critical point on the density map of the structure in the framework of the atoms-in-molecules theory. Nonetheless, a given structural conformation may be simply favored by electrostatic interactions. The present review surveys the in-solution competition of the conformations with intramolecular vs. intermolecular hydrogen bonds for different types of small organic molecules. In their most stable gas-phase structure, an intramolecular hydrogen bond is possible. In a protic solution, the intramolecular hydrogen bond may disrupt in favor of two solute-solvent intermolecular hydrogen bonds. The balance of the increased internal energy and the stabilizing effect of the solute-solvent interactions regulates the new conformer composition in the liquid phase. The review additionally considers the solvent effects on the stability of simple dimeric systems as revealed from molecular dynamics simulations or on the basis of the calculated potential of mean force curves. Finally, studies of the solvent effects on the type of the intermolecular hydrogen bond (neutral or ionic) in acid-base complexes have been surveyed.

Gaussian analysis of Raman spectroscopy of acetic acid reveals a significant amount of monomers that effectively cooperate with hydrogen bonded linear chains

Gaussian analysis of Raman spectroscopy reveals three hydrogen bonding structures in the liquid acetic acid (AA): linear chains, cyclic dimers and dissociated monomers that effectively cooperate with hydrogen bonded stacks of linear AA or polymer chains.

A dispersion-corrected density functional theory study of hexamers of formic acid

Density functional theory with dispersion-correcting effective potentials is used to examine the low-lying isomers of the formic acid hexamer. The lowest-energy structure is a chairlike ring of six Z monomers. π-Stacked structures consisting of a dimer and tetramer lie higher in energy.

Raman spectroscopic studies and DFT calculations on tribromoacetic acid and tribromoacetic acid-d

The Raman spectra of crystalline tribromoacetic acid (CBr(3)COOH), its deuterated analog (CBr(3)COOD) and a partially deuterated acid (CBr(3)COOD/H) have been measured using Raman spectroscopy. Density functional theory (DFT) calculations have been carried out in order to compare the measured spectra with the calculated ones and the bands have been assigned. The theoretical frequencies are close to the ones of the cyclic dimer in the crystal and this fact implies the "oriented gas" model for this compound. The three Raman active intermonomeric modes have been assigned. An extremely weak and broad (~500 cm(-1)) νOH band for (CBr(3)COOH)(2) centred at ~3000 cm(-1) could be detected. In addition, this band shows relatively sharp submaxima, irregularly spaced, assigned to overtones/summation bands of the COOH group. For the deuterated analog, (CBr(3)COOD)(2) the OD stretching band is centred at ~2230cm(-1) and shows sharp submaxima as well. In the solid state the tribromoacetic acid consists of dimers while in aqueous solutions the tribromoacetic acid is in monomeric form.

Formic and acetic acid aggregation in the liquid state

The microscopic structure of neat formic and acetic acid have been measured by neutron diffraction with H/D substitution on SANDALS at the ISIS neutron spallation source. These data, together with complementary x-ray data, have been modeled via the empirical potential structure refinement (EPSR) method, which integrates information obtained from the diffraction data in a Monte Carlo simulation in order to provide a three-dimensional model of the system under study compatible with the measured structure factors. Two models have been generated for each acid, in order to test their consistency, with positive results. The final structure obtained is that of two liquids that are very similar to each other, with high connectivity although rather disordered. They present a hierarchy of probability for hydrogen bond formation, where weaker bonds involving the carbonyl hydrogen for formic acid or the methyl hydrogen for acetic acid are more abundant than the stronger bonds involving the hydroxyl hydrogen. Cooperative effects are found to be fundamental for the description of aggregation of formic and acetic acid, but the structure in the liquid presents a greater variety of bonds than in the solid state.

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.

Vibrational dynamics of carboxylic acid dimers in gas and dilute solution

Ultrafast mid-IR transient absorption spectroscopy has been used to study the vibrational dynamics of hydrogen-bonded cyclic dimers of trifluoroacetic acid and formic acid in both the gas and solution phases (0.05 M in CCl(4)). Ultrafast excitation of the broad O-H cyclic dimer band leads, in the gas phase, to large-scale structural changes of the dimer creating a species with a distinct free O-H stretching band on 20 ps and 200 ps timescales. These timescales are assigned to ring-opening and dissociation of the dimer, respectively. In the solution phase, no such structural rearrangement occurs and our results are consistent with previous studies. The gas phase dynamics are insensitive to both the specific excitation energy (over a span of 550 cm(-1)) and the chemical identity of the dimer.

Infrared spectroscopy of acetic acid and formic acid aerosols: pure and compound acid/ice particles

Acetic acid aerosol particles, formic acid aerosol particles and mixed acid/ice particles were generated in a collisional cooling cell at a temperature of 78 K and investigated using in situ rapid scan Fourier transform infrared spectroscopy. The infrared spectra reveal that the internal structure of the particles critically depends on the particle formation conditions and, especially for the mixed particles, on the composition. The acetic acid particles are likely to have only a partially crystalline structure whereas the formic acid particles are likely to have an overall crystalline structure. The existence of acid in the mixed acid/ice particles prevents the ice from crystallization even at low acid concentrations (less than 10%). Mid-infrared refractive index data were derived from the different particle spectra, which can be helpful for remote sensing of such systems.

Inner-shell excitation spectroscopy and fragmentation of small hydrogen-bonded clusters of formic acid after core excitations at the oxygen K edge

Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K-edge region by time-of-flight fragment mass spectroscopy. In addition to several fragment cations smaller than the parent molecule, we have identified the production of HCOOH.H+ and H3O+ cations characteristic of proton transfer reactions within the clusters. Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations. Resonance transitions of O1s(C[double bond]O/OH) electrons into pi(CO)* orbital in the preedge region were found to shift in energy upon clusterization. A blueshift of the O1s(C[double bond]O)-->pi(CO)* transition by approximately 0.2 eV and a redshift of the O1s(OH)-->pi(CO)* by approximately 0.6 eV were observed, indicative of strong hydrogen-bond formation within the clusters. The results have been compared with a recent theoretical calculation, which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer andor trimer units. Specifically labeled formic acid-d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HDO+ was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that HD2O+ was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster. Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place, producing deuteronated HCOOD.D+ (protonated HCOOH.H+) cations within the clusters.

IR plus vacuum ultraviolet spectroscopy of neutral and ionic organic acid molecules and clusters: Acetic acid

Infrared (IR) vibrational spectroscopy of acetic acid (A) neutral and ionic monomers and clusters, employing vacuum ultraviolet (VUV), 10.5 eV single photon ionization of supersonically expanded and cooled acetic acid samples, is presented and discussed. Molecular and cluster species are identified by time of flight mass spectroscopy: the major mass features observed are A(n)H(+) (n=1-9), ACOOH(+) (VUV ionization) without IR radiation present, and A(+) with both IR and VUV radiation present. The intense feature ACOOH(+) arises from the cleavage of (A)(2) at the beta-CC bond to generate ACOOH(+)+CH(3) following ionization. The vibrational spectrum of monomeric acetic acid (2500-7500 cm(-1)) is measured by nonresonant ionization detected infrared (NRID-IR) spectroscopy. The fundamentals and overtones of the CH and OH stretches and some combination bands are identified in the spectrum. Mass selected IR spectra of neutral and cationic acetic acid clusters are measured in the 2500-3800 cm(-1) range employing nonresonant ionization dip-IR and IR photodissociation (IRPD) spectroscopies, respectively. Characteristic bands observed at approximately 2500-2900 cm(-1) for the cyclic ring dimer are identified and tentatively assigned. For large neutral acetic acid clusters A(n)(n>2), spectra display only hydrogen bonded OH stretch features, while the CH modes (2500-2900 cm(-1)) do not change with cluster size n. The IRPD spectra of protonated (cationic) acetic acid clusters A(n)H(+) (n=1-7) exhibit a blueshift of the free OH stretch with increasing n. These bands finally disappear for n> or =6, and one broad and weak band due to hydrogen bonded OH stretch vibrations at approximately 3350 cm(-1) is detected. These results indicate that at least one OH group is not involved in the hydrogen bonding network for the smaller (n< or =5) A(n)H(+) species. The disappearance of the free OH stretch feature at n> or =6 suggests that closed cyclic structures form for A(n)H(+) for the larger clusters (n> or =6).

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.

Car−Parrinello Molecular Dynamics Simulation of Liquid Formic Acid

First-principles molecular dynamics has been used to investigate the structural, vibrational, and energetic properties of formic acid, formic acid-formate anion dimers, and liquid formic acid in a periodically repeated box with 32 formic acid molecules. We found that in liquid formic acid the hydrogen-bonded clusters mainly consist of linear branching chains. From our simulation, we got good agreement with the available structural and dynamical data. We also studied the proton transfer in the cis-formic acid-formate anion dimer, and we showed that this proton transfer does not have any potential barrier. The hydrogen bonding statistics as well as the mean lifetime of the hydrogen bonds are analyzed.

Structure of Liquid Formic Acid Investigated by First Principle and Classical Molecular Dynamics Simulations

The structure of liquid formic acid has been investigated by Car-Parrinello and classical molecular dynamics simulations, focusing on the characterization of the H-bond network and on the mutual arrangement of pairs of bonded molecules. In agreement with previous computational studies, two levels of H-bonded structures have been found. Small clusters, characterized by O-H...O bonds, are held together by weak C-H...O bonds to form large branched structures. From the ab initio simulation we infer the importance of cyclic H-bond dimer configurations, typical of the gas phase. Most of these dimer structures are however found to be embedded into H-bonded chains. When only O-H...O bonds are taken into account, linear H-bond chains are detected as basic structures of the liquid. More branched structures occur when C-H...O bonds are also considered. Regarding the arrangement of molecular pairs, we observed that O-H...O bonds favor the occurrence of configurations with parallel molecular planes, whereas no preferential orientation is observed for molecules forming C-H...O bonds.

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.