Formic Acid Dimer Research Articles

Terahertz and Mid-infrared Spectra of Cold Formic Acid Aerosol Particles

Fourier transform infrared spectra of formic acid aerosol particles in situ generated in a collisional cooling cell at temperatures ranging between 90 and 210 K are recorded in the mid-infrared and THz/far-infrared regions. Infrared spectroscopic features are used to identify the formic acid dimer above 200 K, the crystalline β1 phase in the 110–200 K temperature range, and amorphous solid formic acid at ∼90 K. Density functional calculations of discrete clusters and the full periodic β1 crystalline structure help to assign the low wavenumber intermolecular modes by comparison with experimental far IR and Raman data from the literature. Refractive indices of the formic acid crystal in the mid-IR range are retrieved. These data are essential for radiative transfer modeling of solid formic acid condensed in the terrestrial atmosphere. THz/far IR spectra, in addition to the well documented mid-infrared spectra, may be useful for the identification of solid formic acid in astrophysical environments such as the young stellar object W33A.

Proton exchange in ammonia, water and formic acid dimers: quantum-chemical calculation

Proton exchange in ammonia, water and formic acid dimers: quantum-chemical calculation

IR Spectra of (HCOOH)2 and (DCOOH)2: Experiment, VSCF/VCI, and Ab Initio Molecular Dynamics Calculations Using Full-Dimensional Potential and Dipole Moment Surfaces.

We report quantum VSCF/VCI and ab initio molecular dynamics (AIMD) calculations of the IR spectra of (HCOOH)2 and (DCOOH)2, using full-dimensional, ab initio potential energy and dipole moment surfaces (PES and DMS). These surfaces are fits, using permutationally invariant polynomials, to 13 475 ab initio CCSD(T)-F12a electronic energies and MP2 dipole moments. Here "AIMD" means using these ab initio potential and dipole moment surfaces in the MD calculations. The VSCF/VCI calculations use all (24) normal modes for coupling, with a four-mode representation of the potential. The quantum spectra align well with jet-cooled and room-temperature experimental spectra over the spectral range 600-3600 cm-1. Analyses of the complex O-H and C-H stretch bands are made based on the mixing of the VSCF/VCI basis functions. The comparisons of the AIMD IR spectra with both experimental and VSCF/VCI ones provide tests of the accuracy of the AIMD approach. These indicate good accuracy for simple bands but not for the complex O-H stretch band, which is upshifted from experimental and VSCF/VCI bands by roughly 300 cm-1. In addition to testing the AIMD approach, the PES, DMS, and VSCF/VCI calculations for formic acid dimer provide opportunities for testing other methods to represent high-dimensional data and other methods that perform postharmonic vibrational calculations.

On the Computational Characterization of Charge-Transfer Effects in Noncovalently Bound Molecular Complexes.

Charge-transfer (CT) is an important binding force in the formation of intermolecular complexes, and there have been a variety of theoretical models proposed to quantify this effect. These approaches, which typically rely on a definition of a "CT-free" state based on a partition of the system, sometimes yield significantly different results for a given intermolecular complex. Two widely used definitions of the "CT-free" state, the absolutely localized molecular orbitals (ALMO) method (where only on-fragment orbital mixings are permitted) and the constrained density functional theory (CDFT) approach (where fragment electron populations are fixed), are carefully examined in this work. Natural bond orbital (NBO) and the regularized symmetry-adapted perturbation theory (SAPT) are also briefly considered. Results for the ALMO and CDFT definitions of CT are compared on a broad range of model systems, including hydrogen-bonding systems, borane complexes, metal-carbonyl complexes, and complexes formed by water and metal cations. For most of these systems, CDFT yields a much smaller equilibrium CT energy compared to that given by the ALMO-based definition. This is mainly because the CDFT population constraint does not fully inhibit CT, which means that the CDFT "CT-free" state is in fact CT-contaminated. Examples of this contamination include (i) matching forward and backward donation (e.g., formic acid dimer) and (ii) unidirectional CT without changing fragment populations. The magnitude of the latter effect is quantified in systems such as the water dimer by employing a 3-space density constraint in addition to the orbital constraint. Furthermore, by means of the adiabatic EDA, it is shown that several observable effects of CT, such as the "pyramidalization" of the planar BH3 molecule upon the complexation with Lewis bases, already appear on the "CT-free" CDFT surface. These results reveal the essential distinctions between the ALMO and CDFT definitions of CT and suggest that the former is more consistent with accepted understanding of the role of CT in intermolecular binding.

High-dimensional fitting of sparse datasets of CCSD(T) electronic energies and MP2 dipole moments, illustrated for the formic acid dimer and its complex IR spectrum.

We present high-level, coupled-mode calculations of the infrared spectrum of the cyclic formic acid dimer. The calculations make use of full-dimensional, ab initio potential energy and dipole moment surfaces. The potential is a linear least-squares fit to 13 475 CCSD(T)-F12a/haTZ (haTZ means aug-cc-pVTZ basis set for O and C, and cc-pVTZ for H) energies, and the dipole moment surface is a fit to the dipole components, calculated at the MP2/haTZ level of theory. The variables of both fits are all (45) internuclear distances (actually Morse variables). The potential, which is fully permutationally invariant, is the one published recently and the dipole moment surface is newly reported here. Details of the fits, especially the dipole moment, and the database of configurations are given. The infrared spectrum of the dimer is calculated by solving the nuclear Schrödinger equation using a vibrational self-consistent field and virtual-state configuration interaction method, with subsets of the 24 normal modes, up to 15 modes. The calculations indicate strong mode-coupling in the C-H and O-H stretching region of the spectrum. Comparisons are made with experiments and the complexity of the experimental spectrum in the C-H and O-H stretching region is successfully reproduced.

Permutationally Invariant Potential Energy Surfaces.

Over the past decade, about 50 potential energy surfaces (PESs) for polyatomics with 4-11 atoms and for clusters have been calculated using the permutationally invariant polynomial method. This is a general, mainly linear least-squares method for precise mathematical fitting of tens of thousands of electronic energies for reactive and nonreactive systems. A brief tutorial of the methodology is given, including several recent improvements. Recent applications to the formic acid dimer (the current record holder in size for a reactive system), the H2-H2O complex, and four protonated water clusters [H+(H2O)n=2,3,4,6] are given. The last application also illustrates extension to large clusters using the many-body representation.

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 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.

Comparison of Degrees of Potential-Energy-Surface Anharmonicity for Complexes and Clusters with Hydrogen Bonds

Previously calculated multidimensional potential-energy surfaces of the MeOH monomer and dimer, water dimer, malonaldehyde, formic acid dimer, free pyridine-N-oxide/trichloroacetic acid complex, and protonated water dimer were analyzed. The corresponding harmonic potential-energy surfaces near the global minima were constructed for series of clusters and complexes with hydrogen bonds of different strengths based on the behavior of the calculated multidimensional potential-energy surfaces. This enabled the introduction of an obvious anharmonicity parameter for the calculated potential-energy surfaces. The anharmonicity parameter was analyzed as functions of the size of the analyzed area near the energy minimum, the number of points over which energies were compared, and the dimensionality of the solved vibrational problem. Anharmonicity parameters for potential-energy surfaces in complexes with strong, medium, and weak H-bonds were calculated under identical conditions. The obtained anharmonicity parameters were compared with the corresponding diagonal anharmonicity constants for stretching vibrations of the bridging protons and the lengths of the hydrogen bridges.

Quantum and classical IR spectra of (HCOOH)2, (DCOOH)2 and (DCOOD)2 using ab initio potential energy and dipole moment surfaces.

An accurate quantum mechanical description of the vibrational dynamics and IR spectra of molecules is illustrated here for the formic acid dimer, (HCOOH)2 and its isotopologues (DCOOH)2 and (DCOOD)2 in full dimensionality. The calculations make use of recent full-dimensional ab initio potential energy and dipole moment surfaces and are done with the code MULTIMODE. IR spectra are reported for the three dimers and also compared to available experimental spectra. In addition, standard classical and "semiclassically prepared" quasiclassical molecular dynamics calculations of the IR spectra of these complexes are reported and compared to the quantum spectra and also experiment. These comparisons indicate good accuracy of the MD spectra for sharp bands but not for the complex O-H stretch band, where the complex molecular dynamics band is upshifted from experiments by roughly 300 cm-1. For the fully deuterated dimer (DCOOD)2, the quantum spectral band for the O-D stretch sharpens relative to the O-H spectral bands in (HCOOH)2 and (DCOOH)2; however, the molecular dynamics OD stretch band does not exhibit this sharpening.

Insights into the Mechanism of Ground and Excited State Double Proton Transfer Reaction in Formic Acid Dimer.

The mechanism of ground and excited state double proton transfer reaction in formic acid dimer has been analyzed with the help of reaction force and the reaction electronic flux. The separation of reaction electronic flux in terms of electronic activity and reactivity, NBO, and dual descriptor lends additional support for the mechanism. Interestingly we found that the ground state double proton transfer mechanism is concerted synchronic, whereas the excited state double proton transfer is concerted asynchronic in nature.

Liquid Microjet Measurements of the Entry of Organic Acids and Bases into Salty Water

We explore collisions of hydrogen-bonding molecules with salty water using gas–microjet scattering experiments. Two aqueous solutions, 8 molal (m) LiBr/H2O and ∼4 m K2SO3/H2O at 253 K were exposed to seven organic gases representing different functional groups. These gases comprise weak acids (formic and acetic), weak bases (dimethylamine and piperidine), and an alcohol, ether, and ester (ethanol, dimethyl ether, and methyl formate). The scattering experiments are used to monitor the disappearance of each gas into the aqueous solutions over a ∼100 μs observation time. They demonstrate that formic acid and piperidine disappear into both solutions on almost every collision. Dimers of formic and acetic acid are also captured by the solutions on every collision, despite their pre-existing double hydrogen bonds. The methylene ring of piperidine, (CH2)5NH, also does not interfere with uptake. At the opposite extreme, methyl formate and dimethyl ether are so weakly soluble that they evaporate completely within the observation window, precluding the measurement of their entry probability. Ethanol and dimethylamine represent intermediate cases in which dimethylamine interacts more strongly with dissolved Li+ ions than K+ ions. Collectively, the experiments imply that organic acids and bases reach hydrogen-bonding configurations following nearly every collision, enabling them to be captured by surface water molecules.

Dimers of formic acid: Structures, stability, and double proton transfer.

A stochastic search of the potential energy surface for the formic acid dimers results in 21 well-defined minima. A number of structures are reported here for the first time, others have already been experimentally detected or computationally predicted. Four types of different hydrogen bonds (HBs) are at play stabilizing the clusters: primary C=O⋯ H-O and H-O⋯ H-O and secondary C=O⋯ H-C and H-O⋯ H-C HBs corresponding to well-characterized bonding paths are identified. A novel C=O⋯ C stabilizing interaction is also reported. The double proton transfer reaction is calculated to occur in a synchronous fashion, with an energy barrier smaller than the energy needed to break up the dimers.

Noncovalent Interactions by Fixed-Node Diffusion Monte Carlo: Convergence of Nodes and Energy Differences vs Gaussian Basis-Set Size

Convergence of fixed-node (FN) shape and FN diffusion Monte Carlo (FNDMC) interaction energies is studied vs the Gaussian basis set saturation level in HF and CH4 dimers and one-determinant Slater-Jastrow trial wave functions (ΨT). The tested 25 distinct basis sets obtained by stepwise trimming of aug-VDZ and aug-VTZ bases suggest minimum basis set requirements to achieve reasonable results. A single selected trimmed basis set, about 2 times smaller in size than aug-VTZ, is extensively tested on a set of 12 noncovalent complexes including formic acid dimer, benzene-methane, or coronene-H2. The results indicate that equivalent noncovalent FNDMC energy differences are available at costs lower than assumed before. Additional insights from electron density differences and comparison of dimer vs monomer ΨT nodes explain this observation.

Effective Fragment Potential Method for H-Bonding: How To Obtain Parameters for Nonrigid Fragments.

Accuracy of the effective fragment potential (EFP) method was explored for describing intermolecular interaction energies in three dimers with strong H-bonded interactions, formic acid, formamide, and formamidine dimers, which are a part of HBC6 database of noncovalent interactions. Monomer geometries in these dimers change significantly as a function of intermonomer separation. Several EFP schemes were considered, in which fragment parameters were prepared for a fragment in its gas-phase geometry or recomputed for each unique fragment geometry. Additionally, a scheme in which gas-phase fragment parameters are shifted according to relaxed fragment geometries is introduced and tested. EFP data are compared against the coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) method in a complete basis set (CBS) and the symmetry adapted perturbation theory (SAPT). All considered EFP schemes provide a good agreement with CCSD(T)/CBS for binding energies at equilibrium separations, with discrepancies not exceeding 2 kcal/mol. However, only the schemes that utilize relaxed fragment geometries remain qualitatively correct at shorter than equilibrium intermolecular distances. The EFP scheme with shifted parameters behaves quantitatively similar to the scheme in which parameters are recomputed for each monomer geometry and thus is recommended as a computationally efficient approach for large-scale EFP simulations of flexible systems.

A density functional theory study of double proton transfer reactions in formamide, formamide-formic acid and formic acid dimers

We have performed a theoretical study on the double proton transfer (DPT) reactions for the formamide-formamide, formamide-formic acid and formic acid-formic acid dimeric complexes by using the ωB97XD/aug-cc-pVDZ level of density functional theory (DFT). The DFT calculated reaction energy barriers are calibrated by the high-level MP2 and CCSD(T) methods at the complete basis set limit. The reaction energy profiles along the intrinsic reaction coordinates have been analyzed using the symmetry adapted perturbation theory. The reaction mechanism is elucidated by examining the reaction force, the chemical potential and the reaction electronic flux. Our results show that the DPT reactions are essentially synchronous which can form an alternative reaction pathway to monomer tautomerization. The reactions proceed with significant intermolecular bond shortening and charge accumulation, yielding large increment in the exchange energy toward the formation of the reaction energy barrier. Once the transition state is reached, the energetic constraint is released by the attractive energies.

Full- and reduced-dimensionality instanton calculations of the tunnelling splitting in the formic acid dimer.

The ring-polymer instanton approach is applied to compute the ground-state tunnelling splitting of four isotopomers of the formic acid dimer using the accurate PES of Qu and Bowman [Phys. Chem. Chem. Phys., 2016, 18, 24835]. As well as performing the calculations in full dimensionality, we apply a reduced-dimensionality approach to study how the results converge as successively more degrees of freedom are included. The instanton approximation compares well to exact quantum results where they are available but shows that nearly all the modes are required to quantitatively obtain the tunnelling splitting. The full-dimensional instanton calculation reproduces the experimental results, with an error of only about 20 percent.

A Reinvestigation of the Ionic Liquid Diisopropylethylammonium Formate by NMR and DFT Methods.

The complex between diisopropylethylamine (DIPEA) and formic acid has been reinvestigated. Mixing the compounds in the ratio 1:1 leads to a phase separation in which the upper phase is DIPEA and the lower phase is the "ionic liquid" named DIPEF. A combined NMR and DFT study shows that the lower phase primarily is formic acid:formate and diisopropylammonium ions in the ratio 2:1 (acid:base) plus the formic acid dimer. Addition of more acid leads to more and more of the acid dimer. The proton transfer in the system is 65-80%. The structural picture presented in this paper is very different from that presented elsewhere. However, the present picture should be considered using acids and bases with a pKa difference less than 8. The formic acid content in the DIPEF ionic liquid causes desorption of the dye-sensitized solar cell (DSC) dye N719 from the photo anode, and DIPEF is therefore not a suitable electrolyte for DSCs.

Apparatus for testing gas-surface reactions for epicatalysis.

Recently, a new mode of gas-surface heterogeneous catalysis (epicatalysis) has been identified, having potential applications ranging from industrial and green chemistry to novel forms of power generation. This article describes an inexpensive, easily constructed, vacuum-compatible apparatus by which multiple candidate gas-surface combinations can be rapidly screened for epicatalytic activity. In exploratory experiments, candidate surfaces (teflon, kapton, glass, and gold) and gases (helium, argon, cyclohexane, water, methanol, formic acid, and acetic acid) were tested for epicatalytic activity. Kapton and teflon displayed small but reproducible differences in formic acid and methanol dimer desorption, thereby demonstrating the first examples of room-temperature epicatalysis. Other gas-surface combinations showed smaller or inconclusive evidence for epicatalysis.

Effect of cluster environment on the electron attachment to 2-nitrophenol

Effect of cluster environment on the electron attachment to 2-nitrophenol (2NP) is studied in homogeneous 2NP clusters and heterogeneous clusters of 2NP, argon and water. The cluster environment significantly reduces fragmentation of 2NP after electron attachment. Parent cluster anions 2NPn - are primary reaction products in both, homogeneous and heterogeneous clusters. Non-dissociative electron attachment to homogeneous clusters proceeds at low energies <2 eV, presumably via dipole-supported states. In heterogeneous clusters, the interaction with low energy (<2 eV) electrons is shielded by the solvent. Surprisingly, the energetic threshold for the electron attachment rises with the number (n) of 2NP molecules in the cluster (2NP)n -. This rise can be either due to a strong change of the 2NP conformation induced by the cluster environment or due to the the competition with electron autodetachment after proton transfer that has been first observed by Allan in the formic acid dimer [M. Allan, Phys. Rev. Lett. 98, 123201 (2007)]. We observe the same threshold rise for complex Ar m ·(2NP)n - and H2O·(2NP)n - anions. This indicates that the electron attachment to 2-nitrophenol in cluster environment is more influenced by the solute − solute interaction compared to the solute − solvent interaction.