Ab Initio Investigation of Vibrational Spectra of Water−(CO2)n Complexes (n = 1, 2)

The Structure of Carbon Dioxide at the Air-Water Interface and its Chemical Implications

The efficient reduction of CO2 into valuable products is a challenging task in an international context marked by the climate change crisis and the need to move away from fossil fuels. Recently, the use of water microdroplets has emerged as an interesting reaction media where many redox processes which do not occur in conventional solutions take place spontaneously. Indeed, several experimental studies in microdroplets have already been devoted to study the reduction of CO2 with promising results. The increased reactivity in microdroplets is thought to be linked to unique electrostatic solvation effects at the air-water interface. In the present work, we report a theoretical investigation on this issue for CO2 using first-principles molecular dynamics simulations. We show that CO2 is stabilized at the interface, where it can accumulate, and that compared to bulk water solution, its electron capture ability is larger. Our results suggest that reduction of CO2 might be easier in interface-rich systems such as water microdroplets, which is in line with early experimental data and indicate directions for future laboratory studies. The effect of other relevant factors which could play a role in CO2 reduction potential is discussed.

Quantum Chemical Study of the Carbon Dioxide-Philicity of Surfactants: Effects of Tail Functionalization

Carbon dioxide (CO₂)-philic surfactants have broad application prospects in organic synthesis, fracture-enhanced oil recovery, polymerization, extraction, and other fields and can be used to enhance the viscosity of supercritical CO₂ (scCO₂). In this work, the relationship between the functional group of the surfactant tail and CO₂-philicity is studied from a new perspective using density functional theory. Three common functional group types (fluorinated, oxidative, and methyl groups) were investigated. The analysis of binding energy demonstrates that all three types of functional groups can improve the CO₂-philicity of the surfactant. Among these three kinds of functional groups, the strongest interaction with CO₂ molecules is observed for oxidative functional groups followed by semifluorinated, fluorinated, and methyl groups. However, the CO₂ molecules tend to be adsorbed onto the middle segment of the oxidative group, and the intrusion of the CO₂ molecules results in the low solubility of oxidative surfactants. In contrast, fluorinated and methyl groups interact with CO₂ at the end of the surfactant tail. As a result, the fluorinated surfactants show the best solubility in CO₂. Therefore, the solubility of a surfactant in CO₂ is not only related to the interaction strength between the surfactant and CO₂, it also depends on the interaction structure. The results of this study provide a new strategy for evaluating surfactant CO₂-philicity and provide guidance for the design of surfactants with high solubility in scCO₂.

THz spectroscopy of weakly bound cluster molecules in solid para-hydrogen: a sensitive probe of van der Waals interactions

The present work demonstrates that 99.9% enriched solid para-H2 below 3 K provides an excellent inert and transparent medium for the exploration of large-amplitude intermolecular vibrational motion of weakly bound van der Waals cluster molecules in the THz spectral region. THz absorption spectra have been generated for CO2/H2O and CS2/H2O mixtures embedded in enriched solid para-H2 and numerous observed transitions associated with large-amplitude librational motion of the weakly bound binary CO2H2O and CS2H2O van der Waals cluster molecules have been assigned together with tentative assignments for the ternary CS2(H2O)2 system. The interaction strength, directionality and anharmonicity of the weak van der Waals "bonds" between the molecules can be characterized via these THz spectral signatures and yield rigorous benchmarks for high-level ab initio methodologies. It is suggested that even a less stable linear conformation of the ternary CS2(H2O)2 system, where one H2O molecule is linked to each S atom of the CS2 subunit, may be formed due to the kinetics associated with the mobility of free H2O molecules in the soft para-H2 medium. In addition, the spectroscopic observations confirm a linear and planar global intermolecular potential energy minimum for the binary CS2H2O system with C2v symmetry, where the O atom on the H2O molecule is linked to one of the S atoms on the CS2 subunit. A semi-experimental value for the vibrational zero-point energy contribution of 1.93 ± 0.10 kJ mol-1 from the class of large-amplitude intermolecular vibrational modes is proposed. The combination with CCSD(T)/CBS electronic energy predictions provides a semi-experimental estimate of 5.08 ± 0.15 kJ mol-1 for the binding energy D0 of the CS2H2O van der Waals system.

Structure-Property Relationships in CO2-philic (Co)polymers: Phase Behavior, Self-Assembly, and Stabilization of Water/CO2 Emulsions

This Review provides comprehensive guidelines for the design of CO2-philic copolymers through an exhaustive and precise coverage of factors governing the solubility of different classes of polymers. Starting from computational calculations describing the interactions of CO2 with various functionalities, we describe the phase behavior in sc-CO2 of the main families of polymers reported in literature. The self-assembly of amphiphilic copolymers of controlled architecture in supercritical carbon dioxide and their use as stabilizers for water/carbon dioxide emulsions then are covered. The relationships between the structure of such materials and their behavior in solutions and at interfaces are systematically underlined throughout these sections.

Communication: THz absorption spectrum of the CO2-H2O complex: observation and assignment of intermolecular van der Waals vibrations

Terahertz absorption spectra have been recorded for the weakly bound CO2-H2O complex embedded in cryogenic neon matrices at 2.8 K. The three high-frequency van der Waals vibrational transitions associated with out-of-plane wagging, in-plane rocking, and torsional motion of the isotopic H2O subunit have been assigned and provide crucial observables for benchmark theoretical descriptions of this systems' flat intermolecular potential energy surface. A (semi)-empirical value for the zero-point energy of 273 ± 15 cm(-1) from the class of intermolecular van der Waals vibrations is proposed and the combination with high-level quantum chemical calculations provides a value of 726 ± 15 cm(-1) for the dissociation energy D0.

Assessing the non-ideality of the CO2-CS2 system at molecular level: a Raman scattering study

The dense phase of CO2-CS2 mixtures has been analysed by Raman spectroscopy as a function of the CO2 concentration (0.02-0.95 mole fractions) by varying the pressure (0.5 MPa up to 7.7 MPa) at constant temperature (313 K). The polarised and depolarised spectra of the induced (ν2, ν3) modes of CS2 and of the ν1-2ν2 Fermi resonance dyad of both CO2 and CS2 have been measured. Upon dilution with CO2, the evolution of the spectroscopic observables of all these modes displays a "plateau-like" region in the CO2 mole fraction 0.3-0.7 never previously observed in CO2-organic liquids mixtures. The bandshape and intensity of the induced modes of CS2 are similar to those of pure CS2 up to equimolar concentration, after which variations occur. The preservation of the local ordering from pure CS2 to equimolar concentration together with the non-linear evolution of the spectroscopic observables allows inferring that two solvation regimes exist with a transition occurring in the plateau domain. In the first regime, corresponding to CS2 concentrated mixtures, the liquid phase is segregated with dominant CS2 clusters, whereas, in the second one, CO2 monomers and dimers and CO2-CS2 hetero-dimers coexist dynamically on a picosecond time-scale. It is demonstrated that the subtle interplay between attractive and repulsive interactions which provides a molecular interpretation of the non-ideality of the CO2-CS2 mixture allows rationalizing the volume expansion and the existence of the plateau-like region observed in the pressure-composition diagram previously ascribed to the proximity of an upper critical solution temperature at lower temperatures.

The water–boryl radical as a proton-coupled electron transfer reagent for carbon dioxide, formic acid, and formaldehyde — Theoretical approach

The thermochemistry of the hydrogen atom transfer reactions from the H2O–BX2 radical system (X = H, CH3, NH2, OH, F) to carbon dioxide, formic acid, and (or) formaldehyde, which produce hydroxyformyl, dihydroxymethyl, and hydroxymethyl radicals, respectively, were investigated theoretically at ROMP2/6–311+G(3DF,2P)//UB3LYP/6–31G(D) and UG3(MP2)-RAD levels of theory. Surprisingly, in the cases of a strong Lewis acid (X = H, CH3, F), the spin transfer process from the water–boryl radical to the carbonyl compounds was barrier-free and associated with a dramatic reduction in the B–H bond dissociation energy (BDE) relative to that of isolated water–borane complexes. Examining the coordinates of these reactions revealed that the entire hydrogen atom transfer process is governed by the proton-coupled electron transfer (PCET) mechanism. Hence, the elucidated mechanism has been applied in the cases of weak Lewis acids (X = NH2, OH), and the variation in the accompanied activation energy was attributed to the stereoelectronic effect interplaying in CO2 and HCOOH compared with HCHO. We ascribed the overall mechanism as a SA-induced five-center cyclic PCET, in which the proton transfers across the so-called complexation-induced hydrogen bond (CIHB) channel, while the SOMOB–LUMOC=O′ interaction is responsible for the electron migration process. Owing to previous reports that interrelate the hydrogen-bonding and the rate of proton-coupled electron-transfer reactions, we postulated that “the rate of the PCET reaction is expected to be promoted by the covalency of the hydrogen bond, and any factor that enhances this covalency could be considered an activator of the PCET process.” This postulate could be considered a good rationale for the lack of a barrier associated with the hydrogen atom transfer from the water-boryl radical system to the carbonyl compounds. Light has been shed on the water–boryl radical reagent from the thermodynamic perspective.

On the Perturbation of the Intramolecular H-Bond in Diols by Supercritical CO2: A Theoretical and Spectroscopic Study

The role played by supercritical carbon dioxide used as a dispersant medium in the synthesis of polyurethane particles has been investigated. High-temperature-high-pressure in situ infrared spectroscopic measurements combined with ab initio calculations were performed to investigate the hydroxyl stretching vibrations of ethylene glycol (EG) and 1,4-butanediol (BD), two monomers commonly used in the field of step growth polymerization. Specific interactions between the diols and CO2 have been put in evidence. While the structural characteristics of EG and BD are very similar--both diols have a gauche conformation due to an internal H-bond between the two hydroxyl functions--they behave differently in the presence of dense CO2. In the case of EG, this internal H-bond is broken, allowing the diol and CO2 to form a complex through the conjunction of a Lewis acid-Lewis base (LA-LB) interaction and a new H-bond. When BD complexes to CO2, this internal H-bond remains and is even reinforced indirectly by the LA-LB interaction occurring between the two moieties. In both cases, such a complex formation induces a polarization of the hydroxyl groups and consequently an increase of their nucleophilicity.

Counterpoise corrected geometries of hydrated complexes

We have calculated the equilibrium geometries of the hydrated complexes, H2O.CO2, H2O.CS2,H2O.OCS, H2O.SO2, and H2O.SO3, in the electronic ground state. We have used the coupled cluster with singles, doubles, and perturbative triples ab initio method with a correlation consistent augmented triple-zeta basis set. We find that a counterpoise corrected optimization scheme is important for an accurate description of the geometries. These high level ab initio calculated geometries are of comparable quality to those obtained experimentally.

Density Functional Theory (DFT) Calculations of the Infrared Absorption Spectra of Acetaminophen Complexes Formed with Ethanol and Acetone Species

We have investigated the infrared (IR) vibrational spectra of acetaminophen (N(4-hydroxyphenyl) acetamide or paracetamol) complexes formed with ethanol and acetone in relation to the nature of the specific intermolecular interactions involved in the stabilization of the complexes. The structures and binding energies of the complexes have been determined using Hartree-Fock (HF) and DFT-B3PW91 procedures and different Pople's basis sets as well. The main results are presented and discussed by considering the hydroxyl (OH), amino (NH), and carbonyl (CO) chemical groups of acetaminophen interacting with the acetone or ethanol molecules either separately or in conjunction in the complex formation. The frequency shifts and IR intensity variations associated with the internal modes of acetaminophen (namely nu(OH), nu(NH), and nu(CO)) as well as the most pertinent vibrational probes of ethanol (nu(OH)) and acetone (symmetric nu(CO) and nu(CCC) stretching modes) interacting with acetaminophen have been analyzed. The predicted spectral changes have been critically discussed in comparison with IR absorption measurements of acetaminophen dissolved as a solute in ethanol or acetone CO2 expanded solutions. It is argued that the exchange-correlation contribution taken into account in DFT calculations is likely significant in determining the main IR spectral features of acetaminophen complexes formed with acetone or involving hydrogen-bonded as with ethanol.

Infrared and molecular-dynamics studies of the rotational dynamics of water highly diluted in supercritical CO2

Far-infrared (FIR) and mid-infrared (MIR) profiles of D2O infinitely dilute in supercritical CO2 have been studied using molecular-dynamics simulations. For this purpose, we have proposed an intermolecular potential model taking implicitly into account electron donor-acceptor (EDA) interactions between water and CO2 evaluated from ab initio calculations of the intermolecular potential-energy surface (IPS). Interaction-induced dipole mechanisms have been also taken into account in addition to the water permanent dipole to evaluate the simulated FIR profiles of water and CO2 polarizable molecules. They were found to play a minor role in the genesis of the FIR profiles of water/CO2 under supercritical conditions. The analysis of the reorientational dynamics of D2O shows that the rotational dynamics of water is weakly anisotropic due to the EDA interactions which affect more specifically the reorientational motions of the C2 symmetry axis of solute. These results have been used to assess the contribution of the vibrational relaxation in the experimental mid-infrared profiles associated with the nu1 symmetric and nu3 antisymmetric stretching and nu2 bending modes of D2O. It was found that the rotational dynamics mainly contribute to the broadening of the infrared (IR) profiles. Nevertheless, the vibrational processes play a role in the frequency shifts of the band centers and the relative intensity enhancements of the nu1 and nu3 modes of D2O. In particular, the EDA interactions between water and CO2 lead to the appearance of a well-defined IR band of the nu1 mode of D2O. Finally, a comparison with another model taking only into account dipole-quadrupole electrostatic interactions between water and CO2 molecules clearly reveals that EDA interactions have to be considered to reproduce both MIR and FIR measurements. From this point of view CO2 can be classified on a hydrophilic solvent scale based upon the solubility criterion as an intermediate solvent between "inert" xenon and carbon tetrachloride.