Exploring High-Pressure Polymorphism in Carbonic Acid through Direct Synthesis from Carbon Dioxide Clathrate Hydrate

Carbon dioxide (CO2) is widespread in astrochemically relevant environments, often coexisting with water (H2O) ices and thus triggering a great interest regarding the possible formation of their adducts under various thermodynamic conditions. Amongst them, solid carbonic acid (H2CO3) remains elusive, yet being widely studied. Synthetic routes followed for its production have always been characterised by drastic irradiation on solid ice mixtures or complex procedures on fluid samples (such as laser heating at moderate to high pressures). Here we report about a simpler yet effective synthetic route to obtain two diverse carbonic acid crystal structures from the fast, cold compression of pristine clathrate hydrate samples. The two distinct polymorphs we obtained, differing in the water content, have been deeply characterised via spectroscopic and structural techniques to assess their composition and their astonishing pressure stability, checked up to half a megabar, also highlighting the complex correlations between them so to compile a detailed phase diagram of this system. These results may have a profound impact on the prediction and modelisation of the complex chemistry which characterises many icy bodies of our Solar System.

Conformer-Specific Photoelectron Spectroscopy of Carbonic Acid: H2CO3

Carbonic acid (H2CO3) is a fundamental species in biological, ecological, and astronomical systems. However, its spectroscopic characterization is incomplete because of its reactive nature. The photoionization (PI) and the photoion mass-selected threshold photoelectron (ms-TPE) spectra of H2CO3 were obtained by utilizing vacuum ultraviolet (VUV) synchrotron radiation and double imaging photoelectron photoion coincidence spectroscopy. Two carbonic acid conformers, namely, cis-cis and cis-trans, were identified. Experimental adiabatic ionization energies (AIEs) of cis-cis and cis-trans H2CO3 were determined to be 11.27 ± 0.02 and 11.18 ± 0.03 eV, and the cation enthalpies of formation could be derived as ΔfH°0K = 485 ± 2 and 482 ± 3 kJ mol-1, respectively. The cis-cis conformer shows intense peaks in the ms-TPES that are assigned to the C=O/C-OH stretching mode, while the cis-trans conformer exhibits a long progression to which two C=O/C-OH stretching modes contribute. The TPE spectra allow for the sensitive and conformer-selective detection of carbonic acid in terrestrial experiments to better understand astrochemical reactions.

In Situ Electrochemistry of Formate on Cu Thin Films Using ATR-FTIR Spectroscopy and X-ray Photoelectron Spectroscopy

Formate (HCOO-) is the most dominant intermediate identified during carbon dioxide electrochemical reduction (CO2ER). While previous studies showed that copper (Cu)-based materials that include Cu(0), Cu2O, and CuO are ideal catalysts for CO2ER, challenges to scalability stem from low selectivity and undesirable products in the -1.0-1.0 V range. There are few studies on the binding mechanism of intermediates and products for these systems as well as on changes to surface sites upon applying potential. Here, we use an in situ approach to study the redox surface chemistry of formate on Cu thin films deposited on Si wafers using a VeeMAX III spectroelectrochemical (SEC) cell compatible with attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Spectra for surface species were collected in real time as a function of applied potential during cyclic voltammetry (CV) experiments. Results showed the reproducibility of CV curves on freshly prepared Cu/Si wafers with relatively high signal-to-noise ATR-FTIR absorbance features of surface species during these electrochemical experiments. The oxidation reaction of HCOO- to bicarbonate (HCO3-) was observed using ATR-FTIR at a voltage of 0.27 V. Samples were then subjected to reduction in the CV, and the aqueous phase products below the detection limit of the SEC-ATR-FTIR were identified using ion chromatography (IC). We report the formation of glycolate (H3C2O3-) and glyoxylate (HC2O3-) with trace amounts of oxalate (C2O42-), indicating that C-C coupling reactions proceed in these systems. Changes to the oxidation state of surface Cu were measured using X-ray photoelectron spectroscopy, which showed a reduction in Cu(0) and an increase in Cu(OH)2, indicating surface oxidation.

Effects of carbon dioxide and oxygen on the growth rate of various food spoilage bacteria

The growth of six bacterial species (Carnobacterium maltaromaticum, Bacillus weihenstephanensis, Bacillus cereus, Paenibacillus spp., Leuconostoc mesenteroides and Pseudomonas fragi) was studied in various gas compositions. Growth curves were obtained at various oxygen concentrations (between 0.1 and 21%), or various carbon dioxide concentrations (between 0 and 100%). Decreasing the O₂ concentration from 21% to about 3-5% has no effect on the bacterial growth rates, which are only affected by low oxygen levels. For each strain studied, the growth rate decreased linearly with carbon dioxide concentration, except for L. mesenteroides which remained insensible to this gas. Conversely, the most sensitive strain was totally inhibited by 50% of carbon dioxide in the gas phase at 8 °C. Predictive models were fitted, and the parameters characterizing the inhibitory effect of these two gases were estimated. This study provides new tools to help the food industry design suitable packaging for MAP storage.

Mechanistic In Situ ATR-FTIR Studies on the Adsorption and Desorption of Major Intermediates in CO2 Electrochemical Reduction on CuO Nanoparticles

Increasing levels of carbon dioxide (CO₂) from human activities is affecting the ecosystem and civilization as we know it. CO₂ removal from the atmosphere and emission reduction by heavy industries through carbon capture, utilization, and storage (CCUS) technologies to store or convert CO₂ to useful products or fuels is a popular approach to meet net zero targets by 2050. One promising process of CO₂ removal and conversion is CO₂ electrochemical reduction (CO₂ER) using metal and metal oxide catalysts, particularly copper-based materials. However, the current limitations of CO₂ER stem from the low product selectivity of copper electrocatalysts due to existing knowledge gaps of the reaction mechanisms using surfaces that normally have native oxide layers. Here, we report systematic control studies of the surface interactions of major intermediates in CO₂ER, formate, bicarbonate, and acetate, with CuO nanoparticles in situ and in real time using attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). Spectra were collected as a function of concentration, pH, and time in the dark and the in absence of added electrolytes. Isotopic exchange experiments were also performed to elucidate the type of surface complexes from H/D exchange. Our results show that the organics and bicarbonate form mostly outer-sphere complexes mediated by hydrogen bonding with CuO nanoparticles with Gibbs free energy of adsorption of about -25 kJ mol^-1. The desorption kinetics of the surface species indicated relatively fast and slow regions reflective of the heterogeneity of sites that affect the strength of hydrogen bonding. These results suggest that hydrogen bonding, whether intermolecular or with surface sites on CuO nanoparticles, might be playing a more important role in the CO₂ER reaction mechanism than previously thought, contributing to the lack of product selectivity.

Theoretical Characterization of Carbonic Acid Clusters in the UV

Two theoretical structural motifs are proposed to match two experimental solid carbonic acid UV spectra from previous literature ( Astron. Astrophys. 2021, 646, A172): a linear ribbon structure as a single octamer and nonplanar orientations of carbonic acid clusters. The latter have some contribution from approximated amorphous solid carbonic acid in the form of 40 different clusters of 8 carbonic acid molecules ensemble-averaged together, but unoptimized pairs of optimized dimers oriented perpendicular to one another give the strongest intensities of lower energy UV transitions. The linear ribbon structure's predicted spectrum computed with CAM-B3LYP/6-311G(d,p) agrees well with Experimental Solid B─the β-carbonic acid experimental data in the UV region. Meanwhile, the 40 amorphous clusters are built with a randomization script, and the electronically excited states are calculated with both CAM-B3LYP/6-311G(d,p) and ωB97XD/6-311G(d,p). The resulting theoretical spectrum is constructed by employing a Boltzmann distribution of the intensities and artificially broadening the simulated spectra. The nonplanar dimer pairs are computed with CAM-B3LYP and B3LYP with the 6-311G(d,p) basis set. The results of the amorphous simulation weakly correspond with the Experimental Solid A spectrum, but the fully nonplanar motif matches the experiment much more convincingly. As a result, the previous work appears to have observed the traditional crystalline phase of solid carbonic acid in Experimental Solid B, whereas the nonplanar orientations of the carbonic acids in the clusters appear to correlate with Experimental Solid A. This spectral classification will aid in future laboratory work exploring the role that carbonic acid can play in low temperature, low pressure desorbed environments with potential application to astrochemistry.

Electron Flow Characterization of Charge Transfer for Carbonic Acid to Strong Base Proton Transfer in Aqueous Solution

Protonation of the strong base methylamine CH₃NH₂ by carbonic acid H₂CO₃ in aqueous solution, HOCOOH···NH₂CH₃ → HOCOO^-···⁺HNH₂CH₃, has been previously studied ( J. Phys. Chem. B 2016, 109, 2271-2280; J. Phys. Chem. B 2016, 109, 2281-2290) via Car-Parinnello molecular dynamics. This proton transfer (PT) reaction within a hydrogen (H)-bonded complex was found to be barrierless and very rapid, with key reaction coordinates comprising the proton coordinate, the H-bond separation R_ON, and a solvent coordinate, reflecting the water solvent rearrangement involved in the neutral to ion pair conversion. In the present work, the reaction's charge flow aspects are analyzed in detail, especially a description via Mulliken charge transfer for PT (MCTPT). A natural bond orbital analysis and some extensions of them are employed for the complex's electronic structure during the reaction trajectories. Results demonstrate that consistent with the MCTPT picture, the charge transfer (CT) occurs from a methylamine base nonbonding orbital to a carbonic acid antibonding orbital. A complementary MCTPT reaction product perspective of CT from the antibonding orbital of the HN⁺ moiety to the nonbonding orbital of the oxygen in the H-bond complex is also presented. σ_OH and σ_HN⁺ bond order expressions show this CT to occur within the H-bond OHN triad, an aspect key for simultaneous bond-breaking and -forming in the PT reaction.

Linear and Helical Carbonic Acid Clusters

Crystallization of carbonic acid likely begins with a linear or ribbon-esque oligomerization, but a helical spiral is shown here to be a new, competing motif for this process. The present combined density functional theory and coupled-cluster theory work examines both the ribbon and the new helical spiral motifs in terms of relative energies, sequential binding energies, and electronic spectra which could potentially aid in distinguishing between the two forms. The helix diverges in energy from the ribbon by roughly 0.2 eV (∼4 kcal/mol) per dimer addition, but the largest intensity absorption features at 9.16 eV (135 nm) and 7.11 eV (175 nm), respective of the ribbon and spiral, will allow these to be separately observed and classified via electronic spectroscopy to determine more conclusively which motif holds in the earliest formation stages of solid carbonic acid.

Observation of Carbonic Acid Formation from Interaction between Carbon Dioxide and Ice by Using In Situ Modulation Excitation IR Spectroscopy

Carbonic acid, H₂ CO₃ , is of fundamental importance in nature both in living and non-living systems. Providing direct spectroscopic evidence for carbonic acid formation is however a challenge. Here we provide clear evidence by in situ attenuated total reflection IR spectroscopy combined with modulation excitation spectroscopy and phase-sensitive detection that CO₂ adsorption on ice surfaces is accompanied by carbonic acid formation. We demonstrate that carbonic acid can be formed from CO₂ on ice in the absence of high-energy irradiation and without protonation by strong acids. The formation of carbonic acid is favored at low temperature, whereas at high temperature it rapidly dissociates to form bicarbonate (HCO₃ ^- ) and carbonate (CO₃ ^2- ). The direct formation of carbonic acid from adsorption of CO₂ on ice could play a role in the upper troposphere in cirrus clouds, where all the necessary ingredients to form carbonic acid, that is, low temperature, CO₂ gas, and ice, are present.

Dynamic CO2 and pH levels in coastal, estuarine, and inland waters: Theoretical and observed effects on harmful algal blooms

Rising concentrations of atmospheric CO₂ results in higher equilibrium concentrations of dissolved CO₂ in natural waters, with corresponding increases in hydrogen ion and bicarbonate concentrations and decreases in hydroxyl ion and carbonate concentrations. Superimposed on these climate change effects is the dynamic nature of carbon cycling in coastal zones, which can lead to seasonal and diel changes in pH and CO₂ concentrations that can exceed changes expected for open ocean ecosystems by the end of the century. Among harmful algae, i.e. some species and/or strains of Cyanobacteria, Dinophyceae, Prymnesiophyceae, Bacillariophyceae, and Ulvophyceae, the occurrence of a CO₂ concentrating mechanisms (CCMs) is the most frequent mechanism of inorganic carbon acquisition in natural waters in equilibrium with the present atmosphere (400 μmol CO₂  mol^-1 total gas), with varying phenotypic modification of the CCM. No data on CCMs are available for Raphidophyceae or the brown tide Pelagophyceae. Several HAB species and/or strains respond to increased CO₂ concentrations with increases in growth rate and/or cellular toxin content, however, others are unaffected. Beyond the effects of altered C concentrations and speciation on HABs, changes in pH in natural waters are likely to have profound effects on algal physiology. This review outlines the implications of changes in inorganic cycling for HABs in coastal zones, and reviews the knowns and unknowns with regard to how HABs can be expected to ocean acidification. We further point to the large regions of uncertainty with regard to this evolving field.

Molecular Hydration of Carbonic Acid: Ab Initio Quantum Chemical and Density Functional Theory Investigation

Molecular hydration of carbonic acid (H₂CO₃) is investigated in terms of bonding patterns in H₂CO₃···(H₂O) _n [ n = 1-4] hydrogen-bonded clusters within ab initio quantum chemical and density functional theory (DFT) frameworks. Successive addition of water molecules to H₂CO₃···H₂O entails elongation of O-H (hydroxyl) bond as well as contraction of specific intermolecular hydrogen bonds signifying hydration of carbonic acid; these structural features get markedly enhanced under the continuum solvation framework. A comparison between the structurally similar clusters H₂CO₃···(H₂O) _n and HCOOH···(H₂O) _n [ n = 1-3] brings out the structural stability of the former. The present investigation in conjunction with the binding energy behavior of approaching water molecule(s) should serve as a precursor for pathways exploring aqueous dissociation of H₂CO₃ for larger clusters, as well as development of force-field potentials for acid dissociation process.

Novel Stable Compounds in the C-H-O Ternary System at High Pressure

The chemistry of the elements is heavily altered by high pressure, with stabilization of many new and often unexpected compounds, the emergence of which can profoundly change models of planetary interiors, where high pressure reigns. The C-H-O system is one of the most important planet-forming systems, but its high-pressure chemistry is not well known. Here, using state-of-the-art variable-composition evolutionary searches combined with quantum-mechanical calculations, we explore the C-H-O system at pressures up to 400 GPa. Besides uncovering new stable polymorphs of high-pressure elements and known molecules, we predicted the formation of new compounds. A 2CH4:3H2 inclusion compound forms at low pressure and remains stable up to 215 GPa. Carbonic acid (H2CO3), highly unstable at ambient conditions, was predicted to form exothermically at mild pressure (about 1 GPa). As pressure rises, it polymerizes and, above 314 GPa, reacts with water to form orthocarbonic acid (H4CO4). This unexpected high-pressure chemistry is rationalized by analyzing charge density and electron localization function distributions, and implications for general chemistry and planetary science are also discussed.

Tunnelling in carbonic acid

The cis,trans-conformer of carbonic acid (H₂CO₃), generated by near-infrared radiation, undergoes an unreported quantum mechanical tunnelling rotamerization with half-lives in cryogenic matrices of 4–20 h, depending on temperature and host material.