Predicting the Formation of Sulfur-Based Brønsted Acids from the Reactions of SOx with H2O and H2S

The sulfur dioxide-water complex: CCSD(T)/CBS anharmonic vibrational spectroscopy of stacked and hydrogen-bonded dimers

This study examines the structures, energies, and IR vibrational spectra of the sulfur dioxide-water SO2(H2O) complexes by employing coupled cluster theory CCSD(T) with Dunning style correlation consistent type basis sets aug-cc-pV(n+d)Z (n = D, T, Q, 5). Complete basis set (CBS) extrapolations have been carried out to predict binding energies for two isomers of the SO2(H2O) complex: a stacked global minimum (1A) structure and a hydrogen-bonded local minimum (1B) structure. The CCSD(T)/CBS extrapolation predicts an intermolecular S-O distance rS⋯O = 2.827 Å for the stacked isomer, which is in excellent agreement with an experimental measurement of 2.824 Å [K. Matsumura et al., J. Chem. Phys., 91, 5887 (1989)]. The CCSD(T)/CBS binding energy for the stacked dimer 1A and hydrogen-bonded form 1B is De = -4.37 kcal/mol and De = -2.40 kcal/mol, respectively. This study also employs anharmonic VPT2 MP2/aug-cc-pV(n+d)Z level corrections to CCSD(T)/aug-cc-pV(n+d)Z vibrational frequencies in both forms of SO2(H2O). The anharmonic CCSD(T)/aug-cc-pV(Q+d)Z OH stretching frequencies in the stacked structure 1A are 3743 cm-1 (ν3) and 3647 cm-1 (ν1), and these align well with the recorded IR spectroscopic values of 3745 and 3643 cm-1, respectively [C. Wang et al., J. Phys. Chem. Lett., 13, 5654 (2022)]. If we combine CCSD(T)/aug-cc-pV(n+d)Z De values with VPT2 vibrational frequencies, we obtain a new CCSD(T)/aug-cc-pV(Q+d)Z anharmonic dissociation energy D0 = -3.48 kcal/mol for 1A and D0 = -1.74 kcal/mol for 1B. In summary, the results presented here demonstrate that the application of CCSD(T) calculations with aug-cc-pV(n+d)Z basis sets and CBS extrapolations is critical in probing the structure and IR spectroscopic properties of the sulfur dioxide-water complex.

Effect of Linker Structure and Functionalization on Secondary Gas Formation in Metal-Organic Frameworks

Rare-earth terephthalic acid (BDC)-based metal-organic frameworks (MOFs) are promising candidate materials for acid gas separation and adsorption from flue gas streams. However, previous simulations have shown that acid gases (H₂O, NO₂, and SO₂) react with the hydroxyl on the BDC linkers to form protonated acid gases as a potential degradation mechanism. Herein, gas-phase computational approaches were used to identify the formation energies of these secondary protonated acid gases across multiple BDC linker molecules. Formation energies for secondary protonated acid gases were evaluated using both density functional theory (DFT) and correlated wave function methods for varying BDC-gas reaction mechanisms. Upon validation of DFT to reproduce wave function calculation results, rotated conformational linkers and chemically functionalized BDC linkers with -OH, -NH₂, and -SH were investigated. The calculations show that the rotational conformation affects the molecule stability. Double-functionalized BDC linkers, where two functional groups are substituted onto BDC, showed varied reaction energies depending on whether the functional groups donate or withdraw electrons from the aromatic system. Based on these results, BDC linker design must balance adsorption performance with degradation via linker dehydrogenation for the design of stable MOFs for acid gas separations.

Predicting the Mechanism and Products of CO2 Capture by Amines in the Presence of H2O

An extensive correlated molecular orbital theory study of the reactions of CO₂ with a range of substituted amines and H₂O in the gas phase and aqueous solution was performed at the G3(MP2) level with a self-consistent reaction field approach. The G3(MP2) calculations were benchmarked at the CCSD(T)/CBS level for NH₃ reactions. A catalytic NH₃ reduces the energy barrier more than a catalytic H₂O for the formation of H₂NCOOH and H₂CO₃. In aqueous solution, the barriers to form both H₂NCOOH and H₂CO₃ are reduced, with HCO₃^- formation possible with one amine present and H₂NCOO^- formation possible only with two amines. Further reactions of H₂NCOOH to form HNCO and urea via the Bazarov reaction have high barriers and are unlikely in both the gas phase and aqueous solution. Reaction coordinates for CH₃NH₂, CH₃CH₂NH₂, (CH₃)₂NH, CH₃CH₂CH₂NH₂, (CH₃)₃N, and DMAP were also calculated. The barrier for proton transfer correlates with amine basicity for alkylammonium carbamate (ΔG^‡_aq < 15 kcal/mol) and alkylammonium bicarbonate (ΔG^‡_aq < 30 kcal/mol) formation. In aqueous solution, carbamic acids, carbamates, and bicarbonates can all form in small amounts with ammonium carbamates dominating for primary and secondary alkylamines. These results have implications for CO₂ capture by amines in both the gas phase and aqueous solution as well as in the solid state, if enough water is present.

Anharmonic fundamental vibrational frequencies and spectroscopic constants of the potential HSO2 radical astromolecule

The recent report that HSO₂ is likely kinetically favored over the HOSO thermodynamic product in hydrogen addition to sulfur dioxide in simulated Venusian atmospheric conditions has led to the need for reference rotational, vibrational, and rovibrational spectral data for this molecule. While matrix-isolation spectroscopy has been able to produce vibrational frequencies for some of the vibrational modes, the full infrared to microwave spectrum of 1 ²A' HSO₂ is yet to be generated. High-level quantum chemical computations show in this work that the >2.5 D dipole moment of this radical makes it a notable target for possible radioastronomical observation. Additionally, the high intensity antisymmetric S-O stretch is computed here to be 1298.3 cm^-1, a 13.9 cm^-1 blueshift up from H₂ matrix analysis. In any case, the full set of rotational and spectroscopic constants and anharmonic fundamental vibrational frequencies is provided in this work in order to help characterize HSO₂ and probe its kinetic favorability.

Preferential SOx Adsorption in Mg-MOF-74 from a Humid Acid Gas Stream

The preferential adsorption of SO_x versus water in Mg-MOF-74 from a humid SO_x gas stream has been investigated via materials studies and nuclear magnetic resonance (NMR). Mg-MOF-74 has been synthesized and subsequently loaded simultaneously with water vapor and SO_x (62-96 ppm) in an adsorption chamber at room temperature over a time period of 4 days with a sample taken every 24 h. Each sample was analyzed by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA)-mass spectrometry, and scanning electron microscopy-energy-dispersive spectroscopy. The metal-organic framework (MOF) showed retained crystallinity and peak intensity in PXRD, and after 2 days, it showed no obvious degradation to the structure. Use of multiple techniques, including TGA, identified 10% by weight of SO_x species, specifically H₂S and SO₂, within the MOF. ¹H solid-state NMR shows a substantial reduction of H₂O when SO_x is present, which is consistent with SO_x preferentially binding to the oxophilic metal site of the framework. After 14 weeks aging, the sulfur remains present in the three-dimensional MOF, with only half being desorbed after 23 weeks in air.