IR Spectra of Protonated Carbonic Acid and Its Isomeric H3O+⋅CO2 Complex

A Two Step Postsynthetic Modification Strategy: Appending Short Chain Polyamines to Zn-NH2-BDC MOF for Enhanced CO2 Adsorption

Functionalizing metal-organic frameworks (MOFs) with amines is a commonly used strategy to enhance their performance in CO₂ capture applications. As such, in this work, a two-step strategy to covalently functionalize NH₂-containing MOFs with short chain polyamines was developed. In the first step, the parent MOF, Zn₄O(NH₂-BDC)₃, was exposed to bromoacetyl bromide (BrAcBr), which readily reacts with pendant -NH₂ groups on the 2-amino-1,4-benzenedicarboxylate (NH₂-BDC^2-) ligand. ¹H NMR of the digested MOF sample revealed that as much as 90% of the MOF ligands could be functionalized in the first step. Next, the MOF samples 60% of the ligands functionalized with acetyl bromide, Zn₄O(NH₂-BDC)_1.2(BrAcNH-BDC)_1.8, was exposed to several short chain amines including ethylenediamine (ED), diethylenetriamine (DETA), and tris(2-aminoethyl)amine (TAEA). Subsequent digested ¹H NMR analysis indicated that a total of 30%, 28%, and 19% of the MOF ligands were successfully grafted to ED, DETA, and TAEA, respectively. Next, the CO₂ adsorption properties of the amine grafted MOFs were studied. The best performing material, TAEA-appended-Zn₄O(NH₂-BDC)_1.2(BrAcNH-BDC)_1.8, exhibits a zero-coverage isosteric heat of CO₂ adsorption of -62.5 kJ/mol, a value that is considerably higher than the one observed for the parent framework, -21 kJ/mol. Although the boosted CO₂ affinity only leads to a slight increase in the CO₂ adsorption capacity in the low-pressure regime (0.15 bar), which is of interest in postcombustion carbon dioxide capture, the CO₂/N₂ (15/85) selectivity at 313 K is 143, a value that is ∼35 times higher than the one observed for Zn₄O(NH₂-BDC)₃, 4.1. Such enhancements are attributed to accessible primary amines, which were grafted to the MOF ligand. This hypothesis was further supported via in situ DRIFTS measurements of TAEA-Ac-Zn₄O(NH₂-BDC)_1.2(BrAcNH-BDC)_1.8 after exposure to CO₂, which revealed the chemisorption of CO₂ via the formation of hydrogen bonded carbamates/carbamic acid and CO₂^δ- species; the latter are adducts formed between CO₂ and [amineH]⁺Br^- salts that are produced during the amine grafting step.

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.

Infrared Spectrum of the Adamantane+ -Water Cation: Hydration-Induced C-H Bond Activation and Free Internal Water Rotation

Diamondoid cations are reactive intermediates in their functionalization reactions in polar solution. Hydration is predicted to strongly activate their C-H bonds in initial proton abstraction reactions. To study the effects of microhydration on the properties of diamondoid cations, we characterize herein the prototypical monohydrated adamantane cation (C₁₀ H₁₆ ⁺ -H₂ O, Ad⁺ -W) in its ground electronic state by infrared photodissociation spectroscopy in the CH and OH stretch ranges and dispersion-corrected density functional theory (DFT) calculations. The water (W) ligand binds to the acidic CH group of Jahn-Teller distorted Ad⁺ via a strong CH⋅⋅⋅O ionic H-bond supported by charge-dipole forces. Although W further enhances the acidity of this CH group along with a proton shift toward the solvent, the proton remains with Ad⁺ in the monohydrate. We infer essentially free internal W rotation from rotational fine structure of the ν₃ band of W, resulting from weak angular anisotropy of the Ad⁺ -W potential.

IR Spectrum and Structure of Protonated Monosilanol: Dative Bonding between Water and the Silylium Ion

We report the spectroscopic characterization of protonated monosilanol (SiH₃ OH₂⁺ ) isolated in the gas phase, thus providing the first experimental determination of the structure and bonding of a member of the elusive silanol family. The SiH₃ OH₂⁺ ion is generated in a silane/water plasma expansion, and its structure is derived from the IR photodissociation (IRPD) spectrum of its Ar cluster measured in a tandem mass spectrometer. The chemical bonding in SiH₃ OH₂⁺ is analyzed by density functional theory (DFT) calculations, providing detailed insight into the nature of the dative H₃ Si⁺ -OH₂ bond. Comparison with protonated methanol illustrates the differences in bonding between carbon and silicon, which are mainly related to their different electronegativity and the different energy of the vacant valence p_z orbital of SiH₃⁺ and CH₃⁺ .