Guanidine as a strong CO2 adsorbent: a DFT study on cooperative CO2 adsorption

Among the various carbon capture and storage (CCS) technologies, the direct air capture (DAC) of CO2 by engineered chemical reactions on suitable adsorbents has attained more attention in recent times. Guanidine (G) is one of such promising adsorbent molecules for CO2 capture. Recently Lee et al. (Phys. Chem. Chem. Phys., 2015, 17, 10925-10933) reported an interaction energy (ΔE) of -5.5 kcal mol-1 for the GCO2 complex at the CCSD(T)/CBS level, which was one of the best non-covalent interactions observed for CO2 among several functional molecules. Here we show that the non-covalent GCO2 complex can transform to a strongly interacting G-CO2 covalent complex under the influence of multiple molecules of G and CO2. The study, conducted at M06-2X/6-311++G** level density functional theory, shows ΔE = -5.7 kcal mol-1 for GCO2 with an NC distance of 2.688 Å while almost a five-fold increase in ΔE (-27.5 kcal mol-1) is observed for the (G-CO2)8 cluster wherein the N-C distance is 1.444 Å. All the (G-CO2)n clusters (n = 2-10) show a strong N-CO2 covalent interaction with the N-C distance gradually decreasing from 1.479 Å for n = 2 to 1.444 Å for n = 8 ≅ 9, 10. The N-CO2 bonding gives (G+)-(CO2-) zwitterion character for G-CO2 and the charge-separated units preferred a cyclic arrangement in (G-CO2)n clusters due to the support of three strong intermolecular OHN hydrogen bonds from every CO2. The OHN interaction is also enhanced with an increase in the size of the cluster up to n = 8. The high ΔE is attributed to the large cooperativity associated with the N-CO2 and OHN interactions. The quantum theory of atoms in molecules (QTAIM) analysis confirms the nature and strength of such interactions, and finds that the total interaction energy is directly related to the sum of the electron density at the bond critical points of N-CO2 and OHN interactions. Further, molecular electrostatic potential analysis shows that the cyclic cluster is stabilized due to the delocalization of charges accumulated on the (G+)-(CO2-) zwitterion via multiple OHN interactions. The cyclic (G-CO2)n cluster formation is a highly exergonic process, which reveals the high CO2 adsorption capability of guanidine.

Splitting of multiple hydrogen molecules by bioinspired diniobium metal complexes: a DFT study

Splitting of molecular hydrogen (H₂) into bridging and terminal hydrides is a common step in transition metal chemistry. Herein, we propose a novel organometallic platform for cleavage of multiple H₂ molecules, which combines metal centers capable of stabilizing multiple oxidation states, and ligands bearing positioned pendant basic groups. Using quantum chemical modeling, we show that low-valent, early transition metal diniobium(ii) complexes with diphosphine ligands featuring pendant amines can favorably uptake up to 8 hydrogen atoms, and that the energetics are favored by the formation of intramolecular dihydrogen bonds. This result suggests new possible strategies for the development of hydrogen scavenger molecules that are able to perform reversible splitting of multiple H₂ molecules.

Formation of large clusters of CO2 around anions: DFT study reveals cooperative CO2 adsorption

The cooperative O⋯C secondary interactions compensate for the diminishing effect of primary anion⋯C interactions in anionic clusters of CO₂ molecules.

Anion-encapsulating fullerenes behave as large anions: a DFT study

M06L/6-311++G(d,p)//M06L/6-31G(d,p) level density functional theory studies show that the endohedral reaction of C60 with X- (X = F, Cl, Br, OH, NH2, NO2, CN, and ClO) is exothermic by 37.8-65.2 kcal mol-1. The exothermic character of the reaction is drastically reduced in polar and nonpolar solvents due to the lack of direct solvation influence on the encapsulated anion. In all X-@C60, the occupied frontier molecular orbitals (FMOs) are located on X- while the energy levels of FMOs centered on C60 are very similar to those of the C60- radical anion. Molecular electrostatic potential (MESP) analysis of X-@C60 revealed that the negative character of the MESP minimum (Vmin) on the carbon cage increases by ∼72 fold compared to C60, which is very similar to the enhancement in the negative MESP observed on the C60- radical anion. The MESP data and quantum theory of atoms in molecules (QTAIM) analysis of charge, electron delocalization index, and Laplacian of bond critical point (bcp) support significant electron sharing from the anion to the carbon atoms of the fullerene cage, which makes the cage behave like a very large anion in a closed shell configuration. The data are also supportive of a multicenter charge-shift type of bonding interaction between the anion and the carbon cage. The anionic nature of the fullerene cage has been verified in the cases of larger systems such as Cl-@C70, Cl-@C84, and Cl-@C90. The binding of a counter cation K+ with X-@C60 is found to be highly exothermic (∼72 kcal mol-1) and very similar to the binding of K+ with the C60- radical anion (72.9 kcal mol-1), which suggests that C60 in X-@C60 behaves as a closed shell anion.

A Noncovalent Binding Strategy to Capture Noble Gases, Hydrogen and Nitrogen

A molecular design strategy to develop receptor systems for the entrapment of noble gases, H₂ and N₂ is described using M06L-D3/6-311++G(d,p)//M06L/6-311++G(d,p) DFT method. These receptors made with two-, three-, four- and five-fluorinated benzene cores, linked with methelene units viz. R_I , R_II , R_III and R_IV as well as the corresponding non-fluorinated hydrocarbons viz. R_IH , R_IIH , R_IIIH and R_IVH show a steady and significant increase in binding energy (E_int ) with increase in the number of aromatic rings in the receptor. A stabilizing "cage effect" is observed in the cyclophane type receptors R_IV and R_IVH which is 26-48% of total E_int for all except the larger sized Kr, Xe and N₂ complexes. E_int of R_IV …He, R_IV …Ne, R_IV …Ar, R_IV …Kr, R_IV …H₂ and R_IV …N₂ is 4.89, 7.03, 6.49, 6.19, 8.57 and 8.17 kcal/mol, respectively which is 5- to9-fold higher than that of hexafluorobenzene. Similarly, compared to benzene, multiple fold increase in E_int is observed for R_IVH receptors with noble gases, H₂ and N₂ . Fluorination of the aromatic core has no significant impact on E_int (∼ ±0.5 kcal/mol) for most of the systems with a notable exception of the cage receptor R_IV for N₂ where fluorination improves E_int by 1.61 kcal/mol. The E_int of the cage receptors may be projected as one of the highest interaction energy ranges reported for noble gases, H₂ and N₂ for a neutral carbon framework. Synthesis of such systems is promising in the study of molecules in confined environment. © 2018 Wiley Periodicals, Inc.

Periodic Trends Manifested through Gas-Phase Generation of Anions Such as [AlH4]-, [GaH4]-, [InH4]-, [SrH3]-, [BaH3]-, [Ba(0)(η2-O2CH)1]-, [Pb(0)H]-, [Bi(I)H2]-, and Bi- from Formates

Metal-hydride anions of main group elements, such as BaH₃ ^- and InH₄ ^-, were generated by dissociating formate adducts of the respective metal formates. Upon activation, these adducts fragment by formate-ion ejection or by decarboxylation. For adducts of alkali-metal formates, the formate-ion ejection is the preferred pathway, whereas for those of alkaline-earth and group 13-15 metals, the expulsion of CO₂ is the more favorable pathway. Decarboxylation is deemed to yield a metal-hydrogen bond presumably by a hydride transfer to the metal atom. For example, the decarboxylation of Al(η-OCOH)₄ ^- and Ga(η-OCOH)₄ ^- generated AlH₄ ^- and GaH₄ ^-, respectively. The initial fragment-ion with a H-M bond formed in this way from adducts of the heavier metals of group 13 (Ga, In, and Tl) undergo a unimolecular reductive elimination, ascribable to the "inert-pair" effect, to lower the metal-ion oxidation state from +3 to +1. As group 13 is descended, the tendency for this reductive elimination process increases. PbH₃ ^-, generated from the formate adduct of lead formate, reductively eliminated H₂ to form PbH^-, in which Pb is in oxidation state zero. In the energy-minimized structure [H-Pb(η²-H₂)]^-, proposed as an intermediate for the process, a H₂ molecule is coordinated with PbH^- as a dihapto ligand. The formate adducts of strontium and barium produce monoleptic ions such as [M(0)(η²-O₂CH)₁]^-, in which the formate ion is chelated to a neutral metal atom. The bismuth formate adduct undergoes a double reductive elimination process whereby the oxidation state of Bi is reduced from +3 to +1 and then to -1. Upon activation, the initially formed [H-Bi-H]^- ion transforms to an anionic η²-H₂ complex, which eliminates dihydrogen to form the bismuthide anion (Bi^-).

Sumanene: an efficient π-bowl for dihydrogen storage

The outstanding ability of sumanene, its anionic and dianionic forms and sumanene–M⁺ion-pair complexes (M = Li, Na, K) to bind dihydrogen has been revealed using density functional theory calculations pointing out that these systems could be employed for developing new H₂storage systems.

Dihydrogen Binding Affinity of Polyatomic Anions: A DFT Study

The dihydrogen-binding ability of polyatomic oxohalo anions ClO^-, ClO₂ ^-, ClO₃ ^-, ClO₄ ^-, BrO^-, BrO₂ ^-, BrO₃ ^-, and BrO₄ ^- has been studied at the M06L/6-311++G(d,p) density functional theory and the CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVDZ ab initio theory. The maximum number of dihydrogen adsorbed by the anions (n _max) varies from 17 to 24 in the first coordination shell. As the number of H₂ adsorbed varies from 1 to n _max, the oxochloro and oxobromo anions show a wide range for interaction energy (E _int), namely, 1.5-45.4 kcal/mol for the former and 1.4-46.0 kcal/mol for the latter. These results indicate that both series of anions show very similar and high affinity to bind with several dihydrogen molecules. Further, an increase in the coordination ability and a decrease in the strength of the dihydrogen interaction are observed with an increase in the number of oxygen atoms in the polyatomic anion. In contrast, the neutral oxohaloacids show negligible interaction with dihydrogen. The anion···H₂ noncovalent interactions along with H···H dihydrogen interactions within the complex are ascertained by locating the bond critical points (bcps) in the quantum theory of atoms in molecules analysis. The electron density at the bcp summed up for all of the anion···H₂ interactions (∑ρ_bcp) showed a strong linear relationship with E _int, indicating that the stability of the complex is due to the formation of a large network of noncovalent bonds in the complex. The amount of electron density donated by the anion to the dihydrogen during complex formation is also gauged from the molecular electrostatic potential values at the nuclei (V _n) of all of the atoms in the anion. The hydrogen uptake leads to a significant reduction in the negative character of V _n, and the total change in V _n from all of the anion atoms (∑ΔV _n) is found to be directly proportional to E _int. The polyatomic anions have a very high affinity toward dihydrogen binding, which can be utilized for the development of new hydrogen storage systems.