Polyanionic cyano-fullerides for CO2 capture: a DFT prediction

The reaction of C₆₀ fullerene with 'n' molecules (n = 1 to 6) of 1,3-dimethyl-2,3-dihydro-2-cyano-imidazole (IMCN) results in the exothermic formation of imidazolium cation-polyanionic fulleride complexes, (IM⁺)_n⋯((C₆₀(CN)_n)^n-). The binding energy of IM⁺ with (C₆₀(CN)_n)^n- in the imidazolium-fulleride ionic complexes increased from -69.6 kcal mol^-1 for n = 1 to -202.9 kcal mol^-1 for n = 6. The energetics of the complex formation and cation-anion interaction energy data suggest the formation of imidazolium-fulleride ionic liquid (IL) systems. Furthermore, the dimer formation of such ionic complexes showed more exergonic nature due to multiple cooperative electrostatic interactions between oppositely charged species and suggested improved energetics for higher order clusters. The molecular electrostatic potential (MESP) analysis has revealed that the extra 'n' electrons in the ionic complex as well as that in the bare (C₆₀(CN)_n)^n- are delocalized mainly on the unsaturated carbon centers of the fullerene unit, while the CN groups remain as a neutral unit. The MESP minimum (V_min) values of (C₆₀(CN)_n)^n- on the carbon cage have shown that the addition of each CN^- unit on the cage enhances the negative character of V_min by ∼54.7 kcal mol^-1. This enhancement in MESP is comparable to the enhancement observed when one electron is added to C₆₀ to produce (^-62.5 kcal mol^-1) and suggests that adding 'n' CN^- groups to the fullerene cage is equivalent to supplying 'n' electrons to the carbon cage. Also the high capacity of the fullerene cage to hold several electrons can be attributed to the spherical delocalization of them onto the electron deficient carbon cage. The interactive behavior of CO₂ molecules with (IM⁺)_n⋯(C₆₀(CN)_n)^n- systems showed that the interaction becomes stronger from -2.3 kcal mol^-1 for n = 1 to -18.6 kcal mol^-1 for n = 6. From the trianionic fulleride onwards, the C⋯CO₂ noncovalent (nc) interaction changes to C-CO₂ covalent (c) interaction with the development of carboxylate character on the adsorbed CO₂. These results prove that cyano-fullerides are promising candidates for CO₂ capture.

Hydration patterns of rings in drugs and relationship to lipophilicity: A DFT study

Rings are one of the major scaffold components of drugs in medicinal chemistry, due to their unique electronic distribution, scaffold rigidity, and three-dimensionality while lipophilicity is considered as a vital parameter of rings that can influence the reactivity, metabolic stability, and toxicity. We have analyzed the electronic features, hydration patterns, solvation effect and lipophilicity data for 51 most widely used ring systems in drugs. Molecular electrostatic potential (MESP) topology analysis has been used to assess the electronic distribution in rings which provided an easy interpretation of the most suitable hydration patterns of the ring with H₂ O molecule. Further, the global minimum of ring…H₂ O complex has been utilized to predict lipophilicity (logP) with the incorporation of implicit solvation effect. Classification of ring systems based on their molecular weight into four categories, viz. small ring 'sr', medium ring 'mr', large ring 'lr' and extra large ring 'xlr' systems has led to the finding of strong correlations between logP and hydration energy with R = 0.942, 0.933, 0.968 and 0.933, respectively. The micro solvation model is found to be useful for locating the hydrophobic-hydrophilic border for each category of rings in terms of hydration energy whereas the implicit solvation model used for two solvents, n-octanol and water on the most stable hydrated structure led to a global correlation between logP and solvation energy ratio. This correlation predicts a limiting logP value -7.03 for the most hydrophilic ring system and also suggests a clear partitioning of the ring molecules into hydrophobic and hydrophilic classes. The MESP topology-guided approach to understand the electronic features and hydration patterns of rings in drugs lead to powerful predictions on their lipophilicity behavior.

Imidazolium-fulleride ionic liquids - a DFT prediction

Ionic liquids (ILs) exhibit tunable physicochemical properties due to the flexibility of being able to select their cation-anion combination from a large pool of ions. The size of the ions controls the properties of the ILs in the range from ionic to molecular, and thus large ions play an important role in regulating the melting temperature and viscosity. Here, we show that the exohedral addition of anionic X^- moieties to C₆₀ (X = H, F, OH, CN, NH₂, and NO₂) is a thermodynamically viable process for creating large X-fulleride anions (C₆₀X)^-. The addition of X^- to C₆₀ is modelled by locating the transition state for the reaction between C₆₀ and 1,3-dimethyl-2X-imidazole (IMX) at the M06L/6-311++G(d,p)//M06L/6-31G(d,p) level. The reaction yields the ion-pair complex IM⁺⋯(C₆₀X)^- for X = H, F, OH, CN, NH₂, and NO₂ and the ordered pair of (activation free energy, reaction free energy) is found to be (14.5, 1.1), (6.1, 3.1), (16.7, 2.3), (14.7, -7.9), (27.9, 0.5) and (11.9, 12.4), respectively. The low barrier of the reactions suggests their feasibility. The reaction is slightly endergonic for X = H, F, OH, and NH₂, while X = CN shows a significant exergonic character. The X-fulleride formation is not observed when X = Cl and Br. The ion-pair interactions (E_ion-pair) observed for IM⁺⋯(C₆₀X)^- range from -64.0 to -73.0 kcal mol^-1, which is substantially lower (∼10%) than the typically reported values for imidazolium-based ionic liquids such as [EMIm]⁺[trz]^-, [EMIm]⁺[dc]^-, [EMIm]⁺[dtrz]^-, and [EMIm]⁺[NH₂tz]^-. The quantum theory of atoms in molecules (QTAIM) analysis showed that the C-X bonding in (C₆₀X)^- is covalent, while that in (IM⁺⋯X^-)⋯C₆₀ (for X = Cl and Br) is non-covalent. Furthermore, molecular electrostatic potential (MESP) analysis showed that the X-fulleride could behave as a large spherical anion due to the delocalization of the excess electron in the system over the entire carbon framework. The large anionic character of the X-fulleride is also revealed by the identification of several close lying local energy minima for the IM⁺⋯(C₆₀X)^- ion-pair. The low E_ion-pair value, the significant contribution of dispersion to the E_ion-pair and the spherical nature of the anion predict low-melting point and highly viscous IL formation from X-fullerides and the imidazolium cation.

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.

Growth Pattern, Stability, and Properties of Complexes of C2H5OH and nCO2 (n = 1-5) Molecules: A Theoretical Study

This work is dedicated to theoretically investigate the formation process of C2H5OH···nCO2 (n = 1-5) complexes and to shed light on the nature of interactions formed under the variation of CO2 concentration. It is found that CO2 molecules tend to locate around the polarized -OH group to interact with the lone pairs of the O atom. The interaction of ethanol with three CO2 molecules (C2H5OH···3CO2) induces the most stable structure in the sequence considered. The atoms in molecules (AIM), NCIplot, and natural bond orbital (NBO) analyses point out that the Oethanol···CCO2 tetrel bond overcomes hydrogen, chalcogen, and CO2···CO2 tetrel-bonded interactions and mainly contributes to the strength of C2H5OH···nCO2 (n = 1-5) complexes. All intermolecular interactions in the examined complexes are weakly noncovalent, and their positive cooperativity is evaluated to be slightly weaker than that of CO2 pure systems. SAPT2+ and molecular electrostatic potential (MEP) calculations indicate that the electrostatic force is the main factor underlying the attractive interplay in the complexes of C2H5OH and CO2.

Dissociative detachment of the fluoroformate anion

3D fragment imaging of the fluoroformate anion (FCO₂⁻) dissociative photodetachment products shows reductive fragmentation, forming FCO + O, as well as a dominant cleavage of the CF bond.