Cucurbit[6]uril: A Possible Host for Noble Gas Atoms

Density functional and ab initio molecular dynamics studies are carried out to investigate the stability of noble gas encapsulated cucurbit[6]­uril (CB[6]) systems. Interaction energy, dissociation energy and dissociation enthalpy are calculated to understand the efficacy of CB[6] in encapsulating noble gas atoms. CB[6] could encapsulate up to three Ne atoms having dissociation energy (zero-point energy corrected) in the range of 3.4–4.1 kcal/mol, whereas due to larger size, only one Ar or Kr atom encapsulated analogues would be viable. The dissociation energy value for the second Ar atom is only 1.0 kcal/mol. On the other hand, the same for the second Kr is −0.5 kcal/mol, implying the instability of the system. The noble gas dissociation processes are endothermic in nature, which increases gradually along Ne to Kr. Kr encapsulated analogue is found to be viable at room temperature. However, low temperature is needed for Ne and Ar encapsulated analogues. The temperature–pressure phase diagram highlights the region in which association and dissociation processes of Kr@CB[6] would be favorable. At ambient temperature and pressure, CB[6] may be used as an effective noble gas carrier. Wiberg bond indices, noncovalent interaction indices, electron density, and energy decomposition analyses are used to explore the nature of interaction between noble gas atoms and CB[6]. Dispersion interaction is found to be the most important term in the attraction energy. Ne and Ar atoms in one Ng entrapped analogue are found to stay inside the cavity of CB[6] throughout the simulation at 298 K. However, during simulation Ng₂ units in Ng₂@CB­[6] flip toward the open faces of CB[6]. After 1 ps, one Ne atom of Ne₃@CB­[6] almost reaches the open face keeping other two Ne atoms inside. At lower temperature (77 K), all the Ng atoms in Ng_<i>n</i>@CB­[6] remain well inside the cavity of CB[6] throughout the simulation time (1 ps)

On the Validity of the Maximum Hardness Principle and the Minimum Electrophilicity Principle during Chemical Reactions

Hardness and electrophilicity values for several molecules involved in different chemical reactions are calculated at various levels of theory and by using different basis sets. Effects of these aspects as well as different approximations to the calculation of those values vis-à-vis the validity of the maximum hardness and minimum electrophilicity principles are analyzed in the cases of some representative reactions. Among 101 studied exothermic reactions, 61.4% and 69.3% of the reactions are found to obey the maximum hardness and minimum electrophilicity principles, respectively, when hardness of products and reactants is expressed in terms of their geometric means. However, when we use arithmetic mean, the percentage reduces to some extent. When we express the hardness in terms of scaled hardness, the percentage obeying maximum hardness principle improves. We have observed that maximum hardness principle is more likely to fail in the cases of very hard species like F^–, H₂, CH₄, N₂, and OH appearing in the reactant side and in most cases of the association reactions. Most of the association reactions obey the minimum electrophilicity principle nicely. The best results (69.3%) for the maximum hardness and minimum electrophilicity principles reject the 50% null hypothesis at the 2% level of significance

MNgCCH (M = Cu, Ag, Au; Ng = Xe, Rn): The First Set of Compounds with M–Ng–C Bonding Motif

Although Ng–M (M = Cu, Ag, Au; Ng = noble gas) and Ng–C bonds are known to exist in different viable species, we report here a series of systems with formula MNgCCH (Ng = Xe, Rn) in which both bonds coexist. These compounds possess reasonably high kinetic stability (free energy barrier, Δ<i>G</i>^‡ of 14.0–34.8 kcal/mol) along an exergonic isomerization channel, MNgCCH → NgMCCH. For a given M, the Δ<i>G</i>^‡ associated with this channel increases from Xe to Rn, whereas for a given Ng, it increases along Ag < Cu < Au. No other possible dissociation channel is feasible at standard condition, except for the Ag–Xe analogue, where one three-body neutral dissociation channel, AgXeCCH → Ag + Xe + CCH, is slightly exergonic by 2.4 kcal/mol. Examination of the thermochemical stability of the Ng–M bonds in noninserted compounds against the dissociation, NgMCCH → Ng + MCCH reveals that Kr–Rn bound Cu and Au analogues, and Xe and Rn bound Ag analogues would be viable at 298 K. The natural bond order analysis indicates the formation of M–Ng covalent bond and Ng–C ionic bonds in these compounds having an ionic representation of (MNg)⁺(CCH)⁻. Energy decomposition analysis reveals a significant contribution of the electrostatic term in the M–Ng covalent bonds

Comparative Study on the Noble-Gas Binding Ability of BeX Clusters (X = SO₄, CO₃, O)

Ab initio computations are carried out to assess the noble gas (Ng) binding capability of BeSO₄ cluster. We have further compared the stability of NgBeSO₄ with that of the recently detected NgBeCO₃ cluster. The Ng–Be bond in NgBeCO₃ is somewhat weaker than that in NgBeO cluster. In NgBeSO₄, the Ng–Be bond is found to be stronger compared with not only the Ng–Be bond in NgBeCO₃ but also that in NgBeO, except the He case. The Ar–Rn-bound BeSO₄ analogues are viable even at room temperature. The Wiberg bond indices of Be–Ng bonds and the degree of electron transfer from Ng to Be are somewhat larger in NgBeSO₄ than those in NgBeCO₃ and NgBeO. Electron density and energy decomposition analyses are performed in search of the nature of interaction in the Be–Ng bond in NgBeSO₄. The orbital energy term (Δ<i>E</i>^orb) contributes the maximum (ca. 80–90%) to the total attraction energy. The Ar/Kr/Xe/Rn–Be bonds in NgBeSO₄ could be of partial covalent type with a gradual increase in covalency along Ar to Rn

In Quest of Strong Be–Ng Bonds among the Neutral Ng–Be Complexes

The global minimum geometries of BeCN₂ and BeNBO are linear BeN–CN and BeN–BO, respectively. The Be center of BeCN₂ binds He with the highest Be–He dissociation energy among the studied neutral He–Be complexes. In addition, BeCN₂ can be further tuned as a better noble gas trapper by attaching it with any electron-withdrawing group. Taking BeO, BeS, BeNH, BeNBO, and BeCN₂ systems, the study at the CCSD­(T)/def2-TZVP level of theory also shows that both BeCN₂ and BeNBO systems have higher noble gas binding ability than those related reported systems. Δ<i>G</i> values for the formation of NgBeCN₂/NgBeNBO (Ng = Ar–Rn) are negative at room temperature (298 K), whereas the same becomes negative at low temperature for Ng = He and Ne. The polarization plus the charge transfer is the dominating term in the interaction energy

Selectivity in Gas Adsorption by Molecular Cucurbit[6]uril

The relative preference in adsorption among 19 common gas molecules, namely, C₂H₂, C₂H₄, C₂H₆, CH₄, X₂, HX (X = F, Cl, Br), CO₂, CS₂, CO, H₂, H₂O, H₂S, N₂, NO₂, and NO within the cavity of cucurbit[6]­uril (CB[6]) is investigated via density functional theory computations. Energies associated with the dissociation of gas@CB[6] producing CB[6] and gas molecules show the order of the efficacy to be encapsulated within CB[6], C₂H₂@CB­[6] being the most viable system. However, the dissociation free energy change implies that CB[6] is most efficient in accommodating Cl₂ followed by C₂H₂ among the considered gas molecules. In general, guest molecules having large surface contact with the host and/or high polarizability and/or having acidic hydrogen to make hydrogen bond with >CO show larger propensity to be encapsulated within CB[6] cavitand. Functionalized CB[6] are better candidates for gas adsorption than CB[6]. However, the nature of functionalization needed to improve the adsorption ability varies with the change in the guest molecule. While full −C₂H₅ substitution improves C₂H₂ and CO₂ adsorption ability of CB[6] the most, the −CN functionalized CB[6] is the best candidate to encapsulate C₂H₄ and C₂H₆ among the studied −OH, −C₂H₅, and −CN substituted analogues. The interaction is mostly of van der Waals type, except in the cases of C₂H₂, H₂O, H₂S, and HX (X = F, Cl, Br), in which both the electrostatic and dispersion contributions are important owing to the interaction between acidic hydrogen of these guest molecules and oxygen centers of the host moiety

Carbo-Cages: A Computational Study

Inspired by their geometrical perfection, intrinsic beauty, and particular properties of polyhedranes, a series of carbo-cages is proposed in silico via density functional theory computations. The insertion of alkynyl units into the C–C bonds of polyhedranes results in a drastic lowering of the structural strain. The induced magnetic field shows a significant delocalization around the three-membered rings. For larger rings, the response is paratropic or close to zero, suggesting a nonaromatic behavior. In the carbo-counterparts, the values of the magnetic response are shifted with respect to their parent compounds, but the aromatic/nonaromatic character remains unaltered. Finally, Born–Oppenheimer molecular dynamics simulations at 900 K do not show any drastic structural changes up to 10 ps. In the particular case of a carbo-prismane, no structural change is perceived until 2400 K. Therefore, although carbo-cages have enthalpies of formation 1 order of magnitude higher than those of their parent compounds, their future preparation and isolation should not be discarded, because the systems are kinetically stable, explaining why the similar systems like carbo-cubane have already been synthesized