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)

DataSheet1_OsB9−: An Aromatic Osmium-Centered Monocyclic Boron Ring.docx

Transition-metal-centered monocyclic boron wheels are important candidates in the family of planar hypercoordinate species that show intriguing structure, stability and bonding situation. Through the detailed potential energy surface explorations of MB9− (M = Fe, Ru, Os) clusters, we introduce herein OsB9− to be a new member in the transition-metal-centered borometallic molecular wheel gallery. Previously, FeB9− and RuB9− clusters were detected by photoelectron spectroscopy and the structures were reported to have singlet D9h symmetry. Our present results show that the global minimum for FeB9− has a molecular wheel-like structure in triplet spin state with Cs symmetry, whereas its heavier homologues are singlet molecular wheels with D9h symmetry. Chemical bonding analyses show that RuB9− and OsB9− display a similar type of electronic structure, where the dual σ + π aromaticity, originated from three delocalized σ bonds and three delocalized π bonds, accounts for highly stable borometallic molecular wheels.</p

Donor–Acceptor vs Electron-Shared Bonding: Triatomic Si_<i>n</i>C_3–<i>n</i> (<i>n</i> ≤ 3) Clusters Stabilized by Cyclic Alkyl(amino) Carbene

SinC3–n (n ≤ 3) clusters are interstellar species that are transient in nature at ambient conditions. Herein, the structure, stability, and nature of bonding in cyclic alkyl­(amino) carbene (cAAC) protected SinC3–n (n ≤ 3) clusters are studied in silico. The Si3(cAAC)3 complex was previously reported to be synthesized in large scale. The present results indicate that because the C–CcAAC bond is stronger than the Si–CcAAC bond, C3(cAAC)3 and SiC2(cAAC)3 complexes have significantly larger stability with respect to ligand dissociation than the Si3(cAAC)3 complex, while Si2C­(cAAC)3 has almost the same stability as in the latter complex. Moreover, considering the Si3(cAAC)3 complex as a precursor, the hypothetical successive single Si substitution process by a single C atom in Si3(cAAC)3 complex is exergonic in nature. The bonding situation is analyzed by employing natural bond orbital (NBO), electron density, and energy decomposition analyses in combination with the natural orbital for chemical valence theory. These studies show that the nature of bonding in C–CcAAC and Si–CcAAC bonds differs significantly from each other. The former bonds are best described as an electron-shared double bond, whereas the latter bonds are of donor–acceptor type consisting of two components, Si←CcAAC σ-donation and Si→CcAAC π-back-donation. Nevertheless, in the former bonds, covalent character is larger than the ionic one but in the latter bonds the reverse is true. For some Si–CcAAC bonds, the π-natural orbital cannot be located by the NBO method, presumably because of slightly lower occupancy than the cutoff values, but the electron density analysis confirms that different Si–CcAAC bonds in a given complex are almost equivalent in terms of electron density distribution. This paper reports an interesting change in bonding pattern when one replaces Si by a C atom in triatomic silicon carbide clusters stabilized by a ligand

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

Noble Gas Inserted Metal Acetylides (Metal = Cu, Ag, Au)

Metal acetylides (MCCH, M = Cu, Ag, Au) were already experimentally detected in molecular form. Herein, we investigate the possibility of noble gas (Ng) insertion within the C–H bond of MCCH and their stability is compared with those of the reported MNgCCH and HCCNgH molecules. Our coupled-cluster-level computations show that MCCNgH (Ng = Kr, Xe, Rn) systems are local minima on the corresponding potential energy surfaces, whereas their lighter analogues do not remain in the chemically bound form. Further, their stability is analyzed with respect to all possible dissociation channels. The most favorable dissociation channel leads to the formation of free Ng and MCCH. However, there exists a high free energy barrier (29.3–46.9 kcal/mol) to hinder the dissociation. The other competitive processes against their stability include two-body and three-body neutral dissociation channels, MCCNgH → MCC + NgH and MCCNgH → MCC + Ng + H, respectively, which are slightly exergonic in nature at 298 K for Ng = Kr, Xe and M = Cu, Ag, and for AuCCKrH. However, the Xe analogues for Cu and Ag and AuCCKrH would be viable at a lower temperature. AuCCNgH (Ng = Kr–Rn) molecules are the best candidates for experimental realization, since they have higher dissociation energy values and higher kinetic protection in the case of feasible dissociation channels compared to the Cu and Ag systems. A detailed bonding analysis indicates that the Ng–H bonds are genuine covalent bonds and there is also a substantial covalent character in Ng–C contacts of these molecules. Moreover, the possibility of insertion of two Xe atoms in AuCCH resulting in AuXeCCXeH and the stability of XeAuXeCCXeH are also tested herein

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

Bonding in Binuclear Carbonyl Complexes M₂(CO)₉ (M = Fe, Ru, Os)

Quantum-chemical density functional theory calculations using the BP86 functional in conjunction with a triple-ζ basis set and dispersion correction by Grimme with Becke-Johnson damping D3­(BJ) were performed for the title molecules. The nature of the bonding was examined with the quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) methods and with the energy decomposition analysis in conjunction with the natural orbital for chemical valence (EDA-NOCV) analysis. The energetically lowest-lying form of Fe₂(CO)₉ is the triply bridged <i>D</i>_3<i>h</i> structure, whereas the most stable structures of Ru₂(CO)₉ and Os₂(CO)₉ are singly bridged <i>C</i>₂ species. The calculated reaction energies for the formation of the cyclic trinuclear carbonyls M₃(CO)₁₂ from the dinuclear carbonyls M₂(CO)₉ are in agreement with experiment, as the iron complex Fe₂(CO)₉ is thermodynamically stable in these reactions, but the heavier homologues Ru₂(CO)₉ and Os₂(CO)₉ are not. The metal–CO bond to the bridging CO ligands is stronger than the bonds to the terminal CO ligands. This holds for the triply bridged <i>D</i>_3<i>h</i> structures as well as for the singly bridged <i>C</i>₂ or <i>C</i>_2<i>v</i> species. The analysis of the orbital interactions with the help of the EDA-NOCV method suggests that the overall M→CO π backdonation is always stronger than the M←CO σ donation. The bridging carbonyls are more strongly bonded than the terminal CO ligands, and they are engaged in stronger σ donation and π backdonation, but the formation of bridging carbonyls requires reorganization energy, which may or may not be compensated by the stronger metal–ligand interactions. The lower-lying <i>D</i>_3<i>h</i> form of Fe₂(CO)₉ and <i>C</i>₂ structures of Ru₂(CO)₉ and Os₂(CO)₉ are due to a delicate balance of several forces

Data_Sheet_1_Modified Particle Swarm Optimization Algorithms for the Generation of Stable Structures of Carbon Clusters, Cn (n = 3–6, 10).doc

Particle Swarm Optimization (PSO), a population based technique for stochastic search in a multidimensional space, has so far been employed successfully for solving a variety of optimization problems including many multifaceted problems, where other popular methods like steepest descent, gradient descent, conjugate gradient, Newton method, etc. do not give satisfactory results. Herein, we propose a modified PSO algorithm for unbiased global minima search by integrating with density functional theory which turns out to be superior to the other evolutionary methods such as simulated annealing, basin hopping and genetic algorithm. The present PSO code combines evolutionary algorithm with a variational optimization technique through interfacing of PSO with the Gaussian software, where the latter is used for single point energy calculation in each iteration step of PSO. Pure carbon and carbon containing systems have been of great interest for several decades due to their important role in the evolution of life as well as wide applications in various research fields. Our study shows how arbitrary and randomly generated small Cn clusters (n = 3–6, 10) can be transformed into the corresponding global minimum structure. The detailed results signify that the proposed technique is quite promising in finding the best global solution for small population size clusters.</p

Structural Characterization and Bonding Analysis of [Hg{Fe(CO)₅}₂]²⁺ [SbF₆]⁻₂

The non-classical carbonyl complex [Hg{Fe(CO)5}2]2+ [SbF6]−2 is prepared by reaction of Hg(SbF6)2 and excess Fe(CO)5 in anhydrous HF. The single-crystal X-ray structure reveals a linear Fe–Hg–Fe moiety as well as an eclipsed conformation of the eight basal CO ligands. Interestingly, the Hg–Fe bond length of 2.5745(7) Å is relatively similar to the corresponding Hg–Fe bonds in literature-known [Hg{Fe(CO)4}2]2– dianions (2.52–2.55 Å), which intrigued us to analyze the bonding situation in both the dications and dianions with the energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV) method. Both species are best described as Hg(0) compounds, which are also confirmed by the shape of the HOMO-4 and HOMO-5 of the dication and dianion, respectively, in which the electron pair is located mainly at the Hg. Furthermore, for the dication and the dianion, the σ back-donation from Hg into the [Fe(CO)5]22+ or the [Fe(CO)4]22– fragment is the most dominant orbital interaction and surprisingly these interaction energies are also very similar even in absolute values. The fact that both iron-based fragments are missing two electrons explains their prominent σ-acceptor properties

Generation and Characterization of the Charge-Transferred Diradical Complex CaCO₂ with an Open-Shell Singlet Ground State

The CaCO2 complex is generated via the reaction of excited-state calcium atom with carbon dioxide in a solid neon matrix. Infrared absorption spectroscopy and quantum chemical calculations reveal that the complex has a planar four-membered ring structure with a strongly bent CO2 ligand side-on coordinated to the calcium center in an η2–O, O manner. The complex has an open-shell singlet ground state, which can be described as the bonding interactions between a Ca+ (4s1) cation in the doublet ground state and a doublet ground state CO2– anion. The analysis of the bonding situation suggests that the Ca-O2C bonds have a large (75%) electrostatic character. The covalent (orbital) interactions come from the coupling of the unpaired electrons of Ca+ and CO2– giving rise to electron-sharing bonding and a stronger contribution from dative bonding (Ca+)←(CO2–). The atomic orbitals (AOs) of Ca+ that are engaged in the covalent bonds are the 4s AO for the electron-sharing bonds and the 3d AOs for the dative bonds. This is further evidence for the assignment of the heavier alkaline-earth atoms as transition metals rather than main-group elements