Enhancement in the Stability of 36-Atom Fullerene through Encapsulation of a Uranium Atom

With an objective to rationalize the experimentally observed intense U@C₃₆ peak in the mass spectrum of U@C_2<i>n</i> metallofullerene, structural, stability, and spectroscopic aspects of the uranium doped C₃₆ fullerene have been studied in a unified and systematic way using density functional theory (DFT) and its time-dependent variant. Relativistic effects have been taken into account within the framework of zeroth-order regular approximation using scalar and spin–orbit-based approaches. Among all of the 15 possible classical isomers reported for the C₃₆ fullerene cage system, singlet <i>D</i>_2<i>d</i> and triplet <i>D</i>_6<i>h</i> structures are found to be isoenergetic and most stable. Encapsulation of uranium atom into various C₃₆ cages leads to 15 distinct isomers with considerable energy differences. It has also been shown that this encapsulation process results in significant gain in thermodynamic stability. The most stable U@C₃₆ isomer is found to be associated with <i>C</i>_6<i>v</i> symmetry and closed-shell electronic configuration, derived from the open-shell <i>D</i>_6<i>h</i> structure of C₃₆. The next stable isomer is associated with <i>C_s</i> symmetry and obtained from the corresponding singlet <i>D</i>_2<i>d</i> structure of the C₃₆ cage. Distinct changes have also been found in the calculated vibrational and UV–visible spectra of the U@C₃₆ cluster as compared to the corresponding bare C₃₆ cage. All of the calculated quantities reported here suggest that the stability of the U@C₃₆ cluster is high enough for possible formation of cluster-assembled material leading to synthesis of this metallofullerene experimentally

Prediction of a New Series of Thermodynamically Stable Actinide Encapsulated Fullerene Systems Fulfilling the 32-Electron Principle

Density functional theory (DFT) within the framework of zeroth order regular approximation has been used to predict a new class of stable clusters through encapsulation of an actinide or lanthanide atom/ion into the C₂₆ cage. The electronic structures, bonding, stability, aromaticity and spectroscopic properties of these endohedral metallofullerenes, M@C₂₆ (M = Pr^–, Pa^–, Nd, U, Pm⁺, Np⁺, Sm²⁺, Pu²⁺, Eu³⁺, Am³⁺, Gd⁴⁺, and Cm⁴⁺) have been investigated systematically using DFT and its time-dependent variant. On encapsulation of an f-block metal atom/ion with 6 valence electrons, the classical bare open shell C₂₆ cage with <i>D</i>_3<i>h</i> symmetry and ellipsoid shape is transformed to a more spherical closed shell <i>D</i>_3<i>h</i> structures with high HOMO–LUMO gap (in the range of 2.44–3.99 eV for M@C₂₆ clusters as compared to 1.62 eV for the bare C₂₆ cage). Calculated binding energy values imply that all of the M@C₂₆ clusters are stable with respect to dissociation into atomic fragments. Moreover, thermodynamic parameters indicate that the encapsulation process is highly favorable for all of the actinides and some of the lanthanides considered here. A higher stability and nearly spherical shape of M@C₂₆ system is rationalized through the fulfillment of 32-electron principle corresponding to the fully occupied spdf atomic shells for the encapsulated central atom, where considerable amount of overlap between the metal and cage orbitals has been found. Thus, the calculated structural and energetic parameters strongly suggest the possible formation of M@C₂₆ species under appropriate experimental conditions. Furthermore, the present work implies that the 32-electron principle might be important in designing of new materials involving lanthanides and actinides

What Are the Ground State Structures of C₂₀ and C₂₄? An Explicitly Correlated Ab Initio Approach

A new benchmark study has been performed for six isomers of C₂₀ and four isomers of C₂₄ using explicitly correlated methods, together with coupled cluster theory with large basis sets and DFT with advanced functionals. The relative energy trends obtained are extremely sensitive to the methods used. Combining our best CCSD­(T)-MP2 difference with our best MP2 basis set limit, the dehydrocorannulene bowl is found to be the most stable for C₂₀, followed by the cage at about 8 kcal/mol, and the ring at about 46 kcal/mol. For C₂₄, the <i>D</i>_3<i>d</i> cage is found to be the most stable isomer, followed at only a few kilocalories per mole by dehydrocoronene, and at larger separations by then octahedral cage and the ring, respectively. This makes C₂₄ the smallest classical fullerene. The estimated residual basis set error of the estimated CCSD­(T) basis set limit is conservatively expected to be ±1 kcal/mol. In general, DFT exhibits large errors for relative energies with RMSD values in the 8–34 kcal/mol range. However, among the DFT functionals, the DSD-PBEP86-D3BJ double hybrid comes close to our best ab initio results, while the ωB97X-V range-separated hybrid is in semiquantitative agreement

Theoretical Prediction of XRgCO⁺ Ions (X = F, Cl, and Rg = Ar, Kr, Xe)

In this work we have predicted novel rare gas containing cationic molecules, XRgCO⁺ (X = F, Cl and Rg = Ar, Kr, Xe) using ab initio quantum chemical methods. Detail structural, stability, vibrational frequency, and charge distribution values are reported using density functional theory, second-order Møller–Plesset perturbation theory, and coupled-cluster theory based methods. These ions are found to be metastable in nature and exhibit a linear geometry with <i>C</i>_<i>∞v</i> symmetry in their minima energy structures, and the nonlinear transition state geometries are associated with <i>C</i>_<i>s</i> symmetry. Except for the two-body dissociation channel (Rg + XCO⁺), these ions are stable with respect to all other dissociation channels. However, the connecting transition states between the above-mentioned two-body dissociation channel products and the predicted ions are associated with sufficient energy barriers, which restricts the metastable species to transform into the global minimum products. Thus, it may be possible to detect and characterize these metastable ions using an electron bombardment technique under cryogenic conditions

Structure and Stability of Zn, Cd, and Hg Atom Doped Golden Fullerene (Au₃₂)

Structures and properties of various complexes formed between the “golden fullerene”, Au₃₂, and group IIB atoms such as Zn, Cd, and Hg have been investigated using density functional theory (DFT). Binding energy values indicate that the group IIB atoms can form stable clusters in most of the different isomeric forms of the Au₃₂ cage. The HOMO–LUMO gap of the Au₃₂ cage remains almost the same even after doping of Zn, Cd, and Hg atoms for high symmetry clusters, while it decreases for the low symmetry isomers. The highest stable isomer for the Hg-doped Au₃₂ cluster is found to be associated with <i>I</i>_<i>h</i> symmetry with a large energy difference from the other low symmetry isomers, using generalized gradient approximation (GGA) type functionals. However, for the Zn and Cd encapsulated Au₃₂ clusters, the highest stable structures are of <i>C_s</i>[1] and <i>C</i>_5<i>v</i> symmetry, respectively, along with one low symmetry isomer for each of them, having energy very close to the respective most stable isomer. Nevertheless, depending on the energy density functional, the relative energy orderings for the various isomers are found to be modified strongly. In fact, the meta-GGA TPSS functional predicts low symmetry compact isomers to be more stable for all the metal atom doped Au₃₂ clusters. Moreover, low symmetry compact isomers are found to be more stable with the dispersion-corrected GGA type PBE functional for the Zn- and Cd-doped cluster, in agreement with the TPSS results; however, the same dispersion correction fails to reproduce the TPSS results for the Hg-doped Au₃₂ system. Structural data, energetic parameters, and spectral analysis point toward the possible experimental observation of group IIB atom doped golden fullerene, which in turn might help to understand the nature of interactions between the metal atom and the Au₃₂ cage. Furthermore, experimental investigations would likely confirm the predictive ability of the different functionals used in this work

The X40x10 Halogen Bonding Benchmark Revisited: Surprising Importance of (n-1)d Subvalence Correlation

<p>We have re-evaluated the X40x10 benchmark for halogen bonding using conventional and explicitly correlated coupled cluster methods. For the aromatic dimers at small separation, improved CCSD(T)–MP2 “high-level corrections” (HLCs) cause substantial reductions in the dissociation energy. For the bromine and iodine species, (n-1)d subvalence correlation increases dissociation energies, and turns out to be more important for noncovalent interactions than is generally realized; ; (n-1)sp subvalence correlation is much less important. The (n-1)d subvalence term is dominated by core-valence correlation; with the smaller cc-pVDZ-F12-PP and cc-pVTZ-F12-PP basis sets, basis set convergence for the core-core contribution becomes sufficiently erratic that it may compromise results overall. The two factors conspire to generate discrepancies of up to 0.9 kcal/mol (0.16 kcal/mol RMS) between the original X40x10 data and the present revision.</p

Noble-Gas-Inserted Fluoro(sulphido)boron (FNgBS, Ng = Ar, Kr, and Xe): A Theoretical Prediction

The possibility of the existence of a new series of neutral noble gas compound, FNgBS (where Ng = Ar, Kr, Xe), is explored theoretically through the insertion of a Ng atom into the fluoroborosulfide molecule (FBS). Second-order Møller–Plesset perturbation theory, density functional theory, and coupled cluster theory based methods have been employed to predict the structure, stability, harmonic vibrational frequencies, and charge distribution of FNgBS molecules. Through energetics study, it has been found that the molecules could dissociate into global minima products (Ng + FBS) on the respective singlet potential energy surface via a unimolecular dissociation channel; however, the sufficiently large activation energy barriers provide enough kinetic stability to the predicted molecules, which, in turn, prevent them from dissociating into the global minima products. Moreover, the FNgBS species are thermodynamically stable, owing to very high positive energies with respect to other two two-body dissociation channels, leading to FNg + BS and F^– + NgBS⁺, and two three-body dissociation channels, corresponding to the dissociation into F + Ng + BS and F^– + Ng + BS⁺. Furthermore, the Mulliken and NBO charge analysis together with the AIM results reveal that the Ng–B bond is more of covalent in nature, whereas the F–Ng bond is predominantly ionic in character. Thus, these compounds can be better represented as F^–[NgBS]⁺. This fact is also supported by the detail analysis of bond length, bond dissociation energy, and stretching force constant values. All of the calculated results reported in this work clearly indicate that it might be possible to prepare and characterize the FNgBS molecules in cryogenic environment through matrix isolation technique by using a mixture of OCS/BF₃ in the presence of large quantity of noble gas under suitable experimental conditions

The X40×10 Halogen Bonding Benchmark Revisited: Surprising Importance of (<i>n</i>–1)d Subvalence Correlation

We have re-evaluated the X40×10 benchmark for halogen bonding using conventional and explicitly correlated coupled cluster methods. For the aromatic dimers at small separation, improved CCSD­(T)-MP2 “high-level corrections” (HLCs) cause substantial reductions in the dissociation energy. For the bromine and iodine species, (<i>n</i>–1)­d subvalence correlation increases dissociation energies and turns out to be more important for noncovalent interactions than is generally realized; (<i>n</i>–1)­sp subvalence correlation is much less important. The (<i>n</i>–1)­d subvalence term is dominated by core–valence correlation; with the smaller cc-pVDZ-F12-PP and cc-pVTZ-F12-PP basis sets, basis set convergence for the core–core contribution becomes sufficiently erratic that it may compromise results overall. The two factors conspire to generate discrepancies of up to 0.9 kcal/mol (0.16 kcal/mol RMS) between the original X40×10 data and the present revision

The X40×10 Halogen Bonding Benchmark Revisited: Surprising Importance of (<i>n</i>–1)d Subvalence Correlation

We have re-evaluated the X40×10 benchmark for halogen bonding using conventional and explicitly correlated coupled cluster methods. For the aromatic dimers at small separation, improved CCSD­(T)-MP2 “high-level corrections” (HLCs) cause substantial reductions in the dissociation energy. For the bromine and iodine species, (<i>n</i>–1)­d subvalence correlation increases dissociation energies and turns out to be more important for noncovalent interactions than is generally realized; (<i>n</i>–1)­sp subvalence correlation is much less important. The (<i>n</i>–1)­d subvalence term is dominated by core–valence correlation; with the smaller cc-pVDZ-F12-PP and cc-pVTZ-F12-PP basis sets, basis set convergence for the core–core contribution becomes sufficiently erratic that it may compromise results overall. The two factors conspire to generate discrepancies of up to 0.9 kcal/mol (0.16 kcal/mol RMS) between the original X40×10 data and the present revision

Effect of Hydrogen Atom Doping on the Structure and Electronic Properties of 20-Atom Gold Cluster

We test the validity of gold–hydrogen analogy in a hydrogen-atom-doped larger gold cluster, namely, Au₂₀, which has attracted considerable interest in recent years because of its unique nature. For this purpose, we carry out density functional theory based calculations to determine the structures of various possible isomers of Au₁₉H cluster by employing GGA and meta-GGA functionals. To obtain the optimized structures of Au₁₉H cluster, several possible initial geometries have been explored. We find that the structure of Au₁₉H cluster is very close to that of tetrahedral Au₂₀ cluster, and the dopant H atom prefers to sit on one of the vertices of the tetrahedron. On the other hand, for the cases of Li, coinage metal (Cu and Ag), and Pt atom doping, the dopant atom has been shown to preferably sit on the surface site of the tetrahedral Au₂₀ cluster. The structure and HOMO–LUMO gap of the Au₁₉H cluster are found to be very close to that of the pure Au₂₀ cluster. Moreover, we observe that the adsorption energies and the extent of activations of CO and O₂ molecules on Au₁₉H cluster are similar to those on the Au₂₀ cluster. On the other hand, it has been reported in the literature that in the smaller sized gold clusters the catalytic activity of the clusters is found to be enhanced significantly due to the doping with a hydrogen atom. Hence, it is clear from the present study that the structure and the electronic properties of hydrogen-atom-doped 20-atom gold cluster almost remain the same as that of Au₂₀ cluster, thereby demonstrating the existence of gold–hydrogen analogy in a larger sized gold cluster