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

Unprecedented Enhancement of Noble Gas–Noble Metal Bonding in NgAu₃⁺ (Ng = Ar, Kr, and Xe) Ion through Hydrogen Doping

Behavior of gold as hydrogen in certain gold compounds and a very recent experimental report on the noble gas–noble metal interaction in Ar complexes of mixed Au–Ag trimers have motivated us to investigate the effect of hydrogen doping on the Ng–Au (Ng = Ar, Kr, and Xe) bonding through various <i>ab initio</i> based techniques. The calculated results show considerable strengthening of the Ng–Au bond in terms of bond length, bond energy, stretching vibrational frequency, and force constant. Particularly, an exceptional enhancement of Ar–Au bonding strength has been observed in ArAuH₂⁺ species as compared to that in ArAu₃⁺ system, as revealed from the CCSD­(T) calculated Ar–Au bond energy value of 32 and 72 kJ mol^–1 for ArAu₃⁺ and ArAuH₂⁺, respectively. In the calculated IR spectra, the Ar–Au stretching frequency is blue-shifted by 65% in going from ArAu₃⁺ to ArAuH₂⁺ species. Similar trends have been obtained in the case of all Ar, Kr, and Xe complexes with Ag and Cu trimers. Among all the NgM_3–<i>k</i>H_<i>k</i>⁺ complexes (where <i>k</i> = 0–2), the strongest binding in NgMH₂⁺ complex is attributed to significant enhancement in the covalent characteristics of the Ng–M bond and considerable increase in charge-induced dipole interaction, as shown from the topological analysis

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

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

Atom- and Ion-Centered Icosahedral Shaped Subnanometer-Sized Clusters of Molecular Hydrogen

The recently observed “new form of condensed hydrogen” has motivated us to investigate the structures of H@H₂₄^–, H@H₆₄^–, and H@H₈₈^– clusters and to explore their stability by using dispersion-corrected density functional theory. Stability of these clusters has been explained with the help of high values of the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gap and geometrically closed shell of 12, 32, and 44 hydrogen molecules around the central hydride ion, which in turn form electronically closed shell systems. The H@H₂₄^– cluster has been observed as the most stable cluster followed by H@H₆₄^– and H@H₈₈^–. Apart from the hydride-centered clusters, we have also predicted various other metal and nonmetal atom- and ion-centered new clusters with large HOMO–LUMO gap and high binding energy. The structures and stability of some of the smaller clusters have been investigated by using MP2 and CCSD­(T) methods as well, and the MP2-calculated binding energies are found to be very close to the corresponding CCSD­(T) computed values. Calculated results indicate that both electronic shell closing and geometric shell closing are equally important in explaining the structure and stability of these systems. It has been shown that the binding energy of icosahedral H₂₄ to an ionic core is heavily dependent on the encapsulated central ion

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

Structural and Chemical Properties of Subnanometer-Sized Bimetallic Au₁₉Pt Cluster

Structure and chemical reactivity of the bimetallic Au₁₉Pt cluster has been investigated within the framework of the relativistic density functional theory. It is observed that all isomers of the tetrahedral Au₁₉Pt cluster are energetically more stable as compared to pure Au₂₀ as well as cage-like isomers of the Au₁₉Pt cluster. The high stability of the bimetallic Au₁₉Pt cluster can be attributed to the strong interaction of the Au and Pt atoms, which is caused by the hybridization of s- and d-orbitals of guest Pt and the host Au atoms in the energy span of 5 eV below the HOMO level. To explore the chemical reactivity of the isomers of the bimetallic Au₁₉Pt cluster, we investigate the adsorption behavior of a CO molecule on various nonequivalent sites of these isomers. We calculate CO adsorption energy, C–O bond length, and bond stretching frequency for all the possible cluster–CO complexes. We find that a CO molecule is preferably adsorbed on Pt sites when both the Au and Pt sites are exposed for adsorption. Interestingly, we observe that the CO adsorption energy increases by more than 1.3 eV when a CO molecule gets adsorbed on the Pt site in the tetrahedral Au₁₉Pt cluster as compared to the adsorption on corresponding Au atoms in the pure Au₂₀ cluster. Moreover, we have shown that due to the charge transfer from the cluster to the CO molecule C–O bond length increases by around 0.02 Å, which causes a substantial amount of red shift (104–121 cm^–1) in C–O stretching frequency. These results indicate that the electronic structure of the CO molecule is highly disturbed when it is adsorbed on the bimetallic clusters, which in turn suggests that the oxidation of the adsorbed CO molecule becomes easy

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

Combined experimental and theoretical studies of some uranium(VI) complexes derived from imidazole-based carbenes

A series of bidentate, viz., 1,1'-(1,2-ethylene)-3,3'-dimethyldiimidazoline-2,2'-diylidene (L1), 1-methyl-3-(2-pyridylmethyl)-imidazoline-2-ylidene (L2) and tridentate, viz., 1,3-bis(2-pyridyl)-imidazoline-2-ylidene (L3) ligands have been obtained from their corresponding imidazolium salts through deprotonation reactions. Treatment of UO2Cl2(THF)3 with one equivalent L3 produces air-stable U(VI)-carbene complex 3, characterized by elemental analysis, FTIR, NMR spectroscopy as well as extended X-ray absorption fine structure (EXAFS) analysis. EXAFS result indicates that the ligand is bonded through one C and two N-atoms to the uranium atom. Attempts to synthesize uranyl complexes derived from L1 and L2 were also successful but these complexes are decomposed quickly within one hour at room temperature. 3 produces pure UO2 powder when heated under an argon atmosphere from room temperature to 600 °C with constant heating rate of 5 °C/min. The solid-state UV–Vis spectrum of the compound shows absorption peaks at 332 and 450 nm. The excitation spectrum of 3 (at λem = 520 nm) exhibits two almost symmetrical peaks at 273 and 368 nm. Density functional theory-based quantum mechanical calculations indicate that a partial covalent interaction exists between the carbene C and U while a weak non-covalent interaction exists between carbene N and U atoms. </p