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The presence of neutral C60 fullerenes in circumstellar environments has been firmly established by astronomical observations as well as laboratory experiments and quantum-chemistry calculations. However, the large variations observed in the C60 17.4 μ m/18.9 μ m band ratios indicate that either additional emitters should contribute to the astronomical infrared (IR) spectra or unknown physical processes exist besides thermal and UV excitation. Fullerene-based molecules such as metallofullerenes and fullerene-adducts are natural candidate species as potential additional emitters, but no specific specie has been identified to date. Here we report a model based on quantum-chemistry calculations and IR spectra simulation of neutral and charged endo(exo)hedral metallofullerenes, showing that they have a significant contribution to the four strongest IR bands commonly attributed to neutral C60. These simulations may explain the large range of 17.4 μ m/18.9 μ m band ratios observed in very different fullerene-rich circumstellar environments like those around planetary nebulae and chemically peculiar R Coronae Borealis stars. Our proposed model also reveals that the 17.4 μ m/18.9 μ m band ratio in the metallofullerenes simulated IR spectra mainly depends on the metal abundances, ionization level, and endo/exoconcentration in the circumstellar envelopes. We conclude that metallofullerenes are potential emitters contributing to the observed IR spectra in fullerene-rich circumstellar envelopes. Our simulated IR spectra indicate also that the James Webb Space Telescope has the potential to confirm or refute the presence of metallofullerenes (or even other fullerene-based species) in circumstellar environments. [ABSTRACT FROM AUTHOR]

The properties of metal-metal bonding for transition metal homonuclear diatomics from groups 10, 11 and 12 are studied within the framework of the quantum theory of atoms in molecules (QTAIM) at the coupled cluster CCSD and CCSD(T) levels of theory. A novel approximate method developed by Keith and Frisch is used to augment electron densities calculated with pseudopotentials with the missing relativistic core densities to obtain approximations to the total densities of the dimers. The calculated delocalization indices for group 10 dimers are: [Ni.sub.2] (1.6), [Pd.sub.2] (0.44, an outlier in the group), and [Pt.sub.2] (1.8); for group 11 dimers: [Cu.sub.2] and [Ag.sub.2] (1.01), and [Au.sub.2] (1.13), all covalent bonds; for group 12: [Zn.sub.2] (0.06), [Cd.sub.2] (0.08), and [Hg.sub.2] (0.09), all consistent with weak van der Waals complexes. The picture of bonding obtained by examining the values of the electron density at the bond critical points is consistent with the one obtained on the basis of these delocalization indices. A curious linear (instead of exponential) dependence of the delocalization index on the electron density at the bond critical point is presented here as an observation and will be investigated in more depth in later work. Several correlations between bond properties and bond dissociation energies are also explored. It is found that, with the exception of the [Ni.sub.2] dimer that exhibits considerable multi-reference character, there are correlations between the calculated bond dissociation energies of the studied diatomics and several bond critical point properties. These correlations are novel as they span a set of bonds between different pairs of elements, while traditionally these correlations were reported for bonds between the same pair or elements but with different substituents. Key words: topology of the electron density, chemical bonding in transition metals, quantum theory of atoms in molecules, QTAIM, metal-metal bonding. Resume : Les proprietes des liaisons metal-metal pour les dimeres homonucleaires des metaux de transition des groupes 10,11 et 12 sont etudies dans le cadre de la theorie quantique des atomes-dans-les-molecules (QTAIM) en faisant appel aux niveaux de theorie CCSD et CCSD (T). Une nouvelle methode developpee par Keith et Frisch est utilisee pour augmenter des densites electroniques calculees a l'entremise de pseudo potentiels avec les densites de cceur relativistes manquantes pour obtenir des approximations aux densites totales des dimeres. Les indices de delocalisation calcules pour les dimeres du groupe 10 : [Ni.sub.2] (1.6), [Pd.sub.2] (0.44, qui est en ecart du reste du groupe) et [Pt.sub.2] (1.8); pour les dimeres du groupe 11: [Cu.sub.2] et [Ag.sub.2] (1.01) et [Au.sub.2] (1.13), toutes les liaisons etant covalentes; et pour le groupe 12 : [Zn.sub.2] (0.06), [Cd.sub.2] (0.08) et [Hg.sub.2] (0.09) indiquant de faibles liaisons du type van der Waals. L'image qui emerge a propos de la nature de la liaison chimique obtenu en examinant les valeurs de la densite electronique aux points critiques est completement en accord avec celle obtenue sur la base des indices de delocalisation. Une curieuse dependance lineaire (au lieu d'exponentielle) de l'indice de delocalisation sur la densite electronique au point critique est presente ici comme une observation et sera enquete en plus de profondeur dans un travail future. Plusieurs correlations entre les proprietes des liaisons chimiques et les energies de leur dissociation sont aussi explorees. Il est constate que, a l'exception du dimer de [Ni.sub.2], dote d'un caractere multireferenciel considerable, il existe des correlations entre les energies de dissociation des dimeres etudies et plusieurs des proprietes de leur point critiques de liaisons. Ces correlations sont nouvelles car ils s'etendent sur un ensemble de liaisons entre de paires d'elements differentes, quand traditionnellement ces correlations ont ete trouvees pour les liaisons entre une meme paire d'element, mais avec des substituents differents. Mots-cles: la topologie de la densite electronique, la liaison chimique des metaux de transition, la theorie quantique des atomes dans les molecules (QTAIM/TQADM), la liaison metal-metal., Introduction Transition metal dimers have been studied for a number of years (1) and continue to provide challenges for ab initio methods. As noted by Yanagisawa, Tsuneda, and Hirao, nonhybrid [...]

We demonstrate that by combining two robust theoretical quantum chemistry calculation techniques, stepwise ionization of C60 fullerene by UV and extreme UV photons can in principle occur up to a limit as high as C6026+ before coulomb explosion of the cage. Furthermore, these highly ionized forms exhibit a comparable structural and bonding stability as for the neutral fullerene. Certain astrophysical sources like the central stars of planetary nebulae and the hottest white dwarf stars have sufficiently hard UV radiation fields that can result in a series of highly charged C60q+ species from q = 1 up to q = 16. Harsher environments, like hot X-ray bubbles in planetary nebulae, X-ray binaries and other sources, may further push the ionization right up to the q = 26 limit. These remarkable theoretical findings add new avenues to complex ion/molecule reactions, the chemistry of fragmentation products and additional pathways for spreading carbon throughout the universe. The implications for the emerging field of astrochemistry of C60 fullerene in all its possible states could be profound. [ABSTRACT FROM AUTHOR]

Electronic structure and bonding properties of the Group 12 dications M22+ (M = Zn, Cd, Hg) are investigated and electron density‐derived quantities are used to characterize the metastability of these species. Of particular interest are the complementary descriptions afforded by the Laplacian of the electron density ∇2ρ(r) and the one‐electron Bohm quantum potential (Q = ∇2ρr/2ρr) along the bond path. Further, properties derived from the pair density including the localization‐delocalization matrices (LDMs) and the interacting quantum atoms (IQA) energies are analyzed within the framework of the quantum theory of atoms in molecules (QTAIM). From the crossing points of the singlet (ground) and triplet (excited) potential energy curves, the barriers for dissociation (BFD) are estimated to be 25.2 kcal/mol (1.09 eV) for Zn22+, 22.8 kcal/mol (0.99 eV) for Cd22+, and 26.4 kcal/mol (1.14 eV) for Hg22+. For comparison and benchmarking purposes, the case of N22+ is considered as a texbook example of metastability. At the equilibrium geometries, LDMs, which are used here as an electronic fingerprinting tool, discriminate and group together Group 12 M22+ from its isoelectronic Group 11 M2. While "classical" bonding indices are inconclusive in establishing regions of metastability in the bonding, it is shown that the one‐electron Bohm quantum potential is promising in this regard. [ABSTRACT FROM AUTHOR]

Astronomical infrared spectral features at ∼6.6, 9.8 and 20 µm have recently been suggested as being due to the planar graphene form of C24 carbon cluster. Here, we report density functional theory and coupled cluster calculations on wavefunctions stability, relative energies and infrared spectra of four different types of C24 isomers, including the graphene and fullerene forms. The types of vibrational motions under these bands are also discussed. Among the four isomers, we find that the astronomical data are best approximated by the graphene form of C24. [ABSTRACT FROM AUTHOR]

The topology of the electron density ρ(r) of ${\mathrm {H}}_{3}^{+}$ is revisited by series of ultra fine tuned geometry optimizations within Hartree-Fock self-consistent virial scaling (SCVS) approach in combination with correlation consistent cc-pVXZ basis sets. The calculations are extended to approach the Hartree-Fock complete basis set (CBS) limit. It is discussed that within such tuned ab initio calculations, the sources of errors that are mapped to the final density matrix in normal calculations are essentially eliminated. The results of electron density analysis on such error-free ρ(r) function via the quantum theory of atoms in molecules (QTAIM) confirm unambiguously the non-nuclear attractor (NNA) as the fundamental topological building block (together with three H atomic basins) to describe the bonding in ${\mathrm {H}}_{3}^{+}$ ion-molecule. The convergence patterns of the values of different density-dependent properties toward CBS limit are also explored. It is reported that the cc-pVXZ sets are not only energy-consistent but also density-consistent. Therefore, on the basis of this important density consistency behavior, the CBS limit values of different atomic and bonding indexes are estimated and ultimately the structure and bonding pattern of ${\mathrm {H}}_{3}^{+}$ are concluded. [ABSTRACT FROM AUTHOR]

We employ second-order Møller-Plesset perturbation theory level in combination with recently developed pseudopotential-based correlation consistent basis sets to obtain accurate relativistic-consistent electron densities for small coinage metal clusters. Using calculated electron densities, we employ Bader's quantum theory of atoms in molecules (QTALM) to gain insights into the nature of metal-metal bonding in the clusters M2, M4, M4,2-, and M4Na2 (M = Cu, Ag, Au). For the simplest case of the metal dimer, M2, we correlate the strength of the metal-metal bond with the value of the electron density at the bond critical point, the total energy density at the bond critical point, the sharing (derealization) index, and the values of die two principle negative curvatures. We then consider changes to the metal-metal bonding and charge density distribution upon the addition of two metal atoms to form the metal tetramer, M4 and then followed by the addition of an electron pair to form M42- and finally followed by die addition of two alkali metal (sodium) ions to form M4Na2. Using topological properties of die electron density, we present evidence for the existence of a-aromaticity in Au42- . We also report die existence of two non-nuclear attractors in the molecular in the molecular graph of Cu42- and large negative charge accumulation in the nonbonded Cu basins of this cluster. [ABSTRACT FROM AUTHOR]

We respond to the two questions posed by Weinhold, Schleyer, and McKee (WSM) in their study of cis-2-butene (Weinhold et al., J Comput Chem 2014, 35, 1499), in which they solicit explanations for the relative conformational energies of this molecule in terms of the Quantum Theory of Atoms in Molecules (QTAIM). WSM requested answers to the questions: (1) why is cis-2-butene less stable than trans-2-butene despite the presence of a hydrogen-hydrogen (H⋯H) bond path in the former but not in the latter if the H⋯H bond path is stabilizing? (2) Why is the potential well of the conformational global minimum of cis-2-butene only 0.8 kcal/mol deep when the H⋯H bonding is stabilizing by 5 kcal/mol? Both questions raised by WSM are answered by considering the changes in the energies of all atoms as a function of the rotation of one of the two methyl groups from the minimum-energy structure, which exhibits the H⋯H bond path, to the transition state, which is devoid of this bond path. It is found that the stability gained by the H⋯H bonding interaction is cancelled by the destabilization of one of the ethylenic carbon atoms which, alone, destabilizes the system by as much as 5 kcal/mol in the global minimum conformation. Further, it is found that the 1.1 kcal/mol stability of trans-2-butene with respect to the cis-isomer is driven by the considerable destabilization of the ethylenic carbons by 11 kcal/mol, while the changes in the atomic energies of the other corresponding atoms in the two isomers account for the observed different stabilities. The error introduced into QTAIM atomic energies by neglecting the virials of the forces on the nuclei for partially optimized structures is discussed., (© 2015 Wiley Periodicals, Inc.)