A quantum chemical study of [1.1.1.1]pagodane and its related compounds

The PBE0/6-31G** quantum chemical method is used to determine the symmetry and equilibrium structural parameters of the molecules of [1.1.1.1]pagodane (C₂₀H₂₀, D _2h ), two dienes (C₂₀H₂₀, D _2h ), two diradicals (C₂₀H₂₀, C _2ν ), and two dications (C₂₀H₂₀²⁺, D _2h and C _2ν ). The energy of a highly symmetric dication with a rectangular cycle is lower by 36 kcal/mole than that of a low symmetric dication with a trapezoidal cycle. The polarization interaction with liquid methylene chloride causes its decrease by 147 kcal/mole.     

Computational Study of Endohedral IrSi9 + Isomers

Recent ion-trap experiments (See Ref. 2) have demonstrated the existence of endohedral transition metal–silicon clusters of composition MeSi_N ⁺ (Me = Hf, Ta, W, Re, Ir) with a characteristic number of Si cage atoms for each of the Me atom species. In this series, IrSi₉ ⁺ was the smallest of the reported systems. In view of its moderate size, this cluster appears particularly suitable for a systematic study of the bonding and stabilization mechanism prevailing in this newly discovered class of metal–Si_N units. We present a discussion of two highly symmetric pure Si₉ cage structures, namely a trigonal prism and a tricapped trigonal prism, and identify a stable IrSi₉ ⁺ (C_3h) unit with a cage structure intermediate between these two D _3h geometries. An additional endohedral IrSi₉ ⁺ species of lower symmetry (C_s), but higher stability than the C _3h alternative, is found from insertion of the Ir impurity into the C _2v ground-state geometry of Si₉. Comparison is made with an exohedral isomer derived from a substitutional structure based on Si₁₀. The role of electron transfer from the Si₉ cage to the Ir impurity in the stabilization of the endohedral complexes is emphasized.     

The electronic and molecular structure of carbon clusters: C8 and C10

Summary The equilibrium geometries of C₈ and C₁₀ have been determined from electronic structure calculations, using a variety of correlated methods and large basis sets of atomic natural orbitals. For C₈, a cyclic form withC _4h symmetry (¹ A _g) and a linear, cumulene-like form (³ _g ^– ) are isoenergetic candidates for the electronic ground state. For C₁₀, the ground-state equilibrium structure is definitely monocyclic. Three different cyclic structures have been considered here, i.e. cumulenicD _10h , distorted-cumulenicD _5h and acetylenicD _5h . These are all essentially isoenergetic, and are about 50 kcal/mol below the linear³ _g ^– state. The choice of basis sets and methods used has a strong impact on the predicted ground-state structures.Dedicated to Prof. Klaus Ruedenberg     

Electronic structures of multi-decker transition metal sandwich complexes

The electronic structures of multi-decker transition metal sandwich complexes are discussed according to the structure rules for transition metal heterocarboranes. A series of skeletons of the structures Fe₂C₅(D_5h), Ni₂C₅(D_5h), V₂C₆(D_6h), Co₂C₆(D_6h), and Fe₂C₄(D_4h) are calculated using the EHMO method. The calculated results show that the number of valence bonding orbitals (VBO) can vary as the distance between the metal atoms in the metalocenes is increased. This fact can be used to explain the number of valence electrons (VE) in triple-decker sandwich complexes. The conclusions are proved and discussed through a theoretical analysis of the electronic structures of such complexes and through EHMO calculations for actual compounds containing 29–34 valence electrons.     

Quantum-chemical study of hexaazapentalene and its dioxides

Hexaazapentalene and its dioxides were calculated by the MINDO/3 and MNDO methods with full optimization of the geometric parameters. It was shown that the antiaromatic structure with C_2h and not D_2h symmetry is more favorable. The D_2h structure corresponds to the transition state of the automerization of the C_2h structure. For hexaazapentalene 2,5-dioxide, on the other hand, the aromatic structure with D_2h symmetry is more favorable. The possible dissociation paths of hexaazapentalene and its oxides are examined.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1825–1828, August, 1991.     

Intramolecular Rearrangements of Metal β-Diketonates. Ab Initio Study of Sc(MDA)3 (MDA = C3H3O2)

The geometrical parameters, force constants, and vibrational spectra of the C_2v configuration of the Sc(MDA)₃ molecule were calculated in terms of second-order Möller–Plesset perturbation theory with inclusion of electron correlation. Calculations were carried out using effective pseudopotentials (for describing atomic cores) and double-zeta valence basis sets complemented with polarization functions. The C_2v structure corresponds to the first-order saddle point on the potential energy surface (PES) of the ground electronic state. The results of our previous calculations for D₃ and D_3h configurations were used to show that the C_2v and D_3h structures are the transition states of two intramolecular rearrangements, describing transitions between the PES minima corresponding to different equivalent geometrical D₃ configurations of metal -diketonate tris-complexes. In Sc(MDA)₃, the rearrangement that occurs via the C_2v configuration is energetically (9.4 kJ/mole) more favorable.     

Carbodications. I. The structures and energies of C4H isomers

At high levels of ab initio theory (6-31G*//4-31G), the most stable C₄H isomer is indicated to be the nonplanar cyclobutadiene dication ( 1a ); the planar form, 1b , is indicated to be 7.5 kcal/mol less stable. The second most stable C₄H isomer, the methylenecyclopropene dication, is indicated to prefer the perpendicular ( 2a ) over the planar ( 2b ) arrangement by 7 kcal/mol. The “anti van't Hoff” cyclo-(HB)₂C?CH₂ system ( 4 ), isoelectronic with 2 , also prefers the perpendicular conformation ( 4a ), and retains the C?C double bond. The linear butatriene dication ( 3 ) is the least stable C₄H species investigated. The perpendicular (D_2d) arrangement ( 3a ), permitting double allyl cationlike conjugation, is preferred over the planar D_2h form ( 3b ) by 26 kcal/mol. The heat of formation of the most stable form of C₄H, 1a , is estimated to be 623–640 kcal/mol. This species should be thermodynamically stable toward dissociation into smaller charged fragments.     

Ab initio SCF studies of the molecular structure of XeF6, IF 6 − , and TeF 6 2− in non-octahedral geometries

All-electron SCF calculations in contracted large Gaussian basis sets were performed for the molecules in the isoelectronic series XeF₆, IF ₆ ^– , and TeF ₆ ^2– . Molecular equilibrium geometry of these molecules was studied first in O _h symmetry. Then, the gradient minimization technique was used to determine molecular structure of the studied systems near the local minima corresponding to C _3v and C _2v geometries involved in the internal motion.In the O _h symmetry, TeF ₆ ^2– and IF ₆ ^– are bound by 172 and 104 kcal/mol, respectively. The total energy of XeF₆ is larger than the sum of total energies of the constituent atoms by 192 kcal/mol. Lowering the symmetry to C _3v and C _2v results in an energy gain of about 20 kcal/mol for all studied systems.     

Pseudo Jahn–Teller effects of triphenylene dianion

Pseudo Jahn–Teller effects of triphenylene dianion were discussed by molecular orbital method. While the triplet ground state preferred a D_3h geometry, singlet states were subject to symmetry lowering to have C_2v geometries. The resultant geometries were rationalized to amplitude patterns of the degenerate frontier orbitals.     

The structure and stability of the acetylene dication

The dication C₂H has been investigated by ab initio molecular orbital theory. It is found to have a linear (D_∞h), structure with a triplet (³σ^?_g) ground state. Deprotonation to C₂H⁺ is exothermic by 9.8 kcal/mol, but this process is hindered by a large barrier of 65 kcal/mol.     

trans‐Tetra­chloro­bis(4‐methyl­pyridine)­germanium(IV)

The structure of the title compound, [GeCl₄(C₆H₇N)₂], (I), is the first example of an addition compound of GeCl₄ with two aromatic nitro­gen bases. The mol­ecule, with essential D_2h symmetry, has crystallographic C_2h symmetry. The environment around the Ge atom can be described as a slightly distorted octahedron with the 4‐methyl­pyridine ligands occupying axial positions and the four chloro ligands in the equatorial plane. The structure of (I) is isomorphous with the corresponding silicon halides.     

Raman and infrared spectra of6Li2C2O4 and7Li2C2O4

Raman and infrared spectra of polycrystalline⁶Li₂C₂O₄ and⁷Li₂C₂O₄ have been investigated in the wavenumber region from 1,800 to 40 cm^–1. The internal C₂O₄ ^–2 vibrations have been studied on the basis of a D_2h molecular structure and the correlation field splittings have been found to be about 40 cm^–1 for the stretching modes and about 15 cm^–1 for the bending modes. The external vibrations of the Li⁺ and C₂O₄ ^–2 sites have been discussed by considering the results of the factor group analysis and the⁶Li/⁷Li isotope effect on the normal vibrations.

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Raman- und Infrarot-Spektren von⁶Li₂C₂O₄ und⁷Li₂C₂O₄

Zusammenfassung Es wurdenRaman- und IR-Spektren von polykristallinem⁶Li₂C₂O₄ und⁷Li₂C₂O₄ im Bereich der Wellenzahlen von 1800 bis 40 cm^–1 untersucht. Die internen Schwingungen wurden auf der Basis einer D_2h Molekülstruktur analysiert. Für die Streckschwingungen wurde eine Korrelationsaufspaltung von etwa 40 cm^–1 gefunden, für die Deformationsschwingungen etwa 15 cm^–1. Die Diskussion der externen Schwingungen von Li⁺ und C₂O₄ ^–2 erfolgte unter Berücksichtigung der Resultate der Faktorgruppenanalyse und des⁶Li/⁷Li Isotopeneffekts auf die Normalschwingungen.

     

<Emphasis Type="Italic">Ab initio</Emphasis> structural study of M(mda)<Subscript>2</Subscript> complexes (M = Be,Mg, Ca; mda = C<Subscript>3</Subscript>O<Subscript>2</Subscript>H<Subscript>3</Subscript>)

The structure of M(mda)₂ (M = Be, Mg, Ca; mda = C₃O₂H₃) bis-complexes was investigated by the ab initio Hartree-Fock method and by including electron correlation in terms of second order Möller-Plesset perturbation theory; for calculations we used triple-zeta valence basis sets complemented with polarization functions. Two most probable geometrical nuclear configurations (D _2h and D _2d ) are considered for each molecule. The structure with two mutually orthogonal chelate ligands (D _2d symmetry) corresponds to the potential energy surface (PES) minimum. The planar D _2h configuration corresponds to the first order saddle point on PES; consequently, its relative energy determines the height of the barrier to the D _2d D _2h D _2d intramolecular rearrangement. Correlation equations that relate the calculated values of equilibrium internuclear distances, force constants, and rearrangement barrier heights to the value of the ionic radius of the metal atom have been obtained. These correlations were employed to evaluate the molecular constants for Sr(mda)₂ and Ba(mda)₂. The theoretical data are compared with the available experimental literature data.Original Russian Text Copyright © 2004 by V. V. Sliznev, S. B. Lapshina, and G. V. GirichevTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 4, pp. 611–623, July–August, 2004This revised version was published online in April 2005 with a corrected cover date.     

Quantum-chemical investigation of structure and valence isomerizations of perdehydrotetrasilacyclobutadiene

The structure and valence isomerizations of perdehydrotetrasilacyclobutadiene (Si₄) are discussed on the basis of concepts of cyclic ^– and ^–conjugation. Calculations by the MINDO/3 and MNDO methods established two types of geometric configurations with D_4h symmetry for both the singlet and triplet states of perdehydrotetrasilacyclobutadiene, differing in inversion of the HOMO and LUMO. Neither D_4h configuration of the singlet state corresponds to minima on the potential energy surface, and these configurations are distorted, one toward a rectangular structure with D_2h symmetry with alternation of bond lengths in the ring, and the other toward a rhombic bicyclic structure with D_2h symmetry. The latter structure corresponds to the global minimum on the potential energy surface of the Si₄.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 414–420, July–August, 1987.     

Blending Non‐Group‐3 Transition Metal and Rare‐Earth Metal into a C80 Fullerene Cage with D5h Symmetry

Rare‐earth metals have been mostly entrapped into fullerene cages to form endohedral clusterfullerenes, whereas non‐Group‐3 transition metals that can form clusterfullerenes are limited to titanium (Ti) and vanadium (V), and both are exclusively entrapped within an I_h‐C₈₀ cage. Non‐Group‐3 transition‐metal‐containing endohedral fullerenes based on a C₈₀ cage with D_5h symmetry, V_xSc_3?xN@D_5h‐C₈₀ (x=1, 2), have now been synthesized, which exhibit two variable cluster compositions. The molecular structure of VSc₂N@D_5h‐C₈₀ was unambiguously determined by X‐ray crystallography. According to a comparative study with the reported Ti‐ and V‐containing clusterfullerenes based on a I_h‐C₈₀ cage and the analogous D_5h‐C₈₀‐based metal nitride clusterfullerenes containing rare‐earth metals only, the decisive role of the non‐Group‐3 transition metal on the formation of the corresponding D_5h‐C₈₀‐based clusterfullerenes is unraveled.     

A MNDO study of carbon clusters with specifically fitted parameters

Summary A MNDO method with new parameters for carbon clusters is presented. The parameters in the new sets are specifically tuned to fit the properties of small carbon clusters, C₂, C₃, C₅ and C₇–C₁₀, and buckminsterfullerene, C₆₀. The validity of these MNDO parameters is verified by experimental data. The calculated (with new parameters) IR spectra of C₆₀ and the heat of formation, geometry and IR spectra of C₇₀ agree satisfactorily with observed data. Heats of formation of other fullerenes, from C₂₀ to C₈₄, and C₆₀O are evaluated. The resulting heats of formation of the isomers of C₇₆ and C₈₄ are reliable and their relative stability is in excellent agreement with other reports. The predicted IR spectra of several fullerenes, C₂₄(C_6v ), C₂₈(T _d ), C₃₂(D₃), C₃₆(D_6h ), C₅₀(C_5h ) and C₈₀(D_5d ) are provided to aid assignments of experimental spectra.     

Mapping the Metal Positions inside Spherical C80 Cages: Crystallographic and Theoretical Studies of Ce2@D5h‐C80 and Ce2@Ih‐C80

The dynamic positions of the dimetallic cluster inside the mid‐sized spherical cages of C₈₀–C₈₂ have been seldom studied, despite the high abundance of M₂@C_2n (2n=80, 82) species among various endohedral metallofullerenes. Herein, using crystallographic methods, we first unambiguously map the metal positions for both Ce₂@D_5h‐C₈₀ and Ce₂@I_h‐C₈₀, showing how the symmetry or geometrical change in cage structure can influence the motional behavior of the cluster. Inside the D_5h cage, the primary cerium sites have been identified along a cage belt of the contiguous hexagons, which suggests the significant influence of such a cage motif on endohedral cluster motion. Further analysis revealed a distorted D_5h cage owing to the “punch‐out” effect of cerium atoms. The consequence is the presence of two localized electrostatic potential minima inside the cage of (D_5h‐C₈₀)^6?, thus reflecting the primary ionic cerium–cage interaction. In contrast, a different motional behavior of Ce₂ cluster was observed inside the I_h cage. With the major cerium sites, the molecule of Ce₂@I_h‐C₈₀ presented an approximate D_2h configuration. With the combined theoretical study, we propose that the additional unidentified influence of Ni^II(OEP) (OEP=octaethylporphyrin) might be also relevant for the location of cerium sites inside the I_h cage.     

Ab initio study of cyclic siloxanes (H2SiO)n: n = 3, 4, 5

The geometry optimizations for several conformations of tri-, tetra-, and pentacyclosiloxane (H₂SiO)_n (n = 3, 4, and 5) were carried out, and the relative stabilities were compared at the Hartree-Fock (HF) and second order perturbation theory (MP2) levels of theory using the 6–31G* and 6–311G(d, p) basis sets. At the highest levels of theory, the only minimum for n = 4 (D₄) occurs at the highly symmetric D_4h structure. In contrast, several, nearly isoenergetic, minima are found on the D₅ surface. These have C₁, C₂, C_s, and D_5h symmetries. While the C₁ structure has the lowest MP2/6–311G(d, p) energy, this species is predicted to be highly fluxional, and the distribution of isomers is dependent on temperature. © 1996 by John Wiley & Sons, Inc.     

Computationally Designed Families of Flat,Tubular, and Cage Molecules Assembled with “Starbenzene” Building Blocks through Hydrogen‐Bridge Bonds

Using density functional calculations, we demonstrate that the planarity of the nonclassical planar tetracoordinate carbon (ptC) arrangement can be utilized to construct new families of flat, tubular, and cage molecules which are geometrically akin to graphenes, carbon nanotubes, and fullerenes but have fundamentally different chemical bonds. These molecules are assembled with a single type of hexagonal blocks called starbenzene (D_6h C₆Be₆H₆) through hydrogen‐bridge bonds that have an average bonding energy of 25.4–33.1 kcal mol^?1. Starbenzene is an aromatic molecule with six π electrons, but its carbon atoms prefer ptC arrangements rather than the planar trigonal sp² arrangements like those in benzene. Various stability assessments indicate their excellent stabilities for experimental realization. For example, one starbenzene unit in an infinite two‐dimensional molecular sheet lies on average 154.1 kcal mol^?1 below three isolated linear C₂Be₂H₂ (global minimum) monomers. This value is close to the energy lowering of 157.4 kcal mol^?1 of benzene relative to three acetylene molecules. The ptC bonding in starbenzene can be extended to give new series of starlike monocyclic aromatic molecules (D_4h C₄Be₄H₄^2?, D_5h C₅Be₅H₅^?, D_6h C₆Be₆H₆, D_7h C₇Be₇H₇⁺, D_8h C₈Be₈H₈^2?, and D_9h C₉Be₉H₉^?), known as starenes. The starene isomers with classical trigonal carbon sp² bonding are all less stable than the corresponding starlike starenes. Similarly, lithiated C₅Be₅H₅ can be assembled into a C₆₀‐like molecule. The chemical bonding involved in the title molecules includes aromaticity, ptC arrangements, hydrogen‐bridge bonds, ionic bonds, and covalent bonds, which, along with their unique geometric features, may result in new applications.     

A coupled-cluster study of linear and rhombic boron nitride dimers: a revisit

The potential energy surfaces of both singlet and triplet B₂N₂ have been investigated computationally at the coupled-cluster level with a polarized triple zeta basis set augmented with diffuse functions. Calculated vibrational frequencies and intensities are also reported. The triplet species are consistently more stable than their singlet analogs and the stabilities of the linear B₂N₂ isomers increase with increasing number of B–N bonds. The most stable isomer is the linear triplet BNBN isomer with a rhombic form with a short diagonal BB distance close in energy. Our results are consistent with the results of the matrix IR studies of Andrews et al. nucleus-independent chemical shift (NICS) values were calculated for the singlet D_2h rhombic form and its C_2v dication, and these were compared to those of the D_2h cyclobutadiene and its D_2d dication, respectively. Electron density plots for the linear and rhombic B₂N₂ minima showed similar distributions for the singlet and triplet states. These plots confirmed weak BB bonding interactions in both rhombic forms but larger BN bond orders.