A study of electronic structures of some C9H9- carboanions

The electronic structures of five C₉H₉^-, carboanions have been studied by ab initio STO-3G calculations, and some general conclusions on related C₉H₉^- and C₉H₉⁺ structures are presented. Large antibonding interactions in one occupied MO make barbaral-9-yl anion (2) unstable as its cationic counterpart (8). The proposed D_9h-symmetrical cation and D_3h-symmetrical anion (3) do not exist due to Jahn-Teller distortions. A study of the MO correlations confirms that the two tetracyclic anions with C_2v symmetry (5 and 6) are the results of the Jahn-Teller distortions of 3. Anion 5 is identified as the proper intermediate of the Cope rearrangement of anion 2.     

Geometrical and electronic structure of hydrated CH 5 + and CH 5 −

Different conformations of isolated and solvated CH ₅ ⁺ and CH ₅ ^? ions have been studied by CNDO method with Wiberg's parametrization. The anion has been found to have a most stable conformation ofD _3h symmetry both in the gas phase and in solution. AC _s symmetry conformation is the most stable one for the isolated cation, whereas a conformation withC _4v symmetry is energetically preferred in solution.     

Magnetic criteria of aromaticity in a benzene cation and anion: how does the Jahn–Teller effect influence the aromaticity?

The aromatic/antiaromatic behavior of the Jahn–Teller (JT) active benzene cation and anion has been investigated using Density Functional Theory (DFT) calculations of Nuclear Independent Chemical Shifts (NICS) and magnetic susceptibility. NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) for the benzene cation showing antiaromaticity which decreases with increasing deviation from D_6h to D_2h symmetry. Changes in NICS values along the IDP from D_6h to C_2v in the benzene anion revealed non-aromatic character.     

Preparation and Crystalline Structure of Dihydrate of <Emphasis Type="Italic">hemi</Emphasis>-(Diazonium-18-crown-6) Isocyanurate of Isocyanuric Acid

Crystalline dihydrate of hemi-(diazonium-18-crown-6) isocyanurate of isocyanuric acid, 0.5[H₂DA18K6]²⁺?(C₃H₂N₃O₃)^??C₃H₃N₃O₃?2H₂O, was prepared and investigated by X-ray crystallography. In the given structure the dication DA18K6 is center-symmetric and possesses crown conformation of approximate D_3h symmetry. Molecule of isocyanuric acid and its anion are nearly planer. This molecule possesses approximate D_3h symmetry and its anion possesses approximate C_2v symmetry. With sufficient precision were determined geometrical parameters (bond lengths, bond angles etc.) of ions and molecules and was determined their packing in the given crystalline structure in which there is developed system of intermolecular (interion) hydrogen bonds.     

A quantum chemical study of the structure of dodecasilsequioxane H12Si12O18

Quantum chemical B3LYP/cc-pVTZ, PBE0/cc-pVTZ, and MP2(full)/6-311G(d,p) methods are used to calculate the structural parameters of dodecasilsequioxane H₁₂Si₁₂O₁₈ and the H₁₂Si₁₂O ₁₈ ⁺ cation. According to DFT/cc-pVTZ calculations the energy of H₁₂Si₁₂O₁₈ (D _6h ) is 1.3–1.7 kcal/mol higher than the energy of H₁₂Si₁₂O₁₈ (D _2d ). A reduction of the basis set results in a greater energy difference of H₁₂Si₁₂O₁₈ isomers. For the cation ² B _2u and ² B ₁ electronic states are obtained, which correspond to symmetric equilibrium structures H₁₂Si₁₂O ₁₈ ⁺ (D _6h ) and (D ₂) respectively. For the He@H₁₂Si₁₂O₁₈ endocomplex the D _2d symmetry is obtained; for He₂@H₁₂Si₁₂O₁₈ the D _2h symmetry; and for H₂@H₁₂Si₁₂O₁₈ the D _6h symmetry.     

Bridged structures in multiply bonded silicon compounds: Disilyne,protonated disilyne and disilene

Ab initio SCF and electron correlation calculations are reported for the singlet ground state of the title compounds. These calculations confirm earlier findings that non-planar bridged Si₂H₂ is the most stable structure. For protonated disilyne (Si₂H³⁺) a bridged D_3h structure is the global mimimum. Two bridged structures of C_2v and C_2h symmetry are found in the case of disilene (Si₂H₄) which are only 14–17 kcal/mol above the D_2h structure.     

Structural stability and energetics of C,Si, and Ge microclusters: empirical many-body potential energy function calculation

The structural stability and energetics of carbon, silicon, and germanium microclusters containing 3?7 atoms have been investigated by using a recently developed empirical many-body potential energy function (PEF), which comprises two- and three-body atomic interactions. The PEF satisfies both bulk cohesive energy per atom and bulk stability exactly. It has been found that the most stable C_3?4 microclusters are linear withD _∞h symmetry but C_5?7 microclusters are planar withD _nh symmetry. Silicon and germanium microclusters show similar structural stability. TheX _n (X=Si, Ge;n=3?7) microclusters are found to be most stable in the following forms:X ₃ is triangular withD _3h symmetry,X ₄ is tetragonal withT _d symmetry,X ₅ is square pyramidal withD _4h symmetry,X ₆ is bipyramidal square withO _h symmetry, and finallyX ₇ is square pyramidal having two atoms underneath withD _2h symmetry.     

Single‐Electron Self‐Exchange between Cage Hydrocarbons and Their Radical Cations in the Gas Phase

We show that the radical cations of adamantane (C₁₀H₁₆^.+, 1 H^.+) and perdeuteroadamantane (C₁₀D₁₆^.+, 1 D^.+) are stable species in the gas phase. The radical cation of adamantylideneadamantane (C₂₀H₂₈^.+, 2 H^.+) is also stable (as in solution). By using the natural ¹³C abundances of the ions, we determine the rate constants for the reversible isergonic single‐electron transfer (SET) processes involving the dyads 1 H^.+/ 1 H, 1 D^.+/ 1 D and 2 H^.+/ 2 H. Rate constants for the reaction 1 H^.++ 1 D? 1 H+ 1 D^.+ are also determined and Marcus’ cross‐term equation is shown to hold in this case. The rate constants for the isergonic processes are extremely high, practically collision‐controlled. Ab initio computations of the electronic coupling (H_DA) and the reorganization energy (λ) allow rationalization of the mechanism of the process and give insights into the possible role of intermediate complexes in the reaction mechanism.     

The molecular and electronic structure of radical cations derived from tetramethyltin and hexamethylditin: An SCF-MO study

Molecular geometries and energies have been calculated, using the semi-empirical MNDO method for the closed-shell species SnMe₄, Sn₂Me₆, and (SnMe₃)⁺; and using the UHF-MNDO method for the radicals (SnMe₄)⁺, (Sn₂Me₆)⁺ and SnMe₃^.. The radical cation (SnMe₄)⁺ is calculated to have C_3v skeletal symmetry, with a C_2v isomer some 15 kJ mol^?1 higher in energy. The dinuclear radical cation (Sn₂Me₆)⁺ is calculated to be a σ(SnSn) radical, of D_3d skeletal symmetry: although the calculated Sn(5s) spin density is extremely low, the tin atoms are far from planarity. Calculated spin densities are compared with experimental hyperfine couplings.     

Ab initio study of structures and energies of Al2H4 and Al2H 4 −

The low-lying isomers of Al₂H₄ and their anions are investigated with the hybrid density functional B3LYP, the coupled-cluster CCSD and CCSD(T) methods, and the electron propagator theory. The positive adiabatic electron affinities 5,798 and 10,112 cm⁻¹ are predicted for the neutral C_2v and D_2d symmetric isomers, respectively. The D_2h symmetric anion is more stable by 852 cm⁻¹ than the C_2v symmetric anion. The photodetachment spectra for Al₂H₄⁻ anions at the C_2v and D_2h symmetries are simulated on the basis of the Franck–Condon factor calculations, indicating a reasonable way to study the transition state of the intramolecular torsion process     

A classification of metaborate crystals based on Raman spectroscopy

The Raman spectra of crystals built solely of metaborate triangles provide fingerprint identification of three distinct network types. Classified according to increasing cation field strength these are: rings with degenerate intra-annular bonds and D_3h symmetry, distorted rings with alternating intra-annular bonds and C_3h symmetry, as well as chains. The occurrence of each network type has been associated with a characteristic range of cationic field strength. This approach led to the discovery of a hitherto unknown C_3h ring strontium metaborate crystal, with Sr²⁺ cations in 9- or 10-fold coordination to oxygen atoms. The Raman spectra of the mixed cation metaborates Ba₂Ca(B₂O₄)₃ and Ba₂Mg(B₂O₄)₃ confirm their C_3h-ring structure and clearly point to the fact that the synergetic effect of dissimilar cations to the metaborate network cannot be predicted by the additivity of their field strengths.     

Algebraic symmetry and determination of the “imaginary” crystal field parameters from optical spectra of fn-ions. Hexagonal and trigonal symmetry

Algebraic symmetry of the characteristic equation of the crystal field hamiltonian Ĥ_CF for hexagonal (C₆, C_3h, C_6h) and trigonal (C₃, S₆) symmetry is considered. Explicit dependence of the coefficients a_i of the characteristic equation on the CF parameters is derived by ALTRAN. Magnitudes, signs and/or relative signs of the CF parameters which can be determined from fitting the optical spectra of fⁿ-ions at sites of the above symmetry are established. The algebraic results derived here can facilitate the fitting procedures for any hexagonal and trigonal symmetry. The relevant literature is extensively surveyed. The approach C (complete) to fitting the optical spectra, which yields ambiguous and unreliable results in general, has not been employed to hexagonal (C₆, C_3h, C_6h) symmetry. Some misinterpretations on the approach R (reduced) and A (approximated), which merge for this symmetry, are clarified. Crystal field analysis has been carried out for trigonal (C₃, S₆) symmetry in but a few cases only. The survey has revealed one case of the inappropriate C used for Eu³⁺ in the trigonal (C₃) Eu(DBM)₃ · H₂O system.     

Calculation of the thermodynamic characteristics of ions in vapor over sodium fluoride

The geometric parameters, normal vibration frequencies, and thermochemical characteristics of the ions present in vapor over sodium fluoride, Na₂F⁺, Na₃F ₂ ⁺ , NaF ₂ ^? , and Na₂F ₃ ^? , were calculated ab initio by the Hartree-Fock method and taking into account electron correlation. The main equilibrium configuration of all ions was found to be the linear configuration of D _∞h symmetry. Pentaatomic ions could also exist as two isomers, planar cyclic of C _2v symmetry and bipyramidal of D _3h symmetry. Their energies were higher than that of the D _∞h isomers, and their contents in vapor were negligibly low. The energies and enthalpies of dissociation of the ions with the elimination of the NaF molecule were calculated. The enthalpies of formation of the ions were obtained.     

Flattened fullerenes

Quantum-chemical calculations of giant flattened fullerenes C _n (lentil-shaped) have been carried out. The topology, molecular and electronic structure of these fullerenes have been studied. Such molecules consist of two identical coronenoid fragments of a graphite layer, which are arranged one above the other, and a system of polycondensed five- and six-membered cycles, which form a side surface of the cluster. Polyhedral structures with isolated pentagons of three symmetry types (D _6h ,D _6d , andD _3h ) have been considered. The topology of these structures is described in terms of planar molecular graphs. Electronic structures of eleven flattened lentil-shaped C _n clusters (n = 72–216) have been studied in the π approximation. Most of the considered systems have closed or quasi-closed electron shells (according to Hückel) and rather large energy gaps separating the highest occupied and lowest unoccupied MO, which is indicative of their kinetic stability. Fragments of the potential energy surfaces of the C₇₂ and C₉₆ fullerenes have been studied by the MNDO, AM1, and MNDO/PM3 methods. For the C₉₆ cluster, two local energy minima, which correspond to the lentil-shaped isomers withD _6h andD _6d symmetry, have been determined. As a result of optimization of geometric parameters, it was found that all three methods give close values of heights (H = 6.7 Å) and diameters (D = 9.8 Å) for both isomers. The clusters change to quasi-two-dimensional systems (H«D) with increasing sizes of coronenoid fragments.     

Ligands of low electronegativity in the vsepr model: electron-rich hydrides MH3E2, MH4E,MH5E,and MH6E

Hydrides FH₃, ClH₃, and OH^?₃ of type MH₃E₂ are calculated to adopt D_3h structures: NH₃^2?, PH₃^2?, and SH₃^? each have two energy minima, one at D_3h and the other at a T-shaped geometry, of which the D_3his the more stable for SH₃^? but the less stable for NH₃^2? and PH₃^2?. Hydrides NH₄^2?, OH₄, and ClH₄⁺ of type MH₄E have a single energy minimum at T_d: CH₄^2?, SiH₄^2?, PH₄^?, and SH₄ each have two minima, one at T_d (more stable for SH₄ only) and one at an SF₄-like C_2v geometry, which is the more stable for CH₄^2?, SiH₄^2? and PH₄^?. D_3h and C_4V structures are very close in energy for all hydrides of type MH₅E, with no activation barrier between the two configurations: D_3h is the more stable configuration for OH₅^?, FH₅, SH₅, and ClH₅, but C_4V is the more stable for NH₅^2?, SiH₅^3?, and PH₅^?. The T_1u bending force constant in hydrides MH₆E becomes negative, for C_3V distortion, in PH₆^3? and SiH₆^4?. Both the equilibrium geometries and the force constants strongly support an interpretation, in terms of the second-order Jahn-Teller effect, of the observed stereochemical inactivity of non-bonding electrons in the presence of ligands of low electronegativity. Molecular energies, equilibrium geometries, orbital energies and electron populations are reported for all species considered in this study. Three molecular states of ClH₄^?, of type MH₄E₂, were also briefly investigated.     

Die struktur des dreikernigen hexamethylbenzolniob-komplexes [(Me6C6)3Nb3Cl6]+ Cl- · 3 CHCl3

Well developed crystals of [(Me₆C₆)₃Nb₃Cl₆]⁺ Cl^? · 3 CHCl₃ can be obtained from a solution of [(Me₆C₆)₃Nb₃Cl₆] Cl in CHCl₃ (monoclinic, P2₁/c, a 11.850(3), b 15.906(6), c 28.529(8) Å, β 98.14(3)°, Z  4). An X-ray structure determination shows the structure of the complex cation to be highly symmetric (non-crystallographic D_3h symmetry) and to agree within narrow limits with the known structure of the corresponding 2+ cation. Important distances are: NbNb 3.347(4) and NbCl 2.504(2) Å. The C₆ rings of the hexamethylbenzene rings are not planar. The average folding angle of the C₆ groups is 156.6°. In the crystal the Cl^? anion is bonded by weak H-bridges to three CHCl₃ molecules.     

The isotropic and anisotropic EPR spectra of the tetrafluoroethylene radical anion

The tetrafluoroethylene radical anion has been generated in solid solutions by electron addition to the parent molecule. Both its isotropic and anisotropic EPR spectra have been observed, including the isotropic ¹³C satellite lines in natural abundance. The isotropic EPR parameters are a_F = 94.3 G, a_C = 48.7 G, and g = 2.0027. Two possible geometries, planar and chair, are discussed for this radical anion. The magnitude of the ¹⁹F and ¹³C hyperfine coupling constants are consistent with a planar (D_2h) structure provided that the unpaired electron occupies the 5b_1u (σ*) rather than the 2b_1g (π*) molecular orbital which is predicted by ab initio calculations to be the LUMO of the parent molecule. On the other hand, the EPR parameters do not rule out a chair (C_2h) structure if the bending of the CF₂ groups introduces only a small distortion from planarity.     

Ring opening in methylenecyclopropane

The indo molecular orbital calculations on methylenecyclopropane show that the ¹A₁ state of the (90,90) configuration is the most stable one at θ = 64°. The ground state of the ring-opened trimethylenemethane is predicted to be the ³A'₂ state (D_3h symmetry), however the³A″ state of non-planar (0,90) trimethylenemethane is about 10 kcal/mole higher in energy than the planar diradical species. This suggests intermediacy of the orthogonal diradical in the degenerate thermal rearrangement of substituted methylenecyclopropane. The calculated geometry, dipole moment of methylenecyclopropane, and spin densities of trimethylenemethane are in excellent agreement with the experimental values.     

Radical Ions in the Pentalene Series [1]. Part II. Dibenzo [b,f]pentalene and its 5, 10-dimethyl derivative

Proton hyperfine data have been determined for the radical anions and cations of dibenzo [b,f]pentalene (III) and its 5, 10-dimethyl derivative (IV) . The assignment of the coupling constants to pairs of equivalent protons follows from a simple MO model, the use of which enables one to reproduce satisfactorily the experimental values. The proton hyperfine data, a_Hμ

1 The meaning of ^aH_μis ^aH? C(x),H? C(y),whereas only x and y are given in the particular cases.

, for the radical anion III?. correlate fairly well with the π-charge populations ?_μ derived from ¹H-NMR. spectra for the carbon centres μ in the dianion III^2?. The analogous correlation is less good in the case of the radical cation III⊕. and the dication III^2⊕., presumably due to the rough approximations involved in the evaluation of the numbers ?_μ for the latter species. The coupling constants a_H5,10 for III?. and III⊕. are very close to the corresponding values a_H4,6 for the radical ions of 1,3,5-tri-t-butylpentalene (II), in accord with the prediction of the MO model. A similarity is also found between the proton hyperfine data for III?. and those for the radical anions of structurally related 1,4-diphenyl-1,3-butadiene (V). On the other hand, there are striking changes in the coupling constants of the analogous protons on passing from III?. to the radical anions of dibenzo [a,e] cyclooctene (VI) and [16] annulene (VII), as a consequence of raising the symmetry from C_2h to D_2h and D_4h, respectively.