Stabilization of octacoordinate carbon center in metal-containing derivatives of orthocarbonic acid

The electronic and spatial structures of alkali metal compounds CO₃M₂, CO₃M₃ ⁺, and CO₄M₄ (M = Li, Na, K) were investigated by the ab initio (MP2(full)/6-311+G**) and density functional (B3LYP/6-311+G**) methods. The calculated energies of formation decrease in the order E ^Li > E ^Na > E ^K for all structural types, being determined by steric and orbital interactions. Stable structures with octacoordinate carbon are formed in the case of CO₄M₄ salts. Dedicated to Academicians A. L. Buchachenko and N. S. Zefirov on the occasions of their 70th birthdays. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1929–1938, September, 2005.     

Calculations of thermodynamic stability of CpCoC<Subscript>2</Subscript>B<Subscript>9</Subscript>H<Subscript>11</Subscript> cobaltacarborane isomers

Quantum-chemical calculations of the geometry and energies of nine possible isomers of 12-vertex cobaltacarborane CpCoC₂B₉H₁₁ (1) were carried out by the DFT method (PBEPBE/DGDZVP/DGA1). Thermodynamic stability of the isomers increases with increasing distance between the carbon atoms in the cage and is virtually independent of the position of the CpCo vertex. The relative stabilities of the 1,2,3-(17.57 kcal mol⁻¹), 1,2,4-(3.72 kcal mol⁻¹), and 1,2,9-isomers of 1 (0 kcal mol⁻¹) are similar to the corresponding values for the ortho (17.61 kcal mol⁻¹), meta (3.21 kcal mol⁻¹), and para isomers (0 kcal mol⁻¹) of carborane C₂B₁₀H₁₂. The results of the present study confirm a close similarity of the CpCo and BH fragments in metallacarborane chemistry. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1557–1559, July, 2005.     

Simulation of molecular and electronic structure of polyhydrogenated (n,0)-tubulenes and their analogs intercalated with lithium

Molecular and electronic structure of four polyhydrogenated (n,0)-tubulenes, namely, [−C₂₄H₄−] _m (1), two isomers of composition [−C₂₈H₄−] _m (2 and3), and [−C₃₂H₄−] _m (4) withn benzene rings in the cross section (n=6, 7, 7, and 8, respectively), was simulated atm>1 (m is the number of repeating fragemnts). It was assumed that hydrogen atoms are attached to all carbon atoms lying on the two most distant elements of the cylinders of the corresponding tubulenes. The energy band structures of macromolecules1–4 and their Li-intercalated analogs [−C₂₄H₄Li−] _m (5) [−C₂₈H₄Li−] _m (two isomers,6 and7), and [−C₃₂H₄Li−] _m (8), containing one Li atom per repeating unit at each center, were obtained in the EHT approximation by the crystal orbital method. Geometric parameters of repeating units of structures1–8 were found after MNDO/PM3 optimization of the energies of hydrocarbon molecules C₇₂H₂₄, C₈₄H₂₆ (two geometric isomers), and C₉₆H₂₈, containing three repeating units of corresponding tubulenes1–4 each. The conductivity types of polyhydrogenated tubulenes1–4 are the same as those of their precursors, (6,0)-, (7,0)-, and (8,0)-tubulenes. Dispersion curves of systems5–8 are much the same as those of macromolecules1–4; however, electron energy spectra of5–8 possess metallic conductivity type and the positions of Fermi levels for these systems are higher than for compounds1–4. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2061–2067, November, 1999.     

Experimental and theoretical investigation of the electronic absorption spectra of 2-H(methyl, methylthio, phenyl)-3,4-dihydro-4-quinazolinylidenecyanoacetic ester

The effect of substituents (Me, SMe, Ph) in position 2 has been studied on the nature of the absorption bands of 2-H-3,4-dihydro-4-quinazolinylidenecyanoacetic esters and a very high sensitivity on the substituent has been detected for the short wave maxima. On the basis of data calculated by the ZINDO/S method the featureless long-wave absorption band was assigned to an electronic transition from HOMO to LUMO caused by charge transfer from fragments of the benzene ring and the N=C—N bond to the >C=C—C=O fragment. Dedicated to Professor L. I. Belen’kii on his 75th birthday. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 227–232, February, 2006.     

The Quest for PdII–PdII Interactions: Structural and Spectroscopic Studies and Ab Initio Calculations on Dinuclear [Pd2(CN)4(μ‐diphosphane)2] Complexes

Structural and spectroscopic properties of and theoretical investigations on dinuclear [Pd₂(CN)₄(P–P)₂] (P–P=bis(dicyclohexylphosphanyl)methane ( 1 ), bis(dimethylphosphanyl)methane ( 2 )) and mononuclear trans‐[Pd(CN)₂(PCy₃)₂] ( 3 ) complexes are described. Xray structural analyses reveal Pd???Pd distances of 3.0432(7) and 3.307(4) Å in 1 and 2 , respectively. The absorption bands at λ>270 nm in 1 and 2 have 4d →5p_σ electronic‐transition character. Calculations at the CIS level indicate that the two low‐lying dipole‐allowed electronic transition bands in model complex [Pd₂(CN)₄(μ‐H₂PCH₂PH₂)₂] at 303 and 289 nm are due to combinations of many orbital transitions. The calculated interaction‐energy curve for the skewed dimer [{trans‐[Pd(CN)₂(PH₃)₂]}₂] is attractive at the MP2 level and implies the existence of a weak Pd^II–Pd^II interaction.     

Calculations of enthalpies of formation of carbenes in the ground state in the gas phase

Known enthalpies of formation (Δ_f H ^o) of carbenes in the ground state in the gas phase were analyzed; the prospects for the theoretical evaluation of (Δ_f H ^o) were considered. The (Δ_f H ^o) values of carbenes were calculated by the group-addition method, developed previously for free radicals, as well as by the AM1 and PM3 quantum-chemical methods; these methods were compared. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 57–63, January, 1997.     

Quantum-chemical study of the protonation of pyrrolo[2,1-b]thiazole and its selenium and tellurium analogs

The possibility of proton attack on various centers in pyrrolo[2,1-b]thiazole (1) has been evaluated. The results of semiempirical (MNDO, AM1, and PM3) andab initio (6-31G^*) calculations were compared. The MNDO and 6-31G^* methods give “chemically proper” and qualitatively coincident results. Analysis of the intramolecular (geometric and electronic) reorganization of molecule1, depending on the protonation center, has been carried out. The most probable attack centers, depending on the mechanism of electrophilic reaction, have been recognized. The energy parameters of intramolecular prototropic rearrangements in cation1 and the “blocking” factor value of methyl groups reducting the corresponding complex stability have been evaluated. It has been established that the relative stability of the protonated forms does not change on going to pyrrolo[2,1-b]selenium- and telluriumazoles, but the range of variations is considerably narrowed in the series S>Se>Te. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1707–1711, October, 2000.     

Isomerization and decomposition of germiranes: a density functional study

The formation and decomposition pathways of germiranes (germacyclopropanes), i.e., products of reactions of the GeH₂ and GeMe₂ germylenes with ethylene, tetramethylethylene, buta-1,2,3-triene, and tetramethylbuta-1,2,3-triene, were studied using the density functional approach (PBE/TZ2P approximation). The thermodynamic stabilities of the structures under consideration were evaluated by calculating the Gibbs free energies under normal conditions (ΔG°₂₉₈). Addition of germylenes to the C=C bond can proceed as a single-step process without a barrier or involve the formation of a π-complex (the barrier to this process is lower than the sum of the energies of isolated reactants). Stability of the germiranes formed is determined by their stability to retrodecomposition into the initial germylene and olefin and to the three-membered ring opening followed by simultaneous 1,2-migration of the substituent at the Ge atom and formation of the secondary germylene. Alkyl substituents can efficiently block the opening of the three-membered ring and transformation of the cyclic structure into the secondary germylene, simultaneously decreasing the germirane stability to retrodecomposition. Decomposition into germylene and olefin under normal conditions is thermally favorable for hexamethylgermirane (ΔG°₂₉₈ = −5.7 kcal mol⁻¹), being thermally forbidden for the other germiranes studied in this work (Δ G°₂₉₈ > 0). The activation energy (E _a) for the germirane ring opening depends on the substituents at the germanium atom, namely, E _a ≤ 10 kcal mol⁻¹ for unsubstituted germiranes and E _a > 30 kcal mol⁻¹ for methyl-substituted germiranes. Taking the experimentally isolated germirane as an example, it was shown how the introduction of substituents and modification of the carbon skeleton make it possible to stabilize the germacyclopropane system. Dedicated to Academician A. L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1943–1951, September, 2005.     

Tautomerism of rhodanine

Semiempirical (MINDO/3, AM1, PM3, MNDO) and ab initio (4-31G and 4-3IG + dAO/S basis sets) calculations on the relative stabilities and structures of the five potential tautomeric forms of rhodanine are reported. It is shown that all methods (excepting PM3) predict as most stable 2-thioxo-4-thiazolidinone. These results correspond to the known experimental data. The infrared spectrum of rhodanine was recorded for the region 4000-150 cm^–1, and the characteristic bands were compared with AM1 and 4-31G + dAO/S calculated frequencies. The transition states between five pairs of all possible tautomeric forms of the rhodanine were found by the AM1 method.     

A Systematic Quantum Chemistry Study on Cyclodextrins

Summary.  AM1 and PM3 modeling of β-hydroxyethyl ether and α-(1→4)-glucobiose indicated that PM3 is advantageous to AM1 in cyclodextrin (CD) chemistry. The conclusion was supported by direct structure optimization of α- and β-CD with AM1 and PM3, in which AM1 gave badly distorted geometries due to unreasonable hydrogen bonding, whereas PM3 reproduced the crystalline structures rather well. Ab initio calculation was for the first time performed on CD, demonstrating the feasibility of this method for future studies concerning CD chemistry. The results also provided valuable insights into the driving forces in CD molecular recognition. Received January 7, 2000. Accepted (revised) March 22, 2000     

Investigation of UV spectra of isomeric nitropyrazoles by the semiempirical AM1 (CI) method

The absorption bands in the UV spectra of isomeric nitropyrazoles were assigned by the calculations in the semiempirical AMI (CI) approximation. The long-wave absorption of nitropyrazoles is caused by π→π* and η₀→π* transitions. The charge-transfer band is the most intense. The π→π* transitions undergo a considerable bathochromic shift in the deprotonation. The first ionization potential (PI) of the 4-nitropyrazole anion was estimated from the empirical dependence of the energy of the excited π-state on PI of alkyl-substituted 4-nitropyrazoles. The PI of the 4-nitropyrazole anion is 3 eV lower than that of a neutral molecule. This is evidence for a substantial destabilization of the boundary β-orbital in the heterolytic cleavage of the N−H bond. The analysis of the UV and NMR spectra of 3(5)-nitropyrazole confirms the viewpoint that the 3-nitro tautomer predominates in solution. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 2, pp. 310–314, February, 1997.     

MNDO/PM3 study of the mechanism of a model reaction between methane and Br+ cation

The structure of complexes formed in the [CH₄+Br⁺] system was simulated by the MNDO/PM3 method. Along with five local minima, a number of stationary points at which the Hessians have only one negative eigenvalue were found on the potential energy surface of reactions occurring in the [CH₄+Br⁺] system. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1715–1718, September, 1998.     

Structure and coordination of organometallic groups on a chemically modified SiO2 surface

The structural, electronic, and energy parameters of the metal-containing clusters (H₃SiO)₃Si−O−XMe _n (X=H, B, Al, or Zn;n=0, 1, or 2), which model organometallic groups on a SiO₂ surface modified with B-, Al-, and Zn-containing alkyls, have been studied by quantum-chemical methods. Full geometry optimization for these clusters was carried out by the SCF MO LCAO method taking into account the electron correlation within the frameworks of the MP2 and B3LYP schemes using the 6-31G(d) (6-311G(d) for Zn) basis set. The effect of the crystal environment was taken into account in calculations of siliconoxygen clusters containing 10 and 30 silicon atoms using theab initio SCF/6-31G(d) and semiempirical MNDO-PM3 methods. Various modes of coordination and interactions of organometallic groups with oxygen atoms of surface groups were studied. For the organoaluminum group on the surface, two stable conformations were found, namely, the three-coordinate structure with the chain −O−AlMe₂ ligand and the four-coordinate (quasicyclic) structure with the Al atom that forms two nonequivalent bonds with the O atoms at the same Si atom. The four-coordinate structure is energetically more favorable. No stable structures were found for the organoboron and organozinc fragments. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1296–1303, July, 1998.     

Heat-Resistant Properties in the Phosphorescence of trans-Bis[β-(iminomethyl)aryloxy]platinum(II) Complexes: Effect of Aromaticity on d–π Conjugation Platforms

The heat-resistant properties towards thermal emission quenching of trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes bearing 3-iminomethyl-2-naphtholato- ( 1 ), 1-iminomethyl-2-naphtholato- ( 2 ), 2-iminomethyl-1-naphtholato- ( 3 ), and 2-iminomethyl-1-phenolato ( 4 ) moieties, and a mechanistic rationale of these properties, are described in this report. Complex 1 a , with N,N′-dipentyl groups, exhibits intense red emission in 2-methyl-2,3,4,5-tetrahydrofuran (2-MeTHF) at 298 K, whereas the analogues 2 a – 4 a are less or non-emissive under the same measurement conditions. All four complexes are highly emissive at 77 K. The heat-resistant properties toward thermal emission quenching (Φ_{298 K}/Φ_{77 K}) increase in the order 1 a (0.52)> 2 a (0.09)> 3 a (0.02)>> 4 a (0.00). We investigated the emission decay and thermal-deactivation processes using density functional theory (DFT), time-dependent (TD) DFT, and double-hybrid density functional theory (DHDF) calculations of N,N′-diethyl forms 1 b – 4 b , and discuss the results with a focus on the energy levels, molecular structures, and electronic configurations in the triplet excited states. The energy differences between the triplet metal–ligand charge transfer (³MLCT) state and minimum-energy crossing point between the lowest triplet state and singlet ground state (MECP) increase in the order 1 a > 2 a , 3 a > 4 a , consistent with the experimental results for the heat-resistant properties of these complexes. The origin of the present structure dependence of the ³MLCT–MECP energy gap is ascribed to the ease or difficulty of the high-lying d_σ* orbital participating in the MECP upon thermal structural distortion. The structure dependence in energy gaps between the π* and d_σ* orbitals, which is key for facilitating the thermal deactivation process, is rationally correlated with the extent of aromaticity on the coordination platforms ( 1 b >( 2 b , 3 b )> 4 b ).     

Quantum-chemical evidence for the possible existence of a new isomer of dinitrogen tetraoxide

The geometric, electronic, and thermodynamic parameters of six known isomers of the molecular structure of dinitrogen tetraoxide N₂O₄ were calculated by the quantum-chemical DFT/B3LYP density functional method with the 6-311++G(3df) basis set. The structure of a new isomer of dinitrogen tetraoxide NONO₃, which is characterized by a local potential energy minimum and corresponds to a stationary state of the N₂O₄ isomer, was calculated. The DFT calculations showed that this structure NONO₃ is characterized by a significant negative charge on the NO₃ fragment and a positive charge on the NO fragment. The calculated dipole moment of nitrosonium nitrate NO⁺NO₃⁻ in the gas phase is 4.13 D. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 1, pp. 24–29, January–February, 2008.     

Study of the electronic structure of organogermanes with extended coordination sphere of germanium

The spatial and electronic structures of some organogermanes with extended coordination spheres of the metal were calculated using AMPAC and HYPER CHEM 4.0 program packages. Interatomic distances, bond angles, torsion angles, atomic charges, electron densities, bond orders, parameters of localized and canonical molecular orbitals,etc. were determined. The nature of the additional intramolecular O→Ge interaction in the molecules of these compounds was considered. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 383–387, March, 1998.     

Standard enthalpies of formation in the gas phase and relative stability of tautomers of C-nitro-1,2,4-triazole and isomers of N-alkyl-C-nitro-1,2,4-triazole: quantum-chemical studies

Quantum-chemical calculations of the standard enthalpies of formation in the gas phase of C-nitro-1,2,4-triazole and isomers of N-alkyl-C-nitro-1,2,4-triazoles (Alk = Me, Et, i-Pr, t-Bu) were carried out by the B3LYP method using equations for the isodesmic reactions and isomerization reactions. The relative Gibbs free energies of tautomers and isomers in aqueous solution were calculated. For the tautomers of C-nitro-1,2,4-triazole the structural indexes of aromaticity were estimated and the electron population density of the Natural Bond Orbital was analyzed. The relative stabilities of the described tautomers and isomers in the gas phase and in solutions were discussed on the basis of the results of the calculations. Dedicated to Academician B. A. Trofimov in his 70^th jubilee. Translated from Khimiya Geterosiklicheskikh Soedinenii, No. 1, 83-94, January, 2009.     

Theoretical Investigation on the Effect of Different Nitrogen Donors on Intramolecular Se⋅⋅⋅N Interactions

The effect of different donor nitrogen atoms on the strength and nature of intramolecular Se ??? N interactions is evaluated for organoselenium compounds having N,N‐dimethylaminomethyl (dime), oxazoline (oxa) and pyridyl (py) substituents. Quantum chemical calculations on three series of compounds [2‐(dime)C₆H₄SeX ( 1 a – g ), 2‐(oxa)C₆H₄SeX ( 2 a – g ), 2‐(py)C₆H₄SeX ( 3 a – g ); X=Cl, Br, OH, CN, SPh, SePh, CH₃] at the B3LYP/6‐31G(d) level show that the stability of different conformers depends on the strength of intramolecular nonbonded Se ??? N interactions. Natural bond orbital (NBO), NBO deletion and atoms in molecules (AIM) analyses suggest that the nature of the Se ??? N interaction is predominantly covalent and involves nN→σ*_{Se? X} orbital interaction. In the three series of compounds, the strength of the Se ??? N interaction decreases in the order 3 > 2 > 1 for a particular X, and it decreases in the order Cl>Br>OH>SPh≈CN≈SePh>CH₃ for all the three series 1 – 3 . However, further analyses suggest that the differences in strength of Se ??? N interaction in 1 – 3 is predominantly determined by the distance between the Se and N atoms, which in turn is an outcome of specific structures of 1 , 2 and 3 , and the nature of the donor nitrogen atoms involved has very little effect on the strength of Se ??? N interaction. It is also observed that Se ??? N interaction becomes stronger in polar solvents such as CHCl₃, as indicated by the shorter r_{Se ??? N} and higher E_{Se ??? N} values in CHCl₃ compared to those observed in the gas phase.     

Topological aspects of metals in carbon cages: Analogies with organometallic chemistry

Metals can interact with carbon cages in the following ways: (1) stable carbon cages (i.e., fullerenes) function as electronegative olefins in their exohedral η₂ bonding to transition metals; (2) endohedral metallofullerenes with a highly electropositive lanthanide (Ln) inside the carbon cage can be considered to be ionic with lanthanide cations, Ln³⁺, and fullerene anions; (3) fullerenes too small for independent existence can be stabilized by internal covalent bonding to an endohedral metal atom using the central carbon atoms of pentagon triplets,i.e triquinacene, units, in complexes such as M@C₂₈ (M=Ti, Zr, Hf, and U), derived from the tetrahedral fullerene C₂₈; (4) metal atoms can occur as vertices of binary mixed metal-carbon cages in both early transition metal complexes of the types M₁₄C₁₃, M₈C₁₂, and M₁₃C₂₂ (e.g., M=Ti) and copper-carbon cages of the types Cu_2n +1C_2n ⁺ (n≤10), Cu₇C₈ ⁺, Cu₉C₁₀ ⁺ and Cu₁₂C₁₂ ⁺. The presence of metal atoms as vertices of carbon cages changes radically their stoichiometries and thus their structures. Thus, early transition metals form cages such as Ti₁₄C₁₃ assumed to have titanium atoms at the vertices and face midpoints of a 3×3×3 cube and carbon atoms at the edge midpoints and center of the cube and Ti₁₃C₂₂ assumed to have titanium atoms at the edge midpoints and center of a 3×3×3 cube as well as C₂ units and carbon atoms at the vertices and face midpoints, respectively, of the cube. Elimination of the face metal atoms from the Ti₁₄C₁₃ structure as well as the center carbon atom, which has been achieved experimentally by photofragmentation, leads to the Ti₈C₁₂ cluster. The structure of this cluster is based on a tetracapped tetrahedron withT _d symmetry with two distinct quartets of titanium atoms, six distinct C₂ pairs, and 36 direct Ti−C interactions. The copper-carbon cages of various stoichiometries are suggested to have prismatic, antiprismatic, or cuboctahedral structures in which the electronic configurations of the copper atoms approach the favored 18-electron rare gas configuration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 862–869, May, 1998.     

Free electron model in cluster structure theory. Electronic structures of [Mo6S8(CN)6]6?, [Mo6Se8(CN)6]6?, [Re6S8(CN)6]4?, and Rh6(CO)16 clusters

The results of quantum chemical calculations of the electronic structure and geometry of octahedral clusters [Mo₆S₈(CN)₆]⁶⁻, [Mo₆Se₈(CN)₆]⁶⁻, [Re₆S₈(CN)₆]⁴⁻, and Rh₆(CO)₁₆ by the ab initio SCF (RHF) and DFT (B3LYP) methods with various basis sets are presented. The electronic states of the clusters under study in ideal spherically symmetric potential were classified in the orbital quantum number l (1s, 1p, 1d, 1f, 1g, 1h, 1i), l = 0–6. In real crystal field with O_h symmetry these states are split. The calculated new electronic states were matched to the irreducible representations of the point symmetry group O_h. The polarizabilities of the compounds considered are 55–65 ų. A new model for the electronic structure of octahedral clusters containing M₆ groups was proposed. The model is based on the idea of free electrons moving in spherically symmetric potential field. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2617–2624, December, 2005.