Heteroorganic betaines

A method for the synthesis of a new class of betaines,viz., R₃ ¹P⁺CR²R³SiR⁴R⁵S⁻, was developed. The experimental evidence for the intermediate formation of betaines R₃ ¹P⁺−CR²R³−CR⁴R⁵−S⁻ in the Wittig reaction for a series of thiocarbonyl compounds was obtained. A comparative analysis of the NMR spectra of betaines containing the⁺P−C−Si−S⁻ and⁺P−C−C−S⁻ fragments was performed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 922–930, May, 2000.     

Heteroorganic betaines

Photolysis and thermal decomposition of betaines R₃P−CR¹R²−SiR³R⁴−S⁻ (1) follows two main pathways: (a) a Corey—Chaykovsky type reaction with elimination of Ph₃P and generation of silathiirane (2) and (b) a retro-Wittig type reaction accompanied by elimination of R₃P=CR¹R² and generation of silanethione R³R⁴Si=S (3). Highly reactive compounds2 and3 undergo subsequent transformations to give derivatives of tetrahydro-1,4-dithia-2,5-disilin, 1,3-dithia-2,4-disilolane, and phosphonium salts ofsymm-tetraorganodisilthiane dithiolates [Ph₃P⁺CHR¹R²]₂[(R³R⁴SiS⁻)₂S]. The structures of the compounds obtained were established by X-ray diffraction analysis and multinuclear NMR spectroscopy. For part 3, see Ref. 1. The betaines Et₃P⁺CHMeSiMe₂S⁻ and Et₃P⁺CHMeSiPh₂S⁻ with alkyl groups at the phosphorus atom are distinguished by high thermal stability; their spectral characteristics do not change during storage of solutions of these compounds in pyridine-d₅ or metastable solutions in benzene-d₆ for 1–2 years at −20°C in sealed evacuated tubes or on heating (150°C) for 15 h. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1594–1603, September, 2000.     

A DFT study of ethylene polymerization by zirconocene catalysts

A DFT study of ethylene polymerization by zirconocene catalysts was carried out. Stationary points corresponding to intermediates and transition states were located on the potential energy surface of the [Cp₂ZrC₂H₅]⁺+C₂H₄ model system. Three possible reaction mechanisms involving the formation of β-agostic complexes were considered. The energy and thermodynamic characteristics for different reaction pathways were calculated. Corresponding activation energies lie in the range 3.9–6.8 kcal mol⁻¹. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1168–1177, July, 2000.     

Cryosynthesis,structure, and IR spectra of aluminum chloride—nitromethane molecular complexes

The products of codeposition of aluminum chloride and nitromethane were studied by low-temperature IR spectroscopy (80—200 K). Density functional (B3LYP/6-31G* and B3LYP/6-31+G**) quantum-chemical calculations of the geometry and vibrational frequencies of aluminum chloride, nitromethane, and AlCl₃·MeNO₂ and 2AlCl₃·MeNO₂ complexes were carried out. Comparison of the experimental and calculated IR spectra suggests that the preferred geometry of the 2AlCl₃·MeNO₂ complex is such that one AlCl₃ molecule is coordinated to an O atom of the nitro group, while the other AlCl₃ molecule forms an Al...Cl bridge.     

DFT study of the geometrical and electronic structure of substituted cumulenes in netral and cationic forms

The geometrical and basic energy parameters of monosubstituted cumulenes and their singly and doubly charged cations were calculated by the Hartree-Fock and density functional (DFT) methods at a B3LYP level of theory using the 6-31G(d) basis set. The substituent was fluorine, cyan, amino group, phenyl, cyanophenyl, aminophenyl, or dimethylaminophenyl. In extended linear carbon systems based on cumulene, rotation of a terminal fragment depends on the character of the highest occupied molecular orbital (HOMO) from which electrons are removed. The terminal group rotates through 90 only when the contribution of electron density from the π molecular orbital (MO) of unsubstituted cumulene to the HOMO of substituted cumulene is over 70%. Otherwise, the terminal group rotates through a smaller angle; with a contribution of less than 30%, the dication is planar in any substituted cumulene. Thus quantitative criteria have been determined to evaluate the specific structural effect due to ionization of substituted cumulenes.     

The use of the hybrid density functional theory (DFT) approach for calculation of vibrational spectra: nitromethane

The molecular geometry of nitromethane was optimized and its force field and vibrational spectrum were calculated by the BECKE3LYP method. The accuracy of optimization of the geometry of MeNO₂ obtained by this method using the 6–311G(d,p) and 6–311++G(d,p) basis sets is not poorer than that obtained at the second-order Møller-Plesset level of perturbation theory (MP2). The vibrational frequencies of nitromethane and its d₁, d₂, and d₃ isotopomers obtained by the BECKE3LYP method are in much better agreement with the experimental data than those calculated at the MP2 level using the same basis set. The average absolute error of calculations performed without the use of any scaling factors is ～2% for frequencies; the maximum deviation is ～4%.     

Heteroorganic betaines. 6. Structure and reactivity of silicon-containing organophosphorus betaines with the –S—Si—C—P+ fragment: a DFT study

The structures of silicon-containing organophosphorus betaines ^–S—SiR¹ ₂—CR² ₂—P⁺R³ ₃ and their ylide isomers were calculated using the density functional approach with the gradient-corrected PBE functional and extended TZ2P basis set. Three possible pathways of thermal decomposition of these betaines were analyzed. These are (i) cleavage of the central C—Si bond with the formation of a Wittig ylide and silanethione, (ii) intramolecular nucleophilic S _N-substitution with elimination of phosphine PR³ ₃ and the formation of silathiirane (the Corey—Chaikovscky transformation), and (iii) a Wittig-type decomposition followed by the formation of substituted silaethylene.The structures of products and transition states of these reactions were calculated. The cis-gauche conformation of the ^–S—Si—C—P⁺ fragment of betaines was found to be the most stable. This is in agreement with the results of X-ray diffraction study and can be rationalized by strong Coulomb attraction between the cationic and anionic centers. The betaines are stable toward retro-Wittig thermal decomposition. The Corey—Chaikovscky formation of thiirane is preferable under conditions of thermal decomposition. Retro-Wittig-type decomposition of betaines followed by the formation of silanethione is favored by intra- and intermolecular coordination of donor ligands.     

Novel organosilicon betaines

The structure of organosilicon betaines R₃P⁺CMe₂SiMe₂S^– (R = Ph and Me₂N) was determined by X-ray diffraction analysis.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 339–340, February, 1994.     

Molecular structures of trinitromethane derivatives

The molecular structures of trinitromethane derivatives XC(NO₂)₃ (X = F, Cl, Br, NC, NF₂, N₃) were studied using the density functional approach. The rules for changing the configurations of substituents in these compounds were revealed. Acceptability of the method employed for the calculations of trinitromethane derivatives is discussed.     

Induced aromaticity

The influence of counterions on the stabilization of three-, five-, and six-membered cyclic organic and organoboron systems was studied by the ab initio (MP2(full)/6-311+G**) and density functional (B3LYP/6-311+G**) methods. The structures of molecular charge-transfer molecular complexes formed by the interaction with counterions are predicted. A crucial role of counterions in the stabilization of aromatic systems that are unstable in themselves was revealed. Stabilization of these systems involves both charge transfer and covalent bonding.     

Reactions of mono- and bimolecular hydrogen transfer in alkyl radicals: analysis using the parabolic model and density functional calculations

Experimental data on monomolecular hydrogen transfer in the reactions of the type RC^·H(CH₂)_nCH₂R¹ RCH₂(CH₂)_nC^·HR¹ (n = 2—4, R and R¹ are alkyl substituents) were analyzed using the parabolic model (PM). The parameters characterizing this class of reactions were calculated. Isomerization of alkyl radicals via cyclic transition states (TS) is characterized by the following energy barriers to thermoneutral reaction E _e0: 53.5, 65.4, and 63.2 kJ mol^–1 for the six-, five-, and seven-membered TS, respectively. The E _e0 energy and the strain energy change in parallel in the series of cycloparaffins C_nH_2n. Density functional calculations of intramolecular hydrogen transfer in the n-butyl and n-pentyl radicals and of the bimolecular hydrogen abstraction from the ethane molecule by the ethyl radical were performed. The activation energies of the intra- and intermolecular hydrogen transfer were compared. The parameters of the PM were compared with the interatomic distances in the reaction center of the TS calculated by the density functional method.     

Activation of C--H bonds in C1-C3 alkanes by titanium(iv) and zirconium(iv) cationic complexes: a DFT study

A DFT study of a model reaction [(⁵-C₅H₅)₂MCH₃]⁺ + RH [(⁵-C₅H₅)₂MR]⁺ + CH₄ (M = Ti^IV, Zr^IV; R = Me, Et, Pr, Prⁱ) was carried out with the PBE density functional. Exchange of -bonded ligand proceeds through the formation of agostic complexes [Cp₂M(RH)CH₃]⁺ followed by their isomerization into complexes [Cp₂M(CH₄)R]⁺ via an inner-sphere migration of a hydrogen atom. The calculated rate constants for such migrations involving the primary and secondary C--H bonds of propane molecule differ by 930 times for Ti^IV complexes and by 47 times for Zr^IV complexes, which is due to the effect of steric factors.     

DFT calculation of benzoazacrown ethers and their complexes with calcium perchlorate

The complexation constants of several azacrown ethers with Ca(ClO₄)₂ were determined and turned out to be the higher, the large the macrocycle. The structures of free ligands and their complexes and the complexation energies were calculated by the DFT method. In the aza-12(15)-crown-4(5) ether complexes with Ca(ClO₄)₂, the metal cations lie outside the averaged plane of heteroatoms of the macrocycle, and the coordination of both counterions is V-like. In the complexes of aza-18-crown-6 ethers, the counterions are in the axial position relatively to the macrocycle in the center of which the Ca²⁺ ion is localized. The complexation energies increase with an increase in the size of the azacrown ether macrocycle. The involvement of the nitrogen atom in binding with the Ca²⁺ ion decreases with the expansion of the macrocycle. Two methods for quantitative estimation of the degree of pre-organization of ligands to complexation were considered: geometric and energetic methods. Benzoaza-15-crown-5 ether is a ligand which is more pre-organized to complexation than N-phenylaza-15-crown-5 ether.     

Hydrolysis of tetrafluorosilane under neutral conditions: A quantum chemical study

A density functional quantum chemical study of the first step of SiF₄ hydrolysis under neutral conditions in the presence of one or two H₂O molecules was carried out. The reaction is endothermic and can follow three different pathways that involve the formation of penta-coordinated (pathway A) and hexacoordinated (pathways B and C) intermediates and transition states as the key steps. Pathway B is the most energetically favorable. All three pathways of the hydrolysis reaction lead to a product with formal retention of the configuration of substituents at the silicon atom. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 749–753, May, 2006.     

Molecular structure of carbene analogs

The geometrical parameters of all carbene analogs, halocarbenes, and the corresponding tetrahalides obtained from experimental data and high-level quantum-chemical calculations were collected. Trends in their variations are interpreted using the VSEPR model and by consideration of the HOMOs and LUMOs.     

Theoretical study of oxidation of molecular nitrogen with vanadium peroxo complexes

The structure and reactivity toward molecular nitrogen of vanadium diperoxo complexes, as well as the influence of the protonation and coordination of trifluoroacetate on the reactivity, were studied using the density functional method. The most stable form of the starting complex is the ozonide [V(O₃)O₂]^–. The triplet state of the complex is formed with small energy expenses for electron transfer from the peroxo ligand to the vanadium atom to form V^IV. The transfer of the O atom to the N₂ molecule to form N₂O is possible for several transition states. The nature of possible complexes and transition states retains upon protonation but the number of different structures increases depending on the ligand and the site of proton addition. Upon protonation the reactivity increases and the lowest activation barrier decreases from 27 to 20 kcal mol^–1. The coordination of trifluoracetate anion also decreases the activation barrier for the intermolecular transfer of oxygen to the nitrogen molecule.     

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.     

Nitrosonium nitrite isomer of N<Subscript>2</Subscript>O<Subscript>3</Subscript>: Quantum-chemical data

The geometrical, electronic, and thermodynamic parameters of three known isomers of dinitrogen trioxide N₂O₃ were calculated by the density functional theory DFT/B3LYP method using the 6-311++G(3df) basis. The structure of the new isomer, NONO₂, was calculated. From the calculation of vibrational frequencies it follows that the structure of NONO₂ has a local potential energy minimum and corresponds to the stationary state of the N₂O₃ isomer. The molecular structure of NONO₂ is characterized by a substantial negative charge on the NO₂ fragment and positive charge on the NO fragment. The electronic structure of the NO⁺NO ₂ ^? isomer can be characterized as nitrosonium nitrite, which can be oxidized to nitrite and participate in nitrosylation in accordance with the biogenic characteristics of the NO _x intermediate, assumed to be formed in biological systems during the oxidation of NO.