Non-alkane behavior of cyclopropane and its derivatives: characterization of unconventional hydrogen bond interactions

Eight cyclopropane derivatives (Δ − R) have been modeled, with R = −H, −CH₃, −NH₂, −C ≡ CH, −C ≡ CCH₃, −OH, −F and −C ≡ N. All geometries have been fully optimized at the MP2/ AUG-cc-pVTZ level of calculations. Natural bond orbital analyses reveal extra p character (sp^λ, λ > 3) in the C-C bonds of the cyclopropyl rings. The banana-like σ _CC bonds in the rings are described in detail. Alkene-like complexes between Δ − R molecules and hydrogen fluoride are identified. These weakly bonded complexes are formed through unconventional hydrogen bond interactions between the hydrogen atom in the HF molecule and the carbon–carbon bonds in the cyclopropane ring. A topological analysis of the electronic charge density and its Laplacian has been used to characterize the interactions. The possible relevance of such complexes in the modeling of substrate–receptor interactions in some anti-AIDS drugs is discussed. Contribution to the Serafin Fraga Memorial Issue.     

New polyunsaturated organosilicon dendrimers

Spherical dendrimer of regular structure with the spatial characteristic S = 5, tetrakis[tris-(trimethylsilylethynyl)silyl-2-vinyldimethylsilylethynyl]silane, and the unsymmetrical binuclear dendrimer containing simultaneously groups (−CH₂CH₂−), (−CH=CH−), (−C≡C−) and combining the branches of the zero order (G0) and of the second generation (G2), 2-tris(trimethyl-silylethynyl)silyl-1-tris[tris(trimethylsilylethynyl)silylvinyldimethylsilylethynyl]silylethane were isolated from the reaction of hydrosilylation of tetrakis (dimethylethynylsilylethynyl)silane and vinyltris(dimethylethynylsilylethynyl)silane with trichlorosilane and a subsequent treatment of the reaction products with trimethylsilylethynylmagnesium bromide. NMR spectra of the synthesized compounds were studied.     

Radical chain photoreactions involving alkylperoxides grafted onto an activated surface of silicon dioxide

Photolysis of the alkylperoxy radicals ≡SiOCH₂OO and ≡SiOCH₂CH₂OO grafted onto an activated surface of silica was found to be accompanied by photochemical radical chain reactions.     

A study of alkyl radicals in the matrix of polycrystallinen-alkane γ-irradiated at 77 K

The composition of alkyl radicals (AR) formed by γ-radiolysis (T=77 K) of polycrystallinen-alkanes with different lengths of the carbon chain (C(5), C(7), C(10), C(11), and C(18)) and their polymeric analog (polyethylene) was estimated from the ESR spectra. The ESR spectra of the irradiatedn-alkanes are superpositions of the signals from the H₃CC^.HCH₂− and −CH₂C^.HCH₂− radicals, whose HFS constants with α and β protons as well as the equilibrium conformation are independent of the chain length of then-alkane molecule. A dependence of the concentration of the radicals on the chain length ofn-alkane was found. The absence of the −CH₂C^.H₂ radicals that may arise upon H atom elimination from the Me fragments of then-alkane molecules is most likely related to the transfer of excitation energy from the Me group to the neighboring methylene fragment and the transformation of the −CH₂C^.H₂ radicals into H₃CC^.HCH₂− radicals. With account for this, the concentrations of the AR formed were suggested to be proportional to the number of H atoms at the corresponding C atom. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1034–1037, June, 2000.     

Nucleophilic addition of alcohols to the C-N multiple bonds of the nitrilium substituent in the anion [2-B10H9(N≡CMe)]−

The reactions of salts of the anion [2-B₁₀H₉(N≡CMe)]⁻ with aliphatic alcohols ROH (R = C _n H_2n+1 (n = 1–6) CH₂CH₂(OEt), Prⁱ, Buⁱ, Bu^t, i-C₅H₁₁) are studied. These reactions result in hydrolytically stable imidates [2-B₁₀H₉{NH=C(OR)Me}]⁻. Their structures were confirmed by the data from mass spectrometry, IR, ¹H, ¹¹B, and ¹³C NMR spectroscopy. The molecular geometry of [2(Z)-B₁₀H₉{NH=C(OBu)Me}]⁻, which formed in nucleophilic addition reaction of n-butyl alcohol to [2-B₁₀H₉(N≡CMe)]⁻, was established by X-ray diffraction analysis.     

Synthesis and mesomorphism of diacetylene-bridged triphenylene discotic liquid crystal dimers

Connecting two discotic mesogens via a spacer not only stabilizes the columnar mesophase but also leads to the formation of glass columnar phase, and therefore improves the physical properties of discotic liquid crystals as organic semiconductor. Here, we report the synthesis of eight diacetylene-bridged triphenylene discotic liquid crystal dimers, [C18H6(OCnH2n+1)4(OMe)O2C-C8H16-C≡≡ C-]2, 3(n), (n = 4-8), [C18H6(OC6H13)5O2C-C8H16-C≡≡ C-]2, 6 and [C18H6(OC6H13)5O-(CH2)m-C≡≡ C-]2, 8(m), (m = 1, 3) by Eglinto...     

Contributions of hydrocarbon radicals and functional groups of organic molecules to enthalpies of their solvation in mixed water—tert-butyl alcohol solvents

The contributions of hydrocarbon radicals (−CH₃, >CH₂, >CH−, >C<) and functional groups (−OH, −OCOO−, −NO₂, −CN, >SO) to the enthalpies of solvation of organic molecules in mixed water—tert-butyl alcohol solvents were calculated in the whole range of compositions at 298.15 K. The influence of the composition and properties of the mixture on the solvation of different functional groups is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1811–1814, October, 1997.     

Molecular dynamics of alkyl radicals grafted onto silica surface

Correlation times for≡SiOC·X₂ radicals grafted onto activated silica surface were estimated to be 1.3·10⁻⁸s (X=H) and 2.5·10⁻⁸s (X=Me) at room temperature. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 613–616, April, 1998.     

Distonic ions of the “ate” class

The gas phase synthesis, structure, and reactivity of distonic negative ions of the “ate” class are described. “Ate”-class negative ions are readily prepared in the gas phase by addition of neutral Lewis acids, such as BF₃, BH₃, and AlMe₃, to molecular anions, carbene negative ions, and radical anions of biradicals. The ions contain either localized σ- or delocalized π-type radical moieties remote from relatively inert borate and aluminate charge sites. The free radical reactivity displayed by these ions appears to be independent of the charge site. As an example, the distonic alkynyl radical (·C≡CBF₃⁻) is highly reactive and undergoes radical coupling reactions with NO₂, NO, H₂C=CH-CN, and H₂C=CH-CH₃. Radical-mediated group and atom transfers are observed with O₂, CS₂, and CH₃SSCH₃. Furthermore, H-atom abstraction reactions are observed, in accordance with the predicted high C-H bond strength of this species [DH₂₉₈(H-C₂BF₃⁻)=130.8 kcal mol⁻¹]. High level ab initio molecular orbital calculations on the prototype “ate”-class distonic ion · CH₂BH₃⁻ and its conventional isomer CH₃BH₂^·− reveal that CH₃BH₂^·− is 3.2 kcal/mol more stable than the α-distonic form. However, the calculations also show that CH₃BH₂^·− is unstable with respect to electron detachment, and only the α-distonic form ·CH₂BH₃⁻ should be experimentally observed in the gas phase.     

A theoretical study of the interaction between [HB≡CH]<Superscript>−</Superscript>, [H<Subscript>2</Subscript>B=CH<Subscript>2</Subscript>]<Superscript>−</Superscript>, and boratabenzene anions with alkali and alkaline earth metals: properties and structures

The nature of [HB≡CH]⁻, [H₂B=CH₂]⁻, and boratabenzene interactions with alkaline and alkaline earth metals are studied by ab initio calculations. The interaction energies are calculated at the B3LYP/6-311++G(d,p) level. The calculations suggest that the cation size and charge are two influential factors that affect the nature of the interaction. AIM and NBO analyses of the complexes indicate that the variation of densities and the extent of charge transfers upon complexation correlate well with the obtained interaction energies.     

Carbon in silica

The structure of paramagnetic centers (PMC) in carbon-doped silica is studied by the EPR and quantum chemistry methods. Three types of radicals, ≡Si-^·CH₂, (≡Si-)₂ ^·CH and (≡Si-)₃C^·, are identified in which the free valence is localized on the impurity carbon atom. Their structure is determined by using samples enriched by isotopes ²D, ²⁹Si, and ¹³C. The assignment of the obtained radiospectroscopic parameters of radicals is confirmed by quantum-chemical calculations of model systems. Based on the obtained data, a conclusion is made that paramagnetic centers (the so-called EX-centers) discovered earlier by the EPR method in silicon oxidation products have the structure (≡Si-)₃C^·, i.e. are not the intrinsic defects of the material but are related to impurity carbon atoms. Differences in the spectral characteristics of the (≡Si-)₃C^· groups observed in experiments are caused by the amorphous structure of silica. The kinetic nonequivalence of the (≡Si-)₃C^· radicals in the reaction of the hydrogen atom detachment from the H₂ molecule is established (the activation energies for different PMC fractions lie in the range from 10 to 17 kcal/mol). The quantum-chemical calculations of model systems performed in the paper suggest that the differences observed in the reactivity of radicals are related to their spatial structure. It is found that the high-temperature pyrolysis of the (≡Si-)₃C-H and (≡Si-O-)₃Si-H groups is accompanied by the quantitative regeneration of free radicals (≡Si-)₃C^· and (≡Si-O-)₃Si^·. The probable mechanism of carbon atom embedding from. (≡Si-O-)₃Si-O-CH₃ groups to silica accompanied by the formation of (≡Si-)₄C groups is analyzed.     

Kinetics of the peroxy RFO2 radical formation in the RHF-O2-CO2 gaseous mixtures

The kinetic data on the molecular oxygen activity of CH₃CH^·, CH₃CF₂ ^· and CF₃CHF^· radicals are reported. In laboratory, these radicals were generated by pulsed (12 ns) electron beam interaction with the gaseous RHF-O₂-CO₂ mixtures containing large excess of carbon dioxide (RHF = CH₃CH₂F, CH₃CHF₂ or CH₂FCF₃). The transient product (O₃ or RFO₂ ^·) formation was monitored by the UV absorptions at 250 nm and the rate constants of Reactions (4) and (9) were obtained. The values of k ₉ diminished with increasing number of fluorine atoms in RHF molecule. For CH₃CH₂F and CH₃CHF₂ the k ₉’s were equal to (8.8–10.2)^·10⁻¹⁴cm^{3 ·}s⁻¹ and (7.3–8.4)^·10⁻¹⁴cm^{3 ·}s⁻¹, respectively, and seem to be determined for the first time. In the case of CH₂FCF₃ the obtained value of k _CF3CHF+O2 = 5.20±0.76^·10⁻¹⁴cm^{3 ·}s⁻¹ is much higher than the value published in the literature.⁴ The other determined rate constant data are comparable to the literature values.     

Reactions of reducing radicals with 2- and 3-nitroanilines in aqueous solutions: a pulse radiolysis study

Reactions of 2- and 3-nitro anilines (2- and 3-NA) with e_aq⁻, H-atoms and one-electron reductants have been studied using pulse radiolysis in aqueous solutions. Reactions of e_aq⁻ were found to be quite fast with both 2-NA and 3-NA resulting in their corresponding semi-reduced species which are reducing in nature. Reduction potentials for 2-NA/2-Na^•′ have been estimated to be approx. −0.56 Vvs. NHE and that for 3-NA/3-NA^•− was found to be between −0.185 V and −0.45 Vvs. NHE. Semi-reduced 2-NA has main absorption peak at 300 nm with a shoulder in the 350 nm region and a broad weak band in the 470–500 nm region, whereas semi-reduced 3-NA possesses an absorption peak at 520 nm. Reducing radicals such as (CH₃)₂ C^•OH and CO₂^•− reacted with 2-NA, producing semi-reduced species, whereas reactions of these radicals with 3-NA produced their corresponding radical-adduct species.     

Kinetics of the radical reactions in pulse radiolyzed RHF — CO2(SF6)-O2-NO gaseous mixtures; RHF = CH3CH2F, CH3CHF2, CH3CF3, CH2FCF3

The kinetics of the peroxy radicals RHFO₂ reactions with NO has been studied by using pulse radiolysis and UV absorption spectroscopy. The rate constants of interaction of oxygen atoms with NO − k ₂ = 2.2±0.2·10⁻¹² cm³·s⁻¹ and NO₂ − k ₃ = 2.1±0.2·10⁻¹¹ cm³·s⁻¹ were found in agreement with the literature values. The bath gases (SF₆ or CO₂) have got minor effect on the rate constants of RHFO₂+NO→NO₂+prod. reactions; RHFO₂ = CH₃CH₂O₂, CH₃CHFO₂, CH₃CF₂O₂, CF₃CH₂O₂, CF₃CHFO₂. The obtained rate coefficients are in the scope of the literature values, although they are lower than those recommended in NIST database. The reasons are discussed.     

Pulse radiolysis study of dithio-oxamide in aqueous solutions

The reactions of e⁻ _aq, H-atoms, OH radicals and some one electron oxidants and reductants were studied with dithio-oxamide (DTO) in aqueous solutions using pulse radiolysis technique. The transient species formed by the reaction of e⁻ _aq with DTO at pH 6.8 has an absorption band with λ _max at 380 nm and is reducing in nature. H-atom reaction with DTO at pH 6.8 also produced the same transient species. The semi-reduced species was found to be neutral indicating that the electron adduct gets protonated quickly. However at pH 1, the species produced by H-atom reaction had a different spectrum with λ _max at 360 and 520 nm. Reaction of acetone ketyl radicals and CO₂ ⁻ radicals with DTO at pH 6.8 gave transient spectra which were identical to that obtained by e⁻ _aq reaction. However at pH 1, the spectrum obtained by the reaction of acetone ketyl radicals with DTO was similar to that obtained by H-atom reaction at that pH. The transient species formed by OH radical reaction with DTO in the pH range 1–9.2 also has two absorption maxima at 360 and 520 nm. This spectrum was identical with the spectrum obtained by H-atom reaction at pH 1. This means that all these radicals viz. OH, H-atom and (CH₃)₂COH radicals react with DTO at pH 1 by H-abstraction mechanism. The transient species produced was found to be sensitive to the presence of oxygen. One-electron oxidizing radicals such as Br₂ ^−· and SO₄ ^−· radicals reacted with DTO at neutral pH to give the same species as produced by OH radical reaction having absorption maxima at 360 to 520 nm. At acidic pHs, only Br₂ ^−· and Cl₂ ^−· radicals were able to oxidize DTO to give the same species as produced by OH radical reaction. The semioxidized species is a resonance stabilized species with the electron delocalized over the-N-C-S bond. This species was found to be neutral and non-oxidizing in nature.     

Reaction of lanthanide(II) and lanthanide(III) phenylethynyl cuprates with acetyl chloride

Praseodymium and ytterbium phenylethynyl cuprates [(PhC≡C)₃Cu]₃Pr₂(THF)₆ and {[(PhC≡C)₃Cu]·Yb(THF)₂}₂ react with acetyl chloride in tetrahydrofuran with elimination of phenylethynylcopper and formation of alkoxides [PhC≡C-CCl(CH₃)O] _n Ln (n = 3, Ln = Pr; n = 2, Ln = Yb). Then praseodymium alkoxide forms ester [methyl (phenylethynyl)chloromethylethanoate] and praseodymium chloride, alkoxy derivative. Itterbium alkoxide is oxidized to unsymmetrical dialkoxyitterbium chloride PhC≡C-CH(CH₃)-O-Yb(Cl)-O-CCl(CH₃)C≡CPh·2THF.     

Reaction of a naphthalene complex C10H8Yb(THF)2 with cyclopentadienyl-substituted alcohols and amines as a convenient synthetic route to the half-sandwich complexes of divalent ytterbium

The reactions of ytterbium naphthalene complex C₁₀H₈Yb(THF)₂ with 2-cyclopentadienylethanol, 1-cyclopentadienylpropan-2-ol, 3-cyclopentadienyl-1-butoxypropan-2-ol, and cyclopentadienyldimethylsilyl-tert-butylamine were studied. The bivalent ytterbium complexes with chelate bifunctional cyclopentadienyl ligands [(η⁵−C₅H₅)CH₂CH₂(η¹−O)]Yb(THF), [(η⁵−C₅H₅)CH₂CH₂(η¹−O)]Yb(DME). [(η⁵−C₅H₅)CH₂CH(Me)(η¹−O)]Yb(THF), [(η⁵−C₅H₅)CH₂CH(CH₂OC₄H₉)(η¹−O)]Yb(THF), and [(η⁵−C₅H₅)SiMe₂(η¹−N(Bu¹))]Yb(THF) were obtained and characterized. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 742–745, April, 2000.     

Rate constants for reactions of Cl abstraction from CCl4 by CCl3CH2·CHR radicals and Br abstraction from CCl3CH2CHBrR (R=Bun, AcO, OCNC4H8, CN) by ·Re(CO)5 radicals

The rate constants for reactions of Cl abstraction from CCl₄ by CCl₃CH₂·CHR radicals and Br abstraction from CCl₃CH₂CHBrR (R=Buⁿ, AcO, OCNC₄H₈, CN) by·Re(CO)₅ radicals were determined by ESR spectroscopy using spin trapping technique. Replacement of H atoms at the C(β) atom by O or N atoms reduces the reactivity of the radicals in the reactions of Cl abstraction from CCl₄ by approximately an order of magnitude. The presence of two polar groups at the C(β) atom results in appreciable decrease in the strength of the C−Br bond in CCl₃CH₂CHBrR adducts. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 45–48, January, 2000.     

Laser photoemission generation and electrochemical study of methyl radicals as secondary products of OH radicals capture by dimethyl sulfoxide molecules

Electrode reactions of intermediates formed during capture of OH radicals by dimethylsulfoxide molecules were studied by laser photoemission in aqueous buffer solutions and pH range from acidic to basic. The results were compared with those obtained previously for electrochemical behaviour of methyl radicals generated via photoemission from CH₃Cl. The essential similarity was found for parameters of irreversible one-electron transfer from/to these intermediates, i.e. the potentials E _1/2 on time-resolved voltammograms and rate constants at E = E _1/2. Hence, both active particles were concluded to be equivalent and corresponded to methyl radical. The primary product of OH radicals capture by DMSO molecules, i.e. adduct (CH₃)₂SO·(OH), was spontaneously decomposed to form ·CH₃ with time as low as <2 × 10⁻⁵ s. A simultaneous increase of the reduction wave height was observed at pH transition from low basic to low acidic and at illumination times T _m of an electrode with UV light if T _m ≥ 90–300 ms. The increase exceeded considerably the one-electron reduction level. These features were presumably caused by the rather slow formation of organomercury intermediates as interaction products of the components of the system with a mercury electrode.