Dehydroaromatization of quinoxalines: one-step syntheses of trinuclear 1,6,7,12,13,18-hexaazatrinaphthylene titanium complexes

We report the spontaneous coupling of N-heterocycles, initiated by C-H bond activation reactions. The reaction of quinoxalines and the titanocene acetylene complex Cp2Ti{eta2-C2(SiMe3)2}, as an excellent titanocene source, results in the formation of trinuclear 1,6,7,12,13,18-hexaazatrinaphthylene (HATN) titanium complexes. These HATN titanium complexes are thermally stable but sensitive to air and moisture. A three-fold dehydrogenative C-C coupling is proposed as the main step in the presented synthetic procedure. Particularly using commercial starting materials, an efficient route for the dehydrogenative coupling of N-heterocycles, leading to multidentate ligands, has been established.     

Perfluoropoly-ether/peroxide compounds: spectroscopic studies and quantum chemical calculations

Perfluoropolyethers (PFPEs) are a class of high performance materials used in a wide range of applications (refrigeration, lubrication, semiconductor industry, etc.). PFPEs containing peroxidic units are intermediate materials for the preparation of commercial end products. In this work we study the spectroscopic properties of ether and peroxides linkages in this class of compounds; nuclear magnetic resonance (NMR) spectra are discussed, FT-Raman data presented. Quantum chemical calculations on model molecules were used as a tool for the interpretation of the Raman experimental data and physical-chemical properties.     

Vibrational spectroscopy of complex compounds of gold pentafluoride

IR and Raman spectra of X⁺ AuF₆ ^? (X=CIF₂, CIO₂, CIOF₂, CIF₄, CIF₆) were studied. The vibration frequencies of these compounds in the solid phase and in solutions in anhydrous HF were assigned. Peculiar features of the vibrational spectra of solid X⁺ AuF₆ ^?, associated both with structural transformations of cations and anions in the crystal lattice field and cation—anion interactions and also with the Jahn—Teller effect, Fermi resonance, non-rigid intramoleciular rearrangements.,etc., were discussed.     

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.     

Structural studies on ethyl isovalerate by microwave spectroscopy and quantum chemical calculations

We observed the microwave spectrum of ethyl isovalerate by molecular beam Fourier transform microwave spectroscopy. The rotational and centrifugal distortion constants of the most abundant conformer were determined. Its structure was investigated by comparison of the experimental rotational constants with those obtained by ab initio methods. In a first step, the rotational constants of various conformers were calculated at the MP2/6-311++G** level of theory. Surprisingly, no agreement with the experimental results was found. Therefore, we concluded that in the case of ethyl isovalerate more advanced quantum chemical methods are required to obtain a reliable molecular geometry. Ab initio calculations carried out at MP3/6-311++G**, MP4/6-311++G**, and CCSD/6-311++G** levels and also density functional theory calculations using the B3LYP/6-311++G** method gave similar results for the rotational constants, but they were clearly distinct from those obtained at the MP2/6-311++G** level. With use of these more advanced methods, the rotational constants of the lowest energy conformer were in good agreement with those obtained from the microwave spectrum.     

Vibrational spectra and quantum chemical calculations of 3,4-diaminobenzoic acid.

The Fourier Transform Raman and Fourier Transform infrared spectra of 3,4-diaminobenzoic acid (3,4-DABA) were recorded in the solid phase. Geometry optimizations were done without any constraint and harmonic-vibrational wave numbers and several thermodynamic parameters were calculated for the minimum energy conformer at ab initio and DFT levels invoking 6-311++G(d,p) basis set. The results were compared with the experimental values with the help of specific scaling procedures, the observed vibrational wavenumbers in FT-IR and FT-Raman spectra were analyzed and assigned to different normal modes of the molecule. Most of the modes have wavenumbers in the expected range, the error obtained was in general very low. The appropriate theoretical spectrograms for the IR and Raman spectra of the title molecule were also constructed.     

Spectroscopic and quantum chemical studies of isocytosine

The methods of electronic and vibrational (IR) spectroscopy were used to study the spectral properties of isocytosine in H₂O, D₂O, chloroform, and hexane in a wide concentration interval. Quantum chemical calculations of tautomeric forms and dimers of isocytosine were carried out. The bands of the calculated and experimental spectra were assigned. The results of the quantum calculations were compared with the experimental data. The spectral bands were classified according to the type of tautomer or dimer to which they belong.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 29–36, January–February, 1988.The authors are grateful to I. M. Ginzberg and L. F. Strelkova for their participation in the discussion of the infrared spectroscopy results.     

Complexes of dichlorosilylene with allyl chloride and allyl bromide: matrix IR spectroscopy and quantum chemical studies

Formation of donor-acceptor complexes between dichlorosilylene, SiCl₂, and allyl halides, AllHal(Hal= Cl, Br) was detected in Ar matrixes using matrix IR spectroscopy. In agreement with the predictions of the performed quantum chemical calculations, only broad unstructured absorption bands contributed by different conformers of the 1 : 1 complexes between SiCl₂ and AllHal were observed in IR spectra of matrixes after deposition in the regions of characteristic vibrations of starting reactants. Annealing of matrixes resulted in strong narrowing the bands due to conversions of different conformers into the most stable structures. The predominantly formed conformers in both the reaction systems were those of complexes with SiCl₂ coordinated to the Hal atoms of AllHal in the gauche conformations. At the same time, according to the calculations, the complexes with SiCl₂ coordination to the double bonds of AllHal can be only slightly less stable than the complexes with coordination to the Hal atoms, and all these basic centers can be considered as comparable in their activity in the complexation. The only products revealed upon photolysis of complexes were the products of silylene insertion into the C–Hal bonds, viz., AllSiCl₃ and AllSiCl₂Br. Theoretical study of thermal transformations in the SiCl₂ + AllHal systems showed that formal insertion of SiCl₂ in the C–Hal bonds and its addition to the double bonds of AllHal have low activation barriers of 3–8 kcal mol^–1. However, these barriers are too high for these reactions to occur under the matrix isolation conditions.     

Surprisingly slow reaction of dimethylsilylene with dimethylgermane: time-resolved kinetic studies and related quantum chemical calculations

Time-resolved studies of silylene, SiH2, and dimethylsilylene, SiMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to obtain rate constants for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas phase. SiMe2 + Me2GeH2 was studied at five temperatures in the range 299-555 K. Problems of substrate UV absorption at 193 nm at temperatures above 400 K meant that only three temperatures could be used reliably for rate constant measurement. These rate constants gave the Arrhenius parameters log(A/cm3 molecule(-1) s(-1)) = -13.25 +/- 0.16 and E(a) = -(5.01 +/- 1.01) kJ mol(-1). Only room temperature studies of SiH2 were carried out. These gave values of (4.05 +/- 0.06) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + MeGeH3 at 296 K). Rate constant comparisons show the surprising result that SiMe2 reacts 12.5 times slower with Me2GeH2 than with Me2SiH2. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) of the model Si-H and Ge-H insertion processes of SiMe2 with SiH4/MeSiH3 and GeH4/MeGeH3 support these findings and show that the lower reactivity of SiMe2 with Ge-H bonds is caused by a higher secondary barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper. Other, related, comparisons of silylene reactivity are also presented.     

Molecular dynamics and quantum chemical studies on nitroglycerine

A constant temperature molecular dynamics study has been performed on PM3 (RHF) geometry optimized nitroglycerine molecule. The dynamics was carried out by using MM+ method at 550 K which is above the explosion point of nitroglycerine. Some molecular orbital characteristics of nitroglycerine at elevated temperatures were computed.     

Synthesis and studies of tris-indolobenzenes and related compounds

The unsymmetrical N,N,N-trimethyl tris-indolobenzene 3 has been synthesized by several routes, including cyclotrimerization of the O-acetate of indoxyl. This condensation involves a 3→2 rearrangement of the precursor formed in situ. Similar Wagner-Meerwein rearrangements were also prevalent in LAH reductions of some 3,3-diindolyl oxindoles.Treatment of the 3,3-diindolyl indolines with strong acid resulted in a cleavage yielding 3,3'-biindolyls.     

Vibrational spectra and structure of methyl-derivatives of imidazo[4,5-c]pyridine based on DFT quantum chemical calculations and XRD studies

The molecular structures, vibrational energy levels and potential energy distribution of 4-methyl-imidazo[4,5-c]pyridine (4MIPc) and 7-methyl-imidazo[4,5-c]pyridine (7MIPc) are derived from the quantum chemical calculations and compared to the experimental results obtained from the X-ray diffraction (XRD), IR and Raman studies. The B3LYP/6-311G(2d,2p) quantum model and PED contributions have been applied for the assignment of the vibrational modes. 4MIPc crystallizes in an orthorhombic structure, space group Pna2₁ and Z = 4 and 7MIPc crystallizes in a triclinic structure, space group P−1 and Z = 4. The almost planar conformation of the molecules and presence of the N–H?N hydrogen bonds formed with the pyridine and imidazole nitrogen atoms was found to be characteristic for the studied systems. The presence of hydrogen bonds is also confirmed by the results of IR studies.     

Vibrational spectroscopy studies of structural changes in lignin under microwave irradiation

Structural changes that occur in lignin surface-modified with nickel nanoparticles during microwave- assisted dry reforming (DR) are studied via vibrational spectroscopy. IR spectroscopy reveals that the nickel deposition has a considerable effect on the structural characteristics of lignin. It is found that nickel deposition from an acetate salt substantially reduces the intensity of absorption bands at 1700 cm^?1. This finding suggests that Ni(2+) interacts mostly with formate groups, which are subsequently oxidized to carboxylate groups. It is shown that with the deposition of metallic nickel particles from a colloidal nickel solution in toluene prepared via metal vapor synthesis, the nickel particles do not interact with the surface functional groups of the lignin. Deep conversion of an organic mass of lignin by DR to form synthesis gas reduces the intensity of the absorption bands of the identified functional groups and raises the intensity of the absorption bands of the aromatic rings. Raman spectroscopy shows that during lignin conversion, the aromatic rings condense partially to form amorphized graphite. In operando studies reveal that the DR of nickel-modified lignin heated to 200–400°C results in the isolation of vanillic oxygenates that are probably intermediate products of reforming.     

Vibrational spectra of linear oligomers of carbonic acid: a quantum chemical study

Gas phase quantum chemical calculations of linear, hydrogen bonded oligomers of carbonic acid have been carried out to examine the feasibility for such species to be the building blocks of crystalline carbonic acid. Infrared and Raman vibrational spectra have been calculated and are compared against experimentally known spectra for two polymorphs of carbonic acid. The calculated anharmonic frequencies of the linear oligomer agree well with the experimental data for the centrosymmetric β-carbonic acid, rather than with that for the α polymorph. These calculations strongly suggest that β-carbonic acid should consist of one-dimensional hydrogen bonded carbonic acid molecules in the anti-anti conformation.     

Vibrational spectroscopy of implants

The results of systematic spectral studies of polymers in ophthalmology are presented. The spectral criteria of selection of intraocular lens (IOL; implants of long service in an organism have been defined for poly(methylphenylsiloxane). Non-inflammatory implantation of IOL, when the manufacturing of lenses is performed under spectral control at several stages, has been realized. New siloxane–polyurethane block copolymers promising for ophthalmology have been studied and passivated surfaces have been obtained.     

Vibrational spectroscopy of resveratrol

In this article the authors deal with the experimental and theoretical interpretation of the vibrational spectra of trans-resveratrol (3,5,4'-trihydroxy-trans-stilbene) of diverse beneficial biological activity. Infrared and Raman spectra of the compound were recorded; density functional calculations were carried out resulting in the optimized geometry and several properties of the molecule. Based on the calculated force constants, a normal coordinate analysis yielded the character of the vibrational modes and the assignment of the measured spectral bands.