Sergey S. Bukalov

Two Modifications Formed by "Sulflower" C16S8 Molecules, Their Study by XRD and Optical Spectroscopy (Raman, IR, UV-Vis) Methods

Sublimation of sulflower, octathio[8]circulene C 16S 8 ( 1), on heating under high vacuum ( approximately 10 (-5) Torr) leads to successive formation of two modifications: a white film ( 1W) and a red polycrystalline solid ( 1R). When kept at room temperature for several weeks, 1W spontaneously turns pink, reflecting the monotropic phase transition 1W --> 1R. The accurate molecular and crystal structure of 1R has been studied using low-temperature (100 K) high-resolution single crystal X-ray analysis. The C 16S 8 molecule in crystal is strictly planar with nearly equalized bonds of each type (C-C, C-S, and CC). The point symmetry group of the free molecule is D 8 h , and the crystal space group is P2 1/ n. These data allowed group-theoretical analysis of vibrational normal modes to be accomplished. Investigation of the charge density distribution of 1R including Baders AIM approach has revealed rather strong intermolecular S...S, S...C, and C...C interactions of charge transfer and pi-stacking types with overall lattice energy of 28.5 kcal/mol. The charge transfer due to the S...S interactions is the reason for the red coloration of 1R. The latter is reflected by its UV-vis spectrum exhibiting absorption bands in the visible region which are absent from that of 1W. Both modifications were studied comparatively by vibrational (Raman, IR) and electronic spectroscopies as well as XRD powder diffraction. All the results obtained are fully consistent and show that 1W is much less ordered than 1R with significantly weakened intermolecular interactions. Rationalizing of these results has led to an idea that 1W could be soluble, in contrast to 1R. Indeed, 1W appeared soluble in common solvents; this finding opens the way to the study of the chemistry of 1 and investigation of its electrooptical properties.

Vibrational spectra and structure of cesium salts of icosahedral monocarba-closo-dodecarborate anion, [CB11H12]–, and its nido-derivative, [CB100H13]–

The Raman and IR spectra of the cesium salts of monocarbon carboranes, [closo-CB11H12]– and [nido-CB10H13]–, are reported and the assignment of the normal modes is given. Quantum-chemical calculations of the geometry of undistorted closo-anions B12H122– and CB11H12– were carried out and normal coordinate analysis for the latter was performed. Structural parameters and spectral characteristics of isoelectronic closo-polyhedra [B12H12]2–, [CB11H12]–, and p-C2B10H12 and those of the closo- and nido-structures were compared.

Excitation dependence of Raman spectra of various polydialkylsilane conformations and σ-σ conjugation

Abstract Excitation dependence of the Raman spectra in a wide region (from blue to infrared) was investigated for hmw polysilanes [ n Hex 2 Si] n and [ n Pent 2 Si] n at various temperatures. The results obtained along with analogous data for other polydialkylsilanes lead to the conclusion that among all modifications formed by these polymers, there is only one that exhibits pre-resonance Raman scattering, that is, intensity enhancement and its excitation dependence. This is the crystalline modification with the σ–σ conjugated all- A ( anti ) conformation of the silicon backbone. The Raman spectrum of this modification is enhanced by pre-resonance even when ( ν e − ν ), the difference between the frequency of the lowest energy σ–σ* electronic transition ν e and that of irradiating light ν is as much as ∼17 300 cm −1 , whereas Raman spectra of less-ordered modifications do not exhibit such enhancement even at much smaller ( ν e − ν ) values, e.g. ∼9400 cm −1 . These facts are discussed in terms of Shorygins semi-classical treatment of Raman intensity problem. The Raman patterns observed below 800 cm −1 allow clear distinction between polysilane modifications with all- A , helical (all- D ) and TGTG ′ ( AD + AD − ) main chain conformations, whereas helical (all- D ) and disordered conformations are hardly distinguishable.

Molecular Structures of N,N′-Dimethylbenzimidazoline-2-germylene and -stannylene in Solution and in Solid State by Means of Optical (Raman and UV–vis) Spectroscopy and Quantum Chemistry Methods

X-ray data obtained for germylene 1 evidence its monomeric structure, unlike that of stannylene 2, which had been shown previously to form a coordination dimer. Raman spectra of solid and liquid 1 are identical, whereas the Raman spectra of solid 2 and its solution 2a differ significantly. The spectrum of 2 is complicated and contains the lines corresponding to N → Sn coordination bonds forming a dimer. The spectrum of 2a is simpler and close to that of monomeric 1, thus pointing to the rupture of the dimer in solution. The UV-vis spectrum of solid 2 exhibits a band corresponding to a transition involving the N → Sn coordination bonds. Quantum theory of atoms in molecules data estimate the energy of this bond as ∼19 kcal/mol. The aromaticity of 1 and 2 with their 10 π-electron systems including divalent Ge or Sn atoms is confirmed by negative nucleus-independent chemical shift values.

Conformational polymorphism of solid tetramesityldisilene Mes2Si=SiMes2 (Raman, UV-vis, IR and fluorescence study)

Conformational polymorphism of solid tetramesityldisilene (1) has been studied by the methods of optical spectroscopy. The three known modifications of 1: orange unsolvated 1a and two 1:1 solvates with toluene (1b) and THF (1c) have been found to transform under specific conditions to a new, most thermodynamically stable polymorph, yellow unsolvated powder 1d. The latter has been characterized by the Raman, IR, UV-vis and fluorescence data. All forms of 1 exhibit Raman spectra differing in details, which reflect their different crystal and molecular structures. Unsolvated 1a and 1d differ significantly in electronic absorption and fluorescence emission. The yellow form 1d can be converted to the orange form 1a upon illumination with laser light in the region 514-457 nm. Similarity of the Raman and UV-vis spectra of 1d to those of the solutions of 1 provides some evidence for a quasi-trans conformation of 1d.

Vibrational and electronic spectra of tetrahedral molecules (Me3M)4M′ (M=Si, Sn; M′=Si, Ge) with heteronuclear MM′ bonds

Abstract Raman, infrared and UV spectra of tetrahedral molecules (Me 3 Si) 4 Ge ( I ), (Me 3 Sn) 4 Ge ( II ), and (Me 3 Sn) 4 Si ( III ) were obtained. To assign and analyze vibrational modes, model normal coordinate calculations for (C 3 M) 4 M′ moieties were carried out. Stretching vibrations of heteronuclear tetrahedra M 4 M′ were studied comparatively. Solid substances I–III were found to undergo an order–disorder phase transition between an ordered crystalline phase and a plastic mesophase ( II and III on heating to ca. 65°C while I on cooling to ca. −45°C).

Metal oxide–zeolite composites in transformation of methanol to hydrocarbons : do iron oxide and nickel oxide matter?

The methanol-to-hydrocarbon (MTH) reaction has received considerable attention as utilizing renewable sources of both value-added chemicals and fuels becomes a number one priority for society. Here, for the first time we report the development of hierarchical zeolites (ZSM-5) containing both iron oxide and nickel oxide nanoparticles. By modifying the iron oxide (magnetite, Fe3O4) amounts, we are able to control the catalyst activity and the product distribution in the MTH process. At the medium Fe3O4 loading, the major fraction is composed of C9–C11 hydrocarbons (gasoline fraction). At the higher Fe3O4 loading, C1–C4 hydrocarbons prevail in the reaction mixture, while at the lowest magnetite loading the major component is the C5–C8 hydrocarbons. Addition of Ni species to Fe3O4–ZSM-5 leads to the formation of mixed Ni oxides (NiO/Ni2O3) positioned either on top of or next to Fe3O4 nanoparticles. This modification allowed us to significantly improve the catalyst stability due to diminishing coke formation and disordering of the coke formed. The incorporation of Ni oxide species also leads to a higher catalyst activity (up to 9.3 g(methanol)/(g(ZSM-5) × h)) and an improved selectivity (11.3% of the C5–C8 hydrocarbons and 23.6% of the C9–C11 hydrocarbons), making these zeolites highly promising for industrial applications.

Can Sn(OCH2CH2NMe2)2 behave as a stannylene? Equatorial–axial isomerism in the tin(II)–iron(0) complex (Me2NCH2CH2O)2Sn–Fe(CO)4

Equatorial-axial isomerism of the tin(II)-iron(0) complex (Me2NCH2CH2O)2Sn-Fe(CO)4 (), which indicates that the free Sn(OCH2CH2NMe2)2 () ligand can behave as a stannylene, has been revealed and studied by NMR and IR spectroscopy in solution as well as by Raman spectroscopy and X-ray diffraction analysis in the solid-state.

Variable-temperature UV and Raman study of complicated thermochromic phase transition of order—disorder type in poly(di-n-decylsilane) [(n-C10H21)2Si] n

A complicated thermochromic phase transition (PT) of order—disorder type in poly(di-n-decylsilane) has been studied in detail by variable-temperature UV and Raman spectroscopy. Not less than five polymer modifications were shown to participate in this PT. Above the equilibrium PT temperature Tc (∼60 °C), the polymer exists as a hexagonal columnar mesophase (HCM) with a disordered silicon backbone and disordered side chains. PT proceeds not abruptly but over a temperature range 60—5 °C and does not reach completion, being stopped by glassification. At room temperature, a coexistence in the polymer of HCM and a few crystalline phases with ordered backbone is observed. The latter are separated in space, what was evidenced by Raman micromapping of a polymer film. The temperature intervals of existence of different ordered phases also do not coincide, this enables one to identify in the UV and Raman spectra the components, corresponding to the modifications possessing trans (anti) and, supposedly, AD+AD– main chain conformations. As the relaxation processes in this comb-like polymer occur slowly, the quantitative ratio of different phases is not a single-valued function of temperature but depends on the thermal prehistory of the sample. Ordering of long side chains on cooling was demonstrated by the Raman method.

RAMAN STUDY OF CONFORMATIONAL EQUILIBRIUM IN PLASTIC SOLID DODECAMETHYLCYCLOHEXASILANE SI6ME12

The temperature dependence of the Raman spectrum of solid dodecamethylcyclohexasilane Si6Me12 has been studied in the temperature interval 20–235 °C. At 72 °C the substance undergoes a phase transition of an «order-disorder» type, from the ordered crystalline modification to the plastic mesophase with isotropic molecular reorientations. This phase transition is accompanied by the appearance of a conformational equilibrium in the solid state. An analogous equilibrium is also observed in a benzene solution, where the concentration of the second conformer is already noticeable at 6 °C.