G. A. Gavrilova

(Aroyloxymethyl)trifluorosilanes: a new class of pentacoordinate silicon compounds

Abstract A new class of organosilicon compounds containing a pentacoordinate silicon atom, (aroyloxymethyl)trifluorosilanes (AFS), has been prepared. The presence of an intramolecular coordinate F3Si ← OC bond is supported by X-ray diffraction, IR spectroscopy and dipole moment data. Si ← O coordination in AFS has been shown to remain intact in the gaseous and liquid states below 420 K as well as in solution in most organic solvents. The dielectric constant of the medium significantly affects the nature of the coordinate Si ← O bond and that of the SiF, CO, COC bonds. In pyridine the intramolecular coordinate Si ← O interaction is disturbed or greatly weakened. At 420–500 K a reversible reaction takes place: ArC OOCH 2 Si F3 α ArCOOCH2SiF3 The changes in enthalpy and entropy are equal in this case: −△H  8.1 ± 0.7 kcal/mol; −°S  15.3 ± 0.8 e.u. T  450 K An assignment of bands in the IR spectra of AFS resulting from vibrations of atoms in the SiF3, >CO and COC groups is given. The electron impact-induced primary disintegration of the molecular ion 4-XC6H4COOCH2Si mainly involves abstraction of a fluorine atom.

Intramolecular coordination in methyl(aroyloxymethyl)fluorosilanes

Abstract The IR spectra of solutions of methyl(aroyloxymethyl)fluorosilanes, C 6 H 5 -COOCH 2 Si(CH 3 )F 2 (I), 4-CH 3 C 6 H 4 COOCH 2 Si(CH 3 )F 2 (II), C 6 H 5 -COOCH 2 Si(CH 3 ) 2 F (III) in 11 solvents in the temperature range 223–363 K, and in the gaseous state have been studied. The molecules of the above compounds display an intramolecular coordination between silicon and oxygen of the carbonyl group. Unlike (aroyloxymethyl)trifluorosilanes [1], there exists the equilibrium between the molecules with and without Si ← O bonds in the solutions of I–III. The proportion of intramolecularly coordinated molecules is greater in solvents of low polarity. This is determined not by enthalpy (Δ H ) values of the Si ← O bond, but a change in the entropy (Δ S ) of the system upon coordination. The coordinate Si ← O bond becomes stronger with increasing number of fluorine atoms in the molecule. A strong dependence of the spectroscopic parameters characterizing the intramolecular coordination on the dielectric constant and the solvent refractive index has been established.

Intramolecular coordination in molecules of C6H5COOCH2CH2SiX3 (X = F, Cl)

Silicon-substituted 2-benzoyloxyethylsilanes, C6H5COOCH2CH2SiX3 with X = F, Cl, OCH3 or 13(OCH2CH2)3N, have been synthesized for the first time. The molecules of compounds with X = F and Cl may exist in two forms, acyclic with a tetracoordinated silicon atom and cyclic with a six-membered ring and a pentacoordinated silicon atom. The cyclic form of the molecules C6H5COOCH2CH2SiX3, with X = F, Cl, are present in solutions of these compounds in non-polar solvents, the proportion increasing with decreasing temperature.

Solvatochromism of heteroaromatic compounds: XVIII. Reversible effect of the medium on the UV spectra of 2-(2-benzoyl-1-phenylethenyl)-5-phenylpyrrole

Theoretical (B3LYP/6-31G*) and experimental (NMR, UV, and IR spectroscopy) data on the electronic and steric structure of 2-(2-benzoyl-1-phenylethenyl)-5-phenylpyrrole have been analyzed in the context of the hypothesis of the zwitter ionic nature of its ground state. The molecule involves a strong NH···O bond, and its ground state can be described as a hybrid of the neutral and zwitter ionic canonical forms; this conclusion was confirmed by the observation of a reversible medium effect on the electronic absorption spectrum of the substrate.

Solvatochromism of Heteroaromatic Compounds: XXIV. Solvation Complexes of ( = O→Si)(4-Nitrobenzoyloxymethyl)trifluorosilane in Protophilic Solvents

Abstract(4-Nitrobenzoyloxymethyl)trifluorosilane containing a five-coordinate silicon atom ( = O→Si) is specifically solvated in a protophilic medium to form a complex in which the solvent molecule donates an electron pair and the coordination number of the silicon atom increases to six. Formation of such solvation complexes gives rise to a pronounced positive specific solvatochromic effect in the UV spectra. According to dielectrometric, 19F NMR, and IR data, and also to ab initio calculations (HF/6-31G*), the solvent molecule occupies an equatorial position.

Synthesis and IR spectra of 5-phenyl-2-(1-phenyl-2-trichloroacetylethenyl)pyrrole

Abstract5-Phenyl-2-(1-phenyl-2-trichloroacetylethenyl)pyrrole was synthesized by the reaction of 2-phenylpyrrole with 1-phenyl-2-trichloroacetylacetylene on silicon oxide. The structure of an intramolecular proton-transfer H-complex was assigned to the title compound based on analysis of its IR spectra in solutions in the temperature range of 165–298 K.

Vibrational spectra of (O → Si)-(aroyloxymethyl)trifluorosilanes

Abstract The IR spectra of (aroyloxymethyl)trigfluorosilanes 4-X-C 6 H 4 COOCH 2)SiF(in3 (AFS) containing an intramolecular O → Si coordinate bond, have been examined in the 500-200 cm −1 region. The band near 330 cm −1 is assigned to the O → Si bond stretching vibration because of its λ sensitivity to changes in the polarity of the medium and because of its response to the nature of substituent X as well as to the formation of AFS-pyridine complexes. Normal-coordinate analysis of a model compound HCOOCH 2 SiF 3 has been carried out. By use of a set of frequency partial derivatives by force constants a force field corresponding to the experimental frequencies and equilibrium geometry of 4-F-C 6 H 4 COOCH 2 SiF 3 has been selected. The v (O → Si) stretching vibration is delocalized by three low-frequency motions 360, 330 and 218 cm −1 . The effect of electronic and kinematic factors on the AFS vibrational spectra has been evaluated.

Structure of (O→Si)-(acetoxymethyl)trifluorosilane in three phase states and in solutions

According to the IR spectroscopy data, the molecules of (O→Si)-(acetoxymethyl)trifluorosilane having in the liquid state and in polar media the intramolecular bond C=O→Si, exist in the gas phase in the temperature range 438–538 K in the equilibrium with the molecules with tetracoordinate silicon atom. This allowed to determine experimentally the enthalpy of formation of the intramolecular bond C=O→Si for the gas phase to be ΔH = 2.2±0.1 kcal mol−1. In the solid state at 110 K and in the CS2 solution, along with molecule with the C=O→Si bond, the dimers exist, which include both tetra- and pentacoordinate silicon atom. The data of quantum-chemical calculations (B3LYP/6-311G**) show that the shortest intermolecular bond Si-F→Si is realized in the associate formed by the molecules in the ap,sp- and sp,sp-forms, and the longest one, when both components are in the sp,sp-forms.

Tautomeric transformations of arylidenebarbiturates in solutions

IR and UV spectroscopy and quantum chemistry were employed to investigate the structure of arylidenebarbiturates able to undergo tautomeric transformations. The composition of tautomeric mixtures depends on the properties of the solvent and on the concentrations of the solutions.

SI-SUBSTITUTED 8-QUINOLYLTHIOMETHYLSILANES

Abstract(8-Quinolylthiomethyl)trimethoxy- and methyl(8-quinolylthiomethyl)dimethoxysilanes were synthesized and transformed into the corresponding organyltrifluoro- and methyldifluorosilanes. IR spectroscopy revealed the absence of N→Si donor-acceptor interaction in (8-quinolylthiomethyl)trifluorosilane and methyl(8-quinolylthiomethyl)difluorosilane. Transesterification of (8-quinolylthiomethyl)trimethoxysilane with triethanolamine yielded 1-(8-quinolylthiomethyl)silatrane; the crystal and molecular structures of the latter were determined by X-ray study.