1,1,1-Trimethyl-3,3,3-trichlorodisiloxane as a source and a trapping agent for silanones

The pyrolysis of 1,1,1-trimethyl-3,3,3-trichlorodisiloxane (1) was studied and its mechanism involving the formation of dichloro- and dimethylsilanones was proposed. The composition of the condensate from the co-pyrolysis of siloxane i and hexamethyldisiloxane indicated that under the pyrolysis conditions the simplest siloxanes can be both the sources and the trapping agents of silanones.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1299–1300, May, 1996.     

Photolysis and oxidation of azidophenyl-substituted radicals: delocalization in heteroatom-based radicals

2-(4-Azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (14), 2-(4-azidophenyl)benzimidazole-1-oxide-3-oxyl (16), 2-(4-azidophenyl)-1,2,6-triphenylverdazyl (19), 2-(3-azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (21), and (3-azidophenyl)-N-tert-butyl-N-aminoxyl (25) were photolyzed in frozen solution to give S = 3/2 state ESR spectra of the corresponding nitrenophenyl radicals with the following zero-field splitting parameters: |D/hc| = 0.277 cm(-1), |E/hc| < or = 0.002 cm(-1) (7 from 14); |D/hc| = 0.256 cm(-1), |E/hc| < or = 0.002 cm(-1) (8 from 16); |D/hc| = 0.288 cm(-1), |E/hc| < or = 0.002 cm(-1) (9 from 19); |D/hc| = 0.352 cm(-1), |E/hc| = 0.006 cm(-1) (10 from 21); |D/hc| = 0.336 cm(-1), |E/hc| = 0.004 cm(-1) (11 from 25). UB3LYP/6-31G computations and ESR spectroscopic analyses suggest that these are nitreno radicals, even para-linked systems with possible quinonoidal resonance forms. Neat samples of azidophenyl radicals 14 and 21 showed bulk paramagnetic behavior, consistent with the lack of close contacts in their crystal structures. Efforts to make photolabile coordination complexes of 14 and 21 with paramagnetic transition metal ions were unsuccessful: Cu(ClO4)2 x 6H2O instead oxidized them to the corresponding diamagnetic nitrosonium perchlorate salts.     

The reaction of trifluoromethyl radicals with ammonia

The reaction of trifluoromethyl radicals with ammonia in the gas phase has been studied in the temperature range 30-352°. Product formation is not explicable simply in terms of the reactions: and curvature of the Arrhenius plot at high and low temperatures suggests that there are additional sources of fluoroform.     

Photolysis of 1,1,1-difluorochloroethane (Freon 142) at 147 nm

1,1,1-Difluorochloroethane (Freon 142) was photolyzed at 147 nm in the pressure range of 3.6–20.6 torr. The effects of added NO, H₂S, and CF₄ were investigated. The extinction coefficient at 147 nm and 296°K was determined to be 64 ± 8 atm^?1 · cm^?1. The molecule photodecomposes largely by α,β elimination of HCl to give 1,1-difluoroethylene (Φ = 0.74 ± 0.06). There is no observable elimination of HF, but there is strong evidence for the elimination of the elements of FCl though the relative importance of this process is minor, as are contributions from carbon? carbon and carbon? halogen bond fission. The 1,1-difluoroethylene formed is undoubtedly vibratonally excited and is the source of a pressure-dependent small yield of fluoroacetylene. Over the pressure range studied there is no evidence that the major primary process itself is affected by changes in total pressure as is the case in the 147-nm photolysis of ethyl chloride.     

Hydrogen abstraction reactions from organosilicon compounds. The reactions of methyl,trifluoromethyl, and ethyl radicals with trichlorosilane

Arrhenius parameters have been determined for the hydrogen-abstraction reactions: R + SiHCl₃ + RH + SiCl₃

Photolysis of CF3NO as a source of CF3 radicals,rate constant of the CF3 + CF3NO reaction at 296 K

The linear absorption cross-sections and photodissociation quantum yields for CF₃NO have been measured at the wavelengths 694.3 and 632.8 nm (ruby and He(SINGLEBOND)Ne lasers, respectively) in a wide range of total pressure at T = 296 K. The secondary reactions following the photolysis of CF₃NO have been studied. The rate constant of the CF₃ + CF₃NO reaction has been measured. The results obtained show CF₃NO photolysis be a convenient source of CF₃ radicals for kinetic studies. © 1996 John Wiley & Sons, Inc.     

Hydrogen-abstraction reactions from organosilicon compounds. The reactions of methyl,trideuteromethyl, trifluoromethyl,and ethyl radicals with tetramethylsilane

The following Arrhenius parameters have been determined for the hydrogen-abstraction reactions: R + (CH₃)₄Si → RH + (CH₃)₃SiCH₃

Isonitriles as source and fate of imidoyl radicals: a novel homolytic α-fragmentation

Imidoyl radicals 4a-c react with phenylacetylene to give annulation products and nitrile 12, arising from β-scission of the intermediate iminyl radical that is involved in the rearrangement of azaspirocyclohexadienyl 8. In contrast, imidoyls 4d and 15 do not react with the alkyne and give good yields of the corresponding isonitriles through a novel example of homolytic α-fragmentation.     

Reaction of methoxy radicals with oxygen. I. Using dimethyl peroxide as a thermal source of methoxy radicals

Using dimethyl peroxide as a thermal source of methoxy radicals overthe temperature range of 110–160°C, and the combination of methoxy radicals and nitrogen dioxide as a reference reaction: a value was determined of the rate constant for the reaction of methoxy radicals with oxygen: is independent of nitrogen dioxide or oxygen concentration and added inert gas (carbon tetrafluoride). No heterogeneous effects were detected. The value of k₄ is given by the expression In terms of atmospheric chemistry, this corresponds to a value of 10^5.6 M^?1·sec^?1 at 298 K. Extrapolation to temperatures where the combustion of organic compounds has been studied (813 K) produces a value of 10^7.7 M^?1·sec^?1 for k₄. Under these conditions, reaction (4) competes with hydrogen abstraction or disproportionation reactions of the methoxy radical and its decomposition (3): In particular k₃ is in the falloff region under these conditions. It is concluded that reaction (4) takes place as the result of a bimolecular collision process rather than via the formation of a cyclic complex.     

Thermolyses of O-phenyl oxime ethers. A new source of iminyl radicals and a new source of aryloxyl radicals

Six O-phenyl ketoxime ethers, RR'C=NOPh 1-6, with RR' = diaryl, dialkyl, and arylalkyl, together with N-phenoxybenzimidic acid phenyl ether, PhO(Ph)C=NOPh, 7, have been shown to thermolyze at moderate temperatures with "clean" N-O bond homolyses to yield iminyl and phenoxyl radicals, RR'C=N(*) and PhO(*). Since 1-6 can be synthesized at room temperature, these compounds provide a new and potentially useful source of iminyls for syntheses. The iminyl from 7 undergoes a competition between beta-scission, to PhCN and PhO(*), and cyclization to an oxazole. Rate constants, 10(6) k/s(-1), at 90 degrees C for 1-6 range from 4.2 (RR' = 9-fluorenyl) to 180 (RR' = 9-bicyclononanyl), and that for 7 is 0.61. The estimated activation enthalpies for N-O bond scission are in satisfactory agreement with the results of DFT calculations of N-O bond dissociation enthalpies, BDEs, and represent the first thermochemical data for any reaction yielding iminyl radicals. The small range in k (N-O homolyses) is consistent with the known sigma structure of these radicals, and the variations in k and N-O BDEs with changes in RR' are rationalized in terms of iminyl radical stabilization by hyperconjugation: RR'C=N(*) <--> R(*)R'C[triple bond]N. Calculated N-H BDEs in the corresponding RR'C=NH are also presented.     

Reactions of unsymmetrically substituted allyl radicals with methyl radicals

The Hg(6³P₁) photosensitized decompositions of 3-methyl-1-butene, 2-methyl-2-butene, 3,3-dimethyl-1-butene, and 2,3-dimethyl-1-butene have been used to generate 1-methylallyl, 1,2-dimethylallyl, 1,1-dimethylallyl, and 1,1,2-trimethylallyl radicals in the gas phase at 24 ± 1°C. From a study of the relative yields of the CH₃ combination products, the relative reactivities of the reaction centers in each of these unsymmetrically substituted ambident radicals have been determined. The more substituted centers are found to be the less reactive, and this is ascribed primarily to greater steric interaction at these centers during reaction. Measurement of the ratio of trans- to cis-2-pentene formed from the 1-methylallyl radical, combined with published values for this ratio at higher temperatures, enabled the differences in entropy and heat of formation of the trans- and cis-forms of this radical to be calculated as 0.62 ± 0.85 J mol^?1 K^?1 and - 0.63 ± 0.25 kJ mol^?1, respectively, at 298K. Approximate values of the disproportionation/combination ratios for reaction of CH₃ with 1,1-dimethylallyl and 1-methylallyl have been estimated and used to compute rate constants for the recombinations of tert-butyl and isopropyl radicals that are in agreement with recently published data.     

Disproportionation reactions between alkyl and fluoroalkyl radicals. II. Fluoro- and trifluoromethyl with ethyl radicals

Cross-disproportionation/combination ratios for CFH₂ and CF₃ with C₂H₅ radicals have been determined to be Δ = 0.032 ± 0.012 and δ = 0.098 ± 0.020, respectively, over the temperature range 25–75°C. For the pathway that yields CFH and C₂H₆, δ = 0.020 ± 0.005 at 25°C.     

Chemistry of trifluoromethyl compounds : I. NMR evidence for bis(trifluoromethyl)zinc and methyl(trifluoromethyl)zinc

Bis(trifluoromethyl)zinc and methyl(trifluoromethyl)zinc have been identified by ¹⁹F and ^1H NMR methods. The compounds were formed in the following reactions: (1) dimethylzinc and bis(trifluoromethyl)mercury and (2) dimethylzinc and bis(trifluoromethyl)cadmium.     

Photolysis of alkylperoxy radicals frozen out from the gas phase

Photolysis of alkylperoxy radicals with 250 nm light has been studied and proposed for investigation of heterogeneous-homogeneous reactions by freezing out radicals from the gas phase and their ESR identification. Using amine decomposition reaction in the presence of O₂ as an example, we have shown that photodissociation of frozen peroxy radicals can be used to reduce the structure of initial alkyl radicals.

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Electronic absorption spectra of cation radicals as a source of information regarding their conformations

On the basis of differences in the energy gaps between the molecular orbitals of the type of biphenyl, 2,2-dithienyl, 2-phenyl(thienyl)pyrrole, 2-phenylselenophene (photoelectronic spectroscopy), and their cation radicals (nanosecond laser spectroscopy) it was shown that ionization causes transition to a more planar molecular conformation.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1213–1216, September, 1989.     

Rotation of methyl radicals in a solid argon matrix

Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH3) in a solid argon matrix at 14-35 K temperatures. The radicals were produced by dissociating methane by plasma bursts generated either by a focused 193 nm laser radiation or a radio frequency discharge device during the gas condensation on the substrate. The ESR spectrum exhibits axial symmetry at the lowest temperature and is ascribed to ground state molecules with symmetric total nuclear spin function I=3/2. The hyperfine anisotropy (Aparallel)-Aperpendicular) was found to be -0.01 mT, whereas that of the g value was 2.5x10(-5). The anisotropy is observed for the first time in Ar and is manifested by the splitting of the low-field transition. Elevation of temperature leads reversibly to the appearance of excited state contribution having antisymmetric I=1/2. As a function of the sample temperature, the relative intensities of symmetric and antisymmetric spin states corresponding to ground and excited rotor states, respectively, proton hyperfine and electron g-tensor components, and spin-lattice relaxation rates were determined by a numerical fitting procedure. The experimental observations were interpreted in terms of a free rotation about the C3 axis and a thermal activation of the C2-type rotations above 15 K. The ground and excited rotational state energy levels were found to be separated by 11.2 cm-1 and to exhibit significantly different spin-lattice coupling. A crystal field model has been applied to evaluate the energy levels of the hindered rotor in the matrix, and crystal field parameter varepsilon4=-200 cm-1, corresponding to a 60 cm-1 effective potential barrier for rotation of the C3 axis, was obtained.     

Stable nitroxyl radicals and hydroxylamines as inhibitors of methyl linoleate oxidation in micelles

Russian Chemical Bulletin - The kinetics of methyl linoleate oxidation in micelles inhibited by nitroxyl radicals and hydroxylamines was studied. The regeneration of nitroxyl radicals in this...     

Interaction between methyl and hydroxyl radicals: a theoretical study

In this paper, we conduct a computational quantum mechanic study of two molecules interaction between methyl (·CH₃) and hydroxyl (·OH) radicals. The molecular exploration has been focused on the possibility of finding non-bonding interactions (formation of complexes by weak-bond interaction among C–O–H atoms) and the potential energy reaction between those two molecules. It shows that the formed complexes presents in slightly lower potential energy than that of the reactants and/or products. The existence of these complexes could proceed to the further interaction, e.g. reaction of the molecules, as those complexes formed a particular configuration.     

Theoretical study of reaction of trifluoromethyl radical with hydroxyl and hydrogen radicals

Ab initio molecular orbital theory and density functional theory calculations have been carried out on the reactions of the trifluoromethyl radical with the hydroxyl and the hydrogen radicals. These reactions are key reactions that underlie a new fire extinguishing mechanism of non-bromine-containing halon replacements. The activation energies calculated by the MP2 and QCISD methods are in good agreement with the experimental values. The B3LYP, as well as MP2 and QCISD, give good results for the calculations of the heats of reactions. The GAUSSIAN-1 and GAUSSIAN-2 theory calculations present the most acxcurate results on both the activation energies and the heats of reactions. The effects of the scaling factors on the activation energies and the heats of reactions are also evaluated. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 277–289, 1998