Investigation of organosilicon radicals : II. Methylsilyl radicals

CNDO/2 calculations for a series of Me_xH₃?_xSi (where x = 0,1,2,3) free radicals confirm the suggestion, based on electron spin resonance measurements and chemical evidence, that these radicals are not planar. The deviation from planarity decreases with increasing number of methyl groups, but the angle between the carbonsilicon bond and the plane of an assumed planar configuration is the same for methylsilyl, dimethylsilyl, and trimethylsilyl radicals.     

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.     

Bimolecular self-reactions of ketyl radicals of acetophenone and diphenylaminyl radicals

The rate constants of self-reactions of ketyl radicals of acetophenone in n-heptane [2k = (3.2 ± 0.5) × 10⁹ M^?1 s^?1] and diphenylaminyl radicals in toluene [2k = (3.3 ± 0.5) × 10⁷ M^?1 s^?1] have been determined at 298 K using the flash photolysis technique. The rate constant of ketyl radicals is equal to the calculated diffusion constant and, therefore, this reaction is diffusion-controlled. The aminyl radical recombination rate is independent of the viscosity of the toluene/vaseline oil binary mixture (0.55 ? η ? 12 cP) and this reaction is activation-controlled. Reactivity anisotropy averaging due to the cage effect has been considered for ketyl and some other radicals. On the basis of the analysis it has been proposed that ketyl recombination involves formation of not only pinacol, but also iso-pinacols.     

Conjugated radicals

The naphthalene radical anion was calculated by the open-shell SCF method due to Roothaan combined with a configuration interaction treatment. The effect of the doubly excited configurations on the calculated transition energies was found to be unimportant in view of the interpretation of its electronic spectrum.Part V: Collect. Czechoslov. Chem. Commun. 35, 892 (1970).     

Conjugated radicals

Restricted open shell CNDO calculations have been carried out on butadiene anion radical (M_–), butadiene cation radical (M⁺), and butadiene dimer cation radical (M ₂ ⁺ ). Calculated transition energies are in agreement with the experimental data. Formation of M ₂ ⁺ and unsuccessful attempts to detect M ₂ ^– have been interpreted by the CNDO and extended Hückel calculations, in light of which the M + M⁺ M ₂ ⁺ process is energetically favourable while the M + M^– (M ₂ ^– ) process is connected with an energy loss. CNDO calculations support the assumed sandwich structure of M ₂ ⁺ .

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Zusammenfassung Restricted open shell CNDO-Rechnungen für das Butadien-Radikalanion (M^–), das Butadien-Radikalkation (M⁺) und das dimere Butadien-Radikalkation (M ₂ ⁺ ) wurden durchgeführt. Die berechneten Übergangsenergien stimmen mit den experimentellen Daten überein. Die Bildung von M ₂ ⁺ einerseits und die erfolglosen Versuche, das dimere Radikalanion M ₂ ^– herzustellen, andererseits wurden mittels CNDO- und EHT-Berechnungen gedeutet. Es zeigte sich hierbei, daß der Prozeß M + M⁺ M ₂ ⁺ energetisch möglich sein, die Reaktion M + M ₂ ^– M ₂ ^– dagegen mit einem Energieverlust ablaufen sollte. Die CNDO-Rechnungen sprechen für die Annahme einer Sandwich-Struktur von M ₂ ⁺ .

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Résumé On a effectué des calculs avec restricted open shell d'après la méthode CNDO sur l'anion radical du butadiène (M^–) sur le cation radical du butadiene (M⁺) et sur le cation radical du dimère du butadiène (M ₂ ⁺ ). Les énergies de transition calculées sont en accord avec les données expérimentales. La formation de M ₂ ⁺ et les efforts sans succès à prouver la présence de M ₂ ⁺ ont été interprétés par la méthode de CNDO et par les calculs étendus d'après Hückel. De ce point de vue le procès M + M⁺ M ₂ ⁺ est énergétiquement favorable pendant que le procès M + M^– M ₂ ^– est accompagné d'une perte d'énergie. Les calculs d'après la méthode de CNDO soutiennent la structure supposée deM ₂ ⁺ .

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Part XI: ársky, P., Hobza, P., Zahradnik, R.: Collect. Czechoslov. Chem. Commun., in press.     

Hydrogen transfer from ketyl radicals to nitroxyl radicals

The transfer of an H atom from a ketyl radical (KR) to a nitroxyl radical is the sole reaction occurring between benzophenone or acetone KR and nitroxyl radicals (NR). The rate constants (k_H) of the reaction of the KR of substituted benzophenones with the NR-4-hydroxy-2,2,6,6-tetramethylpiperidine-1oxyl and 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl in various solvents were measured by the pulse photolysis method. In low-viscosity solvents (up to 1-2 cP), the values of k_H are not limited by the diffusion of the reagents. The k_H values decrease with increase in the Hammet's -constants of the substituents in the KR and with decrease in the reduction potential of the NR, which indicates a charge transfer from the KR to NR in the transition state of the reaction. A cyclic structure was proposed for the transition state. The reaction is characterized by a low isotopic effect (k_H/k_D=1.4–1.5). The dependence of log k_H on the solvation parameter of the solvent E_t(30) is V-shaped in character.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 999–1003, May, 1990.     

Combinations of alkyl radicals with alkyl and substituted allyl radicals

In contrast with expectations based upon simple collision theory, reactions of alkyl radicals with substituted allyl radicals yield cross-combination ratios of =2. Reactions of CH ₃ ^\ with some C₅ radicals give >2, in accordance with theory.

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, =2. CH₃ ^\ C₅ >2, .

     

Homoallenyl radicals

Several homoallenyl radicals have been generated by the reaction of the corresponding iodides with Bu₃SnH and found to produce small amounts of vinylcyclopropanes in addition to allene hydrocarbons establishing that homoallenyl radicals cyclize to isomeric 1-cyclopropylvinyl radicals.     

Imidyl radicals

Under reaction conditions which prevent halogen atom chains, imidyl radicals are the chain carriers in reactions of N-haloimides. Characteristic reactions are l) ring-opening of certain imidyl radicals, 2) hydrogen abstractions with selectivities similar to chlorine atoms, 3) additions to alkenes and 4) substitutions of arenes. The absolute rate constant of reaction between imidyl radicals and most substrates is in a range of ?10⁵ ?/mol. sec.     

Investigation of organosilicon radicals : I. Methylphenylsilane anion radicals

CNDO/2 calculations with an spd basis set have been carried out on methyl-phenylsilane anion radicals, and the calculated spin density values compared with the experimental hyperfine coupling constants. The CNDO method overestimates the role of d orbitals and the partial charges on hydrogen atoms attached to silicon atom. The partial charge distribution and the carbonsilicon bond order in the anion radicals and the corresponding neutral molecules are discussed. The equilibrium carbonsilicon bond distance in the trimethylphenylsilane molecule and corresponding anion radical have also been investigated.     

Activation barriers in the homolytic cleavage of radicals and ion radicals

As revealed by several experimental examples, radicals and ion radicals may, in contrast with closed-shell molecules, undergo exothermic homolytic cleavages (.A..B --> A: +.B) with substantial activation barriers. A two-state semiclassical model is proposed for explaining the existence of the barrier and estimating its magnitude. It is based on the intersection of the potential energy surfaces characterizing the dissociation of a bonding state, .A..B -->.A. +.B, on one hand, and the approach to bonding distance of a repulsive state, A: +.B --> A therefore B, on the other. After inclusion of the bond cleavage and formation as Morse curves in the normal-mode analysis, a simple activation driving force relationship is obtained, the two main ingredients of the intrinsic barrier being the triplet excitation energy of the A moiety and the pi*--> sigma* excitation energy in .A-B. The model is then tested by quantum chemical calculations, first on a simplified system to evaluate the calculation techniques and then on a real system. A comparison of the model predictions with experiment is finally performed using the rate data recently gathered for the cleavage of 4-cyanophenyl alkyl ether anion radicals, which cover a respectable range of driving forces, showing satisfactory agreement between theoretical predictions and experimental data.     

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.     

Anomeric effect of radicals

The glucosyl radical 1 is attacked predominately at the axial position. This can be explained by an anomeric effect that stabilizes σ-radical 5.