The effects of chemically induced polarisation in the cage recombination products of the methyl radical

A study of the CIDNP effects on methyl protons was carried out for the cage recombination products formed during the thermal decomposition reactions of peracetyls and peracetylcarbonates. The increase of both the cage recombination product yield and the enhancement of CIDNP on the CH₃ protons is shown to be connected with the RCO₂ radical lifetime, where R is an alkyl or alkoxy radical. A general theory of CIDNP is presented and the calculated data prove to be in satisfactory agreement with the experimental results.     

Emulsion polymerizations. I. A novel approach to the evaluation of the molecular weight distributions

A simple, probabilistic approach describing the number distribution of the molecular weight (MWD) of polymers formed by emulsion polymerization is presented. The radical populations of the reacting system are formally split into infinite, quickly convergent series of particularly tagged radical populations. These populations are characterized by the nature and the number of the state transitions of the associated latex particles. For each kind of single radical or radical pairs a distribution function is then defined. These functions can be specified on the basis of α_i and Q_i, the growth probabilities, within a latex particle in state i, of a single radical and a radical pair respectively. The overall MWDs are then given by summation over all the distributions of the radical populations and over all the allowed states. Only one set of ordinary differential equations (the Smith–Ewart equations) are involved in the mathematical formulation, single distribution functions being obtained by solving simple exponential-type integrals. In this paper, analytic solutions are presented for the zero-one, zero-one-two and general systems. Analogies and differences between our approach and previously reported treatments are critically discussed.     

Effect of mercaptan on photoreduction of acetone. Non-repair hydrogen transfer reactions

Abstract— Ultraviolet (u.v.) irradiation of solutions of benzhydrol in acetone leads to formation of -2-propanol, benzpinacol and some benzophenone, apparently from the free radicals (CH₃)₂COH, II, and (C₆H₅)₂COH, I. 2-Propanol is formed more rapidly and benzophenone is formed to a much larger extent and persists longer when the solution contains mesityl mercap-tan, as radical II is reduced by mercaptan and radical I is oxidized by thiyl radical. The same hydrogen atom transfer reactions, which retard by a repair mechanism the photoreduction of benzophenone by 2-propanol, accelerate and alter the course of photoreduction of acetone by benzhydrol. Irradiation of acetone leads to 2-propanol, and this is formed more rapidly in the presence of mercaptan. Irradiation of benzophenone in acetone leads to no apparent reaction. The courses of reaction of the several systems are discussed.     

Radiation chemistry of polysaccharides: 4. Free-radical mechanisms of formaldehyde formation

Based on an analysis of author’s experimental results and published data on the radiolysis of polysaccharides, three main reaction paths of CH₂O formation related to the transformations of primary radicals with unpaired electrons on C1, C5, and C6 atoms were revealed. The formaldehyde molecule was formed in the primary event of C5 radical conversion or through the steps of formyl radical formation in the other two cases.     

Fvp of bis- and tris-(pyrazol-1-yl)methane. A radical reaction

Flash vacuum pyrolysis of bis- and tris-(pyrazol-1-yl) methane was carried out. Alpha, gamma and radical eliminations were considered. The products actually formed correspond to a radical reaction. In the case of bis-(pyrazol-1-yl) methane, working at higher temperatures, pyrimidine was obtained. This compound is formed by rearrangement of a PzCH₂ radical.     

Triggering Redox Activity in a Thiophene Compound: Radical Stabilization and Coordination Chemistry

The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L⁻ are presented. This π‐conjugated anion displayed pronounced redox activity, undergoing facile one‐electron oxidation to the acyclic, metal‐free, neutral radical L ^. on reaction with FeBr₂. In contrast, the reaction of L⁻ with CuI formed the unique, neutral Cu₂I₂( L ^.) complex of a ligand‐centered radical, whereas reaction with the stronger oxidant AgBF₄ formed the metal‐free radical dication L ^.2+.     

A detailed chemical kinetic model for the supercritical water oxidation of methylamine: The importance of imine formation

A detailed chemical kinetic model has been developed for supercritical water oxidation (SCWO) of methylamine, CH₃NH₂, providing insight into the intermediates and final products formed in this process as well as the dominant reaction pathways. The model was adapted from previous mechanisms, with a revision of the peroxyl radical chemistry to include imine formation, which has recently been identified as the dominant gas-phase pathway in amine oxidation. The developed model can reproduce previous experimental data on methylamine consumption and major product formation to reasonable accuracy, although with deficiencies in describing the induction time. Our simulations indicate that oxidation of the ^•CH₂NH₂ radical to methanimine, CH₂NH, is the major channel in methylamine SCWO, with subsequent hydrolysis of CH₂NH providing the experimentally observed reaction products ammonia and formaldehyde. Integral-averaged reaction rates were used to identify major reaction pathways, and a first-order sensitivity analysis indicated that the concentration of CH₃NH₂ is most sensitive to OH radical kinetics. Overall, this work clarifies the importance of imine chemistry in the oxidation of nitrogen-containing compounds and indicates that they are necessary to model these compounds in SCWO processes.     

Chemical polarization of nuclei. Proton polarization during thermal decomposition reaction of dimethyl perdiglutarate

Proton chemical polarization was observed in methyl butyrate, γ-butyrolactone and in some other products of the dimethyl perdiglutarate decomposition reaction. The polarization pattern of the α-CH₂ and β-CH₂ groups in butyrolactone is just as one would expect in the products of the CH₃OCH₂CH₂?H₂ radical which has left the pair of identical radicals. Thus butyrolactone is formed from the CH₃OCH₂CH₂?H₂ radical by means of intramolecular cyclization and elimination of the methyl radical.     

The gas-phase decomposition of methylsilane. Part II. Mechanisms of decomposition under static system conditions

The static system pyrolysis of methylsilane (T ～ 700 K, P_T ～ 150 torr), pure and in the presence of ethylene, propylene, and acetylene, has been investigated. It is proposed that in the uninhibited system, the major products (silane and dimethylsilane) are produced by free radical processes, and that the free radicals are formed at the walls from methylsilylenes. In the presence of olefins, the free radicals are trapped to form methylsilane adducts. In acetylene, trapping of methylsilylenes prevents free radical production and eliminates the free radical produced products of the pure and the olefin inhibited systems. Rates of initiation correlate with rates of reactant loss in acetylene inhibited systems, and with rates of hydrogen formation in olefin inhibited systems. Rough estimates of primary dissociation process yields give for the 1,1-H₂ elimination ?_1,1 ? 0.78, for the 1,2-H elimination ?_1,2 ? 0.16, and for the methane elimination ?CH₄ ? 0.06 at 700 K. Deuteration lowers initial step kinetics by about 15%.     

The molecular structure of trifluoromethyl manganese pentacarbonyl. A study by gas-phase electron diffraction

The results of an SCF-MO calculation on the CH₂CCH radical are presented: population analysis indices and several one-electron properties are reported and the electronic structure of the radical is discussed. The spin density is almost equally associated with the terminal carbon atoms, and there is a large negative spin density associated with the central carbon atom.     

Ene reactions of olefins. I. The addition of ethylene to 2-butene and the decomposition of 3-methylpentene-1

The pyrolysis of ethylene–butene-2 mixtures has been studied in a static system over the temperature range of 689°-754°k and for initial pressures of each olefin of 20–200 torr. The two main addition products were cyclopentene and 3-methylpentene-1. Kinetic evidence indicated that cyclopentene was formed from radical processes while 3-methylpentene-1 was formed by the molecular “ene¨?” addition of ethylene to butene-2 through a six-center transition state. The following rate constants were obtained: The pyrolysis of 3-methylpentene-1 has been studied over the same temperature range and for initial pressures of 20–100 torr. Kinetic evidence showed that the products ethylene and butenes were formed in both radical and molecular processes. Estimates of the rate constant k_?1t and k_?1c were, however, in reasonable agreement with the measurements of k_1t and k_1c. The mechanism of the ene reaction is discussed, and it is concluded that the transition state does not involve the formation of a biradical.     

Chain transfer in ethylene polymerization. V. The effect of temperature

In order to determine the effect of temperature on the chain-transfer reaction in the free-radical polymerization of ethylene, chain-transfer constants were measured for sixteen transfer agents at 130°C and 200°C at 1360 atm. The results were interpreted as ΔE*, the activation energy of the chain-transfer constant. This value is equal to the difference in activation energy between the transfer step (hydrogen abstraction) and the propagation step (addition to the monomer double bond): ΔE* = E_s* ? E_p*. Excellent agreement was found between measured ΔE* values determined at 1360 atm pressure and (E_s* ? E_p*) data for ethyl radical determined in vacuum gas-phase reactions. Apparently, the ethyl radical is a good model for polyethyl radical. The chain-transfer constant of ethylbenzene was found to be insensitive to temperature changes, indicating that E_p* = E_s* for this compound.     

Investigations on the current oscillations during the electrolytic reduction of dichromates in acetic solutions

The reduction mechanism at a mercury electrode of Eriochrome Cyanine R, in a 0.9 M NaClO₄+0.1 M HClO₄ supporting electrolyte, has been investigated by several electrochemical techniques. By means of coulometry at constant potential and cyclic voltammetry it was demonstrated that a radical is formed, which suffers disproportionation, after the first electron transfer. The results of the cyclic voltammetric investigation and the impedance measurements evidenced the intrinsic reversible nature of the electron transfers. Apparent irreversible polarographic behaviour is a consequence of the existence of chemical reactions following the electron transfers. The impedance measurements demonstrated the strong adsorption of the Ox and Red form of Eriochrome Cyanine R as well as of the radical formed. On the basis of the experimental data a reduction mechanism is proposed.     

The nominal butyl ester ion in the mass spectra of long-chain n-alkyl esters: A postscript

n-Octadecyl benzoate, taken as a model for long-chain n-alkyl carboxylates generally, loses C₁₄H₂₈ under electron impact to yield a product with the same elemental composition as the butyl benzoate molecular ion. This product retains quantitatively one hydrogen from C-6, and seems to be formed as an oxygen-protonated 4-benzoyloxybutyl radical. It reacts further to lose H₂O, in which deuterium labeling demostrates that the second hydrogen atom comes predominantly from C-4. The intermediate reorganization, for which the driving force is presumably furnished by the instability associated with a primary radical, is pictured in terms of cyclization via bonding between the C-4 radical site and the benzoyl carbon concerted with hydrogen migration via a 4-membered quasicyclic transition state.     

Products of the gas-phase OH radical-initiated reactions of 4-methyl-2-pentanone and 2,6-dimethyl-4-heptanone

The gas-phase reactions of the OH radical with 4-methyl-2-pentanone and 2,6-dimethyl-4-heptanone have been investigated in the presence of NO_x. Acetone and 2-methylpropanal were identified and qualified as products of both reactions. The acetone yield from 2,6-dimethyl-4-heptanone increased after addition of NO to reacted mixtures, indicating that acetone is formed through the intermediary of an acyl radical. The acetone and 2-methylopropanal formation yields were determined to be 0.78 ± 0.06 and 0.071 ± 0.011, respectively, from 4-methyl-2-pentanone and 0.68 ± 0.11 and 0.385 ± 0.034, respectively, from 2,6-dimethyl-4-heptanone. The possible reaction mechanisms are discussed and compared with these product data, and it is concluded that the experimental data provide direct evidence for isomerization of the (CH₃)₂CHCH₂C(O)CH₂C(O) (CH₃)₂ alkoxy radical formed from 2,6-dimethyl-4-heptanone. However, the isomerization rates of the alkoxy radicals formed from the ketones depend on whether the H-atom abstracted is on a carbon atom α or β to the >C?O group, with H-atom abstraction from C? H bonds on the β carbon atoms being significantly faster than from C? H bonds on the α carbon atoms. © 1995 John Wiley & Sons, Inc.     

The sulphate radical is not involved in aqueous radiation oxidation processes

The H₂O₂/persulphate systems are of enormous environmental and commercial importance with the sulphate radical (SO₄⁻) being assumed as the oxidizing/bleaching species. We show that under normal conditions (air-saturated) no SO₄⁻ is produced and, most likely, a much longer-lived species, the adduct of O₂ and the persulphate radical, is formed.     

Polymerization of allyl esters of unsaturated acids. V. Cyclopolymerization of methyl allyl fumarate

The kinetics of radical polymerization of methyl allyl fumarate (MAF) is discussed in terms of cyclopolymerization and compared with the polymerization results of methyl allyl maleate (MAM) as a cis isomer. In the polymerization of MAF, the rate and degree of polymerization were quite enhanced compared with MAM, and gelation occurred at low conversion. The content of the unreacted allylic double bonds of the MAF polymer was quite large; whereas those of the unreacted fumaric double bonds and the cyclic structural units showed reverse tendencies. Only a slight presence of a five-membered ring was observed in the MAF polymer. The cyclization constants K_A and K_V, the ratios of the rate constants of the unimolecular cyclization reaction to those of the bimolecular propagation reaction of the uncyclized allylic and fumaric radicals, were estimated to be 2.73 and 1.48 mole/liter, respectively. These values suggest the great difference in the cyclopolymerization behavior between two isomeric monomers. These results are discussed in detail in connection with the high reactivity of the fumaric double bond compared to the maleic double bond. In addition, the formation mode and the sequence distribution of the structural units of the polymer produced are discussed on the basis of these analytical results. Thus, for the MAF polymer obtained in the bulk polymerization, about 60% of the cyclic structure can be formed via the intramolecular attack of the uncyclized fumaric radical on the allylic double bond, as opposed to the case of MAM via the predominant intramolecular attack (ca. 90%) of the uncyclized allylic radical on the maleic double bond; these results and the low probability for the succession of cyclic structures and the rather high probability of a vinyl-to-vinyl addition are presented.     

Characterization of the transient species generated by the photoionization of Berberine: A laser flash photolysis study

Using 266 nm laser flash photolysis it has been demonstrated that Berberine (BBR) in aqueous solution is ionized via a mono-photonic process giving a hydrated electron, anion radical that formed by hydrated electron react with steady state of BBR, and neutral radical that formed from rapid deprotonation of the radical cation of BBR. The quantum yield of photoionization is determined to be 0.03 at room temperature with KI solution used as a reference. Furthermore utilizing pH changing method and the SO₄⁻ radical oxidation method, the assignment of radical cation of BBR was further confirmed, the pK_a value of it was calculated, and the related set up rate constant was also determined.     

The photolysis of benzyltin compounds investigated by 1H CIDNP

The photochemical decomposition of (PhCH₂)₃SnMe and (PhCH₂)₃SnCl has been investigated by steady-state and time-resolved chemically induced nuclear polarization (CIDNP) ¹H spectroscopy of the methylene protons using 308 nm pulses of an excimer laser and a 250 MHz NMR spectrometer. From the sign of the polarization it is concluded that (PhCH₂)₃SnMe reacts like other comparable tin compounds via triplet radical pairs; the CIDNP effects are not influenced by the solvent. In contrast the CIDNP effects of (PhCH₂)₃SnCl are solvent dependent: in C₆D₆ the polarization is accounted for by a singlet radical pair precursor, while in CDCl₃ both singlet and triplet radical pairs are formed. The formation of singlet radical pairs during the reaction of (PhCH₂)₃SnCl in C₆D₆ may be interpreted as evidence for stannylene formation.     

The effect of 1,3-diphenylisobenzofuran on the photo-oxidative degradation of cis-1,4-polybutadiene

1,3-Diphenylisobenzofuran (DPBF) is easily photo-oxidized by two mechanisms viz free radical oxidation and singlet oxygen oxidation. The final products of DPBF oxidation by these two mechanisms are the same. Using light in the range 280–480 nm, DPBF is an effective sensitizer of photooxidative degradation of polybutadiene in the solid and in solution. In a system with methylene blue (MB) in methanol-benzene solution (0.5:9.5) where free radicals from MB and ¹O₂ are formed during irradiation with visible light, DPBF is oxidized by both ¹O₂ and free radical mechanisms. DPBF cannot stop free radical degradation of PB initiated by MB radicals in MB-methanol-benzene solution. These results show that the DPBF is an ineffective stabilizer for polydienes against ¹O₂ and free radical oxidation. It rather acts as a sensitizer for photo-oxidation of polydienes.