Salvatore S. Emmi

Addition and elimination kinetics in OH radical induced oxidation of phenol and cresols in acidic and alkaline solutions

Abstract The rates of the two consecutive reactions, OH radical addition and H 2 O/OH − elimination, were studied by pulse radiolysis in highly acidic (pH=1.3–1.9) and alkaline (pH≈11) solutions, respectively, for phenol and for the three cresol isomers. The rate coefficient of the addition as measured by the build-up of phenoxyl radical absorbance and by a competitive method is the same (1.4±0.1)×10 10 mol −1 dm 3 s −1 both in acidic and alkaline solution. The rate coefficient of the H 2 O elimination in acidic solution is (1.6±0.2)×10 6 s −1 , whereas the coefficient of the OH − elimination in alkaline solutions is 6–8 times higher. The kinetics of the phenoxyl radical formation was described by the two-exponential equation of the consecutive reactions: the first exponential is related to the pseudo-first-order addition, while the second to the elimination reaction. No considerable structure dependence was found in the rate coefficients, indicating that the methyl substitutent in these highly acidic or alkaline solutions influences neither the addition nor the elimination rate.

The generation and spectral characterization of oligothiophenes radical cations. A pulse radiolysis investigation

Abstract Conjugated polymers and oligomers of thiophene have been employed in a number of electronic devices due to the change in their conductivity by several orders of magnitude when oxidized to their cationic states. The radical cations of oligothiophenes (nT), with number of rings n =1–6, have been produced by pulse radiolysis for the first time and their spectra have been characterized. Electronic structure and transition energies are briefly discussed in the light of semiempirical theoretical calculations.

The spectral characterization of thiophene radical cation generated by pulse radiolysis

Pulse radiolysis matched with kinetic spectrometry has been employed to produce and characterize the radical cations of the series of unsubstituted oligothiophenes from one to six rings in dichloromethane dilute solutions. The concentration of radicals has been tuned to low values so as to slow down dimerization processes. The spectra obtained for the radical cations with 2 to 6 rings agree with previous literature reports. The novel isolation of the radical cation of thiophene itself and its spectral characterization, both experimental and theoretical, stresses its close relationship to the electronic structure of oligothiophenes. This fact strongly recommends that the charge carrier properties of oligothiophenes be interpreted on the basis of the molecular orbital theory rather than of the polaron model. The state responsible for the main UV absorption band is found to be described by a mixture of configurations where the HOMO→LUMO transition is present with different spin couplings.

Mechanism of OH radical-induced oxidation of p-cresol to p-methylphenoxyl radical

The OH radical-induced oxidation of p-cresol to p-methylphenoxyl radical was studied in aqueous solution in a wide pH range by means of pulse radiolysis combined with optical spectroscopy. OH-adduct cyclohexadienyl type radicals were identified as intermediates of the reaction. In the acidic pH range the first-order rate coefficient of phenoxyl radical formation was found linearly dependent on the H3O+ concentration yielding a bimolecular rate coefficient of 1.8 × 108 mol–1 dm3 s–1. In the alkaline range a linear dependence was found on the OH– concentration with rate coefficient of 4.9 × 1010 mol–1 dm3 s–1. These findings were interpreted in terms of acid-base catalysis of the H2O elimination from the OH-adduct. With the time resolution applied, 30 ns, the radical cation p-CH3C6H4OH+. was not observed as intermediate.

Electrochemical polymerization of thionaphthene−indole

The polymer deriving from thionaphthene-indole has been prepared by electrochemical oxidation of the monomer in methylene chloride on platinum electrode. The oxidized polymer so obtained has been reduced on the same electrode to a neutral polymer, which is processable and soluble in several solvents. Both oxidized and neutral polymers have been characterized by UV, IR and mass spectrometry. The electrical conductivity of the oxidized material has been found to be nine orders of magnitude greater than that of the reduced product.

Electrochemical polymerization of N-methyl-10,10-dimethylphenazasiline

N-methyl-10,10-dimethylphenazasiline (SIN) was polymerized by electrochemical oxidation at +1.3 V in methylene chloride. Acid-base equilibria in solution between the heterocyclic nitrogen and the protons produced by the coupling reaction were detected. The oxidized polymer deposits on platinum or ITO electrodes to give strongly adherent and electrochromic coatings.

Protonation kinetics of acrylate radical anions

The radical anions of acrylamide, methacrylamide and 2-hydroxypropyl acrylate were produced by pulse radiolysis of moderately acidic aqueous solutions well below the pKa of the radical anion. The rate coefficients of protonation were determined by UV kinetic spectroscopy at wavelengths where the protonated and ionic forms have considerably different absorbances. The rate coefficients of protonation were measured as (6.0±0.9)×1010, (6.6±1.0)×1010 and (5.0±1.0)×1010 mol−1 dm3 s−1. Using these rate coefficients and the pKa values of the radical anion, 7.9, 8.0 and 5.5, the rate coefficients of deprotonation were determined as 7.5×102, 6.6×102 and 1.6×105 s−1. The results clearly showed that in acidic solution the protonation occurred with H3O+ and not with H2O. The rate coefficients determined were discussed and compared with relevant data in the literature.

Study of polymerization kinetics of acrylic and methacrylic acid esters at low conversions

The kinetics of high-energy radiation initiated polymerization of acrylate and methacrylate monomers were studied in dilute cyclohexane solutions. The absorption spectra of the intermediates were obtained by pulse radiolysis with optical detection. In solutions of acrylates at longer times after the pulse the presence of oligomer radicals was observed. In solutions of methacrylates the beginning of the oligomerization reaction was detected at higher monomer concentrations. From the kinetic curves the rate coefficients of termination were calculated.

Transient species in the pulse radiolysis of methylene chloride and the self-reaction of chloromethyl radicals

Abstract Chlorine atoms formed during the pulse radiolysis of deaerated methylene chloride at room temperature react with the solvent in the first 70 ns from the pulse at a bimolecular rate constant k4 ≈ 6 × 106 M-1s-1 and are available to otther reactions only at solute concentrations higher than 10-3M. A u.v.-vis. spectrum is detected, the main features of which are a peak at 350 nm, a broad absorption in the vis. and a remarkable band in the u.v. The “350” species undertakes a fast first order decay (k = 9.0 × 107s-1) which is followed by a slower decay (k = 5.3 × 104s-1). The “u.v.” species is a mixing of mono-and dichloromethyl radicals. These radicals recombine and cross-combine as if they were a single species; a rate constant 2 k9 = 2 k10 less than 2.4 × 109M-1s-1 for the combination reactions can be evaluated from the observed decay rate. Configurational factors are considered in connection with the reactivity of chlorosubstituted methyl radicals.

Pulse radiolysis study on polymerization kinetics in organic solvent: 2-ethylhexyl acrylate

Abstract Pulse radiolysis with kinetic spectroscopic detection was applied to study the first steps of the polymerization of 2-ethylhexyl acrylate (EHA) in cyclohexane solution. The absorption spectra and the kinetic characteristics of the intermediates were investigated in solutions of different monomer concentrations. Immediately after the pulse the maximum of the spectrum was at ∼250 nm that is characteristic of the cyclohexyl radical. This maximum was gradually shifted in time to above 300 nm indicating the appearance of carboxyalkyl radicals. For higher concentration solutions a further shift was observed that can be attributed to the absorbance of the oligomer radicals formed. From the kinetic curves the rate coefficients of initiation and termination were calculated and the rate coefficient of propagation was estimated.