Quantum Yield Of Radicals Research Articles

ChemInform Abstract: A QUANTITATIVE STUDY OF ALKYL RADICAL REACTIONS BY KINETIC SPECTROSCOPY. IV. THE FLASH PHOTOLYSIS OF AZOPROPANES

The flash photolysis of azo‒n‒propane and of azoisopropane has been studied by kinetic spectroscopy. Transient absorption spectra in theregion of 220–260 nm have been assigned to the n-propyl and isopropyl radicals. For the n-propyl radical, ϵmax = 744 ± 39 l/mol cm at 245 nm and the rate constants for the mutual reactions were measured to be kc = (1.0 ± 0.1) × 1010 l/mol sec (combination) and kd = (1.9 ± 0.2) × 109 l/mol sec (disproportionation). For the isopropyl radical, ϵmax = 1280 ± 110 l/mol cm at 238 nm, with kc = (7.7 ± 1.6) × 109 l/mol sec and kd = (5.0 ± 1.2) × 109 l/mol sec The rate constant for the dissociation of the vibrationally excited triplet state of the azopropanes into radicals was measured from the variation in the quantum yield of radicals with pressure. For azo-n-propane k = (6.6 ± 1.3) × 107 sec−1, and for azoisopropane k = (1.6 ± 0.4) × 108 sec−1. Collisional deactivation of the vibrationally excited singlet and triplet states was found to occur on every collision for n-pentane; but nitrogen and argon were inefficient with a rate constant of 1.1 × 1010 l/mol sec. Spectra observed in the region of 220–260 and 370–400 nm areattributed to the cis isomers of the parent trans-azopropanes. These are formed, as permanent products, in increasing amounts as the pressure is increased.

A quantitative study of alkyl radical reactions by kinetic spectroscopy. IV. The flash photolysis of azopropanes

AbstractThe flash photolysis of azo‒n‒propane and of azoisopropane has been studied by kinetic spectroscopy. Transient absorption spectra in theregion of 220–260 nm have been assigned to the n‐propyl and isopropyl radicals. For the n‐propyl radical, ϵmax = 744 ± 39 l/mol cm at 245 nm and the rate constants for the mutual reactions were measured to be kc = (1.0 ± 0.1) × 1010 l/mol sec (combination) and kd = (1.9 ± 0.2) × 109 l/mol sec (disproportionation). For the isopropyl radical, ϵmax = 1280 ± 110 l/mol cm at 238 nm, with kc = (7.7 ± 1.6) × 109 l/mol sec and kd = (5.0 ± 1.2) × 109 l/mol secThe rate constant for the dissociation of the vibrationally excited triplet state of the azopropanes into radicals was measured from the variation in the quantum yield of radicals with pressure. For azo‐n‐propane k = (6.6 ± 1.3) × 107 sec−1, and for azoisopropane k = (1.6 ± 0.4) × 108 sec−1. Collisional deactivation of the vibrationally excited singlet and triplet states was found to occur on every collision for n‐pentane; but nitrogen and argon were inefficient with a rate constant of 1.1 × 1010 l/mol sec.Spectra observed in the region of 220–260 and 370–400 nm areattributed to the cis isomers of the parent trans‐azopropanes. These are formed, as permanent products, in increasing amounts as the pressure is increased.

Free radical polymerization in the presence of non‐solvents, 1. Styrene

AbstractDuring styrene (STY) polymerization, initiated by radicals formed by thermal or photochemical decomposition of 2,2′‐azoisobutyronitrile (AIBN) the overall polymerization rate constant K defined by relation K = Rp/([AIBN]0,5 [STY] η) and the ratio kp/(2kt0) increase with decreasing styrene concentration by hexane or benzene (Rp is the polymerization rate and ηMIX the viscosity of the reaction system). In the thermally initiated polymerization K = kp (2f kd/(2kt0))0,5 and in the photochemically initiated polymerization K = kp (2,303 ϕ I0ϵ d/(2kt0))0,5 where kd, kp, and kt0 are respectively, the rate constants of AIBN decomposition, of propagation, and of termination (for a system of the viscosity 1 mPa·s) reactions, ϕ is the quantum yield of radicals entering into reaction with the monomer, I0 the intensity of the incident light, ϵ the molar absorption coefficient of AIBN, and d the path length of the light. The increase of K and of kp/(2kt0) with decreasing monomer concentration is more marked for the system styrene/hexane than for styrene/benzene and this increase is greater at 30°C than at 60°C. For Θ‐systems formed by binary mixtures like styrene/hexane, styrene/decane and styrene/C1 – C4 alcohols the values of kp and kt0 at 30°C range between 57 and 91 dm3·mol−1·s−1 and (0,9 to 2,2)·107 dm3·mPa·mol−1, i.e. they are in principle identical with the tabulated values of these rate constants for styrene bulk polymerization.

Primary quantum yields of ketyl radicals in photoreduction by amines. Abstraction of hydrogen from nitrogen

Results of laser flash photolysis studies of the primary reaction of benzophenone triplet with aliphatic amines in benzene solution are reported. Quantum yield of formation of benzophenone ketyl radical was 0.9 - 1.0. Quantum yields for reduction of ketone also were determined for various amines, and the effects of tert-butyl alcohol on radical formation was investigated. Data indicated that H is not abstracted from -CH/sub 3/ but is abstracted efficiently from -NH/sub 2/. The very high quantum yields observed with tertiary and secondary amines were thought to imply exciplex formation, but lower quantum yields with primary amines were conditionally attributed to higher ionization potentials. (BLM)

The photolysis of t-butyl, isopropyl and ethyl nitrites at 254 nm

The main primary photochemical process for alkyl nitrites is fission to nitric oxide and alkoxy radicals, a proportion of which may be sufficiently excited to decompose spontaneously. The photolysis of t-butyl, of isopropyl and of ethyl nitrite at 254 nm, in the gas phase, each yields nitrosomethane and an aldehyde or ketone. The overall reaction, in presence of nitric oxide, corresponds to: ▪ where R′ and R″ are either H or CH 3. The dependence of the quantum yield of nitrosomethane and of other products on the pressure of alkyl nitrite, of nitric oxide and of other added gases was investigated to seek confirmation of the excited alkoxy mechanism, to determine primary yield and to obtain information about the deactivation of excited radicals. The results for t-butyl nitrite fully confirm the excited alkoxy mechanism. A value of 0.87 is derived for the quantum yield of t-butoxy radicals sufficiently excited to undergo decomposition, in excellent agreement with a previous estimate. The results for ethyl nitrite also support the excited alkoxy mechanism, with the quantum yield of excited ethoxy radicals estimated to be 0.45. The results for isopropyl nitrite differ in several respects from those for the other two systems. A contribution from a primary process involving an intramolecular rearrangement to nitrosomethane and acetaldehyde cannot be excluded, although an alternative explanation in terms of an excited alkoxy mechanism is suggested.

ChemInform Abstract: PHOTOLYSIS OF DI‐TERT.‐BUTYL PEROXIDE IN SOLUTION

Di-tert-butyl peroxide has been photolyzed at 3130 A in carbon tetrachloride solution in the presence of cyclohexane. The quantum yield of free tert-butoxy radicals was 1.9 ± 0.1, independent of the hydrocarbon concentration. Unlike results obtained in the gas phase at 2537 A, no evidence for the quenching of the excited peroxide molecule could be detected.

Photolysis of Di-tert-butyl Peroxide in Solution

Di-tert-butyl peroxide has been photolyzed at 3130 Å in carbon tetrachloride solution in the presence of cyclohexane. The quantum yield of free tert-butoxy radicals was 1.9 ± 0.1, independent of the hydrocarbon concentration. Unlike results obtained in the gas phase at 2537 Å, no evidence for the quenching of the excited peroxide molecule could be detected.

FREE RADICALS PRODUCED IN A RIGID SOLUTION OF TRYPTOPHAN BY A BIPHOTONIC PROCESS

Abstract— Free radicals were trapped by irradiation of tryptophan in an ethylene glycol/water glass with 250 nm U.V. light at 77°K. The ESR‐spectrum was attributed to approximately equal amounts of trapped electrons and ethyl radicals. The electrons could be transformed into the ethyl type radicals by heating or bleaching with near u.v. or visible light. The quantum yield of trapped radicals increased with increasing intensity of the irradiation by more than one order of magnitude over the range of intensities studied. A kinetic model was formulated on the assumption that the radicals were produced by the successive absorption of two light quanta, the absorption of the first giving rise to a transient intermediate which subsequently absorbs a second quantum to produce the radicals. The experiments confirmed the applicability of this model to the present case. The decay time obtained for the intermediate was 6.8 sec which equals the phosphorescence decay time of tryptophan under similar conditions, indicating that the intermediate involved was the lowest triplet level of this molecule. The absorption cross section of this intermediate at 250 nm with regard to the formation of trapped radicals was found to be 8000 l./mol cm.