Limiting Quantum Yield Research Articles

Infrared induced reactions in methanol-bromine mixtures: A negative result

Suggestions have been made that infrared induced reactions in CH 3OH-Br 2 mixtures can lead to isotope separation. A careful quantitative study has been made of limiting quantum yields of the infrared induced reactions in such mixtures and they are found to be very low, φ ⩽ 10 −3, both with radiation from a laser and from a black body source.

Photochemical studies of unimolecular processes V. The photolysis and quenching of cis- and trans hexa-1, 3, 5-triene and of cyclohexa-1, 3-diene

The photolyses of cis- and trans -hexa-1, 3, 5-triene at wavelengths of 228.8 and 253.7 nm and of cyclohexa-1, 3-diene at 265.4 and 280.4 nm have been studied in the gas phase at various pressures and in the presence of added gases. At low pressures, the limiting quantum yield for hexatriene photolysis is unity within experimental error, and it is shown that the pressure dependence of all the observed interconversions of C 6 H 8 isomers and of benzene formation can be explained by reactions of vibrationally excited molecules produced by very rapid internal conversion.

Reaction of Oxygen Atoms, O(3P), with Ethylene in Liquid Nitrogen Solution

The reaction of ground-state oxygen atoms, O(3P), with ethylene and ethylene-d4 in liquid nitrogen solution at 77° K has been studied. The major and perhaps the exclusive products are ethylene oxide and acetaldehyde. The ratio of acetaldehyde to ethylene oxide is 1.2 ± 0.1 for ethylene and 0.91 ± 0.09 for ethylene-d4. Much smaller amounts of formaldehyde and trace quantities of cyclopropane are also observed.The effect of the concentration of ethylene on the quantum yields of addition products was measured. A limiting quantum yield of oxygen atoms scavenged by ethylene to form the addition products was 0.12 + 0.01. The low quantum yield suggests an appreciable cage recombination of the ground-state oxygen atoms with the trace amounts of molecular oxygen present. The effect of the concentration of added oxygen on the product yields and the relative rate constant for the addition of O(3P) to molecular oxygen and to ethylene were determined. The ratio [Formula: see text] is 6.0 ± 1.0 and the relative rate constant for the addition of O(3P) to ethylene and to ethylene-d4, [Formula: see text], is 2.0 ± 0.1.The type of products formed and the isotope effects observed are discussed in terms of the mechanism of addition of O(3P) atoms to ethylene.

Excited State Chemistry of the Ferrocyanide Ion in Aqueous Solution. I. Formation of the Hydrated Electron

Excitation of the Fe(CN)64− ion in aqueous solution into the 1T1u or 1T2g excited singlet state, below 313 nm, leads to hydrated electron formation in competition with internal conversion to the lowest excited singlet 1T1g state. The limiting quantum yield of hydrated electron formation is a linear function of quantum energy between 313 and 228 nm, reaching φe≃ 0.9 at 228 and 214 nm. Above 313 nm hydrated electron formation is not observed, but photoaquation is. Dependence of Φe on scavenger (N2O) concentration is observed. The involvement of CTTS character in the process, and the role of rapid solvent rearrangement in the dissociation of the excited state, are discussed.

Photoreduction of the nπ* singlet of 9,10-diazaphenanthrene by triethylamine

The photoreduction of 9,10-diazaphenathrene by triethylamine in benzene has a limiting quantum yield of ca. 0·8, occurs substantially from the n,π* singlet state, and gives 9,10-dihydro-9,10-diazaphenanthrene.

Effect of Medium on Transients and on Observed Quantum Yields in Photoreduction of Benzophenone by 2‐Propanol

AbstractThe quantum yield for photoreduction of 0.1 M benzophenone by 2‐propanol, at 313 mμ, light intensity 1 × 1016 quanta/sec cm2 is −1.16. The quantum yield increases on dilution with benzene to 1.70 at 3.13 M 2‐propanol. A Stern‐Volmer plot of data below 3 M 2‐propanol leads to an extrapolated limiting quantum yield of ˜1.9. The quantum yield in 2‐propanol decreases with extent of conversion from 1.1 at 17% to 0.74 at 37% photoreduction. Quantum yields for photoreduction of 0.1 M benzophenone by 1 M 2‐propanol and by 1 M benzhydrol in benzene remain constant with conversion. A light absorbing transient builds up strongly and progressively during photoreduction in 2‐propanol, and to a much lesser extent in 1 M 2‐propanol and 1 M benzhydrol in benzene. Photoreduction in 1 M 2‐propanol in t‐butyl alcohol appears to behave like that in 2‐propanol; 1 M 2‐propanol in acetonitrile shows behavior intermediate between that in alcoholic and in hydrocarbon solutions. Efficiency of quenching by naphthalene in alcoholic and in hydrocarbon media indicates that kr is about twice as great in the hydrocarbon as in the alcohol solvents. The effect of the hydrocarbon in diminishing build up of the light absorbing intermediate, which may mask and possibly quench, is largely responsible for the increase in quantum yield on dilution with hydrocarbon.

Photolysis of Liquid Water at 1470 Å

The photolysis of water and of dilute aqueous solutions of O2, methanol, ethanol, and isopropanol was studied at 1470 Å. The results are consistent with the reaction scheme[Complex chemical formula]The limiting quantum yield for the formation of the geminate radical pair φ=0.7±0.1 at room temperature. Scavengers compete in an inhomogeneous kinetics with geminate recombination. The direct formation of molecular products other than by radical mechanisms is excluded. Results on D2O solutions of ethanol support the conclusions. The evidence does not favor direct interaction of scavengers with the excited water molecules H2O‡.

The Limiting Quantum Yield for the Photoreduction of Benzophenone with Isopropyl Alcohol

The Limiting Quantum Yield for the Photoreduction of Benzophenone with Isopropyl Alcohol

REACTIONS OF IODINE ATOMS WITH N-BUTENES: I. CIS–TRANS ISOMERIZATION OF BUTENE-2

In the interaction of iodine atoms with butene-2 at 65 °C no addition products are observed although cis–trans isomerization of the butene occurs readily. Iodine atoms have been generated by photolyzing iodine in three spectral regions. The limiting quantum yield of the isomerization reaction is about 0.1 at 4358 Å. The low quantum yield shows that chain reactions, similar to those observed in olefin photochlorinations and photobrominations, do not take place. A mechanism involving initial formation of π-complexes between iodine atoms and butene-2 is tentatively considered.