McClelland, Ahmad 1 Oxocarbonium Ion Hydrolysis in Sulfuric Acid Solutions 703 1Kinetic and spectral investigations were carried out using a Unicam SP 1800 spectrometer with cell block thermostated at 25.0 f 0.1 OC or a Durrum-Gibson stopped-flow spectrophotometer, also operating at 25.0 f 0.1 "C. The dead time of the latter was determined using a published procedure.26 First-order rate constants were obtained from plots of In ( A , -A m ) or In ( A , -A , ) vs. time.Acknowledgment. Financial support from the National Research Council of Canada is gratefully acknowledged.

No abstract

The quenching by a large number of carboxylic acids of the luminescence of excited uranyl ion has been examined. Acetic and propionic acids display weak quenching as measured by the Stern-Volmer constant KBV, but branching or lengthening of R in RC02H increases K, , to figures of up to 1 5 dm3 mol-l. Cycloalkanecarboxylic acids show considerably higher Ksv. The introduction of halogen atoms introduces diverse effects : CF,CO,H and CC13C0,H increase the luminescence intensity very strongly, CICH,CO,H and BrCH,CO,H weaken it very slightly, ICH,CO,H powerfully quenches it (KNV ca. 2 000 dm3 mol-l). Introduction of CI or Br a t the 2-or 3-positions of propionic acid exerts moderate to strong quenching which is ascribed to a physical, rather than a chemical mechanism, a result also indicated by studies of quenching by various alkyl halides. The introduction of alkoxy-groups increases K,, by a factor of 200, but this has been found to be due to a remsrkably high sensitivity of ether groups to (U02,+) * as further evinced by a high quantum yield for UIV production. The introduction of C=C bonds in strategic positions also enormously increases Ksv (up to 1 600 dm3 mol-I) but this w a s also found to be due to the C=C bond itself, and a number of alkenes were found to exhibit quenching rates approaching diffusion control, presumably via an exciplex mechanism. Substituted benzoic acids quench (UO,,+) most effectively (Ksv up to 6 000 dm3 mol-l) and the rates follow a good Hammett correlation, ( p -0.88 f 0.04). In some instances, the quenching data have been augmented by quantum yield measurements, and absolute constants for the reaction between (UOZ2+) * and substrate have been obtained directly by monitoring the absorption of (UO,") at 590 nm using ns flash p hotol ysis.THE several detailed studies of the quenching of the well known, strong emission from aqueous uranyl ion (U022+) by a variety of aliphatic alcohols due to Matsushima and his colleagues 1-4 has enabled a comprehensive picture to be built up of the primary interaction between (UOZ2+)* and substrate in the photochemical reaction. The effects of structural variation and isotopic substitution indicate the quenching to be chemical in character, involving the abstraction of an a-hydrogen atom from a :CHOH or -CH,OH group to give a transient alcohol radical together with the intermediate aquouranium(v) specie^.^This mechanism has been supported by solution6 and matrix e.s.r. experiments on the U0,2+-alcohol system in addition to spin-trapping experiments a and laser flash photolysis studies of the absorption of (UOZ2+)* in this s y ~t e m . ~ In view of the several previous studies of the UVI photo-oxidation of carboxylic acids.1° including product analysis,ll and e.s.r. ~o r k , ~. ~ which present quite different features from those of the alcohols, we have made a systematic examination of the quenching of the luminescence of (UOz2+)* by carboxylic acids. Additional factors of interest were (i) the nature of the firimary radical produced from RCH,CO,H, which mig...

References and Notes(1) Portions of this work were presented at the 9th International Symposium on the Chemistry of Natural Products, Ottawa, June 1974. (2) (a) J. Romo and P.

No abstract