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The cage effect in the photolysis of (S)-(+)-.alpha.-methyldeoxybenzoin: can triplet radical pairs undergo geminate recombination in nonviscous homogeneous solution?
| Content Provider | Semantic Scholar |
|---|---|
| Author | Step, E. N. Buchachenko, Anatolii L. Turro, Nicholas J. |
| Copyright Year | 1992 |
| Abstract | The photolysis of (S)-(+)-a-methyldeoxybenzoin ((S)-(+)-MDB) in benzene at room temperature causes a small decrease in enantiomeric purity of the starting ketone recovered after photolysis. The photoracemization of (S)-(+)-MDB, measured by circular dichroism (CD), can be related to the geminate recombination probability, PI, of benzoyl/sec-phenethyl triplet radical pairs (RP) formed during photolysis. The probability, PI, is determined from chemical yields of the products only; is is not a quantum yield and does not require the measurement of photon intensity. The experimental value of PI is found to be ca. 0.04. The racemization of (S)-(+)-MDB remaining in a partially converted sample can be completely suppressed by the addition of relatively low concentrations of radical scavengers such as dodecanethiol and a stable nitroxide radical (TEMPO). Since, at low concentrations employed, the scavengers effectively eliminate only free radicals, the absence of products of recombination of triplet bemyl/sec-phenethyl RP under conditions of complete scavenging is consistent with the absence of geminate cage recombination of this RP. The observed value of ca. 0.04 for PI in the absence of scavenger results from the combination of radicals which have made diffusive excursions out of the primary solvent cage, i.e., random radical pairs. This conclusion is supported by CIDNP data, since the 'H polarization of starting ketone and the geminate recombination product, along with polarizations of benzaldehyde and styrene and the products of the reaction of disproportionation of benzoyl/sec-phenethyl RP, are all completely eliminated by the addition of dodecanethiol. |
| Starting Page | 7018 |
| Ending Page | 7024 |
| Page Count | 7 |
| File Format | PDF HTM / HTML |
| DOI | 10.1021/jo00052a009 |
| Volume Number | 57 |
| Alternate Webpage(s) | http://turroserver.chem.columbia.edu/PDF_db/publications_501_550/526.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
