P. Das, M. Encinas, J. Scaiano
Jul 15, 1981
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Journal
Journal of the American Chemical Society
Abstract
The quenching of aromatic carbonyl triplets by phenols is a very fast process for both n,..pi..* and ..pi..,..pi..* states. Representative rate constants for benzophenone triplets in benzene are 1.3 x 10/sup 9/ (phenol) and 8.1 x 10/sup 8/ (m-fluorophenol) M/sup -1/ s/sup -1/, while for p-methoxypropiophenone the values are 4.9 x 10/sup 9/ and 5.0 x 10/sup 9/ M/sup -1/ s/sup -1/ for the same pair of substrates; 18 different phenols were examined. The rate constants are smaller in wet acetonitrile as a result of hydrogen bonding; for example for the benzophenone-phenol system we measured 8.0 x 10/sup 7/ M/sup -1/ s/sup -1/. Isotope effects are larger for p-methoxypropiophenone than for benzophenone: for example in the case of phenol the values of k/sub H//k/sub D/ are 3.9 and 1.2, respectively (in wet acetonitrile). The quenching reaction leads to the efficient, but not quantitative, formation of phenoxy and ketyl radicals, with the only exception of p-nitrophenl where the process involves electronic energy transfer. The temperature dependence of a few representative systems, as well as the quenching by methyl ethers of similar structure, has also been examined. The photochemistry of p-hydroxypropiophenone is characterized by efficient self-quenching involving a head-to-tail hydrogen-transfer reaction.