The 355 nm time-of-flight negative ion photoelectron spectra of (o-, m-, and p-chlorophenyl)nitrene radical anions are reported. Electron affinities are obtained from the photoelectron spectra, and are 1.79 +/- 0.05, 1.82 +/- 0.05, and 1.72 +/- 0.05 eV for the (o-, m-, and p-chlorophenyl)nitrenes, respectively. Singlet-triplet splittings are determined to be 14 +/- 2, 15 +/- 1, and 14 +/- 2 kcal/mol, respectively. The shapes of the photoelectron bands indicate resonance interactions in the singlet states for the ortho- and para-substituted isomers, which is attributed to quinoidal structures of the open-shell singlet states. Reanalysis of the photoelectron spectrum of phenylnitrene anion leads to a revised experimental singlet-triplet splitting of 14.8 kcal/mol in the unsubstituted phenylnitrene.

Tetramethylpiperidinyloxy (TEMPO, TO*) reacts with ketenes RR(1)C=C=O generated by either Wolff rearrangement or by dehydrochlorination of acyl chlorides to give products resulting from addition of one TEMPO radical to the carbonyl carbon and a second to the resulting radical. Reactions of phenylvinylketenes 4b and 4f, phenylalkynylketene 4c, and the dienylketene AcOCMe=CHCH=CHCMe=C=O (11) occur with allylic or propargylic rearrangement. Even quite reactive ketenes were generated as rather long-lived species by photochemical Wolff rearrangement in isooctane solution, characterized by IR and UV, and used for kinetic studies. The rate constants of TEMPO addition to eight different ketenes have been measured and give a qualitative correlation of log k(2)(TEMPO) = 1.10 log k(H(2)O) -3.79 with the rate constants for hydration of the same ketenes. Calculations at the B3LYP/6-311G//B3LYP/6-311G level are used to elucidate the ring opening of substituted cyclobutenones leading to vinylketenes and of 2,4-cyclohexadienone (17) forming 1,3,5-hexatrien-1-one (18).

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