Time-resolved laser flash spectroscopy with transient detection by time-resolved UV−vis, IR, and EPR has been employed to investigate the photochemistry and photophysics of a series of derivatives of 2-hydroxy-2-methyl-1-phenyl propanone, a model photoinitiator for free radical polymerization. H, Cl, and F substituents at the para position promote the n,π* nature of the lowest triplet state and favor fast and efficient α-cleavage from T1 upon irradiation. In contrast, dimethylamino and thioether substitution in the para position of the benzoyl moiety change the configuration of the lowest triplet states into a π,π* and are characterized by a lack of α-cleavage from T1. Alkoxy substitution in the para position represents an intermediate for which α-cleavage occurs efficiently, but at a relatively slow rate. Alkylation of the 2-hydroxy group promotes fast α-cleavage upon irradiation (picosecond time scale), in contrast to the analogous ester derivative for which slow cleavage was observed (microsecond time scale). The conclusions are consistent with phosphorescence characterization of the triplet states and are supported by photopolymerization studies.

Dedicated to Prof. A. Jerry Kresge on the occasion of his 75th birthdayThe equilibrium between 10H-anthr-9-one and 9-anthrol favors the ketone, which ionizes as a carbon acid in aqueous base. Rates of equilibration were measured over the pH range 1 ± 13 in aqueous solution (258, ionic strength I 0. m). Five independent thermodynamic and kinetic parameters were determined by analysis of the pH-rate profile: the equilibrium constant of enolization, pK E 2.17, the ionization quotient of anthrol, pQ E a 7.84, and the rate constants of enolization catalyzed by acid, k E H 2.2 ¥ 10 À4 m À1 s À1 , base, k E HO À 51.0 m À1 s À1 , and water, k E 0 1.21 ¥ 10 À5 s À1 . Structure-reactivity relationships strongly support the view that pH-independent enolization of anthrone in water proceeds by rate-determining ionization of the C-acid.Introduction. ± KetoÀenol tautomerism is a cornerstone of organic reactivity. Ninety years ago, Meyer [1] achieved the separation of solid 9-anthrol 2 ) (E) from its more stable tautomer 10H-anthr-9-one 3 ) (K). He also determined the equilibrium constant of enolization in EtOH solution,. Subsequent studies have dealt with the solvent [3 ± 9] and temperature [10] dependence of the anthroneanthrol equilibrium. Apolar solvents favor the ketone, hydrogen-bond acceptor solvents such as DMSO favor the enol. Kinetic data were reported for the enolization of anthrone induced by base catalysts in acetonitrile solution [6] and in apolar solvents [5]. The photophysical properties and photochemical reactions of anthrone were also investigated [11 ± 18].Over the last two decades, accurate equilibrium constants of enolization covering thirty orders of magnitude have been determined kinetically, i.e., as ratios of enolization and ketonization rate constants in aqueous solution [19]. Few data are available for compounds with K E close to unity [20,21]. Such data are of interest for free-energy relationships and to determine the intrinsic barrier to proton transfer involving C-atoms. To fill the gap between ketones (pK E % 8) and carboxylic acids or esters (pK E % 18), which strongly favor the keto form, and phenol at the other extreme {pK E (cyclohexa-2,4-dienone) À 12.7 [22]}, we have determined the kinetic and thermodynamic parameters of the anthrone-anthrol equilibrium in H 2 O.

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A real-time analysis of the flash-vacuum pyrolysis products of 1-acetylbenzotriazole (1) and I-benzoylbenzotriazole (2) was performed by tandem mass spectrometry. In the temperature range 50(MOO0C, these compounds lose nitrogen, yielding N-acetyl-and N-benzoylcyclopenta-2,4-dienylidenemethaneimines (10 and 17, respectively). At higher pyrolysis temperatures, 1 gives 2-methylbenzoxazole, cyanocyclopentadiene, methylcyanocyclopentadiene(s), benzonitrile and ketene, which were identified by collision-activated dissociation mass spectrometry. Lowtemperature infrared experiments confirmed the pyrolytic transformation l(2) -+ lO(17) at mediated temperatures.

Pyrolysis of benzotriazoles. Relationships between 1-and 2-vinylbenzotriazoles, and -azidostyrenes, N-phenylketenimine and indole. Pitfalls in the use of pyrolysis-mass spectrometry in mechanistic studies