We present a detailed CASSCF study of the mechanism of excited-state intramolecular proton transfer (ESIPT) in the o-hydroxyphenyl triazine class of photostabilizers. The valence-bond analysis of the ground state and the two pipi* excited states permits a simple chemical interpretation of the mechanistic information. Our results show that the barrier to enol-keto tautomerism on the ground-state...

Physical and coding properties of poly (5-aminouridylic acid) and of 5-aminouridine-containing trinucleotides. Crystal structure and conformation of 3-deaza-4-deoxyuridine. 5-Substituents in the uridine moiety and their effect on the conformation of ApU type dinucleoside phosphates. Restriction or amplification of wobble recognition: The structure of 2-thio-5-methylaminomethyluridine and the in...

In a combined study on the photophysics of 2-(2'-hydroxyphenyl)-benzothiazole (HBT) in polar acetonitrile utilizing ultrafast infrared spectroscopy and quantum chemical calculations, we show that a branching of reaction pathways occurs on femtosecond time scales. Apart from the excited-state intramolecular hydrogen transfer (ESIHT) converting electronically excited enol tautomer into the keto t...

The role of NMR in the study of compounds of biological importance is widely recognized and appreciated. There can be few categories of such substances, however, to which NMR has made such substantive contributions as it has to the understanding of the chlorophylls and the corrins. The chlorophylls are indispensible agents in the conversion of the energy of light to chemical oxidizing and reduc...

The title compound, C(16)H(13)FN(2)OS, has undergone enol-to-keto tautomerism during the crystallization process. The 1H-pyrazole-5-one ring [maximum deviation = 0.0198 (11) Å] is inclined at angles of 33.10 (5) and 79.57 (5)° with respect to the fluoro-phenyl [maximum deviation = 0.0090 (12) Å] and phenyl-thiol [maximum deviation = 0.0229 (3) Å] rings attached to it. In the crystal, neighbouri...

From quantum mechanics calculations we confirm that the naturally occurring enol lancifodilactone G is stable over the keto form (by 2.6 kcal/mol in water), the only known stable aliphatic enol (devoid of conjugated or bulky aromatics and lacking a 1,3-diketone structural motif known to stabilize enols). We determine architectural elements responsible for the enol stabilization and find a mecha...

The enol of a-cyano-a-phenylacetic acid was observed as a short-lived transient species formed upon flash photolytic generation of phenylcyanoketene by a photo-Wolff reaction of a-cyano-a-diazoacetophenone followed by hydration of the ketene. Rates of ketonization of the enol were measured in dilute hydrochloric acid solutions and acetic acid, biphosphate ion, tris(hydroxymethyl)methylamine, an...

this paper is a density functional theory (dft) calculation of intramolecular proton transfer (ipt) in 6-hydroxypicolinic acid (6hpa, 6-hydroxypyridine-2-carboxylic acid) tautomeric forms. the transition state for the enol-to-keto transition is reported in the gas phase and in four different solvents. the planar and non-planar dimer forms of 6hpa keto and enol, respectively, were also studied i...