Determination of Bond Dissociation Enthalpies in Solution by Photoacoustic Calorimetry

NRC publication: Ye

Hole transfer equilibrium in rigidly linked bichromophoric molecules

Two bichromophoric molecules consisting of anthracene and diphenylpolyene moieties linked by two fused norbornyl bridges undergo photoionization upon ultraviolet (UV) pulsed laser irradiation. The simultaneous observation of the cation radicals of both anthracene and polyene groups points to a rapid (nanosecond or faster) intramolecular hole transfer equilibrium between the two chromophores. The existence of an equilibrium is supported by the results of one- and two-laser transient absorption and electrochemical experiments. Equilibrium constants (293 K) were determined by both transient absorption and cyclic voltammetry measurements and were independent of the method used within experimental error. For A-sp-VB, which contains anthracene and vinyldiphenylbutadiene chromophores, Keq = 4.0 ? 2 (transient absorption) and 3.2 ? 2 (electrochemical), favoring the anthracene cation radical. For A-sp-VS, containing anthracene and vinylstilbene groups, Keq = 70 ? 30 (transient absorption) and 105 ? 50 (electrochemical), favoring the anthracene cation radical.Peer reviewed: YesNRC publication: Ye

Electrode-mediated Wacker oxidation of cyclic and internal olefins

An improved method for the electrode-mediated oxidations of olefins by palladium(II) is described. Current efficiencies from 80% to 95% were obtained in oxidations of 1-decene, styrene, trans-2-octene, and cyclohexene in which perchloric acid was added to a chloride-free solution of a palladium(II) acetate catalyst. The palladium(0) was reoxidized to palladium(II) by reaction with catalytic amounts of benzoquinone, which was, in turn, regenerated by anodic oxidation. Addition of varying amounts of perchloric acid did not affect the current efficiency but accelerated the oxidation reaction, up to a concentration of approximately 0.15 M. The current efficiency remained high (>80%) over the course of the electrode-mediated oxidations of 1-decene, trans-2-octene, and cyclohexene. At the end of the reactions, when the substrate was depleted, a drastic decrease in the current was observed, indicating that the catalytic cycle leading to product was primarily responsible for the electrochemical reaction. It also was shown that the rates of the electrochemical reactions were generally slower than those of homogeneous reactions in which a stoichiometric amount of benzoquinone was used, indicating that the electrochemical regeneration of benzoquinone was mass transport limited at the highest concentrations of perchloric acid. This is in contrast to other reports in the literature that suggested that the homogeneous (non-electrochemical) reactions were actually slower. Reasons for the discrepancy between these results are discussed.On d\ue9crit une m\ue9thode am\ue9lior\ue9e d'oxydation des ol\ue9fines par le palladium(II), assist\ue9e par une \ue9lectrode. On a obtenu des efficacit\ue9s de courant allant de 80 \ue0 95% lors d'oxydations du d\ue9c-1-\ue9ne, du styr\ue9ne, du trans-oct-2-\ue9ne. et du cyclohex\ue9ne au cours desquelles on avait ajout\ue9 de l'acide perchlorique \ue0 une solution sans chlorure d'un catalyseur d'ac\ue9tate de palladium(II). Le palladium(0) est r\ue9-oxyd\ue9 en palladium(II) par une r\ue9action impliquant des quantit\ue9s catalytiques de benzoquinone qui est, \ue0 son tour, r\ue9g\ue9n\ue9r\ue9 par oxydation anodique. L'addition de quantit\ue9s variables d'acide perchlorique n'affecte pas l'efficacit\ue9 de courant; toutefois, la r\ue9action d'oxydation est acc\ue9l\ue9r\ue9e jusqu'\ue0 une concentration d'environ 0,15 M. Avec le d\ue9c-1-\ue9ne, le trans-oct-2-ene et le cyclohex\ue9ne, l'efficacit\ue9 du courant demeure \ue9lev\ue9e (>80%) pour l'ensemble des oxydations assist\ue9es par une \ue9lectrode. \ue0 la fin de r\ue9actions, lorsqu'il ne reste plus de substrat, on observe une dramatique diminution du courant qui indique que le cycle catalytique conduisant au produit est le principal responsable de la r\ue9action \ue9lectrochimique. On a montr\ue9 que la vitesse des r\ue9actions \ue9lectrochimiques est g\ue9n\ue9ralement plus lente que les r\ue9actions homog\ue9nes dans lesquelles on utilise une quantit\ue9 stoechiom\ue9trique de benzoquinone; ceci indique que, \ue0 des concentrations \ue9lev\ue9es d'acide perchlorique, la r\ue9g\ue9n\ue9ration \ue9lectrochimique de la benzoquinone est limit\ue9e par un transport de masse. Cette situation est en opposition avec d'autres rapports parus dans la litt\ue9rature qui sugg\ue9raient que les r\ue9actions homog\ue9nes (non-\ue9lectrochimiques) sont de fait plus lentes. On discute des raisons pouvant expliquer ces diff\ue9rences.NRC publication: Ye

Radical-trapping antioxidants in vitro and in vivo

The contributions from urate (58% ? 18%), plasma proteins (21% ? 10%), ascorbate (14% ? 8%) and vitamin E (7% ? 2%) to the Total Radical-trapping Antioxidant Parameter (TRAP) of plasma account for all of the peroxyl radical-trapping antioxidant activity in the majority of samples. Time dependent studies show that vitamin E is regenerated or spared during peroxyl radical attack by the water soluble antioxidants. The rates of uptake of dietary RRR- and SRR-[alpha]-tocopherols have been determined using deuterium labelled [alpha]-tocopherol acetates. There is a preferential uptake of the natural, RRR-[alpha]-tocopherol by all tissues (except the liver in the first month). These studies have potential implications for human nutrition.NRC publication: Ye

Palladium catalyzed indirect electrochemical acetoxylation of olefins

Olefins are acetoxylated to allylic and vinylic unsaturated esters by palladium (II) acetate catalyst in combination with controlled current anodic reoxidation and copper (II) acetate as an electron transfer agent.NRC publication: Ye

Selective catalytic debromination by C60 mono-, di-, and trianion

Peer reviewed: YesNRC publication: Ye

Improved method for the Wacker oxidation of cyclic and internal olefins

NRC publication: Ye

Electron paramagnetic resonance spectral parameters for organic free radicals in solution

NRC publication: Ye

New Approaches To Free Radical Thermochemistry [11

A variety of unusual techniques have been used to measure thermodynamic propeties of free radicals and heats of the reactions in which they participate. These include a radical buffer EPR technique, the appearance energy method, photoacoustic spectroscopy, and electrochemical detection of free radicals. The methods can be applied to systems involving organometallic and high molecular weight materials which are normally inaccessible to conventional, gas phase techniques.NRC publication: Ye