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Electroreduction of Dialkyl Peroxides. Activation-Driving Force Relationships and Bond Dissociation Free Energies.
No full textThe electrochemical reduction of five dialkyl peroxides in DMF was studied by cyclic voltammetry. The
electron transfer (ET) to the selected compounds is concerted with the oxygen-oxygen bond cleavage (dissociative
ET) and is independent of the electrode material. Such an electrochemical behavior provided the opportunity to
study dissociative ETs by using the mercury electrode and therefore to test the dissociative ET theory by using
heterogeneous activation-driving force relationships. The convolution voltammetry analysis coupled to the doublelayer
correction led to reasonable estimates of the standard potential (E°) for the dissociative ET to dialkyl peroxides,
as supported, whenever possible, by independent estimates. A thermochemical cycle based on the dissociative ET
concept was employed to calculate the bond dissociation free energies (BDFEs) of the five peroxides, using the
above E°s together with electrochemical or thermochemical data pertaining to the redox properties of the leaving
alkoxide ion. The BDFEs were found to be in the 25-32 kcal/mol range, suggesting a small substituent effect. The
dissociative ET E°s were also used together with the experimental quadratic free energy relationships to estimate the
heterogeneous reorganization energies
Kinetics and Temperature Dependence of the Reduction of Dialkyl Peroxides. New Insights into the Dynamics of Dissociative Electron Transfer
No full textThe concerted dissociative reduction of di-tert-butyl peroxide (DTBP), dicumyl peroxide (DCP), and di-n-butyl peroxide (DNBP) is evaluated by both heterogeneous and homogeneous electron transfer using electrochemical methods. Electrochemical and thermochemical determination of the O−O bond energies and the standard potentials of the alkoxyl radicals allow the standard potentials for dissociative reduction of the three peroxides in N,N-dimethylformamide and acetonitrile to be evaluated. These values allowed the kinetics of homogeneous ET reduction of DTBP and DCP by a variety of radical anion donors to be evaluated as a function of overall driving force. Comparison of the heterogeneous ET kinetics of DTBP and DNBP as a function of driving force for ET allowed the distance dependence on the reduction kinetics of the former to be estimated. Results indicate that the kinetics of ET to DTBP is some 0.8 order of magnitude slower in reactivity than DNBP because of a steric effect imposed by the bulky tert-butyl groups. Experimental activation parameters were measured for the homogeneous reduction of DTBP with five mediators, covering a range of 0.4 eV in driving force over the temperature range −30 to 50 °C in DMF. The temperature dependence of the kinetics leads to unusually low preexponential factors for this series. The low preexponential factor is interpreted in terms of a nonadiabatic effect resulting from weak electronic coupling between the reactant and product surfaces. Finally, the data are discussed in the context of recent advances of dissociative electron transfer reported by Savéant and by German and Kuznestov. In total the results suggest that these peroxides undergo a nonadiabatic dissociative electron transfer and represent the first reported class of compounds where this effect is reported
Azidyl radical reactivity -N -6(Center-dot) as a kinetic probe for the addition-reactions of azidyl radicals with olefins
No full textThe reactivity of the azidyl radical (N-3(.)) With olefins has been studied in acetonitrile solution using time-resolved UV-vis and infrared (TRIR) laser flash photolysis techniques. Azidyl radicals, formed by electron transfer from azide to excited state ketones, combine with excess azide in acetonitrile in an equilibrium reaction to form the pseudohalogen radical anion N-6(.-). The visible absorption of N-6(.-) (lambda(max) = 700 nm) is shown to be an ideal probe for the determination of absolute rate constants for the addition reactions of N-3(.) With a variety of alkenes (kadd) Bimolecular rate constants for the reactions of N-3(.) With a series of ring-substituted styrenes (p-CF3, m-CF3, p-Cl, H, m-CH3, p-CH3, p-CH3O) vary between 1 x 10(6) and 5 x 10(7) M(-1) s(-1). An excellent correlation of log(k(add)) With Hammett sigma(+) constants (rho(+) = -1.2, r = 0.991) is obtained illustrating the electrophilic nature of the azidyl radical. Rate constants for reaction with alpha- and beta-substituted styrenes and simple alkyl- and alkoxy-substituted olefins vary between 1 x 10(6) and 1 x 10(9) M(-1) s(-1) and correlate with the ionization potentials (a reflection of the relative HOMO energies) of the olefin. The addition reactions of N-3(.) were found to be dominated by polar effects with little contribution from steric factors. TRIR was utilized to follow the fate of azide and azidyl radicals by monitoring the bleach of N-3(-)(() over bar v(max) = 1842 cm(-1)); and the concurrent growth of beta-azidyl carbon-centered radicals at ca. 2100 cm(-1). The observation of the latter, which grow in with kinetics identical to the decay of N-6(.-), combined with the lack of evidence for competing hydrogen atom transfer in the reactions with alkyl olefins (even for 1,4-cyclohexadiene) confirms that reaction of N-3(.) with all olefins examined is by addition.PT: J; CR: NRCC37293 1991, NISTI STANDARD REFER ALFASSI ZB, 1985, J PHYS CHEM-US, V89, P3359 ALFASSI ZB, 1987, J PHYS CHEM-US, V91, P2120 ALFASSI ZB, 1990, J PHYS CHEM-US, V94, P8800 ALFASSI ZB, 1993, INT J CHEM KINET, V25, P151 ALFASSI ZB, 1993, J PHYS CHEM-US, V97, P6835 ALKAZWINI AT, 1992, J CHEM SOC PERK APR, P657 ARNOLD BR, 1992, J ORG CHEM, V57, P6469 AVILA DV, 1993, J AM CHEM SOC, V115, P1577 AVILA DV, 1994, J AM CHEM SOC, V116, P99 BERENAK I, 1989, FREE RADICALS SYNTHE, P303 BORS W, 1987, FREE RADICAL RES COM, V2, P289 BURKEY TJ, 1986, J AM CHEM SOC, V108, P2218 BUTLER J, 1982, BIOCHIM BIOPHYS ACTA, V705, P150 BUTLER J, 1984, RADIAT PHYS CHEM, V23, P265 DAVIES MJ, 1991, FREE RADICAL RES COM, V15, P111 DEFELIPPIS MR, 1990, J PHYS CHEM-US, V94, P2420 DOWS DA, 1955, J CHEM PHYS, V23, P1258 FARAGGI M, 1989, J AM CHEM SOC, V111, P5141 FLEMING I, 1976, FRONTIER ORBITALS OR GIESE B, 1983, ANGEW CHEM INT EDIT, V22, P753 GIESE B, 1988, CHEM BER, V121, P2063 GYORGY I, 1992, INT J RADIAT BIOL, V61, P603 JACKSON JE, 1988, J AM CHEM SOC, V110, P5595 JACKSON RL, 1981, J AM CHEM SOC, V103, P1802 JENKINS HDB, 1993, J PHYS CHEM-US, V97, P7876 JONSSON M, 1994, J AM CHEM SOC, V116, P1423 KAPOOR SK, 1993, J RADIOAN NUCL CH AR, V171, P443 KAZANIS S, 1991, J PHYS CHEM-US, V95, P4430 KIM S, 1994, J AM CHEM SOC, V116, P5521 LIEBER E, 1957, ANAL CHEM, V29, P916 LINVIEN D, 1991, HDB INFRARED RAMAN C MISHRA AK, 1994, J AM CHEM SOC, V116, P1414 MUROV SL, 1973, HDB PHOTOCHEMISTRY NAHOR GS, 1991, INT J CHEM KINET, V23, P941 NEAT P, 1988, J PHYS CHEM REF DATA, V17, P1027 PADMAJA S, 1992, J PHYS CHEM-US, V96, P3654 PAUL H, 1978, J AM CHEM SOC, V100, P4520 PEARSON RG, 1989, J ORG CHEM, V54, P1423 RAM MS, 1986, J PHYS CHEM-US, V90, P3691 RAYNER DM, 1986, J PHYS CHEM-US, V90, P2882 RIEMENSCHNEIDER K, 1987, J ORG CHEM, V52, P205 RUCHARDT C, 1970, ANGEW CHEM INT EDIT, V9, P830 SANTI R, 1992, J ORG CHEM, V57, P4250 SCAIANO JC, 1993, J AM CHEM SOC, V115, P8340 SHAIK SS, 1990, J AM CHEM SOC, V112, P1446 SHOUTE LCT, 1991, J PHYS CHEM-US, V95, P3238 SHOUTE LCT, 1994, J PHYS CHEM-US, V98, P5701 SINGH A, 1982, RADIAT PHYS CHEM, V19, P137 STEWART JP, 1986, QCPE, V24 TEDDER JM, 1982, ANGEW CHEM INT EDIT, V21, P401 TREININ A, 1969, J CHEM PHYS, V50, P538 TSANG W, 1986, J PHYS CHEM-US, V90, P1152 WAGNER BD, 1994, J AM CHEM SOC, V116, P6433 WONG MW, 1994, J AM CHEM SOC, V116, P6284 WORKENTIN MS, UNPUB WORKENTIN MS, 1994, J AM CHEM SOC, V116, P1141 WORKENTIN MS, 1995, J PHYS CHEM-US, V99, P94 YU D, 1992, J PHYS CHEM-US, V96, P6031 ZIPSE H, 1991, J AM CHEM SOC, V113, P4324; NR: 61; TC: 31; J9: J AMER CHEM SOC; PG: 8; GA: QC061Source type: Electronic(1
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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