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991.
Jerzy Zakrzewski Jaros?aw Hupko Krzysztof Kryczka 《Monatshefte für Chemie / Chemical Monthly》2003,24(1):843-850
The reaction of thiophosgene with 2,2,6,6-tetramethylpiperidine-1-oxyl (used as a model nitroxyl radical) was examined. 2,2,6,6-Tetramethylpiperidine
and 2,2,6,6-tetramethyl-1-hydroxypiperidine were identified as products. The reaction is not competitive with the reaction
of thiophosgene with an amino group. Thus, three nitroxides with an isothiocyanate group were synthesized from thiophosgene
and the nitroxides containing the amino substituent. 相似文献
992.
Onyido I Swierczek K Purcell J Hengge AC 《Journal of the American Chemical Society》2005,127(21):7703-7711
Earlier work on the hydrolysis of aryl phosphinothioate esters has led to contradictory mechanistic conclusions. To resolve this mechanistic ambiguity, we have measured linear free energy relationships (beta(nuc) and beta(lg)) and kinetic isotope effects for the reactions of oxyanions with aryl dimethylphosphinothioates. For the attack of nucleophiles on 4-nitrophenyl dimethylphosphinothioate, beta(nuc) = 0.47 +/- 0.05 for phenoxide nucleophiles (pK(a) < 11) and beta(nuc) = 0.08 +/- 0.01 for hydroxide and alkoxide nucleophiles (pK(a) >or= 11). Linearity of the plot in the range that straddles the pK(a) of the leaving group (4-nitrophenoxide, pK(a) 7.14) is indicative of a concerted mechanism. The much lower value of beta(nuc) for the more basic nucleophiles reveals the importance of a desolvation step prior to rate-limiting nucleophilic attack. The reactions of a series of substituted aryl dimethylphosphinothioate esters give the same value of beta(lg) with the nucleophiles HO(-) (beta= -0.54 +/- 0.03) and PhO(-) (beta = -0.52 +/- 0.09). A significantly better Hammett correlation is obtained with sigma(-) than with sigma or sigma degrees , as expected for a transition state involving rate-limiting cleavage of the P-OAr bond. The (18)O KIE at the position of bond fission ((18)k = 1.0124 +/- 0.0008) indicates the P-O bond is approximately 40% broken, and the (15)N KIE in the leaving group ((15)k = 1.0009 +/- 0.0003) reveals the nucleofuge carries about a third of a negative charge in the transition state. Thus, both the LFER and KIE data are consistent with a concerted reaction and disfavor a stepwise mechanism. 相似文献
993.
Gordon L. Hug Krzysztof Bobrowski Halina Kozubek Bronislaw Marciniak 《Photochemistry and photobiology》1998,68(6):785-796
Abstract— Oxidation of the triplet state of 4-carboxybenzophenone (CB) by a series of five substituted methionines and three methionine-containing dipeptides was monitored under laser flash photolysis conditions in aqueous solution. Spectral resolution techniques were employed to follow the concentration profiles of the intermediates formed from the quenching events. From these concentration profiles, quantum yields for the intermediates were determined. Branching ratios were evaluated for the decay of the charge-transfer complex by the competing processes of back electron transfer, proton transfer and escape of radical ions. The relative prominence of these processes was discussed in terms of the proton-transfer tendencies of the nominal sulfur-radical-cationic species. A systematic decrease was observed in the quantum yields for the escape of radical ions along with a correlated increase in the proton-transfer yields. The enhanced propensity of the sulfur radical cations to depro-tonate is due to deprotonation at the carbons adjacent to the sulfur-cationic site and at the unsubstituted amino groups when present. This scheme was supported by an observed decrease in the yields of dimeric sulfur radical cations with an increase in the electron-withdrawing abilities of the substituents, making the radical-cationic species stronger acids. The involvement of protons on the amino groups was implicated by the correlation of the quantum yields of ketyl radical formation in the photochemistry experiments with the rate constants for the reaction of the CB radical anion with the sulfur-containing substrates in pulse radiolysis experiments. 相似文献
994.
995.
In this article we prove that many Hamiltonian systems that cannot be separably quantized in the classical approach of Robertson and Eisenhart can be separably quantized if we extend the class of admissible quantizations through a suitable choice of Riemann space adapted to the Poisson geometry of the system. Actually, in this article we prove that for every quadratic in momenta Stäckel system (defined on 2n dimensional Poisson manifold) for which Stäckel matrix consists of monomials in position coordinates there exist infinitely many quantizations–parametrized by n arbitrary functions–that turn this system into a quantum separable Stäckel system. 相似文献
996.
Renata Prusinowska Krzysztof Śmigielski Agnieszka Stobiecka Alina Kunicka-Styczyńska 《Natural product research》2016,30(4):386-393
It was shown that the method for obtaining hydrolates from lavender (Lavandula angustifolia) influences the content of active compounds and the aromatic, antimicrobial and antioxidant properties of the hydrolates. The content of volatile organic compounds ranged from 9.12 to 97.23 mg/100 mL of hydrolate. Lavender hydrolate variants showed low antimicrobial activity (from 0% to 0.05%). The radical scavenging activity of DPPH was from 3.6 ± 0.5% to 3.8 ± 0.6% and oxygen radical absorbance capacity (ORACFL) results were from 0 to 266 μM Trolox equivalent, depending on the hydrolate variant. 相似文献
997.
Anna Bahyrycz Krzysztof Ciepliński 《Journal of Fixed Point Theory and Applications》2016,18(4):737-751
In this paper, we unify the system of functional equations defining a multi-Jensen-quadratic mapping to obtain a single equation. We also prove, using the fixed point method, the generalized Hyers–Ulam stability of this equation both in Banach spaces and in complete non-Archimedean normed spaces. 相似文献
998.
Anna Bahyrycz Krzysztof Ciepliński Jolanta Olko 《Journal of Fixed Point Theory and Applications》2016,18(2):433-444
In this paper we prove, using the fixed point method, the generalized Hyers–Ulam stability of two functional equations in complete non-Archimedean normed spaces. One of these equations characterizes multi-Cauchy–Jensen mappings, and the other gives a characterization of multi-additive-quadratic mappings. 相似文献
999.
Lin CY Coote ML Gennaro A Matyjaszewski K 《Journal of the American Chemical Society》2008,130(38):12762-12774
High-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R-X; R = CH 2CN, CH(CH 3)CN, C(CH 3) 2CN, CH 2COOC 2H 5, CH(CH 3)COOCH 3, C(CH 3) 2COOCH 3, C(CH 3) 2COOC 2H 5, CH 2Ph, CH(CH 3)Ph, CH(CH 3)Cl, CH(CH 3)OCOCH 3, CH(Ph)COOCH 3, SO 2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide. The SRPs of the corresponding alkyl radicals, R (*), are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R-X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer (OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates. Both the homolytic and heterolytic reactions are favored by electron-withdrawing substituents and/or those that stabilize the product alkyl radical, which explains why monomers such as acrylonitrile and styrene require less active ATRP catalysts than vinyl chloride and vinyl acetate. The rate constant of the hypothetical OSET reaction between bromoacetonitrile and Cu (I)/TPMA complex was estimated using Marcus theory for the electron-transfer processes. The estimated rate constant k OSET = approximately 10 (-11) M (-1) s (-1) is significantly smaller than the experimentally measured activation rate constant ( k ISET = approximately 82 M (-1) s (-1) at 25 degrees C in acetonitrile) for the concerted atom transfer mechanism (inner-sphere electron transfer, ISET), implying that the ISET mechanism is preferred. For monomers bearing electron-withdrawing groups, the one-electron reduction of the propagating alkyl radical to the carbanion is thermodynamically and kinetically favored over the one-electron reduction of the corresponding alkyl halide unless the monomer bears strong radical-stabilizing groups. Thus, for monomers such as acrylates, catalysts favoring ISET over OSET are required in order to avoid chain-breaking side reactions. 相似文献
1000.
Typically, ion-selective electrodes used in current triggered electrochemical sensing apply a conducting polymer layer covered with an ion-selective membrane. In this work we propose an ion-selective membrane containing a dispersed conducting polymer. Thus obtained system allows elimination of the Achilles hell of heterogeneous ion-selective membranes containing solid particulates dispersed within the ion-selective polymeric membrane. The herein proposed system, even for high conducting polymer loading equal to 5 % w/w, is characterized with insensitivity towards redox interferences, as well as potentiometric detection limits, selectivity well comparable with that for other ion-selective electrodes constructions. Under voltammetric conditions, with increasing loading of the conducting polymer in the membrane cathodic peak potentials are shifted towards more negative values, yet the linear dependence of the peak potential on logarithm of concentration of the analyte in the solution is preserved. 相似文献