Reactions of thiyl radicals. XIII. Photochemically induced exchange reactions of liquid alkyl disulfides

1980 ◽  
Vol 58 (13) ◽  
pp. 1350-1354 ◽  
Author(s):  
Dinesh Gupta ◽  
Arthur R. Knight
Keyword(s):  
Molecular Weight ◽  
Product Formation ◽  
Quantum Yields ◽  
Light Sources ◽  
Thiyl Radical ◽  
Exchange Reactions ◽  
Thiyl Radicals ◽  
Steric Factors ◽  
Total Molecular Weight ◽  
Symmetrical Disulfide

The liquid phase photolysis of some 35 liquid alkyl disulfide mixtures has been studied. The corresponding unsymmetrical disulfide is the only significant product of the reaction and the System attains a photo-equilibrium state at longer exposure times. Product formation is completely eliminated by the addition of nitric oxide.Quantum yields have been redetermined for the methyl disulfide – ethyl disulfide System using four different light sources. Initial rates have been measured for each of the 35 combinations and have been shown to be a function of total molecular weight of the disulfide mixture and to be strongly influenced by steric factors. These effects are most pronounced for mixtures wherein the exchange reaction between thiyl radical and a symmetrical disulfide is important as indicated by quantum yields above unity. Combinations of isodisulfides give rise to anomalously high rates.

Cyclobutene photochemistry. Involvement of carbene intermediates in the photochemistry of alkylcyclobutenes

1988 ◽  
Vol 66 (7) ◽  
pp. 1571-1578 ◽  
Author(s):  
K. Brady Clark ◽  
William J. Leigh
Keyword(s):  
Ring Opening ◽  
Product Formation ◽  
Methanol Solution ◽  
The Other ◽  
Quantum Yields ◽  
Light Sources ◽  
Far Ultraviolet

The photochemistry of 1,3,3,4- and 1,3,4,4-tetramethylcyclobutene has been investigated in pentane solution with monochromatic far ultraviolet (185, 193, and 214 nm) light sources, as well as in methanol solution with 214-nm excitation. Photolysis of each of the two isomeric cyclobutene derivatives results in competitive electrocyclic ring opening (yielding mixtures of stereoisomeric dienes in each case), fragmentation to yield propyne and methyl-2-butene, and isomerization to the other cyclobutene isomer. Quantum yields for product formation with 185-nm excitation have been measured in each case by cyclooctene actinometry. The occurrence of the interconversion process is offered as evidence for the intermediacy of cyclopropyl carbenes in the photochemistry of simple cyclobutenes in solution. On the basis of the known chemistry of carbenes of this type, their intermediacy in cyclobutene photochemistry may account at least partially for the formation of the formal (2π + 2π) cycloreversion products from photolysis of simple cyclobutene derivatives.

scholarly journals Crotalase, a Fibrinogen-Clotting Snake Venom Enzyme: Primary Structure and Evidence for a Fibrinogen Recognition Exosite Different from Thrombin

1999 ◽  
Vol 81 (01) ◽  
pp. 81-86 ◽  
Author(s):  
Agnes Henschen-Edman ◽  
Ida Theodor ◽  
Brian Edwards ◽  
Hubert Pirkle
Keyword(s):  
Molecular Weight ◽  
Amino Acid ◽  
Amino Acid Sequence ◽  
Spatial Modeling ◽  
Glycosylation Site ◽  
Amino Sugars ◽  
Edman Degradation ◽  
Heparin Binding ◽  
Heparin Binding Site ◽  
Total Molecular Weight

SummaryCrotalase, a fibrinogen-clotting enzyme isolated from the venom of Crotalus adamanteus, and its overlapping fragments were subjected to Edman degradation. The resulting amino acid sequence, VIGGDEC NINEHRFLVALYDYWSQLFLCGGTLINNEWVLTAAHCDRTHI LIYVGVHDRSVQFDKEQRRFPKEKYFFDCSNNFTKWDKDIM LIRLNKPVSYSEHIAPLSLPSSPPIVGSVCRAMGWGQTTSPQET LPDVPHCANINLLDYEVCRTAHPQFRLPATSRTLCAGVLEG GIDTCNRDSGGPLICNGQFQGIVFWGPDPCAQPDKPGLYTK VFDHLDWIQSIIAGEKTVNCP, is characteristic of a serine protein-ase. Comparison with thrombin, the physiological fibrinogen-clotting enzyme, showed that thrombin’s fibrinogen-recognition exosite (FRE) is poorly represented in crotalase. Hirudin, a FRE-dependent inhibitor, had no effect on crotalase. Spatial modeling of crotalase yielded a possible alternative fibrinogen-recognition site comprised of Arg 60F, Lys 85, Lys 87, and Arg 107 (underlined in the sequence above). Crotalase also lacks thrombin’s YPPW loop, as well as its functionally important ETW 146-148, and its heparin-binding site. The enzyme contains a single asparagine-linked glycosylation site, NFT, bearing neutral and amino sugars that account for 8.3% of the enzyme’s total molecular weight of 29,027. The calculated absorbance of crotalase at 280 nm, 1%, cm-1is 15.2.

Reactions of Thiyl Radicals. XI. Further Investigations of Thiol–Disulfide Photolyses in the Liquid Phase

1973 ◽  
Vol 51 (9) ◽  
pp. 1410-1415 ◽  
Author(s):  
Donna D. Carlson ◽  
Arthur R. Knight
Keyword(s):  
Liquid Phase ◽  
Room Temperature ◽  
Liquid Mixtures ◽  
Quantum Yields ◽  
Chain Reaction ◽  
Photochemical Process ◽  
High Quantum Efficiency ◽  
Thiyl Radicals ◽  
Exchange Processes ◽  
A Chain

The photolysis of C2H5SH liquid at 2537 Å has been shown to give H2 and C2H5SSC2H5 at equal rates with a quantum yield of 0.25. The photolysis of ethanethiol – methyl disulfide liquid mixtures leads, via a chain reaction involving propagation by attack of thiyl radicals on the disulfide S—S bond, to the formation with high quantum efficiency of CH3SH, C2H5SSC2H5 and, as an intermediate that is consumed after long exposures, CH3SSC2H5. The net result of the sequence of exchange processes is the essentially irreversible conversion of the methyl disulfide into methanethiol. The same overall reaction occurs thermally at room temperature, but the rate is appreciably less than that of the photochemical process. The quantum yields of formation of the unsymmetrical disulfides arising from the photochemically initiated exchange reaction in equimolar mixtures of CH3SSCH3 + n-C3H7SSC3H7 and C2H5SSC2H5 + n-C3H7SSC3H7 have been shown to be 6.9 and 4.4, compared to 355 for CH3-SSCH3 + C2H5SSC2H5 mixtures. In all three types of system examined in this investigation all thiyl radicals can be accounted for stoichiometrically on the basis of exchange and combination reactions alone, indicating negligible disproportionation of these species in condensed phase.

Recuring Vulcanizates. I. A Novel Way to Study the Mechanism of Vulcanization

1992 ◽  
Vol 65 (1) ◽  
pp. 211-222 ◽  
Author(s):  
Robert W. Layer
Keyword(s):  
Molecular Weight ◽  
High Molecular Weight ◽  
Zinc Salt ◽  
Current View ◽  
Accelerator Complex ◽  
Exchange Reactions ◽  
Model Compounds ◽  
Vulcanization Process ◽  
Weight Model ◽  
Sulfur Donor

Abstract Vulcanizates, to which a curative is added by swelling, can be recured to easily study a variety of aspects of the vulcanization process, such as maturation, reversion, and even how much accelerator remains active as its zinc salt at the end of the cure. In effect, vulcanizates can be viewed as high-molecular-weight model compounds. In this study, we find that recuring SBR/BR vulcanizates, to which sulfur or the sulfur donor, N,N′ -dithiodimorpholine, is added, develops the same state of cure as the same amount of sulfur (or sulfur donor) added for the initial cure. This suggests that exchange reactions occur between crosslinks and the zinc-sulfur-accelerator complex during the cure and that all of the accelerator remains as its zinc salt at the end of the cure. This last result is interesting, since it is not consistent with the current view that most of the accelerator becomes irreversibly bound to the rubber and lost during the vulcanization of polybutadiene rubbers.

scholarly journals Poly(lactic acid) polymer stars built from early generation dendritic polyols

2013 ◽  
Vol 91 (6) ◽  
pp. 392-397 ◽  
Author(s):  
Genny E. Keefe ◽  
Jean-d'Amour K. Twibanire ◽  
T. Bruce Grindley ◽  
Michael P. Shaver
Keyword(s):  
Molecular Weight ◽  
Ring Opening ◽  
Gel Permeation Chromatography ◽  
Poly Lactic Acid ◽  
Reactive Site ◽  
Scanning Calorimetry ◽  
Early Generation ◽  
Sequential Reaction ◽  
Cyclic Esters ◽  
Total Molecular Weight

A family of polymer stars has been prepared from early generation dendritic cores with four, six, and eight arms. Four dendritic cores were prepared from the sequential reaction of a multifunctional alcohol with a protected anhydride, followed by deprotection to afford two or three new alcohol functionalities per reactive site. These cores were used as initiators for the tin-catalyzed ring-opening polymerization of l-lactide and rac-lactide to afford isotactic and atactic degradable stars, respectively. Two series of stars were prepared for each monomer, either maintaining total molecular weight or number of monomer units per arm. The polymers were characterized by NMR spectroscopy, light-scattering gel-permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis. Our results support previous work that suggests that the length of the arms dictates thermal properties rather than the total molecular weight of the star. Little effect was noted between aromatic and aliphatic cores, presumably due to the flexibility of the rest of the core molecule. We have shown that early generation dendrimers can serve as excellent core structures for building core-first polymer stars via the ring-opening of cyclic esters.

scholarly journals Photoinduced Transformation of Carbon Monoxide in Aqueous Solution

1988 ◽  
Vol 43 (5) ◽  
pp. 430-434
Author(s):  
Hyoung Ryun Park ◽  
Nikola Getoff
Keyword(s):  
Aqueous Solution ◽  
Carbon Monoxide ◽  
Product Formation ◽  
Quantum Yields ◽  
Ph Values ◽  
Probable Reaction Mechanism ◽  
Probable Reaction ◽  
Photoinduced Transformation ◽  
Vacuum Uv Radiation ◽  
Vacuum Uv

Abstract The photoinduced conversion of CO in aqueous solution under the influence of vacuum-uv radiation at λ = 185 nm has been investigated at pH values ranging from 2 to 13. Formaldehyde, glyoxal, carboxylic acids and small amounts of glycol were determined as final products. Initial quantum yields for product formation have been determined. A probable reaction mechanism is presented.

Laser flash photolysis studies of the formation and reactivities of phenyl(naphthyl)methyl carbocations generated from phosphonium salt precursors

1992 ◽  
Vol 70 (6) ◽  
pp. 1784-1794 ◽  
Author(s):  
E. O. Alonso ◽  
L. J. Johnston ◽  
J. C. Scaiano ◽  
V. G. Toscano
Keyword(s):  
Rate Constants ◽  
Phosphonium Salt ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Ion Pairs ◽  
Laser Flash ◽  
Product Formation ◽  
Quantum Yields ◽  
Back Reaction ◽  
Transient Experiments

The photolysis of several substituted phenyl(naphthyl)methyl triphenylphosphonium chlorides has been examined using a combination of laser flash photolysis experiments and product studies. Both carbocation and radical intermediates have been characterized in the transient experiments, with the relative yields depending strongly on the solvent. For example, in alcohols, acetonitrile, or aqueous solvents cation formation predominates while acetonitrile/dioxane mixtures (5–10%) are required for the observation of radicals. Quantum yields for cation formation vary from 0.79 in methanol to 0.093 in 1:4 acetonitrile/dioxane, as measured by product studies and transient experiments, respectively. The addition of perchlorate salts leads to dramatic enhancements in the cation lifetimes; the effects are particularly pronounced for acetonitrile/dioxane mixtures where the cation yields also increase by factors of 3–4. In this case the effects are attributed primarily to replacement of chloride by perchlorate in the initial ion pairs. The combined data from both solvent and perchlorate salt effects on the cation lifetimes and yields suggest that the excited state of the phosphonium salt cleaves homolytically, followed by electron transfer within the initial radical/triphenylphosphine radical cation pair to generate carbocation, as opposed to direct heterolytic cleavage. The cation yields also indicate that back reaction to regenerate starting material, as well as product formation within the initial geminate cage, occur in some solvents. The effects of solvent and added perchlorate salts on the rate constants for reaction with nucleophiles have been examined. For example, rate constants that vary by an order of magnitude have been measured for quenching by azide ion in various aqueous acetonitrile and trifluoroethanol mixtures.

Reactions of Thiyl Radicals. VII. Photolysis of Methyl Sulfide Vapor

1972 ◽  
Vol 50 (6) ◽  
pp. 844-852 ◽  
Author(s):  
P. M. Rao ◽  
A. R. Knight
Keyword(s):  
Wavelength Range ◽  
Sulfur Hexafluoride ◽  
Quantum Yields ◽  
Primary Process ◽  
Methyl Radicals ◽  
Methyl Sulfide ◽  
Thiyl Radicals ◽  
Several Variables ◽  
Bond Scission ◽  
Function Of Several Variables

The photolysis of methyl sulfide vapor has been investigated as a function of substrate pressure, exposure time, and temperature in the wavelength range 2000 to 2300 Å. The effects of added propane, sulfur hexafluoride, and 2-methylpentane have been studied. The principal products of the decomposition are CH4, C2H6, CH3SSCH3, and CH3SH. The data indicate direct C—S bond scission in the primary process giving rise to "hot" CH3 and CH3S radicals. A mechanism in which disproportionation of methylthiyl radicals is a very minor process is proposed.The reactions of CH3 and CH3S radicals with methyl sulfide have been examined also by photolyzing CH3SSCH3 and CH3COCH3 in the presence of CH3SCH3. Quantum yields at 2288 Å have been determined as a function of several variables. A number of rate parameters for the reactions of methyl radicals and methylthiyl radicals have been calculated.

Sign in / Sign up

Export Citation Format

Share Document