Generation and reactivity of the radical cations of coniferyl alcohol and isoeugenol in solution

2003 ◽  
Vol 81 (6) ◽  
pp. 799-806 ◽  
Author(s):  
N P Schepp ◽  
Y Rodríguez-Evora
Keyword(s):  
Radical Cation ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Radical Cations ◽  
Phenoxyl Radical ◽  
Laser Flash ◽  
Coniferyl Alcohol ◽  
Nanosecond Laser ◽  
Acid Base Equilibrium

Nanosecond laser flash photolysis of coniferyl alcohol and isoeugenol in acetonitrile leads to the formation of transient species that are identified as the corresponding radical cations. These radical cations decay with rate constants of ca. 1 × 106 s–1 in dry acetonitrile. Both radical cations react rapidly with hydroxylic solvents like water and alcohols to give 4-vinylphenoxyl radicals, indicating that these reagents behave as bases rather than nucleophiles. In addition, anionic reagents (acetate, cyanide, and chloride) react rapidly with the radical cations with second-order rate constants that are close to diffusion controlled. The main products generated in the presence of the anionic reagents are again the 4-vinylphenoxyl radicals, suggesting that these reagents also behave as bases. The lifetime of the radical cations in acidic acetonitrile was found to increase dramatically due to a shift in the radical cation – vinyl phenoxyl radical acid–base equilibrium to the side of the radical cation. An estimate of the pKa of the radical cation in acetonitrile of 4.0 was obtained from the data.Key words: radical cations, laser flash photolysis, lignan, vinylphenols.

Generation and spectroscopic and kinetic characterization of methoxy-substituted phenoxyl radicals in solution and on paper

1997 ◽  
Vol 75 (12) ◽  
pp. 1820-1829 ◽  
Author(s):  
D. Shukla ◽  
N.P. Schepp ◽  
N. Mathivanan ◽  
L.J. Johnston
Keyword(s):  
Radical Cation ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Radical Scavenging ◽  
Radical Cations ◽  
Phenoxyl Radical ◽  
Laser Flash ◽  
Phenoxyl Radicals ◽  
Long Wavelength ◽  
Direct Excitation

A number of methoxy-substituted phenoxyl radicals have been generated and characterized by laser flash photolysis techniques in solution and on paper. The radicals have been produced by three routes in solution: hydrogen abstraction from phenols by tert-butoxyl radical, photolysis of α-aryloxyacetophenones, and direct excitation of phenols. Most of the phenoxyl radicals studied have a characteristic absorption near 400 nm; the ortho-substituted radicals have an additional broad absorption in the visible in non-hydroxylic solvents (e.g., 650 nm for 2-methoxyphenoxyl radical). The relative intensities of the 400 nm and the long-wavelength bands vary with substitution pattern and solvent, with the long wavelength band being much weaker in hydroxylic solvents. Direct excitation of phenols provides an alternate source of phenoxyl radicals in some cases. However, for 4-methoxyphenol there are overlapping absorptions due to triplet phenol and phenol radical cation, both of which absorb in the 400–460 nm region. The 4-methoxyphenol radical cation is formed in a biphotonic process and decays by deprotonation to give the phenoxyl radical. The 4-methoxyphenoxyl radical can also be generated on paper from either the ketone or phenol precursor and decays with complex kinetics that depend on the precursor loading and laser intensity. There is no evidence for the formation of significant amounts of long-lived ketone or phenol triplets on paper. The decay kinetics for the phenoxyl radical are modified by the addition of the radical scavenger dithioerythritol. The results indicate both static and dynamic components to the radical scavenging process. The generation of phenoxyl radicals on paper in the presence and absence of scavengers provides the basis for the development of a new method for screening potential photoyellowing inhibitors and understanding their mode of action. Keywords: phenoxyl radicals, paper photoyellowing, laser flash photolysis, triplets, radical cations.

Dynamics of the transient species generated upon photolysis of diarylmethanes within zeolites — Deprotonation and oxidation reactions

2005 ◽  
Vol 83 (9) ◽  
pp. 1637-1648 ◽  
Author(s):  
Suzanne Shea ◽  
Norman P Schepp ◽  
Amy E Keirstead ◽  
Frances L Cozens
Keyword(s):  
Radical Cation ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Radical Cations ◽  
Laser Flash ◽  
Oxidation Potential ◽  
Oxidation Reactions ◽  
Oxidation Potentials ◽  
Multistep Process ◽  
Acid Zeolites

The oxidation of diarylmethanes is a multistep process involving initial formation of a radical cation, deprotonation of the radical cation to the radical, and oxidation of the radical to the carbocation. The dynamics and efficiency of the last two steps in this process, namely deprotonation and oxidation, in acidic zeolites and non-acid zeolites are examined in the present work as a function of the acidity of the diarylmethane radical cations and the oxidation potential of the diarylmethyl radicals. Our results indicate that rate constants for deprotonation strongly depend on the acidity of the radical cations, but not on the composition of the zeolites. In addition, oxidation of the radicals to the diarylmethyl cations is strongly dependent on both the oxidation potential of the radicals and the oxidizing ability of the zeolite. This dependence allows oxidation potentials of the zeolites to be estimated.Key words: radical cations, carbocations, zeolites, laser flash photolysis.

Time-resolved nanosecond and picosecond absorption studies of excited-state properties of 1-thiobenzoylnaphthalene

1989 ◽  
Vol 67 (6) ◽  
pp. 967-972 ◽  
Author(s):  
R. Minto ◽  
A. Samanta ◽  
P.K. Das
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Picosecond Laser ◽  
Laser Flash ◽  
Quantum Yields ◽  
Pulse Excitation ◽  
Nanosecond Laser Pulse ◽  
Nanosecond Laser ◽  
Quenching Rate

1-Thiobenzoylnaphthalene (TBN), known for its pericyclization reaction from the lowest excited singlet state (S1), has been subjected to nanosecond and picosecond laser flash photolysis studies. The two major transients observed in the course of nanosecond laser pulse excitation are (i) the short-lived triplet characterized by two absorption maxima (400–410 and 740–750 nm) and submicrosecond intrinsic lifetimes (80–130 ns) and (ii) a relatively long-lived species (λmax = 520 nm and τ = 220–240 ns). Various triplet-related photophysical data of TBN, including self-quenching and bimolecular quenching rate constants, have been determined. The existence of a photochemical path from S1 manifests itself in low intersystem crossing quantum yields, particularly in the polar/hydrogen-bonding solvent, methanol. From the build-up of the triplet under picosecond excitation into S1 the lifetime of the latter is estimated to be ≤ 50 ps (in benzene). The fast intrinsic decay of TBN triplet is attributable to facile intra- and intermolecular photochemistry. The 520 nm transient species could not be definitively assigned, except that it is neither a triplet nor a triplet-derived product and that it arises via photochemistry from S1. Keywords: laser flash photolysis, triplet, transients, absorption maxima, lifetimes, quenching rate constants, photochemistry, 1-thiobenzoylnaphthalenes.

Acid–base properties in the triplet state of aromatic ketones studied by nanosecond laser flash photolysis

1984 ◽  
Vol 62 (10) ◽  
pp. 2041-2046 ◽  
Author(s):  
Haruo Shizuka ◽  
Ehichi Kimura
Keyword(s):  
Triplet State ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Nanosecond Laser ◽  
Acid Base ◽  
Aromatic Ketones ◽  
Acid Base Equilibrium ◽  
Nanosecond Laser Flash Photolysis ◽  
Base Properties

Acid–base properties in the triplet state of aromatic ketones in H2O–CH3CN (4:1) mixtures have been studied by means of nanosecond laser flash photolysis. The acidity constants pKa(T) in the triplet state were determined by the Tn ← T1 absorbance titration curve, the Ware plot, and the Rayner–Wyatt plot. Good agreement among them shows that the acid–base equilibrium in the T1 state of aromatic ketones is established during the triplet lifetime. A linear relation between pKa(T) and the Taft σ* values was obtained for phenyl alkyl ketones.

scholarly journals Unusually Fast bis-Histidyl Coordination in a Plant Hemoglobin

2021 ◽  
Vol 22 (5) ◽  
pp. 2740
Author(s):  
Stefania Abbruzzetti ◽  
Alex J. Barker ◽  
Irene Villar ◽  
Carmen Pérez-Rontomé ◽  
Stefano Bruno ◽  
...  
Keyword(s):  
Ligand Binding ◽  
Rate Constants ◽  
Time Course ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Binding Constants ◽  
Laser Flash ◽  
High Reactivity ◽  
Nanosecond Laser ◽  
Ligand Rebinding

The recently identified nonsymbiotic hemoglobin gene MtGlb1-2 of the legume Medicago truncatula possesses unique properties as it generates four alternative splice forms encoding proteins with one or two heme domains. Here we investigate the ligand binding kinetics of MtGlb1-2.1 and MtGlb1-2.4, bearing two hemes and one heme, respectively. Unexpectedly, the overall time-course of ligand rebinding was unusually fast. Thus, we complemented nanosecond laser flash photolysis kinetics with data collected with a hybrid femtosecond–nanosecond pump–probe setup. Most photodissociated ligands are rebound geminately within a few nanoseconds, which leads to rates of the bimolecular rebinding to pentacoordinate species in the 108 M−1s−1 range. Binding of the distal histidine to the heme competes with CO rebinding with extremely high rates (kh ~ 105 s−1). Histidine dissociation from the heme occurs with comparable rates, thus resulting in moderate equilibrium binding constants (KH ~ 1). The rate constants for ligation and deligation of distal histidine to the heme are the highest reported for any plant or vertebrate globin. The combination of microscopic rates results in unusually high overall ligand binding rate constants, a fact that contributes to explaining at the mechanistic level the extremely high reactivity of these proteins toward the physiological ligands oxygen, nitric oxide and nitrite.

Singlet Methylene Removal Rate Constants from the (0,1,0) Vibrational Level: Enhancement via Complex-Mediated Vibrational Relaxation

2001 ◽  
Vol 215 (6) ◽  
Author(s):  
M.A. Buntine ◽  
G.J. Gutsche ◽  
W.S. Staker ◽  
M.W. Heaven ◽  
K.D. King ◽  
...  
Keyword(s):  
Vibrational Level ◽  
Rate Constants ◽  
Vibrational Relaxation ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Removal Rate ◽  
Laser Flash ◽  
Laser Absorption ◽  
Photolysis Laser ◽  
Singlet Methylene

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene,

Laser flash photolysis studies of dibenzosuberenyl cations and radical cations

1989 ◽  
Vol 93 (21) ◽  
pp. 7370-7374 ◽  
Author(s):  
L. J. Johnston ◽  
J. Lobaugh ◽  
V. Wintgens
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Radical Cations ◽  
Laser Flash
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