Mechanism of acid-catalyzed photoaddition of methanol to 3-alkyl2-cyclohexenones

1995 ◽  
Vol 73 (11) ◽  
pp. 2004-2010 ◽  
Author(s):  
David I. Schuster ◽  
Jie-Min Yang ◽  
Jan Woning ◽  
Timothy A. Rhodes ◽  
Anton W. Jensen
Keyword(s):  
Acetic Acid ◽  
Triplet State ◽  
Rate Constant ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Product Formation ◽  
Triplet States ◽  
Quenching Rate ◽  
Acid Catalyzed

Contrary to a previous report, it is concluded that formation of methanol adducts to 3-methyl-2-cyclohexenones and of deconjugated enones on irradiation of the enones in acidified solutions proceeds via protonation of the intermediate enone π,π* triplet excited state and not by protonation of a relatively long-lived ground state trans-cyclohexenone. A rate constant for protonation of the triplet state of 3-methyl-2-cyclohexenone by sulfuric acid of 1.7 × 109 M−1 s−1 was determined by laser flash photolysis in ethyl acetate. Based on quantum efficiencies of product formation, a rate constant of ca. 108 M−1 s−1 was estimated for protonation of the enone triplet by acetic acid, which is too small to cause measurable reduction in the triplet state lifetime in the mM concentration range used in the preparative studies. The intermediate carbocation can be trapped by methanol, or revert to starting enone or the exocyclic deconjugated enone by loss of a proton. Since products revert to starting materials in an acid-catalyzed process, there is an acid concentration at which the yields of products are optimal. This concentration is ca. 6 mM for acetic acid, but is only 0.1 mM for p-toluenesulfonic or sulfuric acids. Product formation could be quenched using 1-methylnaphthalene and cyclopentene as triplet quenchers; in the latter case, formation of [2 + 2] photoadducts was observed to compete with formation of methanol adducts. Quenching rate constants were determined by laser flash studies. Keywords: laser flash photolysis, kinetic absorption spectroscopy (KAS), photoacoustic calorimetry (PAC), protonation of triplet states, trans-cyclohexenones.

A reinvestigation of the interaction between triplet states of cyclohexenones and amines

1988 ◽  
Vol 66 (10) ◽  
pp. 2595-2600 ◽  
Author(s):  
D. Weir ◽  
J. C. Scaiano ◽  
D. I. Schuster
Keyword(s):  
Triplet State ◽  
Rate Constant ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Radical Cations ◽  
Laser Flash ◽  
Triplet States ◽  
New Evidence

Laser flash photolysis studies lead to the conclusion that the short-lived triplet states of cyclohexenones are readily quenched by amines. For example, in the case of 2-cyclohexen-1-one (1) its triplet state (τT = 40 ns in acetonitrile) is quenched by triethylamine with a rate constant of (9.0 ± 0.8) × 107 M−1 s−1. Cyclohexenone triplets are also quenched efficiently by DABCO and by triphenylamine leading to the formation of the corresponding amine radical cations. The new evidence reported rules out the involvement of long-lived detectable exciplexes.

scholarly journals Benzylchlorocarbene: UV Absorption Spectrum, Kinetics for 1,2-H Migration and Mechanism for Reaction With Acetic Acid as Determined by Combined Continuous and Laser-Flash Photolysis

1990 ◽  
Vol 10 (5-6) ◽  
pp. 267-275 ◽  
Author(s):  
R. Bonneau ◽  
M. T. H. Liu ◽  
R. Subramanian ◽  
B. Linkletter ◽  
I. D. R. Stevens
Keyword(s):  
Absorption Spectrum ◽  
Acetic Acid ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Uv Absorption ◽  
Quenching Rate ◽  
Continuous Irradiation ◽  
Insight Into ◽  
Uv Absorption Spectrum

The UV absorption spectrum of benzylchlorocarbene, generated by laser flash photolysis of 3-chloro-3- benzyldiazirine, has been observed in the 290-330 nm range. The lifetime of this species, 18 ns at 25°C, is determined by the rate of the 1,2-H migration to produce chlorostyrenes. Quenching rate constants of this carbene by acetic acid and tetramethylethylene have been measured. Comparison of this kinetic data with the quantitative analysis of the products obtained under continuous irradiation gives further insight into the mechanisms of carbene-acetic acid reactions.

Oxygen sensing based on lifetime of photoexcited triplet state of platinum porphyrin-polystyrene film using time-resolved spectroscopy

2000 ◽  
Vol 04 (03) ◽  
pp. 292-299 ◽  
Author(s):  
YUTAKA AMAO ◽  
KEISUKE ASAI ◽  
ICHIRO OKURA
Keyword(s):  
Triplet State ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Oxygen Sensing ◽  
Laser Flash ◽  
Quenching Rate ◽  
Time Resolved ◽  
Quenching Rate Constant ◽  
Volmer Plot ◽  
Platinum Porphyrin

Optical oxygen-sensing systems based on the quenching of the photoexcited triplet state of platinum porphyrins—platinum octaethylporphyrin (PtOEP) and platinum tetrakis(pentafluorophenyl)porphyrin (PtTFPP)—in polystyrene (PS) using two different time-resolved spectroscopies (luminescence lifetime measurement and diffuse reflectance laser flash photolysis) have been developed. Using both spectroscopies, the same values of Stern-Volmer constant KSV and quenching rate constant kq (KSV = kqτ0) are obtained. The decays of the luminescence and triplet-triplet reflectance of the platinum porphyrins in PS consisted of two components (faster and slower lifetimes) in the absence and presence of oxygen. For both faster and slower components the lifetime decreases with increasing oxygen concentration. For both components a Stern-Volmer plot of the platinum porphyrin-PS films exhibits linearity. However, kq of the faster component is larger than that of the slower component (for PtOEP, three times larger; for PtTFPP, 40 times larger), indicating that two different oxygen-accessible sites exist in the platinum porphyrin-PS films. The faster and slower components are related to oxygen-accessible sites on the surface and in the bulk of the platinum porphyrin films respectively. Concerning the fractional contributions of each lifetime component, the contribution of the faster component is greater than that of the slower component, indicating that the sensing site on the surface is important for optical sensing. The contribution of different oxygen-accessible sites in platinum porphyrin-PS films for oxygen sensing is clarified by these techniques.

Laser flash photolysis study of the photochemistry of ortho-benzoylbenzaldehyde

1993 ◽  
Vol 71 (8) ◽  
pp. 1209-1215 ◽  
Author(s):  
J.C. Netto-Ferreira ◽  
J.C. Scaiano
Keyword(s):  
Steady State ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
High Water ◽  
Laser Flash ◽  
Product Formation ◽  
Deuterated Water ◽  
Quenching Rate ◽  
A New Species ◽  
Volmer Constant

Ketene-enols 4 and 5 have been generated by laser flash photolysis of ortho-benzoylbenzaldehyde (3) and kinetically and spectroscopically characterized. In benzene or acetonitrile, the E ketene-enol, 4, shows absorption at 340 and 400 nm and a lifetime in excess of 1 ms, whereas the Z ketene-enol, 5, shows maxima at 360 and 430 nm and a lifetime of only 1.5 μs. At shorter time scales we observed a weak absorption (λmax = 580 nm) tentatively assigned to biradical 6 with a lifetime of 140 ns. The E ketene-enol is readily quenched by oxygen, dienophiles, methanol, and water, with quenching rate constants ranging from 3.6 × 103 M−1 s−1 (for methanol as a quencher) to 2.2 × 108 M−1 s−1 (for diethylketomalonate). At high water concentrations (typically > 10 M) a new species, 7, was detected showing maximum absoiption at 510 nm and a growth lifetime of 7 μs. In deuterated water and using the same concentration as before we observed a formation lifetime for 7 of 10 μs, which results in an isotope effect of ~ 1.5. It is proposed that 5 is the main precursor for 7. Steady-state irradiation of 3 in deaerated methanol leads to the formation of dihydroanthraquinone (9), a strongly colored and fluorescent (λmax = 475 nm, τn = 29 ns) species, whereas 3-phenylphthalide (2, R = Ph) is the main product when the irradiation is performed in benzene. Steady-state quenching of product formation by diethyl ketomalonate gives a Stern–Volmer constant of 380 M−1 from which we conclude that 5 is the ketene-enol responsible for product formation, in agreement with the laser flash photolysis results.

The absolute kinetics for radical addition and electronic energy transfer to [1.1.1]propellane

1998 ◽  
Vol 76 (10) ◽  
pp. 1474-1489 ◽  
Author(s):  
P F McGarry ◽  
J C Scaiano
Keyword(s):  
Free Radicals ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Electronic Energy ◽  
Triplet States ◽  
Quenching Rate ◽  
Electronic Energy Transfer ◽  
Diffusion Controlled

Free radicals react more readily with [1.1.1]propellane, 1, than with styrene. For example Et3Si· reacts with 1 and styrene with rate constants of 6 × 108 M-1 s-1 and 2 × 108 M-1 s-1, respectively. Fluorenone, phenanthrene, triphenylene, benzophenone, and pyrene transfer electronic energy to 1 with rate constants well below the diffusion-controlled limit. For example, triplet benzophenone is quenched by 1 with a bimolecular rate constant of 9.9 × 106 M-1 s-1. A linear dependence of the log of the quenching rate constants, log kq, upon the excited-state energy of the donors is found.Key words: propellane, laser flash photolysis, free radicals, triplet states.

Kinetic study of the reactions of methoxy-substituted phenacyl radicals

1995 ◽  
Vol 73 (2) ◽  
pp. 223-231 ◽  
Author(s):  
S.V. Jovanovic ◽  
J. Renaud ◽  
A.B. Berinstain ◽  
J.C. Scaiano
Keyword(s):  
Rate Constant ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Chemical Reactivity ◽  
Laser Flash ◽  
Carbonyl Oxygen ◽  
Peroxyl Radicals ◽  
Quantum Yields ◽  
Triplet States ◽  
Singlet States

The photochemistry of various mono- and dimethoxy-substituted α-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-α-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have π,π* character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 × 105 M−1 s−1, while for addition to the double bond in 1,1-diphenylethylene the rate constant is 9.4 × 107 M−1 s−1. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k ≤ 105 M−1 s−1). In contrast, reaction with oxygen occurs with kq = 2.5 × 109 M−1 s−1. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin. Keywords: methoxyphenacyl radicals, α-bromoacetophenones, laser flash photolysis.

Effect of Aggregation on Bacteriochlorin a Triplet-state Formation: A Laser Flash Photolysis Study¶

2002 ◽  
Vol 76 (5) ◽  
pp. 480 ◽  
Author(s):  
Xavier Damoiseau ◽  
Francis Tfibel ◽  
Maryse Hoebeke ◽  
Marie-Pierre Fontaine-Aupart
Keyword(s):  
Triplet State ◽  
State Formation ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash

Mechanistic studies of aromatic ketone-sensitized photoreduction of bis(acetylacetonato)copper(II)

1983 ◽  
Vol 61 (5) ◽  
pp. 801-808 ◽  
Author(s):  
Yuan L. Chow ◽  
Gonzalo E. Buono-Core ◽  
Bronislaw Marciniak ◽  
Carol Beddard
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Decay Rates ◽  
Polynuclear Aromatic Hydrocarbons ◽  
Aromatic Ketone ◽  
Quantum Yields ◽  
Triplet States ◽  
Quenching Rate ◽  
Triplet Energy

Bis(acetylacetonato)copper(II), Cu(acac)2, quenches triplet excited states of ketones and polynuclear aromatic hydrocarbons efficiently, but only aromatic ketones with high triplet energy successfully sensitize photoreduction of Cu(acac)2 in alcohols under nitrogen to give derivatives of aeetylacetonatocopper(I), Cu(acac). For the triplet state benzophenone-sensitized photoreduction of Cu(acac)2, the quantum yields of photoreduction (ΦC) and those of benzophenone disappearance (ΦB) were determined in methanol with various concentrations of Cu(acac)2. The values of the quenching rate constant, kq, determined from these two types of monitors on the basis of the proposed mechanism were in good agreement (6.89 ~ 7.35 × 109 M−1 s−1). This value was higher, by a factor of about two, than that obtained from the monitor of the benzophenone triplet decay rates generated by flash photolysis in the presence of Cu(acac)2. The quenching rate constants of various aromatic ketone and hydrocarbon triplet states by Cu(acac)2 were determined by flash photolysis to be in the order of the diffusion rate constant and the quantum yields of these photoreductions were found to be far from unity. Paramagnetic quenching, with contributions of electron exchange and charge transfer, was proposed as a possible quenching mechanism. For a series of aromatic ketone sensitizers with higher triplet energy, this mechanism was used to rationalize the observed high quenching rate constants in contrast to the low quantum yields of photoreduction.

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