- Unique Pressure Effects on the Absolute Kinetics of Triplet Benzophenone Photoreduction in Supercritical CO2
-
Laser flash photolysis studies of the hydrogen atom abstraction reaction of triplet benzophenone from 2-propanol and 1,4-cyclohexadiene in supercritical CO2 reveal unusual pressure effects on absolute rate constants.Monitoring reactivity close to the critical temperature (Tc) revealed that bimolecular rate constants increase sharply with a decrease in pressure, approaching the critical point.Kinetic investigations along an isotherm further removed from Tc and predictive calculations on the pressure effect expected in supercritical CO2 indicate that enhanced reactivity is due to local substrate clustering.
- Roberts, Christopher B.,Chateauneuf, John E.,Brennecke, Joan F.
-
-
Read Online
- Laser Flash Photolysis Study of the Hydrogen Atom Transfer Reaction from Triplet 1-Naphthol to Ground Benzophenone
-
Laser flash photolyses at 337 nm have been carried out methanol solutions of the 1-naphthol and benzophenone system.It is found that hydrogen atom transfer reaction from the triplet 1-naphthol 3ROH* (produced by triplet sensitization
- Shizuka, Haruo,Hagiwara, Hiroyuki,Fukushima, Masaru
-
-
Read Online
- Benzophenone Triplet Properties in Acetonitrile and Water
-
The technique of time resolved laser flash spectoscopy has been used to study the photophysical and photochemical properties of the lowest triplet of benzophenone in water and acetonitrile.New estimates have been obtained for the triplet extinction coeffi
- Bensasson, Rene V.,Gramain, Jean-Claude
-
-
Read Online
- Hydrogen Abstraction in the Neutral Molecular Cluster of Benzophenone and Hydrogen Domors Formed in a Supersonic Free Jet Expansion
-
Photochemical reactions of benzophenone with hydrogen donors in neutral molecular clusters formed in a supersonic free jet expansion were studied.It was found that benzophenone undergoes photoreduction to form benzophenone ketyl radical in the benzophenon
- Matsushita, Yoshihisa,Kajii, Yoshizumi,Obi, Kinichi
-
-
Read Online
- Laser Flash Photolysis Studies on Proton-Enhanced Hydrogen Atom Transfer Reaction from Triplet Naphthol to Benzophenone via a Triplet Exciplex
-
The laser flash photolysis study at 355 nm in the 1-naphthol (ROH) and benzophenone (BP) system with and without H2SO4 has been carried out in acetonitrile-water (4:1 v/v) or methanol at 290 K.For the ROH (3.0 * 10-3 M)-BP (6.7 * 10-3/sup
- Kaneko, Shouichi,Yamaji, Minoru,Hoshino, Mikio,Shizuka, Haruo
-
-
Read Online
- Magnetic Field Effects on the Hydrogen Abstraction Reactions of Triplet Benzophenone with Thiophenol in Nonviscous Homogenous Solutions
-
The hydrogen abstraction reaction of triplet benzophenone with thiophenol at room temperature was investigated in several nonviscous homogeneous solutions (η = 0.29-3.33 cP) by means of a nanosecond laser flash photolysis technique under magnetic fields o
- Wakasa, Masanobu,Hayashi, Hisaharu
-
-
Read Online
- Photochemical Reaction of Excited Benzophenone in the Gas Phase
-
Photochemical reactions of benzophenone with hydrogen-donating compounds have been studied in the gas phase.It has been found that the photoreduction of excited benzophenone proceeds to form benzophenone ketyl radical without contributions from surroundin
- Matsushita, Yoshihisa,Kajii, Yoshizumi,Obi, Kinichi
-
-
Read Online
- Time-Resolved Fluorescence Monitoring of Aromatic Radicals in Photoinitiated Processes
-
Photolytic initiation of free radical reactions is important to many areas of technology; time-resolved monitoring of submicromolar concentrations of radicals produced during the course of these reactions is needed to provide information about the rate of initiation and its competition with radical recombination. In this work, time-resolved laser-induced fluorescence is evaluated for monitoring of diphenylketyl radicals produced by photoreduction of the triplet state of benzophenone. Fluorescence from the doublet-doublet transition of the radical is excited with a continuous wave laser and provides a sensitive method to detect these intermediates at nanomolar concentrations and to study their kinetics in solution on time scales from a few microseconds to hundreds of milliseconds. The ketyl radical fluorescence measurements of radical initiation reactions allowed the H atom abstraction rate constant by triplet benzophenone from both 2-propanol and benzhydrol to be determined, where kH = (2.1 ± 0.1) × 106 M-1 s-1 for 2-propanol and kH = (4.4 ± 0.1) × 106 M-1 s-1 for benzhydrol. The diphenylketyl radical recombination rate constant was also determined by time-resolved fluorescence monitoring of the decay of the radical population and found to be kr = (1.9 ± 0.2) × 108 M-1 s-1. Formation kinetics could be measured on a microsecond time scale from radical populations as low as 45 nM; decay kinetics could be followed on a millisecond time scale from 20 nM radical concentrations.
- Shield, Stephanie R.,Harris, Joel M.
-
-
Read Online
- Nanosecond Flash Studies of Reduction of Benzophenone by Aliphatic Amines. Quantum Yields and Kinetic Isotope Effects
-
Nanosecond flash photolysis, steady irradiation, and deuterium substitution studies have been carried out on solutions of benzophenone with added redutants.Quantum yields (φketyl) for reduction in benzene of benzophenone triplet to ketyl radica
- Inbar, Shai,Linschitz, Henry,Cohen, Saul G.
-
-
Read Online
- Picosecond and Nanosecond Studies of the Photoreduction of Benzophenone by N,N-Diethylaniline and Triethylamine
-
The photoreduction of benzophenone by N,N-diethylaniline and trimethylamine has been examined in a number of solvents by both nano- and picosecond laser photolysis.With diethylaniline, electron transfer is the primary step and the spectrum of the ion pair has been detected even in nonpolar solvents such as benzene and cyclohexane.Rapid proton transfer then takes place to form a high yield of the ketyl radical.The lifetime of the ion pair in benzene is about 900 ps.In acetonitrile the ion pair dissociates into individual ions which then decay by back electron transfer and proton transfer.A spectral shift to the red occurs over 100 ps as the contact ion pair dissociates.In acidic alcohols such as methanol and trifluoroethanol proton transfer from the alcohol occurs to produce the ketyl radical.In the case of triethylamine, no distinct absorption band for benzophenone anion was seen in picosecond experiments but difference spectra, which removed much of the spectrum of benzophenone triplet, clearly showed some contribution from the anion.For this compound, it is likely that electron transfer occurs first followed by very fast proton transfer.
- Devadoss, C.,Fessenden, Richard W.
-
-
Read Online
- Dispersed fluorescence spectra of jet-cooled benzophenone ketyl radical: Assignment of the low-frequency vibrational modes
-
We measured the laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of jet-cooled benzophenone ketyl radical (BPK), which was generated by the 248 nm photolysis of α-phenyl benzoin or benzhydrol. The several single vibronic level (SVL) DF spectra of BPK were obtained to analyze the low-frequency vibrational modes. We identified three low-frequency vibrations of 47, 74, and 98 cm-1 in the ground state, which are assigned to the out-of-phase torsional motion, the in-phase torsional motion and Ph-C-Ph bending motion, respectively, with the aid of the density functional theory (DFT) calculation. The corresponding vibrational frequencies are 60, 65 and 84 cm-1, respectively, in the excited state. The geometries of BPK in the ground and excited states are discussed based on the experimental results, ab initio calculations, and one-dimensional Franck-Condon simulation.
- Hamatani, Satoshi,Tsuji, Kazuhide,Kawai, Akio,Shibuya, Kazuhiko
-
-
Read Online
- The hydrogen atom transfer reactivity of sulfinic acids
-
Sulfinic acids (RSO2H) have a reputation for being difficult reagents due to their facile autoxidation. Nevertheless, they have recently been employed as key reagents in a variety of useful radical chain reactions. To account for this paradox and enable further development of radical reactions employing sulfinic acids, we have characterized the thermodynamics and kinetics of their H-atom transfer reactions for the first time. The O-H bond dissociation enthalpy (BDE) of sulfinic acids was determined by radical equilibration to be ~78 kcal mol?1; roughly halfway between the RS-H BDE in thiols (~87 kcal mol?1) and RSO-H BDE in sulfenic acids (~70 kcal mol?1). Regardless, RSH, RSOH and RSO2H have relatively similar inherent H-atom transfer reactivity to alkyl radicals (~106 M?1 s?1). Counter-intuitively, the trend in rate constants with more reactive alkoxyl radicals follows the reaction energetics: ~108 M?1 s?1 for RSO2H, midway between thiols (~107 M?1 s?1) and sulfenic acids (~109 M?1 s?1). Importantly, since sulfinic and sulfenic acids are very strong H-bond donors (αH2 ~ 0.63 and 0.55, respectively), their reactivity is greatly attenuated in H-bond accepting solvents, whereas the reactivity of thiols is largely solvent-independent. Efforts to measure rate constants for the reactions of sulfinic acids with alkylperoxyl radicals were unsuccessful. Computations predict these reactions to be surprisingly slow; ~1000-times slower than for thiols and ~10?000?000-times slower than for sulfenic acids. On the other hand, the reaction of sulfinic acids with sulfonylperoxyl radicals-which propagate sulfinic acid autoxidation-is predicted to be almost diffusion-controlled. In fact, the rate-determining step in sulfinic acid autoxidation, and the reason they can be used for productive chemistry, is the relatively slow reaction of propagating sulfonyl radicals with O2 (~106 M?1 s?1).
- Griesser, Markus,Chauvin, Jean-Philippe R.,Pratt, Derek A.
-
-
Read Online
- Reaction of Triplet Benzophenone with Hydroxylamines: Yields of Radicals and Kinetics of Their Formation and Decay
-
Yields of ketyl radicals, ΦKH., triplet-donor reaction rate constants, Kir, and radical decay rate constants, Krd, have been obtained in laser flash irradiation of benzophenone with substituted hydroxylamines in
- Khan, Jamal,Casey, David,Linschitz, Henry,Cohen, Saul G.
-
-
Read Online
- Femtosecond-Picosecond Laser Photolysis Studies on Reduction Process of Excited Benzophenone with N-Methyldiphenylamine in Acetonitrile Solution
-
Photoreduction processes of benzophenone (BP)-N-methyldiphenylamine (MDPA) system in acetonitrile solution were studied by means of femtosecond-picosecond laser photolysis and time-resolved transient absorption spectroscopy.The reaction processes includin
- Miyasaka, Hiroshi,Nagata, Toshihisa,Kiri, Manabu,Mataga, Noboru
-
-
Read Online
- Properties of excited ketyl radicals of benzophenone analogues affected by the size and electronic character of the aromatic ring systems
-
The properties of benzophenone ketyl radical analogues with large aromatic ring systems, such as naphthylphenylketone (2), 4-benzoylbiphenyl (3), and bis(biphenyl-4-yl)methanone (4), were investigated in the excited state by using nanosecond-pico-second t
- Sakamoto, Masanori,Cai, Xichen,Fujitsuka, Mamoru,Majima, Tetsuro
-
-
Read Online
- Reaction of Benzophenone Triplets with Allylic Hydrogens. A Laser Flash Photolysis Study
-
The quenching of benzophenone by olefins having allylic hydrogens leads to hydrogen abstraction in addition to guenching by the ? system.The fraction of quenching events that leads to hydrogen abstraction in benzene at 27 deg C ranges from ca. 0 for conju
- Encinas, M.V.,Scaiano, J.C.
-
-
Read Online
- Effect of nitroxide radicals on chemically induced dynamic electron polarization of spin-correlated radical pairs in aqueous micellar solutions of sodium dodecyl sulfate
-
The multispin systems consisting of spin-correlated radical pairs (SCRPs) and stable nitroxide radicals, localized in micelles of sodium dodecyl sulfate (SDS), were studied by ESR and pulse laser photolysis techniques. In all the systems studied, the stab
- Gorelik,Tarasov,Shakirov,Bagryanskaya
-
experimental part
p. 1416 - 1427
(2009/09/30)
-
- On the mechanism of reaction of radicals with tirapazamine
-
Ketyl radicals produced by photolysis of ketones or di-tert-butyl peroxide (DTBP) in alcohol solvents react rapidly with tirapazamine (TPZ). The acetone ketyl radical (ACOH) reacts with TPZ with an absolute second-order rate constant of (9.7 ± 0.4) × 108 M-1 s-1. The reaction kinetics can be followed by monitoring the bleaching of TPZ absorption at 475 nm or the formation of a reaction product which absorbs at 320 and 410 nm. The ACOD radical reacts with TPZ in 2-propanol-OD with an absolute rate constant of (6.7 ± 0.5) × 108 M-1 s -1 corresponding to a kinetic isotope effect (KIE) of 1.4. Deuteration of the radical on carbon (ACOH-d6) retards the reaction of the radical with TPZ even further (absolute rate constant = (4.8 ± 0.04) × 108 M-1 s-1). This result corresponds to a KIE of 2.0. Radicals derived from dioxane and diisopropyl ether by flash photolysis of DTBP in ethereal solvent react with TPZ more slowly than do ketyl radicals. It is concluded that ketyl radicals react, in part, with TPZ in organic solvents by transfer of a hydrogen atom from the OH and CH 3 groups of the ketyl radical to the oxygen atom at the N4 position of TPZ to form acetone or acetone enol and a radical derivative of TPZ (TPZH). The latter species absorbs at 320 and 405 nm, has a lifetime of hundreds of microseconds in alcohol solvents, and decays by disproportionation to form TPZ and a reduced heterocycle. The reduced heterocycle eventually forms a desoxytirapazamine by a polar mechanism. The results are supported by density functional theory calculations. It is proposed that dioxanyl radical will also react, in part, with TPZ by transfer of a hydrogen atom from the carbon adjacent to the radical center to the oxygen atom at the N4 position of TPZ. This produces the enol ether and the previously mentioned TPZH radical. It is further posited that ether radicals react a bit more slowly than ketyl radicals because they lack the second mode of hydrogen transfer (from the OH group) that is present in the ACOH radical. Our data are permissive of the possibility that ether radicals add to TPZ at a rate that is competitive with β-hydrogen atom transfer.
- Shi, Xiaofeng,Mandel, Sarah M.,Platz, Matthew S.
-
p. 4542 - 4550
(2007/10/03)
-
- Kinetic study of the hydrogen abstraction reaction of the benzotriazole-N-oxyl radical (BTNO) with H-donor substrates
-
The aminoxyl radical (>N-O.) BTNO (benzotriazole-N-oxyl) has been generated by the oxidation of 1-hydroxybenzotriazole (HBT; >N-OH) with a CeIV salt in MeCN. BTNO presents a broad absorption band with λmax 474 nm and e 184
- Brandi, Paolo,Galli, Carlo,Gentili, Patrizia
-
p. 9521 - 9528
(2007/10/03)
-
- Chemistry of the t-butoxyl radical: Evidence that most hydrogen abstractions from carbon are entropy-controlled
-
Absolute rate constants and Arrhenius parameters for hydrogen abstractions (from carbon) by the t-butoxyl radical (tBuO·) are reported for several hydrocarbons and tertiary amines in solution. Combined with data already in the literature, an an
- Finn, Meghan,Friedline, Robert,Suleman, N. Kamrudin,Wohl, Christopher J.,Tanko, James M.
-
p. 7578 - 7584
(2007/10/03)
-
- Absolute rate constants for the quenching of reactive excited states by melanin and related 5,6-dihydroxyindole metabolites: Implications for their antioxidant activity
-
The triplet-excited state of benzophenone and the singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-P) have been employed as kinetic probes to obtain information on the antioxidant activity of the skin and eye pigment melanin and it
- Zhang, Xiangyang,Erb, Carl,Flammer, Josef,Nau, Werner M.
-
p. 524 - 533
(2007/10/03)
-
- Picosecond Dynamics of Nonadiabatic Proton Transfer: A Kinetic Study of Proton Transfer within the Contact Radical Ion Pair of Substituted Benzophenones/N,N-Dimethylaniline
-
Picosecond absorption spectroscopy has been employed in the study of the dynamics of proton transfer within substituted benzophenones/N,N-dimethylaniline contact radical ion pairs. The reactions were investigated in the solvents cyclohexane, benzene, and dimethylformamide. The correlation of the reaction rates with the change in free energy reveals that the reaction pathway corresponds to a nonadiabatic process, that is the reaction proceeds by proton tunneling. In nonpolar solvents, an inverted region is observed in the proton-transfer process.
- Peters, Kevin S.,Cashin, Amanda,Timbers, Peter
-
p. 107 - 113
(2007/10/03)
-
- Laser photolysis investigation of induced quenching in photoreduction of benzophenone by alkylbenzenes and anisoles
-
The quenching processes of triplet benzophenone (JBP) by alkylbenzenes (AB) and anisole derivatives (AD) in benzene (Bz) and a mixture of acetonitriie (ACT-,) and water (4 :1 v/V; have been studied on the basis of rate constants and efficiencies determined by nanosecond laser flash photolysis a; 355 n m at 295 K. It was found that (1) the deactivation of 3BPby ADs in ACN H2O (4 :1 v/v) was governed by electron transfer (ET) to produce the benzophenone anion (BP'~) and corresponding cation (AD' + ) radicals wiih efficiencies, atj 1 whereas no chemical species were formed in Bz; and 2) photoreduction of 3BPby ABs resulted in benzophenone ketyl radical (BPK) formation by benzylic hydrogen abstraction (HA) with efficiencies XHA 1 in 3z and ACN-H2O (4 :1 v/v). The residual efficiency (a: 1 -ET or ! -aH/1) was attributed to a birnolecular process with no photochemical product, which was named 'induced-quenching (IQf. The quenching rate constants (Jcq) of ;'BPby ADs and ABs were less than the diffusion limits of both Bz and AC1~H2O (4 :1 v/v). The net bimolecular rate constants for the ET, HA and IQ processes were estimated from the k values and efficiencies. The rate constants (%T and k,Q) of ET and IQ with AD versus the oxidation potential (£) of AD followed Rchm-Weller behaviour while logarithmic rate constants {/CHA and ki(j) of HA and IQ by ABs increased linearly with a decrease in the Em of AB. It was suggested, for the deactivation mechanism of 3BPby ABs and ADs (RH), that ;1) the IQ process was intersystem crossing (ISC) enhanced by the partial charge transfer (CT) character of the triplet excipiexes, 3(BP"~- A-RHa + )a,e; (2) radical ion formation by ET might be accomplished in a polar solvent by further CT interaction in the excipiex; (3) the process of BPK formation was inferred to be H-atom transfer in the exciplex, where the more protic H-atom was readily mobile, rather than ET followed by proton transfer and (4) the loss of efficiencies of photochemicalproduct formation was derived not from back ET but from the IQ process, inherent to photoreactions, via triplet excipiexes. The deactivation processes of 3BPby RH are illustrated in Scheme 1. I ET BP'- + RH'(3BP' + RHJcoj -3(BPO- RHg,.-BPK 4 R' BP + RH Scheme 1.
- Oekada, Kafsuji,Yamaji, Minora,Smzuka, Haruo
-
p. 861 - 866
(2007/10/03)
-
- The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds
-
A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C+OH + R1COR2 → >C=O + R1R2C+OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C+OH + R1CH=CH2 → >C=O + R1C+ HCH3; >R1CH=CH2 + R2C+HCH2R3 → R2C+HCH3 + R2CH=CHR3. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (aldehydes, ketones, and quinones) from the C-H and O-H bonds were compared.
- Denisov
-
p. 2110 - 2116
(2007/10/03)
-
- One-electron oxidation of sterically hindered amines to nitroxyl radicals: Intermediate amine radical cations, aminyl, α-aminoalkyl, and aminylperoxyl radicals
-
Sterically hindered amines (2,2,6,6-tetramethyl-substituted piperidines) are easily oxidized (i) by electron transfer to parent cations in n-butyl chloride solution, (ii) by the sulfate radical anion in aqueous solution, and (iii) by sensitized electron transfer to carbonyl triplets. In nonpolar surroundings in the nanosecond time range, the radical cations of the tertiary piperidines have been directly observed by optical spectroscopy to exhibit absorption maxima below λ = 300 nm and around 550 nm. Subsequently, they deprotonate to α-alkylamine radicals, which are also the first observable products of oxidation with sulfate radical anions in water. In the case of secondary piperidines, the amine radical cations deprotonate to aminyl radicals in times 8 M-1 s-1) for several alkyl radicals and for the tert-butyloxyl radical and less than 105 M-1 s-1 for alkylperoxyl radicals. Because of the minor importance of radical reactions with the sterically hindered amines (HALS), the antioxidant effect of these compounds ought to be explained by oxidation, primarily via cationic and subsequently radical intermediates to the persistent nitroxyl radicals, which scavenge other radicals very efficiently.
- Brede,Beckert,Windolph,Goettinger
-
p. 1457 - 1464
(2007/10/03)
-
- Diffusion of electrically neutral radicals and anion radicals created by photochemical reactions
-
Diffusion processes of the intermediate radicals created by the photochemical reactions of ketones in alcoholic solvents are investigated by using the transient grating (TG) method. The electrically neutral radicals and the anion radicals of acetophenone, benzaldehyde, xanthone, benzophenone and benzil were created selectively by controlling the concentration of sodium hydroxide (NaOH) in alcoholic solvents. The translational diffusion constants (D) of the anion radicals, the neutral radicals, and the parent stable molecules can be successfully measured under the same conditions by this method. It is found that both the neutral and anion radicals diffuse slower than the parent molecules. Values of D of the anion radicals, the neutral radicals and the parent molecules are compared in detail in wide ranges of solvent viscosities, solute sizes and temperatures. Under any conditions, D values of the charged radicals are similar to those of the neutral radicals. A possible origin of such a similarity is discussed in term of the intermolecular charge polarizabilities of the radicals.
- Okamoto, Koichi,Hirota, Noboru,Terazima, Masahide
-
p. 185 - 194
(2007/10/03)
-
- Direct measurement of ultrafast carbon-carbon cleavage rates via the subpicosecond charge-transfer activation of pinacols
-
Highly transient (benzpinacol) cation radicals (D2+.) and their ultrafast mesolytic fragmentations to the diarylhydroxymethyl radical (D.) and cation (D+) are directly observed on the early picosecond time scale upon the charge-transfer photoactivation of the intermolecular donor-acceptor complexes of pinacol donors with methyl viologen. Ultrashort lifetimes of the cation radicals with τ ~ 10 ps obtain (for the first time) from quantitative kinetics analysis of the time-resolved spectroscopic results. These rapid C-C bond scissions successfully compete with back-electron transfer, which normally predominates on this time scale, and lead to exceptionally high efficiencies for the oxidative fragmentation of pinacols.
- Bockman,Hubig,Kochi
-
p. 6542 - 6547
(2007/10/03)
-
- Photoionization of Diarylmethyl Radicals in Acetonitrile and Alcohol-Water: Laser Flash Production of Diarylcarbenium Ions
-
Diarylmethyl radicals Ar2CH. were produced by 248-nm laser photolysis (20 ns) of diphenylmethane, diphenylmethanol, and diarylmethyl halides in acetonitrile and water:alcohol mixtures.A few microseconds after their generation, the radicals were
- Faria, Joaquim L.,Steenken, S.
-
p. 1924 - 1930
(2007/10/02)
-
- Laser Flash Photolysis Study on the Hydrogen Atom Transfer Reaction from Triplet 2-Naphthylammonium Ion to Benzophenone. Remarkable Proton Effect
-
The laser flash photolysis study at 355 nm in the 2-naphthylammonium ion (RNH3+) and benzophenone (BP) system has been carried out in methanol-water (9:1 v/v) at 290 K.At first, triplet benzophenone (3BP*) produced by fast intersystem crossing of 1BP* excited at 355 nm is quenched by RNH3+ with almost diffusion-controlled rates (1.1x1010 M-1 s-1 at = 0.015 M; 3.9x109 M-1s-1 at = 0.5 M).The triplet 2-naphthylammonium ion (3RNH3+*) is produced in the nanosecond region by triplet energy transfer from 3BP* to RNH3+ with efficiencies of 0.41 ( = 0.015 M) and 0.33 ( = 0.5 M) at +> = 3.0x10-3 M and at = 1.12x10-2 M, competing with hydrogen abstraction of 3BP* from solvent (methanol) or RNH3+ molecules.Subsequently, in the microsecond region the hydrogen atom transfer (HT) reaction from 3RNH3+* to ground BP occurs effectively to give the naphthylamine cation radical (RNH2.+) and the ketyl radical (>C.OH).The decayl rate (τobsd)-1 of 3RNH3+* increases with an increase of the BP concentration, and at higher BP concentrations a leveling off is observed.The (τobsd)-1 value decreases drastically with an increase in acid concentration.The mechanism of the HT reaction can be accounted for by the intracomplex HT reaction of the triplet exciplex 3(RNH3+ *** >CO)* with a rate constant of ca. 5x106 s-1.At higher acid concentrations, the triplet complex is decomposed by protonation to give free 3RNH3+* and BP, resulting in suppression of the rate for the HT reaction.
- Kohno, Seiji,Hoshino, Mikio,Shizuka, Haruo
-
p. 5489 - 5495
(2007/10/02)
-
- Reactions of triplet carbonyl compounds and nitro derivatives with silanes
-
The photoinduced reactions of aromatic and aliphatic carbonyl compounds and nitro derivatives with tetramethylsilane, hexamethyldisilane, and tetrakis(trimethylsilyl)silane were investigated by EPR spectroscopy. The photoreaction of benzophenone with the
- Alberti, Angelo,Dellonte, Sergio,Paradisi, Carmen,Roffia, Sergio,Pedulli, Gian Franco
-
p. 1123 - 1129
(2007/10/02)
-
- Picosecond laser photolysis studies on the photoreduction of excited benzophenone by diphenylamine in solutions
-
The mechanism of photoreduction of benzophenone (BP) by diphenylamine (DPA) in isooctane as well as acetonitrile and other polar solvents has been investigated by means of picosecond laser photolysis and time-resolved transient absorption spectral measurements. The results of measurements have demonstrated clearly that the hydrogen abstraction and charge transfer (CT) or ion pair (IP) state formation by electron transfer are competing at encounter between triplet benzophenone (3BP*) and DPA both in nonpolar and polar solvent, and the CT or IP state relaxed with respect to the donor acceptor configurations and solvation does not contribute to the ketyl radical formation. It has been concluded that the very short-lived CT state at encounter between 3BP* and DPA plays a crucial role in the hydrogen abstraction reaction, i.e. the mutual orientation of 3BP* and DPA in this very short-lived CT state at encounter will determine the successive process, the formation of the ketyl radical or relaxed CT or IP state.
- Miyasaka,Mataga
-
p. 131 - 137
(2007/10/02)
-
- Kinetics of Ketyl Radical Reactions Occurring in the Photoreduction of Benzophenone by Isopropyl Alcohol
-
Benzophenone - isopropyl alcohol - acetonitrile, benzophenone - benzhydrol - acetonitrile and acetone - isopropyl alcohol - acetonitrile systems were studied by laser flash photolysis transient absorption technique and by steady-state quantum yield determinations.Kinetic parameters were derived for reactions of the ketyl radicals formed in the primary photoreduction steps.The room temperature overall rate coefficients at 0.001 mol dm-3 benzophenone and 2.6 mol dm-3 isopropyl alcohol in acetonitrile are in 1E8 dm3 mol-1 s-1 units: 12.8+/-0.6, 8.0+/-0.4 and 0.59+/-0.01 for Me2COH + Me2COH, Me2COH + Ph2COH and Ph2COH + Ph2COH, respectively. (The error limits represent 2 standard deviations).Combination was shown to be the only significant reaction in the interaction of two Ph2COH radicals, while disproportionation is the dominating path in the reactions involving Me2COH radicals.Evidences are presented for the existence of four reaction channels in the cross reaction of the two ketyl radicals.These are combination (4a), disproportionations (4b) and (4c) and formation of a "light absorbing transient" (4d), with branching ratios k4a/k4 = 0.18+/-0.08, k4b/k4 = 0.61+/-0.11, k4c/k4 = 0.05+/-0.02 and k4d/k4 = 0.18+/-0.03, respectively. - The radical transfer reaction Me2COH + Ph2CO --> Me2CO + Ph2COH, which may change considerably the product distribution and quantum yields in the photoreduction of benzophenone by isopropyl alcohol at medium and low light intensities, was investigated by quantum yield determinations and a room temperature rate coefficient of k5 = (7.3+/-1.2)E4 dm3 mol-1 s-1 was extracted through computer model simulation.This rate coefficient is considerably lower than the values reported previously. - Keywords: Chemical Kinetics / Elementary Reactions / Photochemistry / Radicals
- Demeter, A.,Laszlo, B.,Berces, T.
-
p. 1478 - 1485
(2007/10/02)
-
- Photochemistry of Benzophenone in Aliphatic Amines Studied by Laser Photolysis in the Temperature Range 300-77 K
-
Laser photolysis studies of benzophenone in both sec-butylamine and triethylamine were carried out in the temperature range 300-77 K.For the sec-butylamine solution of benzophenone, the transients observed after laser pulsing are found to be the ketyl and anion radicals of benzophenone.The ratio of the yield for the formation of ketyl radical to that of the anion radical is markedly dependent on temperature: the ketyl radical is the major product at high temperatures while the anion radical becomes predominant at low temperatures.On the other hand, the triethylamine solution of benzophenone gives solely the ketyl radical as a photoproduct in the temperature range studied.The photochemical reaction of benzophenone in both sec-butylamine and triethylamine is markedly suppressed on going from high to low temperatures.These results are discussed in detail on the basis of the photochemical reaction mechanism involving the formation of the triplet charge-transfer complex (i.e., triplet exciplex) between triplet benzophenone and an amine molecule.The solvent effects on the photoreaction are also discussed.
- Hoshino, Mikio,Shizuka, Haruo
-
p. 714 - 718
(2007/10/02)
-
- ESR Studies of Photochemical Reactions of Diphenylamines, Phenothiazines, and Phenoxazines
-
Detailed photochemical reactions of diphenylamines, phenothiazines, and phenoxazines have been studied by the ESR technique.Our results show nitroxide radicals are the stable products of the photooxidation of both diphenylamines and phenothiazines, while cation radicals are the stable product of the photolyzed phenoxazines in the presence of air.
- Lin, Tien-Sung,Retsky, Joel
-
p. 2687 - 2689
(2007/10/02)
-
- Investigations on Spin-Trapping Kinetics in Radical Systems with Different Reactivity. Photolyses of α-Phenylbenzoine in the Presence of Benzylidene-tert.-butylamine N-oxide
-
As a model system to study the spin-trapping kinetics of systems forming two radicals of different reactivity α-phenylbenzoine (PhB) was chosen.The photolysis of PhB forms benzoyl (B*) and diphenylketylradicals (K*), respectively, which can be trapped by benzylidene-tert-butyl-amine N-oxide (PBN), as a spin trap.The rate constants of radical addition to PBN and their reactions with the spin adducts were determined by means of flash photolysis and kinetic calculations of the PBN concentration dependence.For B* and K* addition rate constants to PBN of k1 = 8.1E5M-1 s-1 and k2 = 5.8E4M-1s-1, respectively, were found.The rate constants for the reactions of B* and K* with the spin adducts are in the order of 1E9...1E10M-1s-1.
- Baumann, H.,Timpe, H.-J.,Zubarev, V. E.,Fok, N. V.,Mel'nikov, M. Y.,Raskasovskij, Y. W.
-
p. 749 - 758
(2007/10/02)
-
- Picosecond Dynamics of Proton Transfers
-
The dynamics of proton transfer within an acid-base pair, which is created by a photoinduced electron transfer from an amine to a ketone, is investigated through picosecond absorption spectroscopy.Questions relating to molecular motion within the encounter complex and the distance dependence of proton transfer are addressed.Specifically, for substituted benzophenones and diphenylmethylamine, isotope and solvent dependence studies reveal that, although an acid-base pair is formed as an encounter complex, prior reorientation within the complex must take place before proton transfer.Other systems investigated include the transfer of a proton from the radical cation of N-methylacridan to the radical anions of benzophenone and anthraquinone as well as an intramolecular proton transfer for a molecule containing dimethylaniline linked to benzophenone by a methylene bridge.
- Manring, Lewis E.,Peters, Kevin S.
-
p. 6452 - 6458
(2007/10/02)
-
- PHENYL RADICAL KINETICS.
-
The reactions of phenyl radicals with 17 substrates have been examined by using laser flash photolysis techniques. Absolute rate constants were determined for all the reactions in Freon 113 at 298 K by using iodobenzene or benzoyl peroxide as radical precursors. The following are some representative rate constants, which include all possible modes (or sites) of attack on each substrate: benzene, (4. 5 plus or minus 0. 3) multiplied by 10**5 M** minus **1 s** minus **1; chlorobenzene, (1. 18 plus or minus 0. 13) multiplied by 10**6 M** minus **1 s** minus **1; methyl methacrylate, (1. 8 plus or minus 0. 3) multiplied by 10**8 M** minus **1 s** minus **1; carbon tetrachloride, (7,8 plus or minus 0. 7) multiplied by 10**6 M** minus **1 s** minus **1; tetrahydrofuran, (4. 8 plus or minus 0. 6) multiplied by 10**6 M** minus **1 s** minus **1; toluene (1. 7 plus or minus 0. 7) multiplied by 10**6 M** minus **1 s** minus **1.
- Scaiano,Stewart
-
p. 3609 - 3614
(2007/10/02)
-
- Triplet Quenching by tert-Butyl Hydroperoxide
-
Laser flash photolysis studies show that tert-butyl hydroperoxide is an excellent quencher.For example, the rate constants for benzophenone and phenanthrene in benzene at 300 K are 1.8 x 108 and 2.3 x 107 M-1 s-1/sup
- Stewart, Laura C.,Carlsson, D. J.,Wiles, D. M.,Scainano, J. C.
-
p. 3605 - 3609
(2007/10/02)
-
- LASER FLASH SPECTROMETRIC INVESTIGATIONS OF BIRADICALS AND CAGED RADICAL PAIRS
-
The use of laser flash spectroscopic techniques allows the detection, identification and dynamic analysis of biradicals and caged radical pairs in fluid solution at ambient temperatures.The conventional flash spectroscopic device has been advanced with the advent of the Excimer Laser, as an excitation source of the optical multichannel analyzer (OMA) as a detector system and of the transient digitizer (TD) as a data collector and analyzer.The Excimer Laser, in addition to serving as an excitation source, may also induce novel photophysical and/or photochemical events as a result of the unusually high intensities produced by such devices.Examples are given of the use of Excimer Laser-OMA and Excimer Laser-TD systems to study the dynamics of radical pairs that are caged in micelle aggregates and to study novel emissions produced by photoexcitation of benzophenone and substituted benzophenones.
- Turro, Nicholas J.
-
p. 809 - 817
(2007/10/02)
-
- Laser Photolysis Study of the Reactions of Alcoxyl Radicals Generated in the Photosensitized Decomposition of Organic Hyponitrites
-
The triplet photosensitized decomposition of organic hyponitrites produces alkoxy radicals in good yields.For example, the quenching of p-methoxyacetophenone triplets by di-tert-butyl hyponitrite in benzene at 27 deg C occurs with kq = 1.41 * 108 M-1 s-1, and ca.50percent of the quenching events lead to a pair of tert-butoxy radicals.Steric effects play an important role in the kinetics of the quenching process; e.g., phenanthrene triplets are quenched 23 times faster by di-phenylethyl hyponitrite than by di-tert-butyl hyponitrite.The 2-phenylethoxy radicals undergo efficient β-cleavage to yield PhH2 radicals with log (kβ/s-1) = 10.89 - 4830/θ, with θ = 2.3RT in cal/mol.The rate is markedly enhanced by polar solvents.The reaction of alkoxy radicals with diphenylmethanol is also sensitive to steric effects and occurs with rate constants (benzene, 27 deg C) of 5.8 * 106 and 2.6 * 107 M-1 s-1 for tert-butoxy and 2-phenylehoxy radicals, respectively.
- Mendenhall, G. D.,Stewart, L. C.,Scaiano, J. C.
-
p. 5109 - 5114
(2007/10/02)
-
- Absolute Rate Constants for the Reactions of tert-Butoxyl with Ethers: Importance of the Stereoelectronic Effect
-
Absolute rates of hydrogen atom abstraction by tert-butoxyl from variety of cyclic and acyclic ethers, orthoformates, and acetals have been measured at 27 deg C by laser flash photolysis techniques.High rates of abstraction from the C-H bond adjacent to oxygen and forming small dihedral angle (ca. 30 deg) with the ?-type orbital(s) on the oxygen(s) have been observed.For larger dihedral angles (ca. 90 deg) the rate of C-H abstraction is much lower.Arrehenius parameters have also been measured for a few representative cyclic ethers.
- Malatesta, V.,Scaiano, J. C.
-
p. 1455 - 1459
(2007/10/02)
-
- Photosensitized Dissociation of Di-tert-butyl Peroxide. Energy Transfer to a Repulsive Excited State
-
Energy transfer from a variety of aromatic hydrocarbons and ketones to di-tert-butyl peroxide has been examined by using nanosecond laser flash photolysis techniques.Triplet energy transfer to the peroxide leads to its efficient cleavage into two tert-butoxy radicals.Representative rate constants for triplet quenching in benzene at 25 deg C are 7.9*1E6, 3.4*1E6, and 7.0*1E4 M-1 s-1 for p-methoxypropiophenone, benzophenone, and benzanthracene, respectively.The rate of transfer for p-methoxypropiophenone (ET ca. 72.5 kcal/mol) is approximately temperature independent; for lower energy sensitizers ca. 0.17 kcal/mol activation energy is required for each kilocalorie per mole decrease in triplet energy.No evidence indicating exciplex intermediacy was found.A model for energy transfer to a repulsive state of peroxide is proposed in which no activation energy is required if the sensitiser meets the energy requirements at the O-O equilibrium distance.For sensitizers of lower triplet energy, energy transfer to a repulsive state is proposed to occur from a thermally activated ground state having a greater than equilibrium oxygen-oxygen bond lenght.The same mechanism may apply in other systems where the acceptor lacks low-lying excited states.A few rate constants for the quenching of singlet sensitizers have also been determined by using fluorescence techniques.
- Scaiano, J. C.,Wubbels, Gene G.
-
p. 640 - 645
(2007/10/02)
-
- Solvent Effects on the Picosecond Dynamics of the Photoreduction of Benzophenone by Aromatic Amines
-
Solvent effects on the primary processes in the photoreduction of benzophenone by N,N-dimethylaniline and N,N-diethylaniline have been studied by using picosecond absorption spectroscopy.It has been found that the photoreduction proceeds by rapid electron transfer to form the solvent-separated ion pair.This is followed by diffusion to form a contact ion pair.Sebsequent to contact ion-pair formation, proton transfer is observed.Solvent effects on the ion-pair intermediate indicated a dielectric dependent equilibrium between the contact ion pair and the ketyl radical.
- Simon, John D.,Peters, Kevin S.
-
p. 6403 - 6406
(2007/10/02)
-
- Laser Flash Photolysis Study of the Reactions of Carbonyl Triplets with Phenols and Photochemistry of p-Hydroxypropiophenone
-
The quenching of aromatic carbonyl triplets by phenols is very fast process for both n,?* and ?,?* states.Representative rate constants for benzophenone triplets in benzene are 1.3x109 (phenol) and 8.1x108 (m-fluorophenol) M-1 s-1, while for p-methoxypropiophenone the values are 4.9x109 and 5.0x109 M-1 s-1 for the same pair of substrates; 18 different phenols were examined.The rate constants are smaller in wet acetonitrile as a result of hydrogen bonding; for example for the benzophenone-phenol system we measured 8.0x107 M-1 s-1.Isotope effects are larger for p-methoxypropiophenone than for benzophenone: for example in the case of phenol the values of kH/kD are 3.9 and 1.2, respectively (in wet acetonitrile).The quenching reaction leads to the efficient, but not quantitative, formation of phenoxy and ketyl radicals, with the only exception of p-nitrophenol where the process involves electronic energy transfer.The temperature dependence of a few representative systems as well as the quenching by methyl ethers of similar structure, has also been examined.The photochemistry of p-hydroxypropiophenone is characterized by efficient self-quenching involving a head-to-tail hydrogen-transfer reaction.
- Das, P. K.,Encinas, M. V.,Scaiano, J. C.
-
p. 4154 - 4162
(2007/10/02)
-