Mechanism of quenching by oxygen of the excited states of ruthenium(II) complexes in aqueous media. Solvent isotope effect and photosensitized generation of singlet oxygen, O2(1Deltag), by [Ru(diimine)(CN)4]2- complex ions

In this study we report on the photophysical properties of some [RuL(CN)4](2-) complex ions where L = 2,2'-bipyridine (bpy), 5,5'-dimethyl-2,2'-bipyridine (dmb), 1,10-phenanthroline (phen), 1-ethyl-2-(2-pyridyl)benzimidazole (pbe), 2,2':6',2'-terpyridine (tpy) and [RuL3](2+) where L = bpy or phen. Measurements were carried out in H2O and D2O. The effect of the deuterium isotope effect on the lifetime of these complexes is discussed. It has also been found that the presence of cyano groups has a pronounced effect on the lifetime of the excited metal-to-ligand charge transfer ((3)MLCT) of these complexes. Quenching of the (3)MLCT states by oxygen is reported in H2O and D2O. The rate constants, k(q), for quenching of the (3)MLCT states of these ruthenium complex ions by molecular oxygen are in the range (2.55 to 7.01) x 10(9) M(-1) s(-1) in H2O and (3.38 to 5.69) x 10(9) M(-1) s(-1) in D2O. The efficiency of singlet oxygen, O2((1)Delta(g)), production as a result of the (3)MLCT quenching by oxygen, f(Delta)(T), is reported in D2O and found to be in the range 0.29-0.52. The rate constants, k(q)(Delta), for quenching of singlet oxygen by ground state sensitizers in D2O is also reported and found to be in the range (0.15 to 3.46) x 10(7) M(-1) s(-1). The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T1(3)Sigma) and (3)(T1(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O2((1)Delta(g)), and O2((3)Sigmag-). The balance between CT and nCT deactivation channels which is described by the relative contribution p(CT) of CT induced deactivation is discussed. The kinetic model proposed for the quenching of pi-pi* triplet states by oxygen can also be applied to the quenching of (3)MLCT states by oxygen.     

Acrylamide-quenching of Rhizomucor miehei lipase

Steady-state and time-resolved fluorescence-quenching measurements have been performed to study multitryptophan lipase from filamentous fungus Rhizomucor miehei. Using the steady-state acrylamide fluorescence quenching data and the fluorescence-quenching-resolved-spectra (FQRS) method, the total emission spectrum of native ("closed-lid") lipase has been decomposed into two distinct spectral components accessible to acrylamide. According to FQRS analysis, more quenchable component has a maximum of fluorescence emission at about 352 nm whereas less quenchable component emits at about 332 nm. The redder component participates in about 60-64% of the total lipase fluorescence and may be characterized by the dynamic and static quenching constants equal to K(1) = 3.75 M(-1) and V(1) = 1.12 M(-1), respectively. The bluer component is quenchable via dynamic mechanism with K(2) = 1.97 M(-1). Significant difference in the values of acrylamide bimolecular rate quenching constants estimated for redder and bluer component (i.e., k(q) = 1.2 x 10 (9) M(-1)s (-1) vs. k(q) = 4.3 x 10(8) M(-1) s(-1), respectively), suggests that tryptophan residues in fungal lipase are not uniformly exposed to the solvent.     

A highly sensitive method for time-resolved detection of O(1D) applied to precise determination of absolute O(1D) reaction rate constants and O(3P) yields

We demonstrate detection, in the gas-phase, of O(1D2) at concentrations down to 10(7) cm(-3) and develop this new method for time-resolved kinetic studies allowing both the total removal rate of O(1D2), of up to 1.5 x 10(6) s(-1), and the fraction quenched to O(3P(J)) by species X, k(q)/k(X), to be determined precisely from a single time profile: at 295 K we find, k(O(1D2) + N2O) = (1.43 +/- 0.08) x 10(-10) cm3 s(-1) with k(q)/k(N2O) = 0.056 +/- 0.009; k(O(1D2) + C2H2) = (3.1 +/- 0.2) x 10(-10) cm3 s(-1) with k(q)/k(C2H2) = 0.020 +/- 0.010; k(q)/k(H2O) < 0.003 for O(1D2) + H2O.     

A fluorescence-based method for direct measurement of submicrosecond intramolecular contact formation in biopolymers: an exploratory study with polypeptides.

A fluorescent amino acid derivative (Fmoc-DBO) has been synthesized, which contains 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as a small, hydrophilic fluorophore with an extremely long fluorescence lifetime (325 ns in H2O and 505 ns in D2O under air). Polypeptides containing both the DBO residue and an efficient fluorescence quencher allow the measurement of rate constants for intramolecular end-to-end contact formation. Bimolecular quenching experiments indicated that Trp, Cys, Met, and Tyr are efficient quenchers of DBO (k(q) = 20, 5.1, 4.5, and 3.6 x 10(8) M(-1) x s(-1) in D2O), while the other amino acids are inefficient. The quenching by Trp, which was selected as an intrinsic quencher, is presumed to involve exciplex-induced deactivation. Flexible, structureless polypeptides, Trp-(Gly-Ser)n-DBO-NH2, were prepared by standard solid-phase synthesis, and the rates of contact formation were measured through the intramolecular fluorescence quenching of DBO by Trp with time-correlated single-photon counting, laser flash photolysis, and steady-state fluorometry. Rate constants of 4.1, 6.8, 4.9, 3.1, 2.0, and 1.1 x 10(7) s(-1) for n = 0, 1, 2, 4, 6, and 10 were obtained. Noteworthy was the relatively slow quenching for the shortest peptide (n = 0). The kinetic data are in agreement with recent transient absorption studies of triplet probes for related peptides, but the rate constants are significantly larger. In contrast to the flexible structureless Gly-Ser polypeptides, the polyproline Trp-Pro4-DBO-NH2 showed insignificant fluorescence quenching, suggesting that a high polypeptide flexibility and the possibility of probe-quencher contact is essential to induce quenching. Advantages of the new fluorescence-based method for measuring contact formation rates in biopolymers include high accuracy, fast time range (100 ps-1 micros), and the possibility to perform measurements in water under air.     

Kinetics and mechanism of large rate enhancement in the alkaline hydrolysis of N'-morpholino-N-(2'-methoxyphenyl)phthalamide

The apparent second-order rate constant (k OH) for hydroxide-ion-catalyzed conversion of 1 to N-(2'-methoxyphenyl)phthalamate (4) is approximately 10(3)-fold larger than k OH for alkaline hydrolysis of N-morpholinobenzamide (2). These results are explained in terms of the reaction scheme 1 --> k(1obs) 3 --> k(2obs) 4 where 3 represents N-(2'-methoxyphenyl)phthalimide and the values of k(2obs)/k(1obs) vary from 6.0 x 10(2) to 17 x 10(2) within [NaOH] range of 5.0 x 10(-3) to 2.0 M. Pseudo-first-order rate constants (k(obs)) for alkaline hydrolysis of 1 decrease from 21.7 x 10(-3) to 15.6 x 10(-3) s(-1) with an increase in ionic strength (by NaCl) from 0.5 to 2.5 M at 0.5 M NaOH and 35 degrees C. The values of k obs, obtained for alkaline hydrolysis of 2 within [NaOH] range 1.0 x 10(-2) to 2.0 M at 35 degrees C, follow the relationship k(obs) = kOH[HO(-)] + kOH'[HO (-)] (2) with least-squares calculated values of kOH and kOH' as (6.38 +/- 0.15) x 10(-5) and (4.59 +/- 0.09) x 10(-5) M (-2) s(-1), respectively. A few kinetic runs for aqueous cleavage of 1, N'-morpholino-N-(2'-methoxyphenyl)-5-nitrophthalamide (5) and N'-morpholino-N-(2'-methoxyphenyl)-4-nitrophthalamide (6) at 35 degrees C and 0.05 M NaOH as well as 0.05 M NaOD reveal the solvent deuterium kinetic isotope effect (= k(obs) (H 2) (O)/ k(obs) (D 2 ) (O)) as 1.6 for 1, 1.9 for 5, and 1.8 for 6. Product characterization study on the cleavage of 5, 6, and N-(2'-methoxyphenyl)-4-nitrophthalimide (7) at 0.5 M NaOD in D2O solvent shows the imide-intermediate mechanism as the exclusive mechanism.     

Suggested Improvement in the Ing-Manske Procedure and Gabriel Synthesis of Primary Amines: Kinetic Study on Alkaline Hydrolysis of N-Phthaloylglycine and Acid Hydrolysis of N-(o-Carboxybenzoyl)glycine in Aqueous Organic Solvents

A slight modification of the Gabriel synthesis of primary amines is suggested on the basis of the observed and reported values of rate constants for the alkaline and acid hydrolyses of phthalimide, phthalamic acid, benzamide, and their N-substituted derivatives. The suggested procedure requires shorter reactions time and milder acid-base reaction conditions compared with the conventional acid-base hydrolysis in the Gabriel synthesis. A slight modification in the Ing-Manske procedure is also suggested. Pseudo-first-order rate constants, k(obs), for hydrolysis of N-phthaloylglycine, NPG, decrease from 24.1 x 10(-3) to 7.72 x 10(-3) and 6.12 x 10(-3) s(-1) with increasing acetonitrile and 1,4-dioxan contents, respectively, from 2 to 50% v/v (all the percentages given in the paper are vol %), while increasing the organic cosolvents content from 50 to 80% increases k(obs) from 7.72 x 10(-3) to 19.7 x 10(-3) s(-1) for acetonitrile and from 6.12 x 10(-3) to 52.8 x 10(-3) s(-1) for 1,4-dioxan, in aqueous organic solvents containing 0.004 M NaOH at 35 degrees C. The rate constants for NPG hydrolysis decrease from 2.11 x 10(-2) to 1.19 x 10(-4) s(-1) with increasing MeOH content from 2 to 84%, in aqueous organic solvents containing 2% MeCN and 0.004 M NaOH at 35 degrees C.     

Solvent effect on the alpha-effect for the reactions of aryl acetates with butane-2,3-dione monoximate and p-chlorophenoxide in MeCN-H2O mixtures.

Second-order rate constants have been measured spectrophotometrically for the nucleophilic reactions of three substituted phenyl acetates with butane-2,3-dione monoximate (Ox(-)) as an alpha-nucleophile and p-chlorophenoxide (ClPhO(-)) as corresponding normal nucleophile, in MeCN-H2O mixtures of varying compositions at 25.0 +/- 0.1 degrees C. The reactivity of Ox(-) toward the aryl acetates decreases upon addition of MeCN to the reaction medium up to ca. 30 mol % MeCN, followed by a gradual increase in rate upon further addition of MeCN. A similar result has been obtained for the reaction of ClPhO(-) with the aryl acetates. However, the decrease in rate is more significant for the less reactive ClPhO(-) than for the more reactive Ox(-). Thus, for all the aryl acetates studied, Ox(-) exhibits a sizable alpha-effect (k(Ox)-/k(ClPhO)-) whose magnitude increases as the mol % MeCN in the reaction medium increases. The relative basicities (DeltapK(a)) of Ox(-) and ClPhO(-) have been determined spectrophotometrically using piperazine as a reference base. The DeltapK(a) values increase on increasing the mol % MeCN in the medium for both Ox(-) and ClPhO(-). The difference in the relative basicities of these nucleophiles (DeltaDeltapK(a)) becomes larger with increasing mol % MeCN. The plots of log k(Ox)-/k(ClPhO)- vs DeltaDeltapK(a) for the three substrates are linear with near-unit slope, indicating that the difference in the relative basicity of the nucleophiles is largely responsible for the increasing alpha-effect with medium composition in this system.     

Deactivation of singlet molecular oxygen by organo-selenium compounds exhibiting glutathione peroxidase activity and by sulfur-containing homologs.

The bimolecular rate constants (k) of quenching of molecular singlet oxygen 1O2 (1 delta g) by organo-selenium compounds exhibiting glutathione peroxidase activity and by sulfur analogs have been determined by time resolved phosphorescence detection of 1O2 in CD3OD and C6D6, with no solvent effect. The rate constants of quenching by the Se-containing compounds were found to be approximately one order of magnitude higher than those of the S-containing homologs. A linear correlation was observed between log k and the Hammett constant omega ortho with p = -0.89, the rate constant being higher for molecules with an electron-donating substituent and lower for those with an electron-withdrawing substituent. This observation is consistent with the involvement of a charge transfer complex in the deactivation of singlet oxygen.     

Solvent and pressure effects on quenching by oxygen of 9,10-dimethylanthracene fluorescence in liquid n-alkanes: a study on solvent cage effects

The fluorescence quenching by oxygen of 9,10-dimethylanthracene (DMEA) in liquid ethane and propane at pressures up to 60 MPa and 25 degrees C was investigated. The apparent activation volumes for the quenching rate constant, k(q),DeltaV++(q) , were 5.0 +/- 3.4 and 7.4 +/- 1.0 cm(3)/mol, whereas those for the solvent viscosity, eta,DeltaV++(eta) , were 190 +/- 22 and 42 +/- 1 cm(3)/mol in ethane and propane at 6.0 MPa, respectively. These results were discussed together with those in n-alkanes (C(4)-C(7)) and methylcyclohexane (MCH) that were previously reported, and it was found that DeltaV++(q) increases monotonically but DeltaV++(eta) decreases rapidly with increasing the number of carbon atoms in n-alkanes. The plot of ln k(q) against ln eta showed a leveling-off with decreasing eta. These observations were analyzed satisfactorily by the pressure dependence of the solvent viscosity on k(q) coupled with that of the radial distribution function, g(sigma), at contact with a hard sphere assumption. The apparent bimolecular rate constant, k(bim,0), for the quenching in the solvent cage was evaluated by extrapolating to g(sigma)eta = 0 in the plot of g(sigma)/k(q) against g(sigma)eta, and it was found that k(bim,0) decreased with increasing the radius of the solvent molecule. From the solvent size dependence of k(bim,0), the solvent cage effect was discussed phenomenologically.     

Protonated benzofuran,anthracene, naphthalene,benzene, ethene,and ethyne: measurements and estimates of pK(a) and pK(R)

Aqueous solvolyses of acyl derivatives of hydrates (water adducts) of anthracene and benzofuran yield carbocations which undergo competitive deprotonation to form the aromatic molecules and nucleophilic reaction with water to give the aromatic hydrates. Trapping experiments with azide ions yield rate constants k(p) for the deprotonation and k(H2O) for the nucleophilic reaction based on the "azide clock". Combining these with rate constants for (a) the H(+)-catalyzed reaction of the hydrate to form the carbocation and (b) hydrogen isotope exchange of the aromatic molecule (from the literature) yields pK(R) = -6.0 and -9.4 and pK(a) = -13.5 and -16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pK(R) = -6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile-water mixtures, and pK(a) = -20.4 for the 2-protonated naphthalene from combining k(p) with an exchange rate constant. The differences between pK(R) and pK(a) correspond to pK(H2O), the equilibrium constant for hydration of the aromatic molecule (pK(H2O) = pK(R) - pK(a)). For naphthalene and anthracene values of pK(H2O) = +13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pK(a) = -24.3 is derived from an exchange rate constant and an assigned value for the reverse rate constant close to the limit for solvent relaxation. Combining this pK(a) with calculated values of pK(H2O) gives pK(R) = -2.4 and -2.1 for protonated benzenes forming 1,2- and 1,4-hydrates, respectively. Coincidentally, the rate constant for protonation of benzene is similar to those for protonation of ethylene and acetylene (Lucchini, V.; Modena, G. J. Am. Chem Soc. 1990, 112, 6291). Values of pK(a) for the ethyl and vinyl cations (-24.8) may thus be derived in the same way as that for the benzenonium ion. Combining these with appropriate values of pK(H2O) then yields pK(R) = -39.8 and -29.6 for the vinyl and ethyl cations, respectively.     

Choice of solvent (MeCN vs H(2)O) decides rate-limiting step in S(N)Ar aminolysis of 1-fluoro-2,4-dinitrobenzene with secondary amines: importance of Brønsted-type analysis in acetonitrile

A kinetic study is reported for nucleophilic substitution reactions of 2,4-dinitro-1-fluorobenzene (DNFB) with a series of secondary amines in MeCN and H2O at 25.0 degrees C. The reaction in MeCN results in an upward curvature in the plot of k(obsd) vs [amine], indicating that the reaction proceeds through a rate-limiting proton transfer (RLPT) mechanism. On the contrary, the corresponding plot for the reaction in H2O is linear, implying that general base catalysis is absent. The ratios of the microscopic rate constants for the reactions in MeCN are consistent with the proposed mechanism, e.g., the facts that k2/k(-1) < 1 and k3/k2 > 10(2) suggest that formation of a Meisenheimer complex occurs before the rate-limiting step and the deprotonation by a second amine molecule becomes dominant when [amine] > 0.01 M, respectively. The Br?nsted-type plots for k1k2/k(-1) and k1k3/k(-1) are linear with betanuc values of 0.82 and 0.84, respectively, which supports the proposed mechanism. The Br?nsted-type plot for the reactions in H2O is also linear with betanuc = 0.52 which has been interpreted to indicate that the reaction proceeds through rate-limiting formation of a Meisenheimer complex. DNFB is more reactive toward secondary amines in MeCN than in H2O. The enhanced basicity of amines as well as the increased stability of the intermediate whose charges are delocalized through resonance are responsible for the enhanced reactivity in the aprotic solvent.     

On the localization of water-soluble porphyrins in micellar systems evaluated by static and time-resolved frequency-domain fluorescence techniques

Fluorescence quenching of meso-tetrakis-4-sulfonatophenyl (TPPS(4)) and meso-tetrakis-4-N-methylpyridil (TMPyP) porphyrins is studied in aqueous solution and upon addition of micelles of sodium dodecylsulfate (SDS), cetyltrimethylammonium chloride (CTAC), N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and t-octylphenoxypolyethoxyethanol (Triton X-100). Potassium iodide (KI) was used as quencher. Steady-state Stern-Volmer plots were best fitted by a quadratic equation, including dynamic (K(D)) and static (K(S)) quenching. K(S) was significantly smaller than K(D). Frequency-domain fluorescence lifetimes allowed estimating bimolecular quenching constants, k(q). At 25 degrees C, in aqueous solution, TMPyP shows k(q) values a factor of 2-3 higher than the diffusional limit. TPPS(4) shows collisional quenching with pH dependent k(q) values. For TMPyP quenching results are consistent with reported binding constants: a significant reduction of quenching takes place for SDS, a moderate reduction is observed for HPS and almost no change is seen for Triton X-100. Similar data were obtained at 50 degrees C. For CTAC-TPPS(4) system an enhancement of quenching was observed as compared to pure buffer. This is probably associated to accumulation of iodide at the cationic micellar interface. The attraction between CTAC headgroups and I(-), and repulsion between SDS and I(-), enhances and reduces the fluorescence quenching, respectively, of porphyrins located at the micellar interface. The small quenching of TPPS(4) in Triton X-100 is consistent with strong binding as reported in the literature.     

Dye induced quenching of firefly luciferase-luciferin bioluminescence

The quenching of firefly bioluminescence (BL) in presence of xanthene dyes and tetratolylporphyrin was investigated. The BL intensity was quenched with an altered decay pattern in presence of xanthene dyes and tetratolylporphyrin. The electronic absorption spectra indicate that there is no significant interaction occurring between the dyes and the BL components in the ground state. The BL quenching decay rate and fluorescence quenching studies of luciferin by the dyes suggest an energy transfer through an exciplex, involving oxyluciferin, in the excited state and the dyes, in the ground state. The bimolecular quenching rate constant (K(q)) values obtained from fluorescence studies varied between 7.7 x 10(12) and 19.8 x 10(12)M(-1)s(-1). The magnitude of the bimolecular quenching rate constants confirmed the complex formation between dye and excited oxyluciferin. The exciplex subsequently undergoes a non-radiative decay to the ground state via a combination of heavy atom induced and F?rster-type energy transfer. The decay rate constants in presence and in absence of dyes vary between 7.47 x 10(-4) and 7.6 x 10(-2)s(-1). In the presence of dyes the effective decay rate constants (k(eff)) increased while the lifetime of light emitting species decreased. The kinetic studies in presence of singlet oxygen scavengers, like beta-carotene and NaN(3), prove that there is no significant quenching of the firefly BL due to the formation of singlet oxygen.     

Excited-state properties of Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF, PPh(3), py)

The photophysical properties of Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF = tetrahydrofuran, PPh(3) = triphenylphosphine, py = pyridine) were explored upon excitation with visible light. Time-resolved absorption shows that all the complexes possess a long-lived transient (3.5-5.0 micros) assigned as an electronic excited state of the molecules, and they exhibit an optical transition at approximately 760 nm whose position is independent of axial ligand. No emission from the Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF, PPh(3), py) systems was detected, but energy transfer from Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) to the (3)pipi excited state of perylene is observed. Electron transfer from Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) to 4,4'-dimethyl viologen (MV(2+)) and chloro-p-benzoquinone (Cl-BQ) takes place with quenching rate constants (k(q)) of 8.0 x 10(6) and 1.2 x 10(6) M(-1) s(-1) in methanol, respectively. A k(q) value of 2 x 10(8) M(-1) s(-1) was measured for the quenching of the excited state of Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) by O(2) in methanol. The observations are consistent with the production of an excited state with excited-state energy, E(00), between 1.34 and 1.77 eV.     

Vitamin B6 (pyridoxine) and its derivatives are efficient singlet oxygen quenchers and potential fungal antioxidants

Vitamin B6 (pyridoxine, 1) and its derivatives: pyridoxal (2), pyridoxal 5-phosphate (3) and pyridoxamine (4) are important natural compounds involved in numerous biological functions. Pyridoxine appears to play a role in the resistance of the filamentous fungus Cercospora nicotianae to its own abundantly produced strong photosensitizer of singlet molecular oxygen (1O2), cercosporin. We measured the rate constants (kq) for the quenching of 1O2 phosphorescence by 1-4 in D2O. The respective total (physical and chemical quenching) kq values are: 5.5 x 10(7) M-1 s-1 for 1; 7.5 x 10(7) M-1 s-1 for 2, 6.2 x 10(7) M-1 s-1 for 3 and 7.5 x 10(7) M-1 s-1 for 4, all measured at pD 6.2. The quenching efficacy increased up to five times in alkaline solutions and decreased approximately 10 times in ethanol. Significant contribution to total quenching by chemical reaction(s) is suggested by the degradation of all the vitamin derivatives by 1O2, which was observed as declining absorption of the pyridoxine moiety upon aerobic irradiation of RB used to photosensitize 1O2. This photodegradation was completely stopped by azide, a known physical quencher of 1O2. The pyridoxine moiety can also function as a redox quencher for excited cercosporin by forming the cercosporin radical anion, as observed by electron paramagnetic resonance. All B6 vitamers fluoresce upon UV excitation. Compounds 1 and 4 emit fluorescence at 400 nm, compound 2 at 450 nm and compound 3 at 550 nm. The fluorescence intensity of 3 increased approximately 10 times in organic solvents such as ethanol and 1,2-propanediol compared to aqueous solutions, suggesting that fluorescence may be used to image the distribution of 1-4 in Cercospora to understand better the interactions of pyridoxine and 1O2 in the living fungus.     

Kinetic solvent effects on proton and hydrogen atom transfers from phenols. Similarities and differences

Bimolecular rate constants for proton transfer from six phenols to the anthracene radical anion have been determined in up to eight solvents using electrochemical techniques. Effects of hydrogen bonding on measured rate constants were explored over as wide a range of phenolic hydrogen-bond donor (HBD) and solvent hydrogen-bond acceptor (HBA) activities as practical. The phenols' values ranged from 0.261 (2-MeO-phenol) to 0.728 (3,5-Cl(2)-phenol), and the solvents' values from 0.44 (MeCN) to 1.00 (HMPA), where and are Abraham's parameters describing relative HBD and HBA activities (J. Chem. Soc., Perkin Trans. 2 1989, 699; 1990, 521). Rate constants for H-atom transfer (HAT) in HBA solvents, k(S), are extremely well correlated via log k(S) = log k(0) - 8.3 , where k(0) is the rate constant in a non-HBA solvent (Snelgrove et al. J. Am. Chem. Soc. 2001, 123, 469). The same equation describes the general features of proton transfers (k(S) decreases as increases, slopes of plots of log k(S) against increase as increases). However, in some solvents, k(S) values deviate systematically from the least-squares log k(S) versus correlation line (e.g., in THF and MeCN, k(S) is always smaller and larger, respectively, than "expected"). These deviations are attributed to variations in the solvents' anion solvating abilities (THF and MeCN are poor and good anion solvators, respectively). Values of log k(S) for proton transfer, but not for HAT, give better correlations with Taft et al.'s (J. Org. Chem. 1983, 48, 2877) beta scale of solvent HBA activities than with . The beta scale, therefore, does not solely reflect solvents' HBA activities but also contains contributions from anion solvation.     

Reexamination of the Rehm-Weller data set reveals electron transfer quenching that follows a Sandros-Boltzmann dependence on free energy

In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ? A(?-)/D(?+) for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, k(q), to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm-Weller formula rests on an incorrect mechanism. We have remeasured k(q) for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and k(q), including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros-Boltzmann (SB) equation k(q) = k(lim)/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ? exciplex ? A(?-)/D(?+)). The quantity k(lim) represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A(?-)/D(?+) vs A(?-) + D(?+)). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of -0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A(?-)/D(?+) relative to the free A(?-) + D(?+), which opposes the Coulombic stabilization of A(?-)/D(?+). The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A(?-) + D(?+)). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of k(q) on ΔG.     

Study on the kinetic properties of phosphor in deoxycholate aggregates by phosphorescent quenching methodology

Owing to the unique molecular structure and aggregate behaviors in aqueous solution, dihydroxy bile salts can provide phosphorescent probe with a special microenvironment in which the room temperature phosphorescence of probe can be detected in the presence of dissolved oxygen. It, however, is not very clear how the bile salts work in inducing this kind of oxygen-independent phosphorescence. The present work tries to offer with possible more insights by investigating the particular kinetic behaviors of 3-bromoquinoline (3-BrQ) as probe in sodium deoxycholate (NaDC) aggregate based on phosphorescent quenching methodology. The critical aggregate concentration of NaDC is estimated as about 0.5mM based on the enhancement of probe phosphorescence. As the functions of quencher Cu(2+) and NO(2)(-), the rate constants of various photophysical processes for 3-BrQ are obtained in NaDC solution and full aqueous solution, respectively. In NaDC solution, the quenching rate constant k(cu2+) equals to 1.77x10(7)M(-1)s(-1) k(no-2)(mq) 1.62x10(6)M(-1)s(-1). The exit rate k(-) and entrance rate k(+) are determined to be 16-46s(-1) and 10(6)M(-1)s(-1) levels, respectively. The quenching rate constant k(o2)(q) of dissolved oxygen is estimated as 4.15x10(4)M(-1)s(-1) in air-saturated NaDC solution at 1atm.     

A temperature dependence kinetic study of O(1D) + CH4: overall rate coefficient and product yields

Using a recently-developed chemiluminescence technique for monitoring O(1D), the rate coefficient, k1, of the important atmospheric reaction O(1D) + CH4 --> products has been determined over a wide temperature range, 227 to 450 K. The rate coefficient was shown to be independent of temperature, having a value of (1.91 +/- 0.08) x 10(-10) cm3 s(-1); the quoted uncertainties are with 95% confidence. This highly precise value, based on an extended set of determinations with very low scatter, is significantly greater, 26%, than current recommended values. Secondly, the fraction of O(1D) quenched to O(3P) by CH4, k(1q)/k1, was precisely determined from chemiluminescence decays over the temperature range 236 to 340 K. A temperature independent value for k(1q)/k1 of 0.002 +/- 0.003 was found. Finally, LIF detection of OH has been applied to accurately determine the product branching fraction to OH of O(1D) + CH4 at room temperature. Our value, k(1a)/k1 = 0.76 +/- 0.08 (95% confidence), is in line with recent determinations by other groups.