Geminate proton recombination at the surface of SDS and CTAC micelles probed with a micelle-anchored anthocyanin

The functionalized flavylium salt 6-hexyl-7-hydroxy-4-methyflavylium chloride (HHMF) was employed to probe some of the fundamental features of proton transfer reactions at the surface of anionic sodium dodecyl sulfate (SDS) and cationic hexadecyltrimethylammonium chloride (CTAC) micelles. In contrast to most ordinary flavylium salts, HHMF is insoluble in water, but readily incorporates into SDS and CTAC micelles. In the ground state, the rate constant for deprotonation of the acid form (AH+) of HHMF decreases 100-fold upon going from CTAC (kd = 3.0 x 10(6) s(-1)) to SDS (kd = 1.4 x 10(4) s(-1)), consistent with the presence of an activation barrier for proton transfer in the ground state and reflecting, respectively, stabilization or destabilization of the AH+ cation by the micelle. Reprotonation of A is diffusion-controlled in both micelles (kp(SDS) = (2.1 x 10(11))[H+]aq s(-1) and kp(CTAC) = (3.7 x 10(8))[H+]aq s(-1)), the difference reflecting the rate of proton entry into the micelles. In the excited singlet state, the rate constants for deprotonation of the AH+* form of HHMF are similar in the two micelles (2.4 x 10(10) s(-1)), consistent with activationless proton transfer. Reprotonation of the excited A is dominated by fast geminate recombination of the photogenerated (A*-H+) pair at the micelle surface (k(rec)(SDS) = 6.1 x 10(9) s(-1) and k(rec)(CTAC) = 3.4 x 10(10) s(-1)) and the net efficiencies of geminate recombination are quite similar in SDS (0.89) and CTAC (0.86).     

Novel ground- and excited-state prototropic reactivity of a hydroxycarboxyflavylium salt

Synthetic and natural hydroxyflavylium salts are super-photoacids, exhibiting values of the rate constant for proton transfer to water in the excited state as high as 1.5 x 10(11) s(-1). The synthetic flavylium salt 4-carboxy-7-hydroxy-4'-methoxyflavylium chloride (CHMF) has an additional carboxyl group at the 4-position of the flavylium cation that deprotonates in the ground state at a lower pH (pK(a1) = 0.73; AH2+ --> Z) than the 7-hydroxy group (pK(a2) = 4.84; Z --> A-). Ground-state deprotonation of the carboxyl group of the acid (AH2+) to form the zwitterion (Z) is too fast to be detected by nanosecond laser flash perturbation of the ground-state equilibrium, while deprotonation of the hydroxyl group of Z to form the anionic base (A-) occurs in the microsecond time range (k(d2) = 0.6 x 10(6) s(-1) and k(p2) = 4.2 x 10(10) M(-1) x s(-1)). In the excited state, the cationic form (AH2+) deprotonates in approximately 9 ps, resulting in the excited neutral base form (AH), which is unstable in the ground state. Deprotonation of Z occurs in 30 ps (k(d2) = 2.9 x 10(10) s(-1)), to form excited A-, which either reprotonates (k(p3)* = 3.7 x 10(10) M(-1) x s(-1)) or decays in 149 ps, and shows an important contribution from geminate recombination to give the excited neutral base (AH). Predominant reprotonation of A- at the carboxylate group reflects both the presence of the negative charge on the carboxylate and the increase in the excited-state pK(a) of the carboxyl group. Thus, while the hydroxyl pK(a) decreases by approximately 5 units upon going from the ground state (pK(a) = 4.84) to the excited state (pK(a) = -0.2), that of the carboxyl group increases by at least this much. Consequently, the excited state of the Z form of CHMF acts as a molecular proton transporter in the picosecond time range.     

Supramolecular assembly of an amphiphilic Gd(III) chelate: tuning the reorientational correlation time and the water exchange rate

We report the synthesis and characterization of the novel ligand H(5)EPTPA-C(16) ((hydroxymethylhexadecanoyl ester)ethylenepropylenetriaminepentaacetic acid). This ligand was designed to chelate the Gd(III) ion in a kinetically and thermodynamically stable way while ensuring an increased water exchange rate (kappa(ex)) on the Gd(III) complex owing to steric compression around the water-binding site. The attachment of a palmitic ester unit to the pendant hydroxymethyl group on the ethylenediamine bridge yields an amphiphilic conjugate that forms micelles with a long tumbling time (tau(R)) in aqueous solution. The critical micelle concentration (cmc = 0.34 mM) of the amphiphilic [Gd(eptpa-C(16))(H(2)O)](2-) chelate was determined by variable-concentration proton relaxivity measurements. A global analysis of the data obtained in variable-temperature and multiple-field (17)O NMR and (1)H NMRD measurements allowed for the determination of parameters governing relaxivity for [Gd(eptpa-C(16))(H(2)O)](2-); this is the first time that paramagnetic micelles with optimized water exchange have been investigated. The water exchange rate was found to be kappa(298)(ex) = 1.7 x 10(8) s(-1), very similar to that previously reported for the nitrobenzyl derivative [Gd(eptpa-bz-NO(2))(H(2)O)](2-) kappa(298)(ex) = 1.5 x 10(8) s(-1)). The rotational dynamics of the micelles were analysed by using the Lipari-Szabo approach. The micelles formed in aqueous solution show considerable flexibility, with a local rotational correlation time of tau(298)(l0) = 330 ps for the Gd(III) segments, which is much shorter than the global rotational correlation time of the supramolecular aggregates, tau(298)(g0) = 2100 ps. This internal flexibility of the micelles is responsible for the limited increase of the proton relaxivity observed on micelle formation (r(1) = 22.59 mM(-1) s(-1) for the micelles versus 9.11 mM(-1) s(-1) for the monomer chelate (20 MHz; 25 degrees C)).     

Picosecond dynamics of proton transfer of a 7-hydroxyflavylium salt in aqueous-organic solvent mixtures

The intermediacy of the geminate base-proton pair (A*···H(+)) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH(+)* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base-proton pair A*···H(+). The geminate pair becomes detectable because of the increase in the recombination rate constant, k(rec), of (A*···H(+)) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH(+)*, k(d), and recombination, k(rec), of A*···H(+)) from the diffusion controlled rates (dissociation, k(diss), and formation, k(diff)[H(+)], of A*···H(+)), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k(d) and k(rec)) as well as on proton diffusion/migration (k(diss) and k(diff)).     

Formation and stability of enolates of acetamide and acetate anion: an Eigen plot for proton transfer at alpha-carbonyl carbon

Second-order rate constants were determined in D(2)O for deprotonation of acetamide, N,N-dimethylacetamide, and acetate anion by deuterioxide ion and for deprotonation of acetamide by quinuclidine. The values of k(B) = 4.8 x 10(-8) M(-1) s(-1) for deprotonation of acetamide by quinuclidine (pK(BH) = 11.5) and k(BH) = 2-5 x 10(9) M(-1) s(-1) for the encounter-limited reverse protonation of the enolate by protonated quinuclidine give pK(a)(C) = 28.4 for ionization of acetamide as a carbon acid. The limiting value of k(HOH) = 1 x 10(11) s(-1) for protonation of the enolate of acetate anion by solvent water and k(HO) = 3.5 x 10(-9) M(-1) s(-1) for deprotonation of acetate anion by HO(-) give pK(a)(C) approximately 33.5 for acetate anion. The change in the rate-limiting step from chemical proton transfer to solvent reorganization results in a downward break in the slope of the plot of log k(HO) against carbon acid pK(a) for deprotonation of a wide range of neutral alpha-carbonyl carbon acids by hydroxide ion, from -0.40 to -1.0. Good estimates are reported for the stabilization of the carbonyl group relative to the enol tautomer by electron donation from alpha-SEt, alpha-OMe, alpha-NH(2), and alpha-O(-) substituents. The alpha-NH(2) and alpha-OMe groups show similar stabilizing interactions with the carbonyl group, while the interaction of alpha-O(-) is only 3.4 kcal/mol more stabilizing than for alpha-OH. We propose that destabilization of the enolate intermediates of enzymatic reactions results in an increasing recruitment of metal ions by the enzyme to provide electrophilic catalysis of enolate formation.     

Study of valsartan interaction with micelles as a model system for biomembranes

In this study, the interaction of valsartan (VAL), an angiotensin II receptor antagonist, with cationic surfactant cetyltrimethylammonium bromide (CTAB) was investigated. The effect of cationic micelles on spectroscopic and acid-base properties of VAL was carried out using UV spectrophotometry at physiological conditions (pH 7.4). The binding of VAL to CTAB micelles implied a shift in drug acidity constant (pK(a)(water)-pK(a)(micelle)=1.69) proving the great affinity of VAL dianion for the positively charged CTAB micelle surface. To quantify the degree of VAL/CTAB interaction, two constants were calculated by using mathematical models: micelle/water partition coefficient (K(x)) and drug/micelle binding constant (K(b)). The decrease of K(x) with VAL concentration, obtained by using pseudo-phase model, is consistent with an adsorption-like phenomenon. From the dependence of differential absorbance at lambda=295 nm on CTAB concentration, by using mathematical model that treats the solubilization of VAL dianion as its binding to specific sites in the micelles (Langmuir adsorption isotherm), the binding constant (K(b)=(2.50+/-0.49)x10(4)M(-1)) was obtained. Binding constant VAL/CTAB was also calculated using micellar liquid chromatography (MLC).     

Kinetics and mechanism of iron(III)-nitrilotriacetate complex reactions with phosphate and acetohydroxamic acid

The kinetics and mechanism of the substitution of coordinated water in nitrilotriacetate complexes of iron(III) (Fe(NTA)(OH(2))(2) and Fe(NTA)(OH(2))(OH)(-)) by phosphate (H(2)PO(4)(-) and HPO(4)(2)(-)) and acetohydroxamic acid (CH(3)C(O)N(OH)H) were investigated. The phosphate reactions were found to be pH dependent in the range of 4-8. Phosphate substitution rates are independent of the degree of phosphate protonation, and pH dependence is due to the difference in reactivity of Fe(NTA)(OH(2))(2) (k = 3.6 x 10(5) M(-)(1) s(-)(1)) and Fe(NTA)(OH(2))(OH)(-) (k = 2.4 x 10(4) M(-)(1) s(-)(1)). Substitution by acetohydroxamic acid is insensitive to pH in the range of 4-5.2, and Fe(NTA)(OH(2))(2) and Fe(NTA)(OH(2))(OH)(-) react at equivalent rates (k = 4.2 x 10(4) and 3.8 x 10(4) M(-)(1) s(-)(1), respectively). Evidence for acid-dependent and acid-independent back-reactions was obtained for both the phosphate and acetohydroxamate complexes. Reactivity patterns were analyzed in the context of NTA labilization of coordinated water, and outer-sphere electrostatic and H-bonding influences were analyzed in the precursor complex (K(os)).     

CF3CF=CH2 and (Z)-CF3CF=CHF: temperature dependent OH rate coefficients and global warming potentials

Rate coefficients over the temperature range 206-380 K are reported for the gas-phase reaction of OH radicals with 2,3,3,3-tetrafluoropropene (CF(3)CF=CH(2)), k(1)(T), and 1,2,3,3,3-pentafluoropropene ((Z)-CF(3)CF=CHF), k(2)(T), which are major components in proposed substitutes for HFC-134a (CF(3)CFH(2)) in mobile air-conditioning units. Rate coefficients were measured under pseudo-first-order conditions in OH using pulsed-laser photolysis to produce OH and laser-induced fluorescence to detect it. Rate coefficients were found to be independent of pressure between 25 and 600 Torr (He, N(2)). For CF(3)CF=CH(2), the rate coefficients, within the measurement uncertainty, are given by the Arrhenius expression k(1)(T)=(1.26+/-0.11) x 10(-12) exp[(-35+/-10)/T] cm(3) molecule(-1) s(-1) where k(1)(296 K)=(1.12+/-0.09) x 10(-12) cm(3) molecule(-1) s(-1). For (Z)-CF(3)CF=CHF, the rate coefficients are given by the non-Arrhenius expression k(2)(T)=(1.6+/-0.2) x 10(-18)T(2) exp[(655+/-50)/T] cm(3) molecule(-1) s(-1) where k(2)(296 K)=(1.29+/-0.06) x 10(-12) cm(3) molecule(-1) s(-1). Over the temperature range most relevant to the atmosphere, 200-300 K, the Arrhenius expression k(2)(T)=(7.30+/-0.7) x 10(-13) exp[(165+/-20)/T] cm(3) molecule(-1) s(-1) reproduces the measured rate coefficients very well and can be used in atmospheric model calculations. The quoted uncertainties in the rate coefficients are 2sigma (95% confidence interval) and include estimated systematic errors. The global warming potentials for CF(3)CF=CH(2) and (Z)-CF(3)CF=CHF were calculated to be <4.4 and <3.6, respectively, for the 100 year time horizon using infrared absorption cross sections measured in this work, and atmospheric lifetimes of 12 and 10 days that are based solely on OH reactive loss.     

Acetone adsorption on ice surfaces in the temperature range T = 190-220 K: evidence for aging effects due to crystallographic changes of the adsorption sites

The rate and thermodynamics of the adsorption of acetone on ice surfaces have been studied in the temperature range T = 190-220 K using a coated-wall flow tube reactor (CWFT) coupled with QMS detection. Ice films of 75 +/- 25 microm thickness were prepared by coating the reactor using a calibrated flow of water vapor. The rate coefficients for adsorption and desorption as well as adsorption isotherms have been derived from temporal profiles of the gas phase concentration at the exit of the flow reactor together with a kinetic model that has recently been developed in our group to simulate reversible adsorption in CWFTs (Behr, P.; Terziyski, A.; Zellner, R. Z. Phys. Chem. 2004, 218, 1307-1327). It is found that acetone adsorption is entirely reversible; the adsorption capacity, however, depends on temperature and decreases with the age of the ice film. The aging effect is most pronounced at low acetone gas-phase concentrations (< or = 2.0 x 10(11) molecules/cm(3)) and at low temperatures. Under these conditions, acetone is initially adsorbed with a high rate and high surface coverage that, upon aging, both become lower. This effect is explained by the existence of initially two adsorption sites (1) and (2), which differ in nature and number density and for which the relative fractions change with time. Using two-site dynamic modeling, the rate coefficients for adsorption (k(ads)) and desorption (k(des)) as well as the Langmuir constant (K(L)) and the maximum number of adsorption sites (c(s,max)), as obtained for the adsorption of acetone on sites of types (1) and (2) in the respective temperature range, are k(ads)(1) = 3.8 x 10(-14) T(0.5) cm(3) s(-1), k(des)(1) = 4.0 x 10(11) exp(-5773/T) s(-1), K(L) (1) = 6.3 x 10(-25) exp(5893/T) cm(3), c(s,max)(1) < or = 10(14) cm(-2) and k(ads)(2) = 2.9 x 10(-15) T(0.5) cm(3) s(-1), k(des)(2) = 1.5 x 10(7) exp(-3488/T) s(-1), K(L)(2) = 5.0 x 10(-22) exp(3849/T) cm(3), c(s,max)(2) = 6.0 x 10(14) cm(-2), respectively. On the basis of these results, the adsorption of acetone on aged ice occurs exclusively on sites of type (2). Among the possible explanations for the time-dependent two-site adsorption behavior, i.e., crystallographic differences, molecular or engraved microstructures, or a mixture of the two, we tentatively accept the former, i.e., that the two adsorption sites correspond to cubic (1, I(c)) and hexagonal (2, I(h)) sites. The temporal change of I(c) to I(h) and, hence, the time constants of aging are consistent with independent information in the literature on these phase changes.     

Water-exchange study revealed unexpected substitution behavior of [(CO)2(NO)Re(H2O)3]2+ in aqueous media

The pH-dependent water-exchange rates of [(CO)2(NO)Re(H2O(cis))2(H2O(trans))]2+ (1) in aqueous media were investigated by means of 17O NMR spectroscopy at 298 K. Because of the low pK(a) value found for 1 (pK(a) = 1.4 +/- 0.3), the water-exchange rate constant k(obs)(H2O(trans/cis)) was analyzed with a two-pathway model in which k(Re)(H2O(trans/cis)) and k(ReOH)(H2O)(trans/cis)) denote the water-exchange rate constants in trans or cis position to the nitrosyl ligand on 1 and on the monohydroxo species [(CO)2(NO)Re(H2O)2(OH)]+ (2), respectively. Whereas the rate constants k(ReOH)(H2O)(trans)) and k(ReOH)(H2O)(cis)) were determined as (4.2 +/- 2) x 10(-3) s(-1) and (5.8 +/- 2) x 10(-4) s(-1), respectively, k(Re)(H2O)(trans)) and k(Re)(H2O)(cis)) were too small to be determined in the presence of the much more reactive species 2. Apart from the water exchange, an unexpectedly fast C identical with 16O --> C identical withO exchange was also observed via NMR and IR spectroscopy. It was found to proceed through 1 and 2, with rate constants k(Re)(CO) and k(ReOH)(CO) of (19 +/- 4) x 10(-3) s(-1) and (4 +/- 3) x 10(-3) s(-1), respectively. On the other hand, N identical with 16O --> N identical with *O exchange was not observed.     

Ultrafast photoinduced electron transfer in the micelle and the gel phase of a PEO-PPO-PEO triblock copolymer

Ultrafast photoinduced electron transfer (PET) from N,N-dimethylaniline (DMA) to coumarin dyes is studied in the micelle and the gel phase of a triblock copolymer, (PEO)(20)-(PPO)(70)-(PEO)(20) (Pluronic P123) by picosecond and femtosecond emission spectroscopies. The rate of PET in a P123 micelle and gel is found to be nonexponential and faster than the slow components of solvation dynamics. In a P123 micelle and gel, PET occurs on multiple time scales ranging from a subpicosecond time scale to a few nanoseconds. In the gel phase, the highest rate constant (9.3 x 10(9) M(-1) s(-1)) of ET for C152 is about two times higher than that (3.8 x 10(9) M(-1) s(-1)) observed in micelle phase. The ultrafast components of electron transfer (ET) exhibits a bell shaped dependence with the free energy change which is similar to the Marcus inversion. Possible reasons for slower PET in P123 micelle compared to other micelles and relative to P123 gel are discussed.     

Kinetics of Water Exchange on the Dihydroxo-Bridged Rhodium(III) Hydrolytic Dimer

()()Conventional (18)O isotopic labeling techniques have been used to measure the water exchange rates on the Rh(III) hydrolytic dimer [(H(2)O)(4)Rh(&mgr;-OH)(2)Rh(H(2)O)(4)](4+) at I = 1.0 M for 0.08 < [H(+)] < 0.8 M and temperatures between 308.1 and 323.1 K. Two distinct pathways of water exchange into the bulk solvent were observed (k(fast) and k(slow)) which are proposed to correspond to exchange of coordinated water at positions cis and trans to bridging hydroxide groups. This proposal is supported by (17)O NMR measurements which clearly showed that the two types of water ligands exchange at different rates and that the rates of exchange matched those from the (18)O labeling data. No evidence was found for the exchange of label in the bridging OH groups in either experiment. This contrasts with findings for the Cr(III) dimer. The dependence of both k(fast) and k(slow) on [H(+)] satisfied the expression k(obs) = (k(O)[H(+)](tot) +k(OH)K(a1))/([H(+)](tot) + K(a1)) which allows for the involvement of fully protonated and monodeprotonated Rh(III) dimer. The following rates and activation parameters were determined at 298 K. (i) For fully protonated dimer: k(fast) = 1.26 x 10(-)(6) s(-)(1) (DeltaH() = 119 +/- 4 kJ mol(-)(1) and DeltaS() = 41 +/- 12 J K(-)(1) mol(-)(1)) and k(slow) = 4.86 x 10(-)(7) s(-)(1) (DeltaH() = 64 +/- 9 kJ mol(-)(1) and DeltaS() = -150 +/- 30 J K(-)(1) mol(-)(1)). (ii) For monodeprotonated dimer: k(fast) = 3.44 x 10(-)(6) s(-)(1) (DeltaH() = 146 +/- 4 kJ mol(-)(1) and DeltaS() = 140 +/- 11 J K(-)(1) mol(-)(1)) and k(slow) = 2.68 x 10(-)(6) s(-)(1) (DeltaH() = 102 +/- 3 kJ mol(-)(1) and DeltaS() = -9 +/- 11 J K(-)(1) mol(-)(1)). Deprotonation of the Rh(III) dimer was found to labilize the primary coordination sphere of the metal ions and thus increase the rate of water exchange at positions cis and trans to bridging hydroxides but not to the same extent as for the Cr(III) dimer. Activation parameters and mechanisms for ligand substitution processes on the Rh(III) dimer are discussed and compared to those for other trivalent metal ions and in particular the Cr(III) dimer.     

Atmospheric chemistry of perfluorinated aldehyde hydrates (n-C(x)F(2x+1)CH(OH)2, x = 1, 3, 4): hydration, dehydration, and kinetics and mechanism of Cl atom and OH radical initiated oxidation

Smog chamber/Fourier transform infrared (FTIR) techniques were used to measure k(Cl+C(x)F(2x+1)CH(OH)(2)) (x = 1, 3, 4) = (5.84 +/- 0.92) x 10(-13) and k(OH+C(x)F(2x+1)CH(OH)(2)) = (1.22 +/- 0.26) x 10(-13) cm(3) molecule(-1) s(-1) in 700 Torr of N(2) or air at 296 +/- 2 K. The Cl initiated oxidation of CF(3)CH(OH)(2) in 700 Torr of air gave CF(3)COOH in a molar yield of 101 +/- 6%. IR spectra of C(x)F(2x+1)CH(OH)(2) (x = 1, 3, 4) were recorded and are presented. An upper limit of k(CF(3)CHO+H(2)O) < 2 x 10(-23) cm(3) molecule(-1) s(-1) was established for the gas-phase hydration of CF(3)CHO. Bubbling CF(3)CHO/air mixtures through liquid water led to >80% conversion of CF(3)CHO into the hydrate within the approximately 2 s taken for passage through the bubbler. These results suggest that OH radical initiated oxidation of C(x)F(2x+1)CH(OH)(2) hydrates could be a significant source of perfluorinated carboxylic acids in the environment.     

Study of sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles

Sodium dodecyl sulfate (SDS)-poly(propylene oxide) methacrylate (PPOMA) (of molecular weight M(w) = 434 g x mol(-1)) mixtures have been studied using conductimetry, static light scattering, fluorescence spectroscopy, and 1H NMR. It has been shown that SDS and PPOMA form mixed micelles, and SDS and PPOMA aggregation numbers, N(ag SDS) and N(ag PPOMA), have been determined. Total aggregation numbers of the micelles (N(ag SDS) + N(ag PPOMA)) and those of SDS decrease upon increasing the weight ratio R = PPOMA/SDS. Localization of PPOMA inside the mixed micelles is considered (i) using 1H NMR to localize the methacrylate function at the hydrophobic core-water interface and (ii) by studying the SDS-PPO micellar system (whose M(w) = 400 g x mol(-1)). Both methods have indicated that the PPO chain of the macromonomer is localized at the SDS micelle surface. Models based on the theorical prediction of the critical micellar concentration of mixed micelles and structural model of swollen micelles are used to confirm the particular structure proposed for the SDS-PPOMA system, i.e., the micelle hydrophobic core is primarily composed of the C12 chains of the sodium dodecyl sulfate, the hydrophobic core-water interface is made up of the SDS polar heads as well as methacrylate functions of the PPOMA, the PPO chains of the macromonomer are adsorbed preferentially on the surface, i.e., on the polar heads of the SDS.     

Mechanistic studies of nitric oxide reactions with water soluble iron(II), cobalt(II), and iron(III) porphyrin complexes in aqueous solutions: implications for biological activity.

The reactions of nitric oxide and carbon monoxide with water soluble iron and cobalt porphyrin complexes were investigated over the temperature range 298-318 K and the hydrostatic pressure range 0.1-250 MPa [porphyrin ligands: TPPS = tetra-meso-(4-sulfonatophenyl)porphinate and TMPS = tetra-meso-(sulfonatomesityl)porphinate]. Large and positive DeltaS(double dagger) and DeltaV(double dagger) values were observed for NO binding to and release from iron(III) complexes Fe(III)(TPPS) and Fe(III)(TMPS) consistent with a dissociative ligand exchange mechanism where the lability of coordinated water dominates the reactivity with NO. Small positive values for Delta and Delta for the fast reactions of NO with the iron(II) and cobalt(II) analogues (k(on) = 1.5 x 10(9) and 1.9 x 10(9) M(-1) s(-1) for Fe(II)(TPPS) and Co(II)(TPPS), respectively) indicate a mechanism dominated by diffusion processes in these cases. However, reaction of CO with the Fe(II) complexes (k(on) = 3.6 x 10(7) M(-1) s(-1) for Fe(II)(TPPS)) displays negative Delta and Delta values, consistent with a mechanism dominated by activation rather than diffusion terms. Measurements of NO dissociation rates from Fe(II)(TPPS)(NO) and Co(II)(TPPS)(NO) by trapping free NO gave k(off) values of 6.3 x 10(-4) s(-1) and 1.5 x 10(-4) s(-1). The respective M(II)(TPPS)(NO) formation constants calculated from k(on)/k(off) ratios were 2.4 x 10(12) and 1.3 x 10(13) M(-1), many orders of magnitude larger than that (1.1 x 10(3) M(-1)) for the reaction of Fe(III)(TPPS) with NO.     

A kinetic study of the reactions of Ca+ ions with O3, O2, N2, CO2 and H2O

The reactions between Ca(+)(4(2)S(1/2)) and O(3), O(2), N(2), CO(2) and H(2)O were studied using two techniques: the pulsed laser photo-dissociation at 193 nm of an organo-calcium vapour, followed by time-resolved laser-induced fluorescence spectroscopy of Ca(+) at 393.37 nm (Ca(+)(4(2)P(3/2)-4(2)S(1/2))); and the pulsed laser ablation at 532 nm of a calcite target in a fast flow tube, followed by mass spectrometric detection of Ca(+). The rate coefficient for the reaction with O(3) is essentially independent of temperature, k(189-312 K) = (3.9 +/- 1.2) x 10(-10) cm(3) molecule(-1) s(-1), and is about 35% of the Langevin capture frequency. One reason for this is that there is a lack of correlation between the reactant and product potential energy surfaces for near coplanar collisions. The recombination reactions of Ca(+) with O(2), CO(2) and H(2)O were found to be in the fall-off region over the experimental pressure range (1-80 Torr). The data were fitted by RRKM theory combined with quantum calculations on CaO(2)(+), Ca(+).CO(2) and Ca(+).H(2)O, yielding the following results with He as third body when extrapolated from 10(-3)-10(3) Torr and a temperature range of 100-1500 K. For Ca(+) + O(2): log(10)(k(rec,0)/cm(6) molecule(-2) s(-1)) = -26.16 - 1.113log(10)T- 0.056log(10)(2)T, k(rec,infinity) = 1.4 x 10(-10) cm(3) molecule(-1) s(-1), F(c) = 0.56. For Ca(+) + CO(2): log(10)(k(rec,0)/ cm(6) molecule(-2) s(-1)) = -27.94 + 2.204log(10)T- 1.124log(10)(2)T, k(rec,infinity) = 3.5 x 10(-11) cm(3) molecule(-1) s(-1), F(c) = 0.60. For Ca(+) + H(2)O: log(10)(k(rec,0)/ cm(6) molecule(-2) s(-1)) = -23.88 - 1.823log(10)T- 0.063log(10)(2)T, k(rec,infinity) = 7.3 x 10(-11)exp(830 J mol(-1)/RT) cm(3) molecule(-1) s(-1), F(c) = 0.50 (F(c) is the broadening factor). A classical trajectory analysis of the Ca(+) + CO(2) reaction is then used to investigate the small high pressure limiting rate coefficient, which is significantly below the Langevin capture frequency. Finally, the implications of these results for calcium chemistry in the mesosphere are discussed.     

Generation of a peroxynitrato metal complex from nitrogen dioxide and coordinated superoxide

The reaction between photogenerated NO(2) radicals and a superoxochromium(III) complex, Cr(aq)OO(2+), occurs with rate constants k(Cr)(20) = (2.8 +/- 0.2) x 10(8) M(-)(1) s(-)(1) (20 vol % acetonitrile in water) and k(Cr)(40) = (2.6 +/- 0.5) x 10(8) M(-)(1) s(-)(1) (40 vol % acetonitrile) in aerated acidic solutions and ambient temperature. The product was deduced to be a peroxynitrato complex, Cr(aq)OONO(2)(2+), which undergoes homolytic cleavage of an N-O bond to return to the starting materials, the rate constants in the two solvent mixtures being k(H)(20) = 172 +/- 4 s(-)(1) and k(H)(40) = 197 +/- 7 s(-)(1). NO(2) reacts rapidly with 10-methyl-9,10-dihydroacridine, k(A)(20) = 2.2 x 10(7) M(-)(1) s(-)(1), k(A)(40) = (9.4 +/- 0.2) x 10(6) M(-)(1) s(-)(1), and with N,N,N',N'-tetramethylphenylenediamine, k(T)(40) = (1.84 +/- 0.03) x 10(8) M(-)(1) s(-)(1).     

Reactivity of the organometallic fac-[(CO)3ReI(H2O)3]+ aquaion. Kinetic and thermodynamic properties of H2O substitution

The water exchange process on [(CO)(3)Re(H(2)O)(3)](+) (1) was kinetically investigated by (17)O NMR. The acidity dependence of the observed rate constant k(obs) was analyzed with a two pathways model in which k(ex) (k(ex)(298) = (6.3 +/- 0.1) x 10(-3) s(-1)) and k(OH) (k(OH)(298)= 27 +/- 1 s(-1)) denote the water exchange rate constants on 1 and on the monohydroxo species [(CO)(3)Re(I)(H(2)O)(2)(OH)], respectively. The kinetic contribution of the basic form was proved to be significant only at [H(+)] < 3 x 10(-3) M. Above this limiting [H(+)] concentration, kinetic investigations can be unambiguously conducted on the triaqua cation (1). The variable temperature study has led to the determination of the activation parameters Delta H(++)(ex) = 90 +/- 3 kJ mol(-1), Delta S(++)(ex) = +14 +/- 10 J K(-1) mol(-1), the latter being indicative of a dissociative activation mode for the water exchange process. To support this assumption, water substitution reaction on 1 has been followed by (17)O/(1)H/(13)C/(19)F NMR with ligands of various nucleophilicities (TFA, Br(-), CH(3)CN, Hbipy(+), Hphen(+), DMS, TU). With unidentate ligands, except Br(-), the mono-, bi-, and tricomplexes were formed by water substitution. With bidentate ligands, bipy and phen, the chelate complexes [(CO)(3)Re(H(2)O)(bipy)]CF(3)SO(3) (2) and [(CO)(3)Re(H(2)O)(phen)](NO(3))(0.5)(CF(3)SO(3))(0.5).H(2)O (3) were isolated and X-ray characterized. For each ligand, the calculated interchange rate constants k'(i) (2.9 x 10(-3) (TFA) < k'(I) < 41.5 x 10(-3) (TU) s(-1)) were found in the same order as the water exchange rate constant k(ex), the S-donor ligands being slightly more reactive. This result is indicative of I(d) mechanism for water exchange and complex formation, since larger variations of k'(i) are expected for an associatively activated mechanism.     

Bilirubin as an antioxidant: kinetic studies of the reaction of bilirubin with peroxyl radicals in solution, micelles, and lipid bilayers

Bilirubin (BR) showed very weak antioxidant activity in a nonpolar medium of styrene or cumene in chlorobenzene. In contrast, BR exhibited strong antioxidant activity in polar media such as aqueous lipid bilayers or SDS micelles/methyl linoleate (pH 7.4), where the rate with peroxyl radicals, k(inh) = 5.0 x 10(4) M(-)(1) s(-)(1), was comparable to that with vitamin E analogues, Trolox, or PMHC. An electron-transfer mechanism accounts for the effect of the medium on the antioxidant properties of BR.