Kinetics and mechanisms of bromine chloride reactions with bromite and chlorite ions

Chloride ion catalyzes the reactions of HOBr with bromite and chlorite ions in phosphate buffer (p[H(+)] 5 to 7). Bromine chloride is generated in situ in small equilibrium concentrations by the addition of excess Cl(-) to HOBr. In the BrCl/ClO(2)(-) reaction, where ClO(2)(-) is in excess, a first-order rate of formation of ClO(2) is observed that depends on the HOBr concentration. The rate dependencies on ClO(2)(-), Cl(-), H(+), and buffer concentrations are determined. In the BrCl/BrO(2)(-) reaction where BrCl is in pre-equilibrium with the excess species, HOBr, the loss of absorbance due to BrO(2)(-) is followed. The dependencies on Cl(-), HOBr, H(+), and HPO(4)(2)(-) concentrations are determined for the BrCl/BrO(2)(-) reaction. In the proposed mechanisms, the BrCl/ClO(2)(-) and BrCl/BrO(2)(-) reactions proceed by Br(+) transfer to form steady-state levels of BrOClO and BrOBrO, respectively. The rate constant for the BrCl/ClO(2)(-) reaction [k(Cl)(2)]is 5.2 x 10(6) M(-1) s(-1) and for the BrCl/BrO(2)(-) reaction [k(Br)(2)]is 1.9 x 10(5) M(-1) s(-1). In the BrCl/ClO(2)(-) case, BrOClO reacts with ClO(2)(-) to form two ClO(2) radicals and Br(-). However, the hydrolysis of BrOBrO in the BrCl/BrO(2)(-) reaction leads to the formation of BrO(3)(-) and Br(-).     

Active Thermochemical Tables: accurate enthalpy of formation of hydroperoxyl radical, HO2

Through the use of the Active Thermochemical Tables approach, the best currently available enthalpy of formation of HO2 has been obtained as delta(f)H(o)298 (HO2) = 2.94 +/- 0.06 kcal mol(-1) (3.64 +/- 0.06 kcal mol(-1) at 0 K). The related enthalpy of formation of the positive ion, HO2+, within the stationary electron convention is delta(f)H(o)298 (HO2+) = 264.71 +/- 0.14 kcal mol(-1) (265.41 +/- 0.14 kcal mol(-1) at 0 K), while that for the negative ion, HO2- (within the same convention), is delta(f)H(o)298 (HO2-) = -21.86 +/- 0.11 kcal mol(-1) (-21.22 +/- 0.11 kcal mol(-1) at 0 K). The related proton affinity of molecular oxygen is PA298(O2) = 100.98 +/- 0.14 kcal mol(-1) (99.81 +/- 0.14 kcal mol(-1) at 0 K), while the gas-phase acidity of H2O2 is delta(acid)G(o)298 (H2O2) = 369.08 +/- 0.11 kcal mol(-1), with the corresponding enthalpy of deprotonation of H2O2 of delta(acid)H(o)298 (H2O2) = 376.27 +/- 0.11 kcal mol(-1) (375.02 +/- 0.11 kcal mol(-1) at 0 K). In addition, a further improved enthalpy of formation of OH is briefly outlined, delta(f)H(o)298 (OH) = 8.93 +/- 0.03 kcal mol(-1) (8.87 +/- 0.03 kcal mol(-1) at 0 K), together with new and more accurate enthalpies of formation of NO, delta(f)H(o)298 (NO) = 21.76 +/- 0.02 kcal mol(-1) (21.64 +/- 0.02 kcal mol(-1) at 0 K) and NO2, delta(f)H(o)298 (NO2) = 8.12 +/- 0.02 kcal mol(-1) (8.79 +/- 0.02 kcal mol(-1) at 0 K), as well as H(2)O(2) in the gas phase, delta(f)H(o)298 (H2O2) = -32.45 +/- 0.04 kcal mol(-1) (-31.01 +/- 0.04 kcal mol(-1) at 0 K). The new thermochemistry of HO2, together with other arguments given in the present work, suggests that the previous equilibrium constant for NO + HO2 --> OH + NO2 was underestimated by a factor of approximately 2, implicating that the OH + NO2 rate was overestimated by the same factor. This point is experimentally explored in the companion paper of Srinivasan et al. (next paper in this issue).     

Kinetics and mechanisms of S(IV) reductions of bromite and chlorite ions

The reaction of bromite with aqueous S(IV) is first order in both reactants and is general-acid catalyzed. The reaction half-lives vary from 5 ms (p[H+] 5.9) to 210 s (p[H+] 13.1) for 0.7 mM excess S(IV) at 25 degrees C. The proposed mechanism includes a rapid reaction (k(1) = 3.0 x 10(7) M(-1) s(-1)) between BrO(2)(-) and SO(3)(2-) to form a steady-state intermediate, (O(2)BrSO(3))(3-). General acids assist the removal of an oxide ion from (O(2)BrSO(3))(3-) to form OBrSO(3)(-), which hydrolyzes rapidly to give OBr(-) and SO(4)(2-). Subsequent fast reactions between HOBr/OBr(-) and SO(3)(2-) give Br(-) and SO(4)(2-) as final products. In contrast, the chlorite reactions with S(IV) are 5-6 orders of magnitude slower. These reactions are specific-acid, not general-acid, catalyzed. In the proposed mechanism, ClO(2)(-) and SO(3)H(-)/SO(2) react to form (OClOSO(3)H)(2)(-) and (OClOSO(2))(-) intermediates which decompose to form OCl(-) and SO(4)(2-). Subsequent fast reactions between HOCl/OCl(-) and S(IV) give Cl- and SO(4)(2-) as final products. SO(2) is 6 orders of magnitude more reactive than SO(3)H-, where k(5)(SO(2)/ClO(2)(-)) = 6.26 x 10(6) M(-1) s(-1) and k(6)(SO(3)H(-)/ClO(2)(-)) = 5.5 M(-1) s(-1). Direct reaction between ClO(2)(-) and SO(3)(2-) is not observed. The presence or absence of general-acid catalysis leads to the proposal of different connectivities for the initial reactive intermediates, where a Br-S bond forms with BrO(2)(-) and SO(3)(2-), while an O-S bond forms with ClO(2)(-) and SO(3)H-.     

Mechanistic diversity covering 15 orders of magnitude in rates: cyanide exchange on [M(CN)(4)](2-) (M = Ni, Pd, and Pt)

Kinetic studies of cyanide exchange on [M(CN)(4)](2-) square-planar complexes (M = Pt, Pd, and Ni) were performed as a function of pH by (13)C NMR. The [Pt(CN)(4)](2-) complex has a purely second-order rate law, with CN(-) as acting as the nucleophile, with the following kinetic parameters: (k(2)(Pt,CN))(298) = 11 +/- 1 s(-1) mol(-1) kg, DeltaH(2) (Pt,CN) = 25.1 +/- 1 kJ mol(-1), DeltaS(2) (Pt,CN) = -142 +/- 4 J mol(-1) K(-1), and DeltaV(2) (Pt,CN) = -27 +/- 2 cm(3) mol(-1). The Pd(II) metal center has the same behavior down to pH 6. The kinetic parameters are as follows: (k(2)(Pd,CN))(298) = 82 +/- 2 s(-1) mol(-1) kg, DeltaH(2) (Pd,CN) = 23.5 +/- 1 kJ mol(-1), DeltaS(2) (Pd,CN) = -129 +/- 5 J mol(-1) K(-1), and DeltaV(2) (Pd,CN) = -22 +/- 2 cm(3) mol(-1). At low pH, the tetracyanopalladate is protonated (pK(a)(Pd(4,H)) = 3.0 +/- 0.3) to form [Pd(CN)(3)HCN](-). The rate law of the cyanide exchange on the protonated complex is also purely second order, with (k(2)(PdH,CN))(298) = (4.5 +/- 1.3) x 10(3) s(-1) mol(-1) kg. [Ni(CN)(4)](2-) is involved in various equilibrium reactions, such as the formation of [Ni(CN)(5)](3-), [Ni(CN)(3)HCN](-), and [Ni(CN)(2)(HCN)(2)] complexes. Our (13)C NMR measurements have allowed us to determine that the rate constant leading to the formation of [Ni(CN)(5)](3-) is k(2)(Ni(4),CN) = (2.3 +/- 0.1) x 10(6) s(-1) mol(-1) kg when the following activation parameters are used: DeltaH(2)() (Ni,CN) = 21.6 +/- 1 kJ mol(-1), DeltaS(2) (Ni,CN) = -51 +/- 7 J mol(-1) K(-1), and DeltaV(2) (Ni,CN) = -19 +/- 2 cm(3) mol(-1). The rate constant of the back reaction is k(-2)(Ni(4),CN) = 14 x 10(6) s(-1). The rate law pertaining to [Ni(CN)(2)(HCN)(2)] was found to be second order at pH 3.8, and the value of the rate constant is (k(2)(Ni(4,2H),CN))(298) = (63 +/- 15) x10(6) s(-1) mol(-1) kg when DeltaH(2) (Ni(4,2H),CN) = 47.3 +/- 1 kJ mol(-1), DeltaS(2) (Ni(4,2H),CN) = 63 +/- 3 J mol(-1) K(-1), and DeltaV(2) (Ni(4,2H),CN) = - 6 +/- 1 cm(3) mol(-1). The cyanide-exchange rate constant on [M(CN)(4)](2-) for Pt, Pd, and Ni increases in a 1:7:200 000 ratio. This trend is modified at low pH, and the palladium becomes 400 times more reactive than the platinum because of the formation of [Pd(CN)(3)HCN](-). For all cyanide exchanges on tetracyano complexes (A mechanism) and on their protonated forms (I/I(a) mechanisms), we have always observed a pure second-order rate law: first order for the complex and first order for CN(-). The nucleophilic attack by HCN or solvation by H(2)O is at least nine or six orders of magnitude slower, respectively than is nucleophilic attack by CN(-) for Pt(II), Pd(II), and Ni(II), respectively.     

Synthesis and luminescence spectroscopy of a series of [eta(5)-CpFe(CO)2] complexes containing 1,12-dicarba-closo-dodecaboranyl and -ylene ligands

Three new cyclopentadienyliron dicarbonyl compounds, 1-[eta(5)-CpFe(CO)(2)]-1,12-C(2)B(10)H(11), 1-[[eta(5)-CpFe(CO)(2)]-1,12-C(2)B(10)H(10)-12-yl](2)Hg, and 1,12-[eta(5)-CpFe(CO)(2)](2)-1,12-C(2)B(10)H(10), composed of 1,12-dicarba-closo-dodecaborane as a ligand precursor were synthesized and found to be luminescent. The uncoordinated 1,12-C(2)B(10)H(12) bridging ligand precursor is luminescent with a band maximum at 25180 cm(-1), while the iron complexes luminesce at lower energies in the range 13120-14210 cm(-1). The lowest energy excited electronic state in the iron complexes is assigned to a ligand field transition of the iron chromophore. Cyclic voltammetry of 1,12-[eta(5)-CpFe(CO)(2)](2)-1,12-C(2)B(10)H(10) displays two discrete one-electron oxidations, and the luminescence maximum is red shifted from that observed in 1-[eta(5)-CpFe(CO)(2)]-1,12-C(2)B(10)H(11). Both of these observations suggest that the iron-centered chromophores are weakly coupled. In contrast, the 1-[[eta(5)-CpFe(CO)(2)]-1,12-C(2)B(10)H(10)-12-yl](2)Hg complex is uncoupled as is evident from the single oxidation process observed with cyclic voltammetry. The extinction coefficient of 1,12-[eta(5)-CpFe(CO)(2)](2)-1,12-C(2)B(10)H(10) is six times that of 1-[eta(5)-CpFe(CO)(2)]-1,12-C(2)B(10)H(11), while the extinction coefficient of 1-[[eta(5)-CpFe(CO)(2)]-1,12-C(2)B(10)H(10)-12-yl](2)Hg is only twice that of 1-[eta(5)-CpFe(CO)(2)]-1,12-C(2)B(10)H(11). These spectroscopic properties are explained in terms of two coupled antiparallel transition dipole moments.     

Imprinting of chiral molecular recognition sites in thin TiO2 films associated with field-effect transistors: novel functionalized devices for chiroselective and chirospecific analyses

(R)- or (S)-2-Methylferrocene carboxylic acids, (R)-1 or (S)-1, (R)- or (S)-2-phenylbutanoic acid, (R)-2 or (S)-2, and (R)- or (S)-2-propanoic acid, (R)-3 or (S)-3, can be imprinted in thin TiO2 films on the gate surface of ion-sensitive field-effect transistor (ISFET) devices. The imprinting is performed by hydrolyzing the respective carboxylate TiIV butoxide complex on the gate surface, followed by washing off the acid from the resulting TiO2 film. The imprinted sites reveal chiroselectivity only towards the sensing of the imprinted enantiomer. The chiral recognition sites reveal not only chiroselectivity but also chirospecificity and, for example, the (R)-2-imprinted film is active in the sensing of (R)-2, but insensitive towards the sensing of (R)2-phenylpropanoic acid, (R)-3, which exhibits a similar chirality. Similarly, the (R)-3-imprinted film is inactive in the analysis of (R)-2. The chiroselectivity and chirospecificity of the resulting imprinted films are attributed to the need to align and fit the respective substrates in precise molecular contours generated in the cross-linked TiO2 films upon the imprinting process.     

Mono- and dinuclear complexes of chiral tri- and tetradentate Schiff-base ligands derived from 1,1'-binaphthyl-2,2'-diamine

The synthesis and characterization of the bis(bidentate) Schiff-base ligand [(R)-2] formed by the condensation reaction of (R)-1,1'-binaphthyl-2,2'-diamine [(R)-BINAM] with pyridine-2-carboxaldehyde is presented. The coordination chemistry of (R)-2 with Ni(ClO(4))(2).6H(2)O, Co(ClO(4))(2).6H(2)O, CuCl(2), and CuSO(4) has been investigated. Reaction of (R)-2 with the first two metal salts leads to complexes of the type [M((R)-4)(2)](ClO(4))(2) (M = Ni(II), Co(II)), where (R)-4 is a tridentate ligand resulting from the hydrolytic cleavage of one of the pyridyl groups from (R)-2. Both complexes were characterized by X-ray crystallography, which showed that the Lambda absolute configuration of the metal center is favored in both cases. (1)H NMR spectroscopy suggests that the high diastereoselectivity of Lambda-[Co((R)-4)(2)](ClO(4))(2) is maintained in solution. The reaction of (R)-2 with CuCl(2) leads to the dinuclear complex [Cu(2)((R)-2)Cl(4)], which has a [Cu(2)(mu(2)-Cl(2))] core. The reaction of CuSO(4) with (R)-2 gives a dimeric complex, [Cu((R)-4)SO(4)](2), which features a [Cu(2)(mu(2)-(SO(4))(2))] core. This complex can be prepared directly by the reaction of (R)-BINAM with pyridine-2-carboxaldehyde and CuSO(4). The use of rac-BINAM in this synthetic procedure leads to the heterochiral dimer [Cu(2)((R)-4)((S)-4)(SO(4))(2)]; that is, the ligands undergo a self-sorting (self/nonself discrimination) process based on chirality. The reaction of rac-BINAM, pyridine-2-carboxaldehyde, and Co(ClO(4))(2).6H(2)O proceeds via a homochiral self-sorting pathway to produce a racemic mixture of [Co((R)-4)(2)](2+) and [Co((S)-4)(2)](2+). The variable-temperature magnetic susceptibilities of the bimetallic complexes [Cu(2)((R)-2)Cl(4)], [Cu((R)-4)(mu(2)-SO(4))](2), and [Cu(2)((R)-4)((S)-4)(mu(2)-SO(4))(2)] all show weak antiferromagnetic coupling with J = -1.0, -0.40, and -0.67 cm(-)(1), respectively.     

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.     

Nucleophile assistance of electron-transfer reactions between nitrogen dioxide and chlorine dioxide concurrent with the nitrogen dioxide disproportionation

The reaction of chlorine dioxide with excess NO(2)(-) to form ClO(2)(-) and NO(3)(-) in the presence of a large concentration of ClO(2)(-) is followed via stopped-flow spectroscopy. Concentrations are set to establish a preequilibrium among ClO(2), NO(2)(-), ClO(2)(-), and an intermediate, NO(2). Studies are conducted at pH 12.0 to avoid complications due to the ClO(2)(-)/NO(2)(-) reaction. These conditions enable the kinetic study of the ClO(2) reaction with nitrogen dioxide as well as the NO(2) disproportionation reaction. The rate of the NO(2)/ClO(2) electron-transfer reaction is accelerated by different nucleophiles (NO(2)(-) > Br(-) > OH(-) > CO(3)(2-) > PO(4)(3-) > ClO(2)(-) > H(2)O). The third-order rate constants for the nucleophile-assisted reactions between NO(2) and ClO(2) (k(Nu), M(-2) s(-1)) at 25.0 degrees C vary from 4.4 x 10(6) for NO(2-) to 2.0 x 10(3) when H(2)O is the nucleophile. The nucleophile is found to associate with NO(2) and not with ClO(2) in the rate-determining step to give NuNO(2)(+) + ClO(2)(-). The concurrent NO(2) disproportionation reaction exhibits no nucleophilic effect and has a rate constant of 4.8 x 10(7) M(-1) s(-1). The ClO(2)/NO(2)/nucleophile reaction is another example of a system that exhibits general nucleophilic acceleration of electron transfer. This system also represents an alternative way to study the rate of NO(2) disproportionation.     

Photodetachment and photofragmentation pathways in the [(CO2)2(H2O)m]- cluster anions

The mass-selected [(CO(2))(2)(H(2)O)(m)](-) cluster anions are studied using a combination of photoelectron imaging and photofragment mass spectroscopy at 355 nm. Photoelectron imaging studies are carried out on the mass-selected parent cluster anions in the m=2-6 size range; photofragmentation results are presented for m=3-11. While the photoelectron images suggest possible coexistence of the CO(2) (-)(H(2)O)(m)CO(2) and (O(2)CCO(2))(-)(H(2)O)(m) parent cluster structures, particularly for m=2 and 3, only the CO(2) (-) based clusters are both required and sufficient to explain all fragmentation pathways for m>/=3. Three types of anionic photofragments are observed: CO(2) (-)(H(2)O)(k), O(-)(H(2)O)(k), and CO(3) (-)(H(2)O)(k), k6) is attributed to hindrance from the H(2)O molecules.     

Effects of auxiliary complex-forming agents on the rate of metallochromic indicator colour change-IV Mechanism of the colour change of xylenol orange in copper(II)-edta titrations

The rate of ligand substitution of copper(II)-Xylenol Orange (XO) with EDTA (Y) has been determined spectrophotometrically over the pH range 4.8-6.0 at mu = 0.1 (KNO(3)) and at 25 degrees . In 2-(N-morpholino)ethane sulphonic acid buffer, copper forms a 2:1 chelate (Cu(2)XO(2-)) with XO, and the rate-law is expressed as -d[Cu(2)XO(2-)]/dt = 10(2.89)[Cu(2)XO(2)-][Y']- The release of the first copper ion from Cu(2)XO(2-) is the rate-determining step. The resulting CuHXO(3-) or CuH(2)XO(2-) may undergo fast substitution with EDTA. In the presence of hexamine, the copper(II)-XO chelate forms a mixed-ligand complex with hexamine (L). The formation constant K(Cu(2)XOL)(L) = [Cu(2)XOL(2-)]/[Cu(2)XO(2-)] [L] = 10(2.14) (mu = 0.1, 25 degrees ). At 3 x 10(-2)-2 x 10(-1)M hexamine the rate-law is expressed as -d[Cu(2)XOL(2-)]/dt = 5.39[Cu(2)XOL(2-)][Y']/[L']. The dissociation of hexamine from Cu(23)XOL(2-) has to precede the substitution reaction of Cu(2)XO(2-) with EDTA. Hence, hexamine at higher concentrations than 10(-3)M slows down the rate of colour change of XO in the copper-EDTA titration.     

Characterization of acylphosphatidylglycerols from <Emphasis Type="Italic">salmonella typhimurium</Emphasis> by tandem mass spectrometry with electrospray ionization

Acylphosphatidylglycerol (Acyl-PG), a polar lipid class containing three fatty acyl groups, was isolated from Salmonella bacteria and characterized by tandem quadrupole and quadrupole ion-trap mass spectrometric methods with electrospray ionization. The structural characterization of the acyl-PG with various acyl groups (A-B/C-PG, where A not equal B not equal C) is based on the findings that the carboxylate anions (R(x)CO(2)(-)) arising from sn-2 (R(2)CO(2)(-)) is more abundant than that arising from sn-3' (R(3')CO(2)(-)), which is much more abundant than that arising from sn-1 (R(1)CO(2)(-)). This information provides a simple method for determination of the fatty acyl moieties and their positions in the molecule. The structural identification of the molecule can also be achieved by the findings that the fragment ion reflecting the ketene loss at sn-2 is more prominent than that reflecting the acid loss (i.e., [M - H - R'(2)CH=CO](-) > [M - H - R(2)CO(2)H](-)), while the ion arising from acid loss at sn-1 or sn-3' is, respectively, more abundant than the corresponding ketene loss (i.e., [M - H - R(1)CO(2)H](-) > [M - H - R'(1)CH=CO](-); [M - H - R(3')CO(2)H](-) > [M - H -R'(3')CH=CO](-)). The identity of the acyl moiety at sn-3' can be confirmed by an acyl-glycerophosphate anion observed in the product-ion spectrum obtained with a triple-stage quadrupole (TSQ) instrument, but not in that obtained with an ion-trap mass spectrometer (ITMS). However, the MS(2)-spectrum obtained with an ITMS is featured by the ion series that abundances of [M - H - R'(2)CH=CO - R(3)CO(2)H - 74](-) > [M - H - R'(2)CH=CO - R(1)CO(2)H - 74](-) z.Gt; [M - H - R'(1(or 3'))CH=CO - R(3'(or 1))CO(2)H - 74](-). This information also facilitates structural elucidation of the acyl-PG subclass that contains various acyl substituents. Structural identifications of molecular species having two identical fatty acyl substituents at sn-1, sn-2, or sn-3' or consisting of more than one isomeric structures are also demonstrated. The identities of the minor isomeric species in the molecules can be revealed by the aforementioned structural information arising from the various ion series combined.     

Bis(imino)phenoxide complexes of aluminium

Reaction of Me(3)Al (one equivalent) with the bis(imino)phenol, [2,6-(ArNCH)(2)-4-MeC(6)H(2)OH] (I)(Ar = 2,6-Pr(i)(2)C(6)H(3)) in toluene at ambient temperature yields the yellow complex [Me(2)Al[2,6-(ArNCH)(2)-4-MeC(6)H(2)O]](1). Interaction of two equivalents of Me(3)Al in refluxing toluene affords the red complex [(Me(2)Al)(2)[2-ArNCH(Me)-6-(ArNCH)-4-MeC(6)H(2)O]](2). Similar interaction (two equivalents, refluxing toluene) of MeAlCl(2) or (i)Bu(3)Al with [2,6-(ArNCH)(2)-4-MeC(6)H(2)OH] affords [ClAl[2,6-(ArNCH)(2)-4-MeC(6)H(2)O](2)](3) or [(i)Bu(2)Al[2,6-(ArNCH)(2)-4-MeC(6)H(2)O]](4), respectively. Hydrolysis of 2 readily affords the iminoaminophenol ligand [2-(ArN=CH)-6-ArNHCH(Me)-4-MeC(6)H(2)OH](II), which reacts further with Me(3)Al to afford [Me(2)Al[2-ArNCH(Me)-6-(ArNCH)-4-MeC(6)H(2)O]](5). An X-ray study on reveals bidentate imino-alkoxide ligation about the distorted aluminium centre, whereas is a binuclear structure with tetrahedral aluminiums ligated by imino-alkoxide and amido-alkoxide ligand fragments, respectively. For and bidentate imino-alkoxide ligation is observed.     

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).     

Effect of the ribose versus 2'-deoxyribose residue on the metal ion-binding properties of purine nucleotides

The interaction between metal ions and nucleotides is well characterized, as is their importance for metabolic processes, e.g. in the synthesis of nucleic acids. Hence, it is surprising to find that no detailed comparison is available of the metal ion-binding properties between nucleoside 5'-phosphates and 2'-deoxynucleoside 5'-phosphates. Therefore, we have measured here by potentiometric pH titrations the stabilities of several metal ion complexes formed with 2'-deoxyadenosine 5'-monophosphate (dAMP2-), 2'-deoxyadenosine 5'-diphosphate (dADP3-) and 2'-deoxyadenosine 5'-triphosphate (dATP4-). These results are compared with previous data measured under the same conditions and available in the literature for the adenosine 5'-phosphates, AMP(2-), ADP(3-) and ATP(4-), as well as guanosine 5'-monophosphate (GMP(2-)) and 2'-deoxyguanosine 5'-monophosphate (dGMP(2-)). Hence, in total four nucleotide pairs, GMP(2-)/dGMP(2-), AMP(2-)/dAMP(2-), ADP(3-)/dADP(3-) and ATP(4-)/dATP(4-) (= NP/dNP), could be compared for the four metal ions Mg2+, Ni2+, Cu2+ and Zn2+ (= M2+). The comparisons show that complex stability and extent of macrochelate formation between the phosphate-coordinated metal ion and N7 of the purine residue is very similar (or even identical) for the AMP(2-)/dAMP(2-) and ADP(3-)/dADP(3-) pairs. In the case of the complexes formed with ATP(4-)/dATP(4-) the 2'-deoxy complexes are somewhat more stable and show also a slightly enhanced tendency for macrochelate formation. This is different for guanine nucleotides: the stabilities of the M(dGMP) complexes are clearly higher, as are the formation degrees of their macrochelates, than is the case with the M(GMP) complexes. This enhanced complex stability and greater tendency to form macrochelates can be attributed to the enhanced basicity (DeltapKaca. 0.2) of N7 in the 2'-deoxy compound. These results allow general conclusions regarding nucleic acids to be made.     

Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

The kinetics and mechanisms of the oxidation of I (-) and Br (-) by trans-[Ru (VI)(N 2O 2)(O) 2] (2+) have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru (VI)(N 2O 2)(O) 2] (2+) + 3X (-) + 2H (+) --> trans-[Ru (IV)(N 2O 2)(O)(OH 2)] (2+) + X 3 (-) (X = Br, I). In the oxidation of I (-) the I 3 (-)is produced in two distinct phases. The first phase produces 45% of I 3 (-) with the rate law d[I 3 (-)]/dt = ( k a + k b[H (+)])[Ru (VI)][I (-)]. The remaining I 3 (-) is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I (-)], [H (+)], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru (VI) and I (-), which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru (IV)(N 2O 2)(O)(OHI)] (2+), and a one electron transfer to give [Ru (V)(N 2O 2)(O)(OH)] (2+) and I (*). [Ru (V)(N 2O 2)(O)(OH)] (2+) is a stronger oxidant than [Ru (VI)(N 2O 2)(O) 2] (2+) and will rapidly oxidize another I (-) to I (*). In the second phase the [Ru (IV)(N 2O 2)(O)(OHI)] (2+) undergoes rate-limiting aquation to produce HOI which reacts rapidly with I (-) to produce I 2. In the oxidation of Br (-) the rate law is -d[Ru (VI)]/d t = {( k a2 + k b2[H (+)]) + ( k a3 + k b3[H (+)]) [Br (-)]}[Ru (VI)][Br (-)]. At 298.0 K and I = 0.1 M, k a2 = (2.03 +/- 0.03) x 10 (-2) M (-1) s (-1), k b2 = (1.50 +/- 0.07) x 10 (-1) M (-2) s (-1), k a3 = (7.22 +/- 2.19) x 10 (-1) M (-2) s (-1) and k b3 = (4.85 +/- 0.04) x 10 (2) M (-3) s (-1). The proposed mechanism involves initial oxygen atom transfer from trans-[Ru (VI)(N 2O 2)(O) 2] (2+) to Br (-) to give trans-[Ru (IV)(N 2O 2)(O)(OBr)] (+), which then undergoes parallel aquation and oxidation of Br (-), and both reactions are acid-catalyzed.     

New pathways for chlorine dioxide decomposition in basic solution

The product distribution from the decay of chlorine dioxide in basic solution changes as the ClO(2) concentration decreases. While disproportionation reactions that give equal amounts of ClO(2)(-) and ClO(3)(-) dominate the stoichiometry at millimolar or higher levels of ClO(2), the ratio of ClO(2)(-) to ClO(3)(-) formed increases significantly at micromolar ClO(2) levels. Kinetic evidence shows three concurrent pathways that all exhibit a first-order dependence in [OH(-)] but have variable order in [ClO(2)]. Pathway 1 is a disproportionation reaction that is first order in [ClO(2)]. Pathway 2, a previously unknown reaction, is also first order in [ClO(2)] but forms ClO(2)(-) as the only chlorine-containing product. Pathway 3 is second order in [ClO(2)] and generates equal amounts of ClO(2)(-) and ClO(3)(-). A Cl(2)O(4) intermediate is proposed for this path. At high concentrations of ClO(2), pathway 3 causes the overall ClO(3)(-) yield to approach the overall yield of ClO(2)(-). Pathway 2 is attributed to OH(-) attack on an oxygen atom of ClO(2) that leads to peroxide intermediates and yields ClO(2)(-) and O(2) as products. This pathway is important at low levels of ClO(2).     

Synthesis and reduction of 2,2-diaryl-1-nitroethylenes by using a chiral and a non chiral NADH model in the pyrrolopyridine series

Reduction of 2,2-diphenyl-1-nitroethylene ( 1 ) and 2-(2-pyridyl)-2-phenyl-1-nitroethylene ( 5 ) is achieved by using the NADH model in the pyrrolopyridine series 2a to give 2,2-diphenyl-1-nitroethane ( 3 ) and 2-(2-pyridyl)-2-phenyl-1-nitroethane ( 7 ) respectively in 40% yield. The asymmetric reduction of 2-(2-pyridyl)-2-phenyl-1-nitroethylene by the chiral NADH model 2b is studied. Thus, 2-(2-pyridyl)-2-phenyl-1-nitroethane ( 7 ) is obtained in 15 to 32% yield. The stereocontrol of the reduction proved to be dependent on the amount of magnesium ions.     

Kinetics and mechanisms of the ozone/bromite and ozone/chlorite reactions

Ozone reactions with XO(2)(-) (X = Cl or Br) are studied by stopped-flow spectroscopy under pseudo-first-order conditions with excess XO(2)(-). The O(3)/XO(2)(-) reactions are first-order in [O(3)] and [XO(2)(-)], with rate constants k(1)(Cl) = 8.2(4) x 10(6) M(-1) s(-1) and k(1)(Br) = 8.9(3) x 10(4) M(-1) s(-1) at 25.0 degrees C and mu = 1.0 M. The proposed rate-determining step is an electron transfer from XO(2)(-) to O(3) to form XO(2) and O(3)(-). Subsequent rapid reactions of O(3)(-) with general acids produce O(2) and OH. The OH radical reacts rapidly with XO(2)(-) to form a second XO(2) and OH(-). In the O(3)/ClO(2)(-) reaction, ClO(2) and ClO(3)(-) are the final products due to competition between the OH/ClO(2)(-) reaction to form ClO(2) and the OH/ClO(2) reaction to form ClO(3)(-). Unlike ClO(2), BrO(2) is not a stable product due to its rapid disproportionation to form BrO(2)(-) and BrO(3)(-). However, kinetic spectra show that small but observable concentrations of BrO(2) form within the dead time of the stopped-flow instrument. Bromine dioxide is a transitory intermediate, and its observed rate of decay is equal to half the rate of the O(3)/BrO(2)(-) reaction. Ion chromatographic analysis shows that O(3) and BrO(2)(-) react in a 1/1 ratio to form BrO(3)(-) as the final product. Variation of k(1)(X) values with temperature gives Delta H(++)(Cl) = 29(2) kJ mol(-1), DeltaS(++)(Cl) = -14.6(7) J mol(-1) K(-1), Delta H(++)(Br) = 54.9(8) kJ mol(-1), and Delta S(++)(Br) = 34(3) J mol(-1) K(-1). The positive Delta S(++)(Br) value is attributed to the loss of coordinated H(2)O from BrO(2)(-) upon formation of an [O(3)BrO(2)(-)](++) activated complex.     

Reduction of octacyanomolybdate(V) by thioglycolic acid in aqueous media

In aqueous media at 25 degrees C [Mo(CN)(8)](3-) is reduced by thioglycolic acid (HSCH(2)COOH, TGA), and the reaction is strongly accelerated by the presence of trace amounts of copper ions. Dipicolinic acid (dipic) is an effective inhibitor of the copper catalysis. Both with and without dipic the reaction has the stoichiometry 2[Mo(CN)(8)](3-) + 2TGA --> 2[Mo(CN)(8)](4-) + RSSR, where RSSR is the disulfide derived from formal oxidative dimerization of TGA. In the presence of dipic, PBN (N-tert-butyl-alpha-phenyl-nitrone), and with a large excess of TGA the rate law for consumption of [Mo(CN)(8)](3-) is first order in both [TGA] and [Mo(CN)(8)(3-)]. The complex pH dependence is consistent with (-)SCH(2)CO(2)(-) being highly reactive (k = 1.8 x 10(4) M(-1) s(-1)), the monoanion being less reactive, and HSCH(2)CO(2)H being unreactive. A mechanism is proposed in which the dianion undergoes electron transfer to [Mo(CN)(8)](3-), thus generating the thiyl radical. Analysis of the electron-transfer rate constant in terms of Marcus theory yields an effective self-exchange rate constant for the thiolate/thiyl redox couple that is in reasonable agreement with the value derived previously from the reaction of TGA with [IrCl(6)](2-). When copper catalysis is inhibited, the two reactions differ substantially in that the yield of (-)O(3)SCH(2)CO(2)(-) is significant for [IrCl(6)](2-) but undetectable for [Mo(CN)(8)](3-).