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1.
The effects of supporting electrolytes and of pressure on the electrode reactions of the aqueous CoW(12)O(40)(5-/6-) couple at 25 degrees C are reported, together with limited data on PW(12)O(40)(3-)/4-) and PW(12)O(40)(4-/5-). The half-wave potentials E(1/2) for the CoW(12) couple become moderately more positive with increasing electrolyte concentration and cationic charge, and also in the sequences Li(+) approximately Na(+) < NH(4)(+) < or = H(+) < K(+) < Rb(+) < Cs(+) and Na(+) < Mg(2+) < Ca(2+) < Eu(3+). The mean diffusion coefficients for CoW(12) with the 1:1 electrolytes are independent of electrolyte concentration and rise only slightly from Li(+) to Cs(+), averaging (2.4 +/- 0.3) x 10(-6) cm(2) s(-1). Neither the volumes of activation for diffusion Delta V(diff)(++) (average -0.9 +/- 1.1 cm(3) mol(-1)) nor the electrochemical cell reaction volumes Delta V(Ag/AgCl) (average -22 +/- 2 cm(3) mol(-1)) for the CoW(12) couple show significant dependence on electrolyte identity or concentration. For the PW(12)(3-/4-) and PW(12)(4-/5-) couples, Delta V(Ag/AgCl) = -14 and -26 cm(3) mol(-1), respectively, suggesting a dependence on Delta(z(2)) (z = ionic charge number) as predicted by the Born-Drude-Nernst theory of electrostriction of solvent, but comparison with Delta V(Ag/AgCl) for CoW(12) and other anion-anion couples shows that the Born-Drude-Nernst approach fails in this context. For aqueous electrode reactions of CoW(12), as for other anionic couples such as cyanometalates, the standard rate constants k(el) show specific cation catalysis (Na(+) < K(+) < Rb(+) < Cs(+)), and Delta V(el++) is invariably positive, in the presence of supporting electrolytes. For the heavier group 1 cations, Delta V(el++) is particularly large (10-15 cm(3) mol(-1)), consistent with a partial dehydration of the cation to facilitate catalysis of the electron-transfer process. The positive values of Delta V(el++) for the CoW(12) couple cannot be attributed to rate control by solvent dynamics, which would lead to Delta V(el++) < or = Delta V(diff++), i.e., to negative or zero Delta V(el++) values. These results stand in sharp contrast to those for aqueous cationic couples, for which k(el) shows relatively little influence of the nature of the counterion and Delta V(el++) is always negative.  相似文献   

2.
Fundamental information concerning the mechanism of electron transfer from reduced heteropolytungstates (POM(red)) to O2, and the effect of donor-ion charge on reduction of O2 to superoxide anion (O2.-), is obtained using an isostructural series of 1e--reduced donors: alpha-X(n+)W12O40(9-n)-, X(n+) = Al3+, Si4+, P5+. For all three, a single rate expression is observed: -d[POM(red)]/dt = 2k12[POM(red)][O2], where k12 is for the rate-limiting electron transfer from POM(red) to O2. At pH 2 (175 mM ionic strength), k12 increases from 1.4 +/- 0.2 to 8.5 +/- 1 to 24 +/- 2 M-1s-1 as Xn+ is varied from P5+ (3red) to Si4+ (2red) to Al3+ (1red). Variable-pH data (for 1red) and solvent-kinetic isotope (KIE = kH/kD) data (all three ions) indicate that protonated superoxide (HO2.) is formed in two steps--electron transfer, followed by proton transfer (ET-PT mechanism--rather than via simultaneous proton-coupled electron transfer (PCET). Support for an outersphere mechanism is provided by agreement between experimental k12 values and those calculated using the Marcus cross relation. Further evidence is provided by the small variation in k12 observed when Xn+ is changed from P5+ to Si4+ to Al3+, and the driving force for formation of O2.- (aq), which increases as cluster-anion charge becomes more negative, increases by nearly +0.4 V (a decrease of >9 kcal mol-1 in DeltaG degrees ). The weak dependence of k12 on POM reduction potentials reflects the outersphere ET-PT mechanism: as the anions become more negatively charged, the "successor-complex" ion pairs are subject to larger anion-anion repulsions, in the order [(3(ox)3-)(O2.-)]4- < [(2(ox)4-)(O2.-)]5- < [(1(ox)5-)(O2.-)]6-. This reveals an inherent limitation to the use of heteropolytungstate charge and reduction potential to control rates of electron transfer to O2 under turnover conditions in catalysis.  相似文献   

3.
A water-soluble self-assembled supramolecular host molecule catalyzes the hydrolysis of orthoformates in basic solution. Comparison of the rate constants of the catalyzed and uncatalyzed reactions for hydrolysis displays rate accelerations of up to 3900 for tri- n-propyl orthoformate. Kinetic analysis shows that the mechanism of hydrolysis with the supramolecular host obeys the Michaelis-Menten model. Mechanistic studies, including (13)C-labeling experiments, revealed that the resting state of the catalytic system is the neutral substrate encapsulated in the host. Activation parameters for the k cat step of the reaction revealed that upon substrate encapsulation in the assembly, the entropy of activation becomes more negative in contrast to the uncatalyzed reaction. Furthermore, solvent isotope effects reveal a normal k(H 2O)/ k(D 2O) = 1.6, confirming an A-S E2 mechanism in which proton transfer occurs in the rate-limiting step. This is in contrast with the A1 mechanism of the uncatalyzed reaction in which decomposition of the protonated substrate is rate-limiting.  相似文献   

4.
The kinetics and mechanism of the hydrolysis of cysteine sulfenyl thiocyanate (CySSCN) to give cysteine thiosulfinate ester (CyS(=O)SCy) have been investigated between pH 0 and 4. The reaction is reversible. The hydrolysis of CySSCN is second-order in [CySSCN] and inverse first-order in [H+] and [SCN-]. The following mechanism is proposed for the hydrolysis of CySSCN (where the charge depends upon the pH): CySSCN0/+ + H2O <==>CySOH0/+ + SCN- + H+, CySOH0/+ + CySSCN0/+ --> CyS(=O)SCy0/+/2+ + SCN- + H+; k1 = 3.36 +/- 0.01 x 10-3 s-1, K1k2 = 0.13 +/- 0.05 Ms-1 (which yields k2/k-1 = 39 M). The observed rate law rules out alternative mechanisms for 1 0.4 M). The following mechanism is proposed: CyS(=O)SCy2+ + H+ <==> CyS(OH)=SCy3+, Ka; CyS(OH)SCy3+ + SCN- --> CySOH+ + CySSCN+, k-2 = 0.239 +/- 0.007 M-2s-1/Ka M-1. Since cysteine sulfenic acids are known to play an important function in many enzymes, and SCN- exists in abundance in physiologic fluids, we discuss the possible role of sulfenyl thiocyanates in vivo.  相似文献   

5.
Forster RJ 《Inorganic chemistry》1996,35(11):3394-3403
Dense monolayers of [Os(bpy)(2)py(p3p)](2+), where bpy is 2,2'-bipyridyl, py is pyridine, and p3p is 4,4'-trimethylenedipyridine, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Three well-defined waves, corresponding to osmium- and bipyridyl-based redox reactions, are observed in cyclic voltammetry of these monolayers, where the supporting electrolyte is tetrabutylammonium tetrafluoroborate (TBABF(4)) dissolved in acetonitrile. These reactions correspond to the charge states 3+/2+, 2+/1+, and 1+/0, respectively. Chronoamperometry, conducted on a microsecond time scale, has been used to measure the heterogeneous electron transfer rate constant, k/s(-1), for all three redox processes. For concentrations of TBABF(4) above 0.1 M, heterogeneous electron transfer is characterized by a single unimolecular rate constant. Standard heterogeneous electron transfer rate constants, k degrees, have been evaluated by extrapolating Tafel plots of ln k vs overpotential, eta, to zero driving force to yield values of (4.8 +/- 0.3) x 10(4) s(-1), (2.5 +/- 0.2) x 10(5) s(-1), and (3.3 +/- 0.3) x 10(4) s(-1) for k degrees (3+/2+), k degrees (2+/1+), and k degrees (1+/0), respectively. For large values of eta, these Tafel plots are curved for all three redox reactions, and while those corresponding to metal-based electron transfer are asymmetric with respect to eta, those corresponding to ligand-based reactions are symmetric. Temperature-resolved measurements of k reveal that the electrochemical activation enthalpy, DeltaH(), decreases from 43.1 +/- 2.8 kJ mol(-1) for the 3+/2+ reaction to 25.8 +/- 1.9 kJ mol(-1) for the 1+/0 process. Probing the temperature dependence of the formal potential gives the reaction entropy, DeltaS(rc) degrees. The reaction entropy depends on the state of charge of the monolayer with values of 212 +/- 18, 119 +/- 9, and 41 +/- 5 J mol(-1) K(-1) being observed for the 3+/2+, 2+/1+, and 1+/0, redox transformations, respectively. The electronic transmission coefficient, kappa(el), describing the probability of electron transfer once the nuclear transition state has been reached, is considerably less than unity for all three redox processes. However, kappa(el) is larger for the bipyridyl-based reductions, (2.4 +/- 0.9) x 10(-5), than for the metal-based reaction, (1.5 +/- 0.7) x 10(-6). This large difference in electronic transmission coefficients may be a consequence of the redox potentials of the bridging ligand and the remote redox sites being comparable, so that the electronic states of the bridging ligand contribute to the electron tunneling pathway.  相似文献   

6.
6,7-Dimethyl-8-ribityllumazine is the biosynthetic precursor of the vitamin, riboflavin. The biosynthetic formation of the lumazine by condensation of 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione and 3,4-dihydroxy-2-butanone 4-phosphate is catalyzed by the enzyme, lumazine synthase. We show that the condensation reaction can proceed without enzyme catalysis in dilute aqueous solution at room temperature and neutral pH. The reaction rate is proportional to e (pH). The activation energy of the uncatalyzed reaction is E(a) = 46.3 kJ mol(-)(1). The regioselectivity of the uncatalyzed reaction increases with pH and temperature (70% at 65 degrees C and pH 7.75). The data suggest partitioning of the uncatalyzed reaction via two different reaction pathways. The value of k(cat)/k(uncat) may be indicative for an entropy driven process for the enzyme-catalyzed reaction.  相似文献   

7.
The thermal dissociation of SO3 has been studied for the first time in the 1000-1400 K range. The experiments were conducted in a laminar flow reactor at atmospheric pressure, with nitrogen as the bath gas. On the basis of the flow reactor data, a rate constant for SO3 + N2 --> SO2 + O + N2 (R1b) of 5.7 x 10(17) exp(-40000/T) cm3/(mol s) is derived for the temperature range 1273-1348 K. The estimated uncertainty is a factor of 2. The rate constant corresponds to a value of the reverse reaction of k1 approximately 1.8 x 10(15) cm6 mol(-2) s(-1). The reaction is in the fall-off region under the investigated conditions. The temperature and pressure dependence of SO2 + O (+N2) was estimated from the extrapolation of low temperature results for the reaction, together with an estimated broadening parameter and the high-pressure limit determined recently by Naidoo, Goumri, and Marshall (Proc. Combust. Inst. 2005, 30, 1219-1225). The theoretical rate constant is in good agreement with the experimental results. The improved accuracy in k(1) allows a reassessment of the rate constant for SO3 + O --> SO2 + O2 (R2) based on the data of Smith, Tseregounis, and Wang (Int. J. Chem. Kinet. 1982, 14, 679-697), who conducted experiments on a low-pressure CO/O2/Ar flame doped with SO2. At the location in the flame where the net SO3 formation rate is zero, k2 = k1[SO2][M]/[SO3]. A value of 6.9 x 10(10) cm3 mol(-1) s(-1) is obtained for k2 at 1269 K with an uncertainty a factor of 3. A recommended rate constant k2 = 7.8 x 10(11) exp(-3065/T) cm3 mol(-1) s(-1) is consistent with other flame results as well as the present flow reactor data.  相似文献   

8.
A previous approach (Hancock, R. D.; Bartolotti, L. J. Inorg. Chem. 2005, 44, 7175) using DFT calculations to predict log K1 (formation constant) values for complexes of NH3 in aqueous solution was used to examine the solution chemistry of Rg(I) (element 111), which is a congener of Cu(I), Ag(I), and Au(I) in Group 1B. Rg(I) has as its most stable presently known isotope a t(1/2) of 3.6 s, so that its solution chemistry is not easily accessible. LFER (Linear free energy relationships) were established between DeltaE(g) calculated by DFT for the formation of monoamine complexes from the aquo ions in the gas phase, and DeltaG(aq) for the formation of the corresponding complexes in aqueous solution. For M2+, M3+, and M4+ ions, the gas-phase reaction was [M(H2O)6]n+(g) + NH3(g) = [M(H2O)5NH3]n+(g) + H2O(g) (1), while for M+ ions, the reaction was [M(H2O)2]+(g) + NH3(g) = [M(H2O)NH3]+(g) + H2O(g) (2). A value for DeltaG(aq) and for DeltaE for the formation of M = Cu2+ in reaction 1, not obtained previously, was calculated by DFT and shown to correlate well with the LFER obtained previously for other M2+ ions, supporting the LFER approach used here. The simpler use of DeltaE values instead of DeltaG(aq) values calculated by DFT for formation of monoamine complexes in the gas phase leads to LFER as good as the DeltaG-based correlations. Values of DeltaE were calculated by DFT to construct LFER with M+ = H+, and the Group 1B metal ions Cu+, Ag+, Au+, and Rg+, and with L = NH3, H2S, and PH3 in reaction 3: [M(H2O)2]+(g) + L(g) = [M(H2O)L]+g) + H2O(g) (3). Correlations involving DeltaE calculated by DMol3 for H+, Cu+, Ag+, and Au+ could reliably be used to construct LFER and estimate unknown log K1 values for Rg(I) complexes of NH3, PH3, and H2S calculated using the ADF (Amsterdam Density Functional) code. Log K1 values for Rg(I) complexes are predicted that suggest the Rg(I) ion to be a very strong Lewis acid that is extremely "soft" in the Pearson hard and soft acids and bases sense.  相似文献   

9.
Kinetics of reaction between Na2S2O3 and peroxide compound ( H2O2 or Na2S2O8) in a batch reactor and in a continuous stirring tank reactor (CSTR) were studied.Steady oscillations in uncatalyzed reactions in a CSTR were first discovered.In Na2S2O3-H2O2-H2SO4 reaction system,Pt potential and pH of higher and lower flow rutes beyond oscillation flow rates were in around the same extreme values.The reaction catalyeed by Cu2+ corsist of the catalyzed oscillation process and the uncatalyzed osciliation one.On the basis of experiment,a reaction mechanism consisting of three stages was put forward.The three stages are H positive-feedback reactions,proton negative-feedba k (uncatalyzed negative-feedback and catalyzed negative-feedback) reactions and transitional reactions.The mechanism is able to explain reasonably the nonlinear chemical phenomena appearing in the thiosulfatc oxidation reaction by peroxide-compounds.  相似文献   

10.
The first mechanistic study of a spin-forbidden proton-transfer reaction in aqueous solution is reported. Laser flash photolysis of alkaline trioxodinitrate (N(2)O(3)(2)(-), Angeli's anion) is used to generate a nitroxyl anion in its excited singlet state ((1)NO(-)). Through rapid partitioning between protonation by water and electronic relaxation, (1)NO(-) produces (1)HNO (ground state, yield 96%) and (3)NO(-) (ground state, yield 4%), which comprise a unique conjugate acid-base couple with different ground-state multiplicities. Using the large difference between reactivities of (1)HNO and (3)NO(-) in the peroxynitrite-forming reaction with (3)O(2), the kinetics of spin-forbidden deprotonation reaction (1)HNO + OH(-) --> (3)NO(-) + H(2)O is investigated in H(2)O and D(2)O. Consistent with proton transfer, this reaction exhibits primary kinetic hydrogen isotope effect k(H)/k(D) = 3.1 at 298 K, which is found to be temperature-dependent. Arrhenius pre-exponential factors and activation energies of the second-order rate constant are found to be: log(A, M(-)(1) s(-)(1)) = 10.0 +/- 0.2 and E(a) = 30.0 +/- 1.1 kJ/mol for proton transfer and log(A, M(-)(1) s(-)(1)) = 10.4 +/- 0.1 and E(a) = 35.1 +/- 0.7 kJ/mol for deuteron transfer. Collectively, these data are interpreted to show that the nuclear reorganization requirements arising from the spin prohibition necessitate significant activation before spin change can take place, but the spin change itself must occur extremely rapidly. It is concluded that a synergy between the spin prohibition and the reaction energetics creates an intersystem barrier and is responsible for slowness of the spin-forbidden deprotonation of (1)HNO by OH(-); the spin prohibition alone plays a minor role.  相似文献   

11.
过渡元素杂多钨硅酸盐氧化还原性质的研究   总被引:7,自引:0,他引:7  
本文通过极谱和循环伏安法,结合紫外光谱和X射线光电子能谱,研究了过渡元素钨硅杂多酸盐Kn[SiM(H2O)W11O39](M=Mn^2^+,Fe^3^+,Co^2^+,Ni^2^+,Zn^2^+,Cd^2^+)在溶液中的氧化还原性质,提出了它们的还原机理.杂多阴离子的极谱半波还原电位E1/2的顺序为Ni^2^+>Co^2^+>Zn^2^+>Fe^2^+>Mn^2^+,发现杂多阴离子的E1/2与其组分中的过渡元素的电负性X和过渡金属离子与水合电子反应速率常数的对数logke-分别有线性关系,讨论了过渡元素对杂多阴离子氧化还原性的影响.  相似文献   

12.
Electron transfer between Fe(CN)(6)(3-) and Fe(CN)(6)(4-) in homogeneous aqueous solution with K(+) as the counterion normally proceeds almost exclusively by a K(+)-catalyzed pathway, but this can be suppressed, and the direct Fe(CN)(6)(3)(-)-Fe(CN)(6)(4-) electron transfer path exposed, by complexing the K(+) with crypt-2.2.2 or 18-crown-6. Fe((13)CN)(6)(4-)-NMR line broadening measurements using either crypt-2.2.2 or (with extrapolation to zero uncomplexed [K(+)]) 18-crown-6 gave consistent values for the rate constant and activation volume (k(0) = (2.4 +/- 0.1) x 10(2) L mol(-1) s(-1) and Delta V(0) = -11.3 +/- 0.3 cm(3) mol(-1), respectively, at 25 degrees C and ionic strength I = 0.2 mol L(-1)) for the uncatalyzed electron transfer path. These values conform well to predictions based on Marcus theory. When [K(+)] was controlled with 18-crown-6, the observed rate constant k(ex) was a linear function of uncomplexed [K(+)], giving k(K) = (4.3 +/- 0.1) x 10(4) L(2) mol(-2) s(-1) at 25 degrees C and I = 0.26 mol L(-1) for the K(+)-catalyzed pathway. When no complexing agent was present, k(ex) was roughly proportional to [K(+)](total), but the corresponding rate constant k(K)' (=k(ex)/[K(+)](total)) was about 60% larger than k(K), evidently because ion pairing by hydrated K(+) lowered the anion-anion repulsions. Ionic strength as such had only a small effect on k(0), k(K), and k(K)'. The rate constants commonly cited in the literature for the Fe(CN)(6)(3-/4-) self-exchange reaction are in fact k(K)'[K(+)](total) values for typical experimental [K(+)](total) levels.  相似文献   

13.
The binding energies of the first 5 H2O molecules to c-C3H3+ were determined by equilibrium measurements. The measured binding energies of the hydrated clusters of 9-12 kcal/mol are typical of carbon-based CH+...X hydrogen bonds. The ion solvation with the more polar CH3CN molecules results in stronger bonds consistent with the increased ion-dipole interaction. Ab initio calculations show that the lowest energy isomer of the c-C3H3+(H2O)4 cluster consists of a cyclic water tetramer interacting with the c-C3H3+ ion, which suggests the presence of orientational restraint of the water molecules consistent with the observed large entropy loss. The c-C3H3+ ion is deprotonated by 3 or more H2O molecules, driven energetically by the association of the solvent molecules to form strongly hydrogen bonded (H2O)nH+ clusters. The kinetics of the associative proton transfer (APT) reaction C3H3+ + nH2O --> (H2O)nH+ + C3H2* exhibits an unusually steep negative temperature coefficient of k = cT(-63+/-4) (or activation energy of -37 +/- 1 kcal mol(-1)). The behavior of the C3H3+/water system is exactly analogous to the benzene+*/water system, suggesting that the mechanism, kinetics and large negative temperature coefficients may be general to multibody APT reactions. These reactions can become fast at low temperatures, allowing ionized polycyclic aromatics to initiate ice formation in cold astrochemical environments.  相似文献   

14.
The rate constant of the reaction NCN + O has been directly measured for the first time. According to the revised Fenimore mechanism, which is initiated by the NCN forming reaction CH + N(2)→ NCN + H, this reaction plays a key role for prompt NO(x) formation in flames. NCN radicals and O atoms have been quantitatively generated by the pyrolysis of NCN(3) and N(2)O, respectively. NCN concentration-time profiles have been monitored behind shock waves using narrow-bandwidth laser absorption at a wavelength of λ = 329.1302 nm. Whereas no pressure dependence was discernible at pressures between 709 mbar < p < 1861 mbar, a barely significant temperature dependence corresponding to an activation energy of 5.8 ± 6.0 kJ mol(-1) was found. Overall, at temperatures of 1826 K < T < 2783 K, the rate constant can be expressed as k(NCN + O) = 9.6 × 10(13)× exp(-5.8 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (±40%). As a requirement for accurate high temperature rate constant measurements, a consistent NCN background mechanism has been derived from pyrolysis experiments of pure NCN(3)/Ar gas mixtures, beforehand. Presumably, the bimolecular secondary reaction NCN + NCN yields CN radicals hence triggering a chain reaction cycle that efficiently removes NCN. A temperature independent value of k(NCN + NCN) = (3.7 ± 1.5) × 10(12) cm(3) mol(-1) s(-1) has been determined from measurements at pressures ranging from 143 mbar to 1884 mbar and temperatures ranging from 966 K to 1900 K. At higher temperatures, the unimolecular decomposition of NCN, NCN + M → C + N(2) + M, prevails. Measurements at temperatures of 2012 K < T < 3248 K and at total pressures of 703 mbar < p < 2204 mbar reveal a unimolecular decomposition close to its low pressure limit. The corresponding rate constants can be expressed as k(NCN + M) = 8.9 × 10(14)× exp(-260 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(±20%).  相似文献   

15.
The stoichiometry and the kinetics of oxidation of the cyanide complexes M(CN)n4- (M = Fe(II), Ru(II), Os(II), Mo(IV), and W(IV)) by the peroxydisulfate ion, S2O8(2-), and by the much more strongly oxidizing fluoroxysulfate ion, SO4F-, were studied in aqueous solutions containing Li+. Reactions of S2O8(2-) with M(CN)n4- are known to be strongly catalyzed by Li+ and other alkali metal ions, and this applies also to the corresponding reactions of SO4F-. The primary reactions of S2O8(2-) and SO4F- have both been found to be one-electron processes in which the equally strong O-O and O-F bonds are broken. The primary reaction of S2O8(2-) consists of a single step yielding M(CN)n3-, SO4-, and SO42-, whereas the primary reaction of SO4F- comprises two parallel one-electron steps, one leading to M(CN)n3-, SO4-, and F- and the other yielding M(CN)n-1(2-), CN-, SO4- and F-. The relationship between the rate constants and the standard free energies of reaction for the Li+-catalyzed reactions of SO4F- and S2O8(2-) with M(CN)n(4-), and for the uncatalyzed reactions of S2O8(2-) with bipyridyl and phenanthroline complexes MLn2+ (M = Fe(II), Ru(II), and Os(II)) studied previously, suggests that the intrinsic barrier for all three sets of reactions is similar, i.e., unaffected by the Li+ catalysis, and that the electron transfer and the breakage of the O-O and O-F bonds are concerted processes.  相似文献   

16.
Doubly charged lead monohydrate, [Pb(H2O)]2+, was predicted to be unstable in the gas phase, but it has recently been observed to form in low yield via ligand change between [Pb(CH3CN)]2+ and H2O [Shi, T.; Orlova, G.; Guo, J.; Bohme, D. K.; Hopkinson, A. C.; Siu, K. W. M. J. Am. Chem. Soc. 2004, 126, 7975-7980]. Here we report that abundant [Pb(H2O)]2+ is formed in the gas phase by ligand-exchange reaction between [Pb(N2)n]2+ (n = 1-3) and water after collisional activation. Density functional theory has been used to examine the ligand-exchange reaction profile. A comparison of the potential-energy surfaces between [Pb(N2)]2+ and [Pb(CH3CN)]2+ reacting with H2O provides strong evidence that the ligand-exchange reaction of [Pb(N2)]2+ with H2O to form [Pb(H2O)]2+ is more efficient than that of [Pb(CH3CN)]2+ with H2O.  相似文献   

17.
Although the kinetics and mechanism of metal-mediated oxygen atom (oxo) transfer reactions have been examined in some detail, sulfur atom (sulfido) transfer reactions have not been similarly scrutinized. The reactions [M(IV)(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-) + Ph(3)AsQ --> [M(VI)Q(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-) + Ph(3)As (M = Mo, W; Q = O, S) with variable substituent X' have been investigated in acetonitrile in order to determine the relative rates of oxo versus sulfido transfer at constant structure (square pyramidal) of the atom acceptor and of atom transfer at constant structure of the atom donor and metal variability of the atom acceptor. All reactions exhibit second-order kinetics and entropies of activation (-25 to -45 eu) consistent with an associative transition state. At parity of atom acceptor, k(2)(S) (0.25-0.75 M(-1)s(-1)) > k(2)(O) (0.023-0.060 M(-1)s(-1)) with M = Mo and k(2)(S) (4.1-66.7 M(-1)s(-1)) > k(2)(O) (1.8-9.8 M(-1)s(-1)) with M = W. At constant atom donor and X', k(2)(W) > k(2)(Mo) with reactivity ratios k(2)(W)/k(2)(Mo) = 78-184 (Q = O) and 16-89 (Q = S). Rate constants refer to 298 K. At constant M and Q, rates increase in the order X' = Me less, similar OMe < H < Br < COMe < CN; increasing electron-withdrawing propensity accelerates reaction rates. The probable transition state involves significant Ph(3)AsQ...M bond-making (X' rate trend) and concomitant As-Q bond weakening (bond energy order As-O > As-S). Orders of oxo and sulfido donor ability of substrates and complexes are deduced on the basis of qualitative reactivity properties determined here and elsewhere. This work complements previous studies of the reaction systems [M(IV)(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-)/XO where the substrates are N-oxides and S-oxides and k(2)(W) > k(2)(Mo) at constant substrate also applies. The reaction order of substrates is Me(3)NO > (CH(2))(4)SO > Ph(3)AsS > Ph(3)AsO. This research provides the first quantitative information of metal-mediated sulfido transfer.  相似文献   

18.
A range of complexes based on the high-nuclearity [W36] isopolyoxotungstate cluster, [H12W36O120]12-, with a triangular topology has been isolated by using the organic cation, protonated triethanolamine. In analogy to an 18-crown-6 crown ether with six oxygen donors on a ring, the cluster can form alkali and alkaline earth metal complexes [M within W36] (M = K+, Rb+, Cs+, NH4+, Sr2+ and Ba2+, 1-6, respectively). Compounds 1-6 were characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy. Comparisons between the structures of 1-6 and 18-crown-6 as well as the symmetry directing influence of the organo-cations in the isolation of the overall cluster architecture are discussed.  相似文献   

19.
The reactivity of the alkylating agent o-quinone methide (o-QM) toward NH(3), H(2)O, and H(2)S, prototypes of nitrogen-, oxygen-, and sulfur-centered nucleophiles, has been studied by quantum chemical methods in the frame of DF theory (B3LYP) in reactions modeling its reactivity in water with biological nucleophiles. The computational analysis explores the reaction of NH(3), H(2)O, and H(2)S with o-QM, both free and H-bonded to a discrete water molecule, with the aim to rationalize the specific and general effect of the solvent on o-QM reactivity. Optimizations of stationary points were done at the B3LYP level using several basis sets [6-31G(d), 6-311+G(d,p), adding d and f functions to the S atom, 6-311+G(d,p),S(2df), and AUG-cc-pVTZ]. The activation energies calculated for the addition reactions were found to be reduced by the assistance of a water molecule, which makes easier the proton-transfer process in these alkylation reactions by at least 12.9, 10.5, and 6.0 kcal mol(-1) [at the B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p) level], for ammonia, water, and hydrogen sulfide, respectively. A proper comparison of an uncatalyzed with a water-catalyzed reaction mechanism has been made on the basis of activation Gibbs free energies. In gas-phase alkylation of ammonia and water by o-QM, reactions assisted by an additional water molecule H-bonded to o-QM (water-catalyzed mechanism) are favored over their uncatalyzed counterparts by 5.6 and 4.0 kcal mol(-1) [at the B3LYP/6-311+G(d,p) level], respectively. In contrast, the hydrogen sulfide alkylation reaction in the gas phase shows a slight preference for a direct alkylation without water assistance, even though the free energy difference (DeltaDeltaG(#)) between the two reaction mechanisms is very small (by 1.0 kcal mol(-1) at the B3LYP/6-311+G(d,p),S(2df) level of theory). The bulk solvent effect, evaluated by the C-PCM model, significantly modifies the relative importance of the uncatalyzed and water-assisted alkylation mechanism by o-QM in comparison to the case in the gas phase. Unexpectedly, the uncatalyzed mechanism becomes highly favored over the catalyzed one in the alkylation reaction of ammonia (by 7.0 kcal mol(-1)) and hydrogen sulfide (by 4.0 kcal mol(-1)). In contrast, activation induced by water complexation still plays an important role in the o-QM hydration reaction in water as solvent.  相似文献   

20.
The one-electron reduction of [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-) at a glassy carbon electrode was investigated using cyclic and rotating-disk-electrode voltammetry in buffered and unbuffered aqueous solutions over the pH range 3.45-7.50 with an ionic strength of approximately 0.6 M maintained. The behavior is well-described by a square-scheme mechanism P + e(-) <--> Q (E(1)(0/) = -0.275 V, k(1)(0/) = 0.008 cm s(-1), and alpha(1) = 1/2), PH(+) + e(-) <--> QH(+) (E(2)(0/) = -0.036 V, k(2)(0/) = 0.014 cm s(-1), and alpha(2) = 1/2), PH(+) <--> P + H(+) (K(P) = 3.02 x 10(-6) M), and QH(+) <--> Q + H(+) (K(Q) = 2.35 x 10(-10) M), where P, Q, PH(+), and QH(+) correspond to [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-), [alpha(2)-Fe(II)(OH)P(2)W(17)O(61)](9-), [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), and [alpha(2)-Fe(II)(OH(2))P(2)W(17)O(61)](8-), respectively; E(1)(0)' and E(2)(0)' are the formal potentials, k(1)(0)' and k(2)(0)' are the formal (standard) rate constants, and K(P) and K(Q) are the acid dissociation constants for the relevant reactions. The analysis for the buffered media is based on the approach of Laviron who demonstrated that a square scheme with fully reversible protonations, reversible or quasi reversible electron transfers with the assumption that alpha(1) = alpha(2), can be well-described by the behavior of a simple redox couple, ox + e(-) <--> red, whose formal potential, E(app)(0)', and standard rate constant, k(app)(0)', are straightforwardly derived functions of pH, as are the values of E(1)(0)', k(1)(0)', E(2)(0)', k(2)(0)', and K(P) (only three of the four thermodynamic parameters in a square scheme can be specified). It was assumed that alpha(app) = 1/2, and the simulation program DigiSim was used to determine the values of E(app)(0)' and k(app)(0)', which are required to describe the cyclic voltammograms obtained in buffered media in the pH range from 3.45 to 7.52 (buffer-related reactions which effect general acid-base catalysis are included in the simulations). DigiSim simulations of cyclic voltammograms obtained in unbuffered media yielded the values of E(1)(0)' and k(1)(0)'; K(Q) was then directly computed from thermodynamic constraints. These simulations included additional reactions between the redox species and H(2)O. The value of the diffusion coefficient of the [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), 2.92 x 10(-6) cm(2) s(-1), was determined using DigiSim simulations of voltammograms at a rotating disk electrode in buffered and unbuffered media at pH 3.45. The diffusion coefficients of all redox species were assumed to be identical. When the pH is greater than 6, instability of P (i.e., [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-)) led to the loss of the reactant and precluded lengthy experimentation.  相似文献   

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