Influence of terpyridine as pi-acceptor ligand on the kinetics and mechanism of the reaction of NO with ruthenium(III) complexes

The kinetics of the unusually fast reaction of cis- and trans-[Ru(terpy)(NH3)2Cl]2+ (with respect to NH3; terpy=2,2':6',2"-terpyridine) with NO was studied in acidic aqueous solution. The multistep reaction pathway observed for both isomers includes a rapid and reversible formation of an intermediate Ru(III)-NO complex in the first reaction step, for which the rate and activation parameters are in good agreement with an associative substitution behavior of the Ru(III) center (cis isomer, k1=618 +/- 2 M(-1) s(-1), DeltaH(++) = 38 +/- 3 kJ mol(-1), DeltaS(++) = -63 +/- 8 J K(-1) mol(-1), DeltaV(++) = -17.5 +/- 0.8 cm3 mol(-1); k -1 = 0.097 +/- 0.001 s(-1), DeltaH(++) = 27 +/- 8 kJ mol(-1), DeltaS(++) = -173 +/- 28 J K(-1) mol(-1), DeltaV(++) = -17.6 +/- 0.5 cm3 mol(-1); trans isomer, k1 = 1637 +/- 11 M(-1) s(-1), DeltaH(++) = 34 +/- 3 kJ mol(-1), DeltaS(++) = -69 +/-11 J K(-1) mol(-1), DeltaV(++) = -20 +/- 2 cm3 mol(-1); k(-1)=0.47 +/- 0.08 s(-1), DeltaH(++)=39 +/- 5 kJ mol(-1), DeltaS(++) = -121 +/-18 J K(-1) mol(-1), DeltaV(++) = -18.5 +/- 0.4 cm3 mol(-1) at 25 degrees C). The subsequent electron transfer step to form Ru(II)-NO+ occurs spontaneously for the trans isomer, followed by a slow nitrosyl to nitrite conversion, whereas for the cis isomer the reduction of the Ru(III) center is induced by the coordination of an additional NO molecule (cis isomer, k2=51.3 +/- 0.3 M(-1) s(-1), DeltaH(++) = 46 +/- 2 kJ mol(-1), DeltaS(++) = -69 +/- 5 J K(-1) mol(-1), DeltaV(++) = -22.6 +/- 0.2 cm3 mol(-1) at 45 degrees C). The final reaction step involves a slow aquation process for both isomers, which is interpreted in terms of a dissociative substitution mechanism (cis isomer, DeltaV(++) = +23.5 +/- 1.2 cm3 mol(-1); trans isomer, DeltaV(++) = +20.9 +/- 0.4 cm3 mol(-1) at 55 degrees C) that produces two different reaction products, viz. [Ru(terpy)(NH3)(H2O)NO]3+ (product of the cis isomer) and trans-[Ru(terpy)(NH3)2(H2O)]2+. The pi-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern.     

Activation volume measurement for C[bond]H activation. Evidence for associative benzene substitution at a platinum(II) center

The reaction of the platinum(II) methyl cation [(N-N)Pt(CH(3))(solv)](+) (N-N = ArN[double bond]C(Me)C(Me)[double bond]NAr, Ar = 2,6-(CH(3))(2)C(6)H(3), solv = H(2)O (1a) or TFE = CF(3)CH(2)OH (1b)) with benzene in TFE/H(2)O solutions cleanly affords the platinum(II) phenyl cation [(N-N)Pt(C(6)H(5))(solv)](+) (2). High-pressure kinetic studies were performed to resolve the mechanism for the entrance of benzene into the coordination sphere. The pressure dependence of the overall second-order rate constant for the reaction resulted in Delta V(++) = -(14.3 +/- 0.6) cm(3) mol(-1). Since the overall second order rate constant k = K(eq)k(2), Delta V(++) = Delta V degrees (K(eq)) + Delta V(++)(k(2)). The thermodynamic parameters for the equilibrium constant between 1a and 1b, K(eq) = [1b][H(2)O]/[1a][TFE] = 8.4 x 10(-4) at 25 degrees C, were found to be Delta H degrees = 13.6 +/- 0.5 kJ mol(-1), Delta S degrees = -10.4 +/- 1.4 J K(-1) mol(-1), and Delta V degrees = -4.8 +/- 0.7 cm(3) mol(-1). Thus DeltaV(++)(k(2)) for the activation of benzene by the TFE solvento complex equals -9.5 +/- 1.3 cm(3) mol(-1). This significantly negative activation volume, along with the negative activation entropy for the coordination of benzene, clearly supports the operation of an associative mechanism.     

Solvolysis of 1,1-dimethyl-4-alkenyl chlorides: evidence for pi-participation

Tertiary 1,1-dimethyl-4-alkenyl chloride (1) solvolyzes with significantly reduced secondary beta-deuterium kinetic isotope effect (substrate with two trideuteromethyl groups) and has a lower entropy and enthalpy of activation than the referent saturated analogue 4 (k(H)/k(D) = 1.30 +/- 0.03 vs k(H)/k(D) = 1.79 +/- 0.01; Delta Delta H(++) = -9 kJ mol(-1), Delta Delta S(++) = -36 J mol(-1) K(-1), in 80% v/v aqueous ethanol), indicating participation of the double bond in the rate-determining step. Transition structure 1-TS computed at the MP2(fc)/6-31G(d) level of theory revealed that the reaction proceeds through a late transition state with considerably pronounced double bond participation and a substantially cleaved C-Cl bond. The doubly unsaturated compound 3 (1,1-dimethyl-4,8-alkadienyl chloride) solvolyzes with further reduction of the isotope effect, and a drastically lower entropy of activation (k(H)/k(D) = 1.14 +/- 0.01; DeltaS(++) = -152 +/- 12 J mol(-1) K(-1), in 80% v/v aqueous ethanol), suggesting that the solvolysis of 3 proceeds by way of extended pi-participation, i.e., the assistance of both double bonds in the rate-determining step.     

Influence of ligand polarizability on the reversible binding of O2 by trans-[Rh(X)(XNC)(PPh3)2] (X = Cl, Br, SC6F5, C2Ph; XNC = xylyl isocyanide). Structures and a kinetic study

The complexes trans-[Rh(X)(XNC)(PPh 3) 2] (X = Cl, 1; Br, 2; SC 6F 5, 3; C 2Ph, 4; XNC = xylyl isocyanide) combine reversibly with molecular oxygen to give [Rh(X)(O 2)(XNC)(PPh 3) 2] of which [Rh(SC 6F 5)(O 2)(XNC)(PPh 3) 2] ( 7) and [Rh(C 2Ph)(O 2)(XNC)(PPh 3) 2] ( 8) are sufficiently stable to be isolated in crystalline form. Complexes 2, 3, 4, and 7 have been structurally characterized. Kinetic data for the dissociation of O 2 from the dioxygen adducts of 1- 4 were obtained using (31)P NMR to monitor changes in the concentration of [Rh(X)(O 2)(XNC)(PPh 3) 2] (X = Cl, Br, SC 6F 5, C 2Ph) resulting from the bubbling of argon through the respective warmed solutions (solvent chlorobenzene). From data recorded at temperatures in the range 30-70 degrees C, activation parameters were obtained as follows: Delta H (++) (kJ mol (-1)): 31.7 +/- 1.6 (X = Cl), 52.1 +/- 4.3 (X = Br), 66.0 +/- 5.8 (X = SC 6F 5), 101.3 +/- 1.8 (X = C 2Ph); Delta S (++) (J K (-1) mol (-1)): -170.3 +/- 5.0 (X = Cl), -120 +/- 13.6 (X = Br), -89 +/- 18.2 (X = SC 6F 5), -6.4 +/- 5.4 (X = C 2Ph). The values of Delta H (++) and Delta S (++) are closely correlated (R (2) = 0.9997), consistent with a common dissociation pathway along which the rate-determining step occurs at a different position for each X. Relative magnitudes of Delta H (++) are interpreted in terms of differing polarizabilities of ligands X.     

Dissociative photoionization of mono-, di- and trimethylamine studied by a combined threshold photoelectron photoion coincidence spectroscopy and computational approach

Energy selected mono-, di- and trimethylamine ions were prepared by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Below 13 eV, the main dissociative photoionization path of these molecules is hydrogen atom loss. The ion time-of-flight (TOF) distributions and breakdown diagrams for H loss are analyzed in terms of the statistical RRKM theory, which includes tunneling. Experimental evidence, supported by quantum chemical calculations, indicates that the reverse barrier along the H loss potential energy curve for monomethylamine is 1.8 +/- 0.6 kJ mol(-1). Accurate dissociation onset energies are derived from the TOF simulation, and from this analysis we conclude that Delta(f)H degrees (298K)[CH(2)NH(2)(+)] = 750.4 +/- 1.3 kJ mol(-1) and Delta(f)H degrees (298K)[CH(2)NH(CH(3))(+)] = 710.9 +/- 2.8 kJ mol(-1). Quantum chemical calculations at the G3, G3B3, CBS-APNO and W1U levels are extensively used to support the experimental data. The comparison between experimental and ab initio isodesmic reaction heats also suggests that Delta(f)H degrees (298K)[N(CH(3))(3)] = -27.2 +/- 2 kJ mol(-1), and that the dimethylamine ionization energy is 8.32 +/- 0.03 eV, both of which are in slight disagreement with previous experimental values. Above 13 eV photon energy, additional dissociation channels appear besides the H atom loss, such as a sequential C(2)H(4) loss from trimethylamine for which a dissociation mechanism is proposed.     

Synthesis, structure, and substitution mechanism of new Ru(II) complexes containing 1,4,7-trithiacyclononane and 1,10-phenanthroline ligands

Two new Ru complexes containing the 1,10-phenanthroline (phen) and 1,4,7-trithiacyclononane ([9]aneS3, SCH2CH2SCH2CH2SCH2CH2) ligands of general formula [Ru(phen)(L)([9]aneS3)]2+ (L = MeCN, 3; L = pyridine (py), 4) have been prepared and thoroughly characterized. Structural characterization in the solid state has been performed by means of X-ray diffraction analyses, which show a distorted octahedral environment for a diamagnetic d6 Ru(II), as expected. 1H NMR spectroscopy provides evidence that the same structural arrangement is maintained in solution. Further spectroscopic characterization has been carried out by UV-vis spectroscopy where the higher acceptor capability of MeCN versus the py ligand is manifested in a 9-15-nm blue shift in its MLCT bands. The E1/2 redox potential of the Ru(III)/Ru(II) couple for 3 is anodically shifted with respect to its Ru-py analogue, 4, by 60 mV, which is also in agreement with a higher electron-withdrawing capacity of the former. The mechanism for the reaction Ru-py + MeCN--> Ru-MeCN + py has also been investigated at different temperatures with and without irradiation. In the absence of irradiation at 326 K, the thermal process gives kinetic constants of k2 = 1.4 x 10(-5) s(-1) (DeltaH(++) = 108 +/- 3 kJ mol(-1), DeltaS(++) = -8 +/- 9 J K(-1) mol(-1)) and k-2 = 2.9 x 10(-6) s(-1) (DeltaH(++) = 121 +/- 1 kJ mol(-1), DeltaS(++) = 18 +/- 3 J K(-1) mol(-1)). The phototriggered process is faster and consists of preequilibrium formation of an intermediate that thermally decays to the final Ru-MeCN complex with an apparent rate constant of (k1Khnu)app = 1.8 x 10(-4) s(-1) at 304 K, under the continuous irradiation experimental conditions used.     

Synthesis, structure, and molecular dynamics of gallium complexes of schizokinen and the amphiphilic siderophore acinetoferrin

A new general synthesis of the citrate-based siderophores acinetoferrin (Af) and schizokinen (Sz) and their analogues is described. The molecular structure of gallium schizokinen, GaSz, was determined by combined (1)H NMR, Hartree-Fock ab initio calculations, DFT, and empirical modeling of vicinal proton NMR spin-spin couplings. The metal-coordination geometry of GaSz was determined from NOE contacts to be cis-cis with respect to the two chelating hydroxamates. One diaminopropane adopts a single chairlike conformation while another is a mixture of two ring pucker arrangements. Both amide hydrogens are internally hydrogen bonded to metal-ligating oxygen atoms. The acyl methyl groups are directed away from each other with an average planar angle of ca. 130 degrees. The kinetics of GaSz racemization were followed by selective, double spin-echo inversion-recovery (1)H NMR spectroscopy over the temperature range of 10-45 degrees C. The racemization proceeds by a multistep mechanism that is proton independent between pD 5 and 12 (k(0) = 1.47 (0.15 s(-1))) and acid catalyzed below pD 4 (k(1) = 2.25 (0.15) x 10(4) M(-1) s(-1)). The activation parameters found for the two sequential steps of the proton independent pathway were DeltaH(++) = 25 +/- 3 kcal M(-1), DeltaS(++) = 25 +/- 7 cal M(-1) K(-1) and DeltaH(++) = 17.1 +/- 0.2 kcal M(-1), DeltaS(++) = 0.3 +/- 2.7 cal M(-1) K(-1). The first step of the proton-independent mechanism was assigned to the dissociation of the carboxyl group. The second step was assigned to complex racemization. The proton-assisted step was assigned to a complete dissociation of the alpha-hydroxy carboxyl group at pD < 4. The ab initio modeling of gallium acinetoferrin, GaAf, and analogues derived from the structure of GaSz has shown that the pendant trans-octenoyl fragments are oriented in opposite directions with the average planar angle of ca. 130 degrees. This arrangement prevents GaAf from adopting a phospholipid-like structural motif. Significantly, iron siderophore complex FeAf was found to be disruptive to phospholipid vesicles and is considerably more hydrophilic than Af, with an eight-fold smaller partition coefficient.     

Investigation of the mechanism of formation of a thiolate-ligated Fe(III)-OOH

Kinetic studies aimed at determining the most probable mechanism for the proton-dependent [Fe(II)(S(Me2)N(4)(tren))](+) (1) promoted reduction of superoxide via a thiolate-ligated hydroperoxo intermediate [Fe(III)(S(Me2)N(4)(tren))(OOH)](+) (2) are described. Rate laws are derived for three proposed mechanisms, and it is shown that they should conceivably be distinguishable by kinetics. For weak proton donors with pK(a(HA)) > pK(a(HO(2))) rates are shown to correlate with proton donor pK(a), and display first-order dependence on iron, and half-order dependence on superoxide and proton donor HA. Proton donors acidic enough to convert O(2)(-) to HO(2) (in tetrahydrofuran, THF), that is, those with pK(a(HA)) < pK(a(HO(2))), are shown to display first-order dependence on both superoxide and iron, and rates which are independent of proton donor concentration. Relative pK(a) values were determined in THF by measuring equilibrium ion pair acidity constants using established methods. Rates of hydroperoxo 2 formation displays no apparent deuterium isotope effect, and bases, such as methoxide, are shown to inhibit the formation of 2. Rate constants for p-substituted phenols are shown to correlate linearly with the Hammett substituent constants σ(-). Activation parameters ((ΔH(++) = 2.8 kcal/mol, ΔS(++) = -31 eu) are shown to be consistent with a low-barrier associative mechanism that does not involve extensive bond cleavage. Together, these data are shown to be most consistent with a mechanism involving the addition of HO(2) to 1 with concomitant oxidation of the metal ion, and reduction of superoxide (an "oxidative addition" of sorts), in the rate-determining step. Activation parameters for MeOH- (ΔH(++) = 13.2 kcal/mol and ΔS(++) = -24.3 eu), and acetic acid- (ΔH(++) = 8.3 kcal/mol and ΔS(++) = -34 eu) promoted release of H(2)O(2) to afford solvent-bound [Fe(III)(S(Me2)N(4)(tren))(OMe)](+) (3) and [Fe(III)(S(Me2)N(4)(tren))(O(H)Me)](+) (4), respectively, are shown to be more consistent with a reaction involving rate-limiting protonation of an Fe(III)-OOH, than with one involving rate-limiting O-O bond cleavage. The observed deuterium isotope effect (k(H)/k(D) = 3.1) is also consistent with this mechanism.     

Proton transfer to nickel-thiolate complexes. 1. Protonation of [Ni(SC(6)H(4)R-4)(2)(Ph(2)PCH(2)CH(2)PPh(2))] (R = Me, MeO, H, Cl, or NO(2))

The kinetics of the equilibrium reaction between [Ni(SC(6)H(4)R-4)(2)(dppe)] (R= MeO, Me, H, Cl, or NO(2); dppe = Ph(2)PCH(2)CH(2)PPh(2)) and mixtures of [lutH](+) and lut (lut = 2,6-dimethylpyridine) in MeCN to form [Ni(SHC(6)H(4)R-4)(SC(6)H(4)R-4)(dppe)](+) have been studied using stopped-flow spectrophotometry. The kinetics for the reactions with R = MeO, Me, H, or Cl are consistent with a single-step equilibrium reaction. Investigation of the temperature dependence of the reactions shows that DeltaG = 13.6 +/- 0.3 kcal mol(-)(1) for all the derivatives but the values of DeltaH and DeltaS vary with R (R = MeO, DeltaH() = 8.5 kcal mol(-)(1), DeltaS = -16 cal K(-)(1) mol(-)(1); R = Me, DeltaH() = 10.8 kcal mol(-)(1), DeltaS = -9.5 cal K(-)(1) mol(-)(1); R = Cl, DeltaH = 23.7 kcal mol(-)(1), DeltaS = +33 cal K(-)(1) mol(-)(1)). With [Ni(SC(6)H(4)NO(2)-4)(2)(dppe)] a more complicated rate law is observed consistent with a mechanism in which initial hydrogen-bonding of [lutH](+) to the complex precedes intramolecular proton transfer. It seems likely that all the derivatives operate by this mechanism, but only with R = NO(2) (the most electron-withdrawing substituent) does the intramolecular proton transfer step become sufficiently slow to result in the change in kinetics. Studies with [lutD](+) show that the rates of proton transfer to [Ni(SC(6)H(4)R-4)(2)(dppe)] (R = Me or Cl) are associated with negligible kinetic isotope effect. The possible reasons for this are discussed. The rates of proton transfer to [Ni(SC(6)H(4)R-4)(2)(dppe)] vary with the 4-R-substituent, and the Hammett plot is markedly nonlinear. This unusual behavior is attributable to the electronic influence of R which affects the electron density at the sulfur.     

Dissociation dynamics of sequential ionic reactions: heats of formation of tri-, di-, and monoethylphosphine

The sequential ethene (C2H4) loss channels of energy-selected ethylphosphine ions have been studied using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy in which ion time-of-flight (TOF) distributions are recorded as a function of the photon energy. The ion TOF distributions and breakdown diagrams have been modeled in terms of the statistical RRKM theory for unimolecular reactions, providing 0 K dissociation onsets, E0, for the ethene loss channels. Three RRKM curves were used to model the five measurements, since two of the reactions differ only by the internal energy of the parent ion. This series of dissociations provides a detailed check of the calculation of the product energy distribution for sequential reactions. From the determined E0's, the heats of formation of several ethylphosphine neutrals and ions have been determined: Delta(f)H degrees 298K[P(C(2)H(5))3] = -152.7 +/- 2.8 kJ/mol, Delta(f)H degrees 298K[P(C(2)H(5))3+] = 571.6 +/- 4.0 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2] = -89.6 +/- 2.1 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2+] = 669.9 +/- 2.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)] = -36.5 +/- 1.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)+] = 784.0 +/- 1.9 kJ/mol. These values have been supported by G2 and G3 calculations using isodesmic reactions. Coupled cluster calculations have been used to show that the C2H4 loss channel, which involves a hydrogen transfer step, proceeds without a reverse energy barrier.     

Rates of water exchange on the [Fe4(OH)2(hpdta)2(H2O)4]0 molecule and its implications for geochemistry

The ammonium salt of [Fe(4)O(OH)(hpdta)(2)(H(2)O)(4)](-) is soluble and makes a monospecific solution of [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N',N'-tetraacetate). This tetramer is a diprotic acid with pK(a)(1) estimated at 5.7 ± 0.2 and pK(a)(2) = 8.8(5) ± 0.2. In the pH region below pK(a)(1), the molecule is stable in solution and (17)O NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: k(ex)(298) = 8.1 (±2.6) × 10(5) s(-1), ΔH(++) = 46 (±4.6) kJ mol(-1), ΔS(++) = 22 (±18) J mol(-1) K(-1), and ΔV(++) = +1.85 (±0.2) cm(3) mol(-1) for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H(+)] to account for the small pH variation in rate yields a similar value of k(ex)(298) = 8.3 (±0.8) × 10(5) s(-1). These rates are ～10(4) times faster than those of the [Fe(OH(2))(6)](3+) ion (k(ex)(298) = 1.6 × 10(2) s(-1)) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) molecule. As pH approaches pK(a1), the rates decrease and a compensatory relation is evident between the experimental ΔH(++) and ΔS(++) values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between bond lengths and the logarithm of k(ex)(298) is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.     

Stereodynamics of 9,11-Diphenyl-10-azatetracyclo[6.3.0.0.(4,11)0.(5,9)]undecanes. Highly Restricted Nitrogen Inversion and Isolated Phenyl Rotation. X-ray Crystallographic, Dynamic NMR, and Molecular Mechanics Studies

The (1)H NMR spectra of 10-benzyl-9,11-diphenyl-10-azatetracyclo[6.3.0.0.(4,11)0.(5,9)]undecane (BnPh(2)()) and 10-methyl-9,11-diphenyl-10-azatetracyclo[6.3.0.0.(4,11)0.(5,9)]undecane (MePh(2)()) decoalesce due to slowing inversion at nitrogen and to slowing isolated bridgehead phenyl rotation. The high nitrogen inversion barriers in MePh(2)() (DeltaG() = 12.2 +/- 0.1 kcal/mol at 250 K) and BnPh(2)() (DeltaG() = 10.6 +/- 0.1 kcal/mol at 215 K) are typical of tertiary amines in which at least one C-N-C bond angle is constrained to a small value. Compared to the minuscule rotation barriers about sp(2)-sp(3) carbon-carbon bonds in simple molecular systems, the bridgehead phenyl rotation barriers in MePh(2)() (DeltaG() = 9.8 +/- 0.1 kcal/mol at 210 K) and BnPh(2)() (DeltaG() = 9.8 +/- 0.1 kcal/mol at 210 K) are unusually high. Molecular mechanics calculations (MMX force field) suggest that the origin of the high phenyl rotation barriers lies in the close passage of an o-phenyl proton and a methyl (or benzylmethylene) proton in the transition state. BnPh(2)() crystallized from hexane as white needles in the monoclinic system Pn. Unit cell dimensions are as follows: a = 12.198(1) ?, b = 6.1399(6) ?, c = 14.938(2) ?, beta = 107.470(4) degrees, V = 1067.1(2) ?(3), Z = 2. In the crystal molecular structure, the imine bridge CNC bond angle in BnPh(2)() is constrained to a small value (96 degrees ). The benzylic phenyl group is oriented gauche to the nitrogen lone pair.     

Activation energy for the disproportionation of HBrO2 and estimated heats of formation of HBrO2 and BrO2

The kinetics of the reaction HBrO(2) + HBrO(2) --> HOBr + BrO(3)(-) + H(+) is investigated in aqueous HClO(4) (0.04-0.9 M) and H(2)SO(4) (0.3-0.9 M) media and at temperatures in the range 15-38 degrees C. The reaction is found to be cleanly second order in [HBrO(2)], with the experimental rate constant having the form k(exp) = k + k'[H(+)]. The half-life of the reaction is on the order of a few tenths of a second in the range 0.01 M < [HBrO(2)](0) < 0.02 M. The detailed mechanism of this reaction is discussed. The activation parameters for kare found to be E(double dagger) = 19.0 +/- 0.9 kJ/mol and DeltaS(double dagger) = -132 +/- 3 J/(K mol) in HClO(4), and E(double dagger) = 23.0 +/- 0.5 kJ/mol and DeltaS(double dagger) = -119 +/- 1 J/(K mol) in H(2)SO(4). The activation parameters for k' are found to be E(double dagger) = 25.8 +/- 0.5 kJ/mol and DeltaS(double dagger) = -106 +/- 1 J/(K mol) in HClO(4), and E(double dagger) = 18 +/- 3 kJ/mol and DeltaS(double dagger) = -130 +/- 11 J/(K mol) in H(2)SO(4). The values Delta(f)H(29)(8)(0)[BrO(2)(aq)] = 157 kJ/mol and Delta(f)H(29)(8)(0)[HBrO(2)(aq)] = -33 kJ/mol are estimated using a trend analysis (bond strengths) based on the assumption Delta(f)H(29)(8)(0)[HBrO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOBr(aq)] and Delta(f)H(29)(8)(0)[HBrO(3)(aq)] as Delta(f)H(29)(8)(0)[HClO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOCl(aq)] and Delta(f)H(29)(8)(0)[HClO(3)(aq)]. The estimated value of Delta(f)H(29)(8)(0)[BrO(2)(aq)] agrees well with calculated gas-phase values, but the estimated value of Delta(f)H(29)(8)(0)[HBrO(2)(aq)], as well as the tabulated value of Delta(f)H(29)(8)(0)[HClO(2)(aq)], is in substantial disagreement with calculated gas-phase values. Values of Delta(r)H(0) are estimated for various reactions involving BrO(2) or HBrO(2).     

Stabilities of the aqueous complexes Cm(CO3)33- and Am(CO3)33- in the temperature range 10-70 degrees C

The carbonate complexation of curium(III) in aqueous solutions with high ionic strength was investigated below solubility limits in the 10-70 degrees C temperature range using time-resolved laser-induced fluorescence spectroscopy (TRLFS). The equilibrium constant, K(3), for the Cm(CO(3))(2-) + CO(3)(2-) right harpoon over left harpoon Cm(CO(3))(3)(3-) reaction was determined (log K(3) = 2.01 +/- 0.05 at 25 degrees C, I = 3 M (NaClO(4))) and compared to scattered previously published values. The log K(3) value for Cm(III) was found to increase linearly with 1/T, reflecting a negligible temperature influence on the corresponding molar enthalpy change, Delta(r)H(3) = 12.2 +/- 4.4 kJ mol(-1), and molar entropy change, Delta(r)S(3) = 79 +/- 16 J mol(-1) K(-1). These values were extrapolated to I = 0 with the SIT formula (Delta(r)H(3) degrees = 9.4 +/- 4.8 kJ mol(-1), Delta(r)S(3) degrees = 48 +/- 23 J mol(-1) K(-1), log K(3) degrees = 0.88 +/- 0.05 at 25 degrees C). Virtually the same values were obtained from the solubility data for the analogous Am(III) complexes, which were reinterpreted considering the transformation of the solubility-controlling solid. The reaction studied was found to be driven by the entropy. This was interpreted as a result of hydration changes. As expected, excess energy changes of the reaction showed that the ionic strength had a greater influence on Delta(r)S(3) than it did on Delta(r)H(3).     

Octahedral-tetrahedral equilibrium and solvent exchange of cobalt(II) ions in primary alkylamines

The enthalpy differences (Delta H degrees ) of the equilibrium between the octahedral and tetrahedral solvated cobalt(II) complexes were obtained in some primary alkylamines such as propylamine (pa, 36.1 +/- 2.3 kJ mol(-1)), n-hexylamine (ha, 34.9 +/- 1.0 kJ mol(-1)), 2-methoxyethylamine (meea, 44.8 +/- 3.1 kJ mol(-1)), and benzylamine (ba, 50.1 +/- 3.6 kJ mol(-1)) by the spectrophotometric method. The differences in the energy levels between the two geometries of the cobalt(II) complexes in the spherically symmetric field (Delta E(spher)) were estimated from the values of Delta H degrees by offsetting the ligand field stabilization energies. It was indicated that the value of Delta E(spher) is the decisive factor in determining the value of Delta H degrees and is largely dependent on the electronic repulsion between the d-electrons and the donor atoms and the interelectronic repulsion in the d orbitals. The comparison between activation enthalpies (Delta H(++)) for the solvent exchange reactions of octahedral cobalt(II) ions in pa and meea revealed that the unexpectedly large rate constant and small Delta H(++) in pa are attributed to the strong electronic repulsion in the ground state and removal of the electronic repulsion in the dissociative transition state, which can give the small Delta E(spher) between the ground and transition states. Differences in the solvent exchange rates and the DeltaH(++) values of the octahedral metal(II) ions in some other solvents are discussed in connection with the electronic repulsive factors.     

Spin crossover behavior in the iron(II)-2-pyridyl[1,2,3]triazolo[1,5-a]pyridine system: X-ray structure,calorimetric, magnetic,and photomagnetic studies

Compounds [Fe(tzpy)(3)](BF(4))(2) (1), [Fe(tzpy)(2)(NCS)(2)].S (S = 2CHCl(3) (2), H(2)O (3)), and [Fe(tzpy)(2)(NCSe)(2)] (4) (tzpy is 3-(2-pyridyl)[1,2,3]triazolo[1,5-a]pyridine) have been synthesized and characterized. 1 crystallizes in the monoclinic noncentrosymmetric system, Cc space group, Z = 4, with a = 11.4680(6) A, b = 27.449(2) A, c = 12.4510(8) A, beta = 108.860(5) degrees, V = 3709.0(4) A(3), and T = 293(2) K. The structure consists of mononuclear [Fe(tzpy)(3)](2+) diamagnetic species, which stack via pi-interactions. Disordered BF(4)(-) anions fill the voids generated by complex cations. 2 crystallizes in the triclinic system, P one macro space group, Z = 1, with a = 8.3340(4) A, b = 8.6520(4) A, c = 11.6890(6) A, alpha = 89.113(2) degrees, beta = 81.612(2) degrees, gamma = 77.803(2) degrees, V = 814.90(7) A(3), and T = 293(2) K. The structure consists of mononuclear [Fe(tzpy)(2)(NCS)(2)] neutral species, which interact each other via pi-staking defining layers separated by two-dimensional arrays of CHCl(3). The average Fe-N bond distance, 2.176(3) A, corresponds to what is expected for an iron(II) ion in the high-spin state. Compounds 2-4 undergo thermal-driven spin conversion. The regular solution model was applied to account for the corresponding to thermodynamic parameters. The intermolecular interaction parameter, the characteristic temperature, and the enthalpy and entropy changes associated with the spin conversion were estimated as Gamma = 0.86 (2), 0.89 (3), and 3.79 (4) kJ mol(-1), T(1/2) = 75 (2), 118 (3), and 251 K (4), Delta H = 3.67 (2) and 4.08 (3) kJ mol(-1), and Delta S = 34 (2) and 34.5 (3) J K(-1) mol(-1). Delta H = 8.75 kJ mol(-1) and Delta S = 34.8 J K(-1) mol(-1) were estimated from calorimetric measurements and used as fixed parameters for 4. A quantitative light-induced excited spin state trapping (LIESST) effect was observed for 3, and the high-spin to low-spin relaxation was studied in the temperature region 20-63 K.     

The thermodynamics of the transformation of graphite to multiwalled carbon nanotubes

The thermodynamic quantities associated to the transformation from graphite to multiwalled carbon nanotubes (MWCNTs) were determined by electromotive force (emf) and differential scanning calorimetry (DSC) measurements. From the emf versus T data of galvanic cell Mo|Cr(3)C(2), CrF2, MWCNTs|CaF2 s.c.|Cr(3)C(2), CrF2, graphite|Mo with CaF2 as solid electrolyte, Delta(r)H(T) degrees= 8.25 +/- 0.09 kJ mol(-1) and Delta(r)S(T) degrees= 11.72 +/- 0.09 JK(-1) mol(-1) were found at average temperature T = 874 K. The transformation enthalpy was also measured by DSC of the Mn(7)C(3) formation starting from graphite or MWCNTs. Thermodynamic values at 298 K were calculated to be: Delta(r)H(298) degrees = 9.0 +/- 0.8 kJ mol(-1) as averaged value from both techniques and Delta(r)S(298) degrees approximately Delta(r)S(T) degrees. At absolute zero, the residual entropy of MWCNTs was estimated 11.63 +/- 0.09 JK(-1) mol(-1), and transformation enthalpy Delta(r)H(0) degrees approximately Delta(r)H(298) degrees. The latter agrees satisfactorily with the theoretical calculations for the graphite-MWCNTs transformation. On thermodynamic basis, the transformation becomes spontaneous above 704 +/- 13 K.     

Calculated volume and energy profiles for water exchange on t2g6 rhodium(III) and iridium(III) hexaaquaions: conclusive evidence for an Ia mechanism

An I(a) mechanism was assigned for water exchange on the hexaaquaions Rh(OH(2))(6)(3+) and Ir(OH(2))(6)(3+) on the basis of negative Delta V(++) experimental values (-4.2 and -5.7 cm(3) mol(-1), respectively). The use of Delta V(++) as a mechanistic criterion was open to debate primarily because Delta V(++) could be affected by extension or compression of the nonparticipating ligand bond lengths on going to the transition state of an exchange process. In this paper, volume and energy profiles for two distinct water exchange mechanisms (D and I(a)) have been computed using quantum chemical calculations which include hydration effects. The activation energy for Ir(OH(2))(6)(3+) is 32.2 kJ mol(-1) in favor of the I(a) mechanism (127.9 kJ mol(-1)), as opposed to a D pathway; the value for the I(a) mechanism being close to Delta H(++) and Delta G(++) experimental values (130.5 kJ mol(-1) and 129.9 kJ mol(-1) at 298 K, respectively). Volumes of activation, computed using Connolly surfaces and for the I(a) pathway (DeltaV(++)(calc) = -3.9 and -3.5 cm(3) mol(-1), respectively, for Rh(3+) and Ir(3+)), are in agreement with the experimental values. Further, it is demonstrated for both mechanisms that the contribution to the volume of activation due to the changes in bond lengths between Ir(III) and the spectator water molecules is negligible: -1.8 for the D, and -0.9 cm(3) mol(-1) for I(a) mechanism. This finding clarifies the debate about the interpretation of Delta V(++) and unequivocally confirms the occurrence of an I(a) mechanism with retention of configuration and a small a character for both Rh(III) and Ir(III) hexaaquaions.     

Thermal decomposition of HO2NO2 (peroxynitric acid, PNA): rate coefficient and determination of the enthalpy of formation

Rate coefficients for the gas-phase thermal decomposition of HO(2)NO(2) (peroxynitric acid, PNA) are reported at temperatures between 331 and 350 K at total pressures of 25 and 50 Torr of N(2). Rate coefficients were determined by measuring the steady-state OH concentration in a mixture of known concentrations of HO(2)NO(2) and NO. The measured thermal decomposition rate coefficients k(-)(1)(T,P) are used in combination with previously published rate coefficient data for the HO(2)NO(2) formation reaction to yield a standard enthalpy for reaction 1 of Delta(r)H degrees (298K) = -24.0 +/- 0.5 kcal mol(-1) (uncertainties are 2sigma values and include estimated systematic errors). A HO(2)NO(2) standard heat of formation, Delta(f)H degrees (298K)(HO(2)NO(2)), of -12.6 +/- 1.0 kcal mol(-1) was calculated from this value. Some of the previously reported data on the thermal decomposition of HO(2)NO(2) have been reanalyzed and shown to be in good agreement with our reported value.