Thermodynamics and coordination characteristics of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 extraction complex

The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)₂·(H₃O⁺·χH₂O)₂·UO₂Cl₄²⁻ (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (K_ex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log K_ex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol⁻¹. The value of the entropy change, ΔS°, was calculated from ΔH° and K_ex and equals −690 J K⁻¹ mol⁻¹, whereas ΔG° = −6.45 kJ mol⁻¹. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K⁻¹ mol⁻¹, ΔH° = −101 kJ mol⁻¹ and ΔG° = −8.37 kJ mol⁻¹), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H₅O₂⁺ ions in it, in contrast to the H₃O⁺ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H₅O₂⁺ ions is held in the crown ether cavity by four hydrogen bonds. The H₅O₂⁺ ion has a central bond. The uranium atom forms UO₂Cl₄²⁻ as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above.     

Spectroelectrochemistry of ferrioxamine B,ferrichrome, and ferrichrome A

Thin-layer spectroelectrochemical techniques were used to determine the entropy change for the reduction of the three siderophores ferrioxamine B, ferrichrome, and ferrichrome A. The entropy changes were found to be large and negative. The _ΔS° values obtained are: ferrioxamine B. pH 10.2, _ΔS° = ?33.3 ± 0.4 eu; pH 9.0, _ΔS° = ?26.9 ± 0.9 eu; pH 8.0, _ΔS° = ?23.3 ± 1.2 eu; ferrichrome, pH 10.0, ΔS° = ?42.6 ± 0.5 eu; pH 9.1, ΔS° = ?35.8 ± 0.4 eu; pH 7.3, ΔS° = ?74.5 ± 3.4 eu; ferrichrome A, pH 10.1, _ΔS° = ?35.6 ± 0.9 eu; pH 9.1, _ΔS° = ?34.3 ± 0.9 eu; pH 7.9, _ΔS° = ?31.7 ± 0.9 eu. These values are adjusted to the scale on which S°_H + = 0. The large decreases in entropy upon reduction are attributed to an increase in the solvent ordering around the ferrous complex. Upon reduction, the rigid structure of the ferric chelate is loosened and previously sequestered amide groups are made available for solvent interactions. This increased interaction with solvent causes an increase in the order of the water around the molecule and this is responsible for the observed entropy changes. Variations in ΔS° values and the pH dependencies of these values are attributed to structural peculiarities of the individual siderophores.     

Stereoselective synthesis and characterization of the optically labile chiral Cr(III) complex Δ-(+)589{Cr[(−)bdtp]3}

The diastereoisomeric complex Δ-(+)-tris(cyclicO,O′, 1 (R), 2(R)(−)dimethylethylene dithiophosphato)chromium(III), was synthesized stereoselectively in tetrahydrofuran (THF) solution. The complex proves optically labile, [α]_D=+106, in CHCl₃, changing quickly to [α]_D=+211. The CD spectra in THF enable us to characterize the complex and show a configuration inversion which gives the diastereoisomeric equilibrium Λ⇌Δ with an excess of the Λ-(R,R)(R,R)(R,R) diastereoisomeric form. The equilibrium constant K=0.86 at 25 °C is indicative of a different thermodynamic stability between the two diastereoisomers in THF solution, Λ-(R,R)> Δ-(R,R), δΔH°=1.5 kJ mol⁻¹, δΔG°=0.3 kJ mol⁻¹, δΔS°=4 J mol⁻¹ K⁻¹. The kinetic diastereoisomer Δ-(R,R)(R,R)(R,R) is stabilized in CHCl₃, CH₂Cl₂, EtOH solvents where it is highly soluble and optically stable with a maximum negative chirality factor, g=−5×10⁻³, in CHCl₃.     

The kinetics of ligand substitution in bis(N-alkylsalicylaldiminato)nickel(II) Complexes: a study on the reactivity of square-planar versus octahedral coordination

The bis(N-alkylsalicylaldiminato)nickel(II) complexes Ni(R-sal)₂ with R = CH(CH₂OH)CH(OH)Ph (I), R = CH(CH₃)CH(OH)Ph (II) and R = CH₂CH₂Ph (III; Ph = phenyl) were prepared and characterized. In the solid state I and II are paramagnetic (μ = 3.2 and 3.3 BM at 20 °C, respectively), whereas III is diamagnetic. It follows from the UV-Vis spectra that in acetone solution I is six-coordinate octahedral and III is four-coordinate planar, the spectrum of II showing characteristics of both modes of coordination. Vis spectrophotometry and stopped-flow spectrophotometry were applied to study the kinetics of ligand substitution in I–III by H₂salen (= N,N′-disalicylidene-ethylenediamine) in the solvent acetone at different temperatures. The kinetics follow a second-order rate law, rate = k[H₂-salen] [complex]. At 20 °C the sequence of rate constants is k(III):k(II):k(I) = 11 850:40.6:1. The activation parameters are ΔH^≠(I) = 112, ΔH^≠(II) = 40.7, ΔH^≠(III) = 35.7 kJ mol⁻¹ and ΔS^≠(I) = 92, ΔS^≠(II) = −103, ΔS^≠(III) = −89 J K⁻¹ mol⁻¹. The enormous difference in rate between complexes I, II and III, which is less pronounced in methanol, is attributed to the existence of a fast equilibrium planar ⇌ octahedral, which is established in the case of I and II by intramolecular octahedral coordination through the hydroxyl groups present in the organic group R. An A-mechanism is suggested to control the substitution in the sense that the entering ligand attacks the four-coordinate planar complex, the octahedral complex being kinetically inert.     

Kinetics and mechanism of malonate replacement in bis(malonato)oxovanadate(IV) by oxalate

The kinetics of malonate replacement in bis- (malonato)oxovanadate(IV), [VO(mal)₂H₂O]²⁻(hereafter water molecule will be omitted), by oxalate has been studied by the stopped-flow method. The reaction was found to consist of two consecutive steps (k₁ and k₂: first-order rate constants) passing through a mixed ligand complex, [VO(mal)(ox)]²⁻. The rates for each step depended linearly on the concentrations of free oxalate species, Hox⁻ and ox²⁻. The second-order rate constants for the replacement by ox²⁻ were much larger in the k₁ step than in the k₂ step and the activation parameters were determined as follows: ΔH^≠= 43.5 ± 5.6 kJ mol⁻¹, ΔS±-53 ± 19 J K⁻¹ mol⁻¹ and ΔH≠= 43.6 ± 0.5 kJ mol⁻¹, δS≠ = -62 ± 2 J K^−l mol⁻¹ for the k₁ and k₂ steps, respectively. The volume of activation was determined to be -0.65 ± 0.75 cm³ mol⁻¹ at 20.2 °C by the high-pressure stopped-flow method for the apparent rate constants.     

The effect of calcium on the thermotropic properties of bovine blood coagulation factors IX and X and their activation intermediates and products

The thermotropic properties of bovine blood coagulation Factors IX and X, as well as the activation intermediates and products of these proteins, have been investigated by differential scanning microcalorimetry in the presence and absence of Ca²⁺. Bovine Factor IX displays a single thermal-denaturation transition characterized by a temperature midpoint (T_M) of 54.5 ± 0.5 °C and a calorimetric enthalpy (ΔH_c) of 105 ± 15 kcal/mol, in the absence of Ca²⁺. In the presence of Ca²⁺ concentrations sufficient to saturate its sites on Factor IX, the T_m value is increased to 57.0 ± 0.5 °C and the ΔH_c is virtually unchanged. When the activation intermediate, Factor IXα, is similarly analyzed in the absence of Ca²⁺, a broad, diffuse thermogram was obtained which did not lend itself to calculation of thermodynamic parameters. In the presence of Ca²⁺, Factor IXα displayed thermograms characterized by a T_M of 51.0 ± 0.5 °C and a ΔH_c of 109 ± 10 kcal/mol. The activated product, Factor IXaα, in the absence of Ca²⁺ (the values in the presence of saturating Ca²⁺ are given in parentheses), undergoes thermal denaturation with a T_M of 54.5 ± 0.5 °C (57.0 ± 0.5 °C) and a ΔH_c of 158 ±10 kcal/mol (156 ± 10 kcal/mol). Similarly, the terminal-activation product, Factor IXaβ, displays a T_M of 51.5 ± 0.5 °C (54.0 ± 0.5 °C) and a ΔH_c of 85 ± 5 kcal/mol (126 ± 10 kcal/mol). Bovine blood coagulation Factor X has been analyzed in this same fashion, and shows very similar thermal properties to Factor IX. The thermal denaturation of Factor X is represented by a T_M of 54.0 ± 0.5 °C (55.0 ± 0.5 °C) and a ΔH_c of 102 ± 10 kcal/mol (118 ± 10 kcal/mol), whereas its activated form, Factor Xaβ, possesses a T_M of 55.0 ± 0.5 °C (55.0 ± 0.5 °C) and a ΔH_c of 92.0 ± 5 kcal/mol (136 ± 10 kcal/mol). These studies indicate that, for many of these proteins, Ca²⁺ induces a conformational alteration to a more thermally stable form, which also requires the absorption of greater amounts of heat for thermal denaturation.     

Kinetics of imidazole addition to the axial site of the iron(II) complex of 2, 3, 9,10-tetraphenyl-1,4, 8,11-tetraaza-1,3, 8,10-cyclotetradecatetraene in dimethyl sulfoxide. Evidence for a dissociative-interchange mechanism

The axial adduct formation of the iron(II) complex of 2,3,9,10-tetraphenyl-l,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene (L) with imidazole in dimethyl sulfoxide has been investigated spectrophotometrically at various temperatures and pressures. In the presence of a large excess of imidazole the reaction with the two phases has been observed. The first faster reaction is the formation of the monoimidazole complex of FeL²⁺, and the second slower reaction corresponds to the formation of the bisimidazole complex. Activation parameters are as follows: for the first step with k₁ (25.0°C) = (6.8 ±0.2)×10⁵ mol⁻¹ kg s⁻¹, ΔH³₁ = 47.5 ± 4.9 kJ mol⁻¹, ΔS³₁ = 26±16 J K⁻¹ mol⁻¹, and ΔV³₁ (30.0°C) = 27.2±1.5 cm³ mol⁻¹; for the second step with k₂ (25.0°C) = 26.8±0.8 mol⁻¹ kg s⁻¹, ΔH³₂ = 91.6± 0.8 kJ mol⁻¹, ΔS³₂ = 90±3 J K⁻¹ mol⁻¹, and ΔV³₂ (35.0°C) = 21.8±0.9 cm³ mol⁻¹. The large positive activation volumes strongly indicate a dissociative character of the activation process.     

The thermodynamics of the complexation of lanthanides by 1-hydroxy-4,7-disulfo-2-naphthoic acid

The thermodynamic parameters, log β, ΔH and ΔS, for formation of lanthanide-1-hydroxy-4,7- disulfo-2-naphthoic acid complexes have been determined at 25 °C in 0.10 M NaClO₄ solutions by potentiometric and calorimetric titrations. Under the experimental conditions, the data can be explained with the formation of LnL⁻, LnL₂⁵⁻ and LnHL complexes (H₂L²⁻ = 1-hydroxy-4,7-disulfo-2- naphthoic acid anion). At pH < 3 the LnHL complex is the major species, whereas by increasing pH the formation of LnL_n³⁻⁴ⁿ complexes becomes more important. The data are compared to the comparable data for complexing by aromatic carboxylic acids.     

Exchange rates of pyridine on ferro- and ferriprotoporphyrin (IX) dimethylester in chloroform.

Nuclear magnetic resonance line-widths data have been used to determine the rate of solvent exchange from the first coordination sphere of ferro-and ferriprotoporphyrin(IX) dimethylester (Fe-PPD) in pyridine/chloroform. The average values of kinetic parameters for pyridine (PY) exchange indicate an SN2 mechanism tor Fe(III)-PPD(ΔH^&;#; = 36 kJ · mol⁻¹ ; ΔS^&;#; = −53 J·mol⁻¹K⁻¹; T_M(298 K) = 0.07 msec) and an SNI mechanism for Fe(II)-PPD (ΔH^&;#; = 67 kJ·mol⁻¹; ΔS^&;#; = 42 J · mol⁻¹K⁻¹; T_M(298 K) = 0.06 msec). Parallel to the accelerated ligand exchange rate at rising temperatures a redistribution of the electrons causing a transition of the metal porphyrin from the low-spin state to the high-spin state is observed. Enthalpy and entropy of the thermodynamic equilibrium between low- and high-spin Fe-PPD have been determined from experimental values of the average magnetic moment. A mean lifetime of low-spin Fe(III)-PPD was estimated from line. widths changes (T_L→H(298 K)≈ 20 msec) and the corresponding activation parameters have been obtained (ΔH^&;#;_L→H(298 K) = 26 kJ · mol⁻¹; ΔS^&;#;_L→H(298K) = −125 J · mol⁻¹K⁻¹).     

Proton magnetic resonance study of rotation about carbon-nitrogen bonds in pentakis(1,1-dimethylurea)dioxouranium(VI) and its 1,3-dimethylurea analogue

In CD₃CN solutions the kinetic parameters characterising rotation about the CNMe₂ and CNH₂ bonds in [UO₂(1,1-DMU)₅]²⁺ (1,1-DMU = 1,1- dimethylurea) were determined as: k(265 K) = 39.1 ± 0.4 and 2960 ± 60 s^?1, ΔH3 = 49.1 ± 0.76 and 61.1 ±0.5 kJ mol^?1, ΔS2 = ?28.3 ± 2.7 and 53.1 ± 2.2 J K^?1 mol^?1 respectively from ¹H NMR studies. Resonances arising from the three isomeric 1,3-DMU (= 1,3-dimethylurea) ligands were observed for [UO₂(1,3-DMU)₅]²⁺ in CD₃CN solution and the kinetic parameters characterising their isomerisations were also determined. The three isomers of 1,3-DMU have not previously been detected in solution and it appears that coordination of 1,3-DMU to UO₂²⁺ increases the barrier to rotation about the carbon nitrogen bond, as is also shown to be the case for 1,1-DMU.     

Cd(II) complexes with N8 and N4O4 donor macrocyclic ligands

The coordination capability of the octaaza 24-membered (L¹) and the tetraoxotetraaza 28-membered (L²) macrocycle ligands - with different sizes, nature and number of the donor atoms - has been investigated with nitrate and perchlorate Cd(II) salts. The complexes were prepared in 1:1 and 2:1 Cd:L molar ratio. The characterization by elemental analysis, IR, LSI mass spectrometry, conductivity measurements and ¹H NMR spectroscopy, together with the crystal structure of the complexes [CdL¹](NO₃)₂ · 0.5H₂O, [CdL¹](ClO₄)₂ and [CdL²(CH₃CN)₂](ClO₄)₂ · CH₃CN · H₂O confirms the formation of mononuclear complexes in all cases. The [CdL¹](NO₃)₂ · 0.5H₂O and [CdL¹](ClO₄)₂ present a mononuclear endomacrocyclic structure with the metal ion coordinated by the eight donor nitrogen atoms from the macrocyclic backbone in a square antiprism geometry. The complex [CdL²(CH₃CN)₂](ClO₄)₂ · CH₃CN · H₂O is also mononuclear, but the cadmium ion is in an octahedral environment coordinated by four amine nitrogen atoms from the macrocyclic framework and two nitrogen atoms from two acetonitrile molecules. The ether oxygen atoms from the ligand are not coordinated.     

Kinetics and mechanism of the oxidation of L-ascorbic acid by trisoxalatocobaltate(III) in basic aqueous solution

The kinetics and mechanism of the oxidation of L- ascorbic acid by trisoxalatocobaltate(III) were studied as a function of pH, ascorbate concentration, ionic strength and temperature in a weakly basic aqueous solution. The pH dependence of the process can be ascribed to the oxidation of the doubly deprotonated ascorbate ion for which k = 20 M⁻¹ s⁻¹ at 25 °C, ΔH^# = 34 ± 2 kJ mol⁻¹ and ΔS^# = −108 ± 7 J K⁻¹ mol⁻¹. The results are discussed in reference to literature data for this reaction in weakly acidic medium and for the oxidation by a series of other oxidants.     

Synthesis and characterization of triaminecobalt(III) complexes and acid hydrolysis kinetics of fac-[Codpt(H2O)nX3−n]n+ (X = Cl,Br; n = 0,1,2)

The complexes CodptX₃ and [Codpt(H₂O)X₂]ClO₄ (X = Cl, Br; dpt = dipropylenetriamine = NH(CH₂CH₂CH₂NH₂)₂) have been prepared and characterized. Rate constants (s⁻¹) for aqueous solution at 25 °C and μ = 0.5 M (NaClO₄), for the acid-independent sequential ractions.

have been measured spectrophotometrically. For X = Cl: k₁ ⋍ 2 × 10⁻², k₂ = 1.7 × 10⁻⁴ and k₃ = 4.8 × 10⁻⁶, and for X = Br: k₁ ⋍ 2 × 10⁻², k₂ = 5.25 × 10⁻⁴ and k₃ = 2.5 × 10⁻⁵ The primary equation was found to be acid independent, while the secondary and tertiary aquations were acid-inhibited reactions. For the second step, the rate of the reaction was given by the rate equation

where C_t is the complex concentration in the aqua-and hydroxodihalo species, k′₂ is the rate constant for the acid-dependent pathway and K_a is the equilibrium constant between the hydroxo and aqua complex ions. The activation parameters were evaluated, for X = Cl: ΔH^≠₂ = 106.3 ± 0.4 kJ mol⁻¹ and ΔS^≠₂ = 40.2 ± 1.7 J K⁻¹ mol⁻, and for X = Br: ΔH^≠₂ = 91.6 ± 0.4 kJ mol⁻¹ and ΔS^≠₂ = 0.4 ± 1.7 J K⁻¹ mol⁻¹. The results are discussed and detailed comparisons of the reactivities of these complexes with other haloaminecobalt(III) species are presented.     

Mechanisms for acceleration of halide anation reactions of platinum(IV) complexes. REOA versus ligand assistance and platinum(II) catalysis Without central ion exchange

Chloride anation of trans-Pt(CN)₄ClOH₂⁻ has been studied with and without Pt(CN)₄²⁻ present at 25.0°C by use of stopped-flow and conventional spectrophotometry and a 1.00 M perchlorate medium. The rate law in the absence of Pt(CN)₄²⁻ is Rate=(p₁ + p₂ [H⁺] ) [Cl⁻]² [complex]/(1 + q [Cl₋]) with p₁=(3.0 ± 0.1) × 10⁻⁵ M⁻²s⁻¹, p₂=(3.6 ± 0.1) × 10⁻⁵ M⁻³ s⁻¹ and q=(0.62 ± 0.02) M⁻¹. It is compatible with a chloride assistance via an intermediate of the type Cl-Cl-Pt(CN)₄···OH₂²⁻, in which the reactivity of the aqua ligand is enhanced due to a partial reduction of the platinum. This mechanism of halide assistance is in principle the same as the modified reductive elimination oxidative addition (REOA) mechanism proposed by Poë, in which the intermediate is not split into free halogen, platinum(II) and water, and in which electron transfer not necessarily involves complete reduction to platinum(II). To avoid confusion with complete reductive eliminations, reactions without split of the intermediates are here termed halide-assisted reactions. The pH-dependence indicates acid catalysis via a protonated intermediate ClClPt(CN)₄···OH₃⁻.The Pt(CN)₄²⁻accelerated path has the rate law Rate=

[Cl-] [Pt(CN)₄²⁻] [complex] where k=(39.9±0.5) M⁻² s⁻¹ and K_a=(4.0±0.2)10⁻² M is the protolysis constant of trans-Pt(CN)₄ClOH²⁻.Reaction between PtCl₅OH₂⁻ and chloride is accelerated by Pt(CN)₄²⁻ and gives PtCl₆²⁻ as the reaction product. The rate law is Rate=k [Cl⁻] [Pt(CN)₄²⁻] [PtCl₅OH₂⁻] with k=(5.6 ± 0.2)10⁻³ M⁻² s⁻¹ at 35.0°C and for a 1.50 M perchlorate acid medium. The reaction takes place without central ion exchange. Alternative mechanisms with two consecutive central ion exchanges can be excluded. The role of Pt(CN)₄²⁻ in this reaction is very similar to that of the assisting halide in the halide assisted anations. [p ]Reaction between trans-Pt(CN)₄ClOH₂⁻ and PtCl₄²⁻ gives Pt(CN)₄²⁻ and PtCl₅OH₂⁻ as products and has the rate law Rate=k[PtCl₄²⁻] [trans-Pt(CN)₄ClOH₂⁻] with k=(3.32 ± 0.02) M⁻¹ s⁻¹ at 25 °C for a 1.00 M perchloric acid medium. The formation of an aqua complex as the primary reaction product and the rate independent of [Cl⁻] shows that formation of a bridged intermediate of the type Pt(II)Cl₄ClPt(IV)(CN)₄OH₂³⁻ is formed in the initial reaction step, not five-coordinated PtCl₅³⁻.     

Thermodynamics of the glutamate dehydrogenase catalytic reaction

The enthalpy change for the oxidative deamination of glutamate by NADP⁺ catalyzed by bovine liver glutamate dehydrogenase has been determined calorimetrically. The ΔH^o values are 64.6 ± 1.2 $\text{kJ}\text{mol}$ and 70.3 ± 1.2 $\text{kJ}\text{mol}$ at 25 and 35°C respectively. The equilibrium constants for the reaction at the two temperatures were determined spectrophotometrically. This enabled the determination of ΔG^o and ΔS^o of the reaction as well. ΔH^ovalues were also determined for the reaction using an alternative coenzyme and the deuterated substrate.     

Thermodynamic parameters for the helix-coil thermal transition of ribonuclease-S-peptide and derivatives from 1H-NMR data

Values for the thermodynamic quantities, ΔH° = 11.8 ± 2.0 Kcal/mole and ΔS° = 43.6 ± 6.0 e.u., of the 3-13 helix–coil equilibrium of isolated S-peptide (19 residue N-terminal fragment of ribonuclease A) in aqueous solution (3 m M, 1M NaCl, pD 5.4) have been determined from a joint analysis of the Thr 3γ, Ala 6β, Phe 8meta, and Phe 8para ¹H chemical shift vs temperature curves (?7 to 80°C) in several aqueous–trifluorethanol mixtures. Chemical shifts in the coil and in the helix have been determined for up to 16 protons belonging to the 3-13 fragment. Thermodynamic parameters have also been determined for C-peptide (13 residue fragment) and a number of S-peptide derivatives. From the variation of the values of the thermodynamic parameters at pD 2.5, 5.4, and 8.0, a quantitation of the two helix-stabilizing side-chain interactions can be made: (1) Δ(ΔH°) ? 5 Kcal/mole and Δ(ΔS°) ? 18 e.u. for the salt bridge Glu 2^? … Arg 10⁺ and (2) Δ(ΔH°) ? 3 Kcal/mole and Δ(ΔS°) = 9 e.u. for the one in which the His 12⁺ imidazolium group is involved, presumably a partial stacking with the Phe 8 side chain.     

Calorimetric determination of thermodynamic parameters of 2′-dUMP binding to Leishmania major dUTPase

We have investigated the binding of 2′-deoxyuridine 5′-monophosphate (2′-dUMP) to Leishmania major deoxyuridine 5′-triphosphate nucleotide hydrolase (dUTPase) by isothermal titration microcalorimetry under different experimental conditions. Binding to dimeric L. major dUTPase is a non-cooperative process, with a stoichiometry of 1 molecule of 2′-dUMP per subunit. The utilization of buffers with different ionization enthalpies has allowed us to conclude that the formation of the 2′-dUMP–dUTPase complex, at pH 7.5 and 30 °C, is accompanied by the uptake of 0.33±0.05 protons per dUTPase subunit from the buffer media. Moreover, 2′-dUMP shows a moderate affinity for the enzyme, and binding is enthalpically driven across the temperature range studied. Besides, whereas ΔG° remains practically invariant as a function of temperature, both ΔH and ΔS° decrease with increasing temperature. The T_S and T_H were 23.4 and 13.6 °C, respectively. The temperature dependence of the enthalpy change yields a heat capacity change of ΔC_p°=?618.1±126.4 cal·mol^?1·K^?1, a value low enough to discard major conformational changes, in agreement with the fitting model. An interpretation of this value in terms of solvent-accessible surface areas is provided.     

Kinetics of the addition reactions of tetracyanoethylene towards rhodium(I) cationic isocyanide complexes

The kinetics of the addition reactions of tetracyanoethylene (TCNE) to trans-[Rh(RNC)₂(PR′₃)₂]ClO₄, where R = p-CH₃OC₆H₄, p-ClC₆H₄, and C₆H₁₁ and R′ = C₆H₅ and C₆H₅O, in acetonitrile, acetone, and tetrahydrofuran (THF) have been investigated employing stopped-flow techniques. The reaction is first order with respect to both complex and TCNE. The reaction rate increases with increasing solvent polarity in the order of THF < acetone < acetonitrile. The activation parameters for the reactions of [RH(p-CH₃OC₆H₄NC)₂(PPh₃)₂]ClO₄ in the three solvents were: ΔH^*, 7.6, 3.5, 2.2 kcal mol⁻¹; ΔS^*, −15.2, −27.7, −28. e.u. The nature of the transition state and ligand effects on the rate of reaction are discussed.     

Reactions of ruthenium(II) para-substituted tetraphenylporphyrin complexes with carbon monoxide oxygen: A kinetic investigation

The reaction of Ru(XTPP)(DMF)₂, where XTPP is the dianion of para substituted tetraphenylporphyrins and X is MeO, Me, H, Cl, Br, I, F, with O₂ and CO were studied in DMF. The process was found to be first-order in metalloporphyrin, first-order in molecular oxygen and carbon monoxide, and second-order overall. Second-order rate constants for the CO reaction ranged from 0.170 to 0.665 M^?1 s^?1 at 25°C, those for the O₂ reaction from 0.132 to 0.840 M^?1 s^?1 at 25°C. Similar activation parameters (ΔH_CO^± = 87 ± 1 kJ mol^?1, ΔS_CO^± = 22 ± 4 JK^?1 mol^?1; ΔH_O2^± = 81 ± 1 kJ mol^?1, and ΔS_O2^± = 11 ± 5 JK^?1 mol^?1) were found within each series. Reactivities of X substituted metalloporphyrins were found to follow different Hammett σ functions. The CO reactions correlated with σ_? following normal behavior; the O₂ reactions correlated with σ₈° indicating O₂ is π-bonded in the transition states. A dissociative mechanism is postulated for the process.     

Determination of Gibbs free energy of transfer for some univalent ions from water to methanol,acetonitrile, dimethylsulfoxide,pyridine, tetrahydrothiophene and liquid ammonia; standard electrode potentials of some couples in these solvents

The free energy of transfer, ΔG°_tr, for 21 univalent ions are determined from water to methanol, acetonitrile, dimethylsulfoxide (DMSO), pyridine, tetrahydrothiophene and liquid ammonia. These solvents show a wide range of donor properties, whereby water and methanol are regarded as hard donors, dimethylsulfoxide and acetonitrile are on the borderline between hard and soft, and the remaining solvents are regarded as typical soft donors. The ΔG°_tr values of ionic compounds are calculated from solubility product measurements of 1:1 salts. The extrathermodynamic tetraphenylarsonium tetraphenylborate (TATB) assumption has been applied in order to calculate the contributions from the single ions. The TATB assumption implies that the two large ions Ph₄As⁺ and BPh₄⁻ are equally solvated, thus ΔG°_tr(AsPh₄⁺)=ΔG°_tr(BPh₄⁻), for all solvent pairs. Standard electrode potentials in non-aqueous solvents can be calculated from the standard electrode potentials in water and the ΔG°_tr values. The standard electrode potentials calculated from the solubility product measurements, and the potentiometrically determined ones were found to be in excellent agreement. The extrathermodynamic assumption has thereby been experimentally shown to be close to the truth.