Initial state and transition state contributions to reactivity in mercury(II)-catalysed aquation of thetrans-[Rh(en)2Cl2]+, [Cr(NH3)5Cl]2+, andcis-[Cr(NH3)4(OH2)Cl]2+ cations in binary aqueous solvent mixtures

Summary Rate constants are reported for mercury(II)-catalysed aquation of thetrans-[Rh(en)₂Cl₂]⁺, [Cr(NH₃)₅Cl]²⁺, andcis-[Cr(NH₃)₄(OH₂)Cl]²⁺ cations in water and in methanol-, ethanol-, and acetonitrile-water solvent mixtures. In the case oftrans-[Rh(en)₂Cl₂]⁺, the dependence of rate constants on mercury(II) concentration indicates reaction through a binuclear (Rh-Cl-Hg bridged) intermediate. The dependence of the equilibrium constant for the formation of this intermediate and of its rate constant for dissociation (loss of HgCl⁺) on solvent composition have been established. With the aid of measured solubilities, published ancillary thermodynamic data, and suitable extrathermodynamic assumptions, the observed reactivity trends for these mercury(II)-catalysed aquations are dissected into initial state and transition state components. The reactivity patterns for these three complexes are compared with those for mercury(II)-catalysed aquation of other chloro-transition metal complexes, particularlycis-[Rh(en)₂Cl₂]⁺, [Co(NH₃)₅Cl]²⁺, and [ReCl₆]^2–.     

Kinetics and Thermodynamics of Solvolysis of a Cationic and an Anionic Chlorocobalt (III) Complex in Water–Ethanol Mixtures

Rate constants and derived thermodynamic activation parameters are reported for solvolysis of trans-[Co(3Mepy)₄Cl₂]⁺ and [Co(CN)₅Cl]^3– ions in water-rich mixtures of water with ethanol at various temperatures and are analyzed by initial- and transition-state contributions. The variation of enthalpies and entropies of activation with solvent composition show extrema in composition ranges where the physical properties of the mixtures, influenced by changes in solvent structures, also show extrema. From the application of a free-energy cycle to the process of the initial state going to the transition state, it is concluded for the solvolysis of both complexes that the Co(III) species in the transition state is more stable in water + ethanol mixtures than in the initial state.     

Initial state and transition state solvation effects in the cobaltitungstate oxidation of iodide in binary aqueous solvent mixtures

Summary Rate constants are reported for 12-tungstocobaltate(III) [CoW₁₂O₄₀]^5– oxidation of iodide in water and in binary aqueous solvent mixtures containing up to 40% methanol, 40% acetonitrile, or 60% dimethyl sulphoxide. From these kinetic results, solubility measurements on potassium 12-tungstocobaltate(III), and published data on Gibbs free energies of transfer of appropriate ions, it has been deduced that the dominant factor in determining the marked decrease in rate observed on going from water into the binary aqueous solvent mixtures is destabilisation of the transition state for the electron-transfer reaction.     

Kinetic,solvation and reactivity studies of iron(II) complexes of monoxime and dioxime ligands

Complexes of Fe^II with monoxime and dioxime ligands have been isolated and characterised. Kinetic results and rate laws are reported for acid aquation and base hydrolysis of these complexes in H₂O and in MeOH–H₂O mixtures. Kinetics of acid catalysed aquation of Fe^II–monoxime complexes follow a rate law with k_obs = k₂[H⁺] + k₃[H⁺]², while kinetics of acid dissociation and base hydrolysis of the Fe^II–dioxime complex follow rate laws with k_obs = k₂[H⁺] and k_obs = k₂[OH⁻]. Acid aquation and base hydrolysis mechanisms are proposed. The solubilities of Fe^II–monoxime and –dioxime complex salts are reported and transfer chemical potentials of their complex cations are calculated. Solvent effects on reactivity trends have been analysed into initial and transition state components. These are determined from transfer chemical potentials of reactant and kinetic data. Rate constant trends from these complexes are compared and discussed in terms of ligand structure and solvation properties. Our kinetic results give information relevant to the application of these ligands as analytical reagents for trace Fe^II in acidic and neutral media, in water and in aqueous alcohols.     

Kinetic and thermodynamic study of thiolato and sulfenato Co(III) complexes in water–acetonitrile mixtures

Rate constants are reported for peroxodisulfate as well as periodate oxidation of[Co(en)₂{SCH₂CH(COO)NH₂}]⁺, [Co(en)₂(SCH₂CH₂NH₂)]²⁺, and [Co(en)₂(SOCH₂CH₂NH₂)]²⁺ in water–acetonitrile mixtures. The dependence of rate constants on the acetonitrile concentration is established and discussed. Ancillary information relevant to solvation of reactants has been obtained from solvatochromism and from Gibbs transfer functions. The solvent effect is discussed from the viewpoint of change in solvation of initial and transition state on going from water to water–acetonitrile mixtures. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 34–40, 2004     

Kinetics of substitution reactions of transition metal complexes in 1,3-dioxolan-2-one + water and 4-methyl-1,3-dioxolan-2-one + water solvent mixtures

Summary Rate constants are reported and discussed for several substitutions of inorganic complexes in ethylene carbonate (1,3-dioxolan-2-one) + water and in propylene carbonate (4-methyl-1,3-dioxolan-2-one) + water solvent mixtures. The reactions include aquation ofcis- and oftrans-[Co(en)₂Cl₂]⁺, aquation oftrans-[Cr(OH₂)₄Cl₂]⁺, bromide substitution at [Pd(Et₄dien)Cl]⁺, thiourea substitution atcis-[Pt(4-NCpy)₂Cl₂], and aquation and cyanide attack at [Fe(X-phen)₃]²⁺ cations.     

A CAS study on S‐loss and O‐loss dissociation mechanisms of the SO2+ ion in the C,D, and E states

In the present work, we mainly study dissociation of the C ²B₁, D²A₁, and E²B₂ states of the SO₂⁺ ion using the complete active‐space self‐consistent field (CASSCF) and multiconfiguration second‐order perturbation theory (CASPT2) methods. We first performed CASPT2 potential energy curve (PEC) calculations for S‐ and O‐loss dissociation from the X, A, B, C, D, and E primarily ionization states and many quartet states. For studying S‐loss predissociation of the C, D, and E states by the quartet states to the first, second, and third S‐loss dissociation limits, the CASSCF minimum energy crossing point (MECP) calculations for the doublet/quartet state pairs were performed, and then the CASPT2 energies and CASSCF spin‐orbit couplings were calculated at the MECPs. Our calculations predict eight S‐loss predissociation processes (via MECPs and transition states) for the C, D, and E states and the energetics for these processes are reported. This study indicates that the C and D states can adiabatically dissociate to the first O‐loss dissociation limit. Our calculations (PEC and MECP) predict a predissociation process for the E state to the first O‐loss limit. Our calculations also predict that the E²B₂ state could dissociate to the first S‐ and O‐loss limits via the A²B₂ ← E²B₂ transition. On the basis of the 13 predicted processes, we discussed the S‐ and O‐loss dissociation mechanisms of the C, D, and E states proposed in the previous experimental studies. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Ultraviolet Photodissociation Dynamics of m-Bromofluorobenzene at around 240 nm?

The photodissociation dynamics of m-bromofluorobenzene has been experimentally investigated at around 240 nm using the DC-slice velocity map imaging technique. The kinetic energy release spectra and the recoiling angular distributions of fragmented Br(²P_3/2) and Br(²P_1/2) atoms from photodissociation of m-bromofluorobenzene have been measured at different photolysis wavelengths around 240 nm. The experimental results indicate that two dissociation pathways via (pre-)dissociation of the two low-lying ¹ππ^* excited states dominate the production process of the ground state Br(²P_3/2) atoms. Because of the weak spin-orbit coupling effect among the low-lying triplet and singlet states, the spin-orbit excited Br(²P_1/2) atoms are mainly produced via singlet-triplet state coupling in the dissociation step. The similarity between the present results and that recently reported for o-bromofluorobenzene indicates that the substitution position of the fluorine atom does not significantly affect the UV photodissociation dynamics of bromofluorobenzenes.     

Structural variation from trinuclears to 1D chains: Syntheses,structures and properties

Five complexes [Co₃(Hpmad)₆]·(4‐sb)₂·(CH₃COO)₂·(H₂O)₂ ( 1 ), [Co₃(Hpmad)₆]·(3‐sb)₂·(CH₃COO)₂·(H₂O)_0.5 ( 2 ), [Co(Hpmad)₂(4‐sb)]_n ( 3 ), [Co(Hpmad)₂(3‐sb)]_n ( 4 ) and {[Co(Hpmad)(SO₄)(H₂O)₂]·H₂O}_n ( 5 ) [Hpmad is 2‐pyrimidineamidoxime, H₂(4‐sb) is 4‐sulfobenzoic acid and H₂(3‐sb) is 3‐sulfobenzoic acid], were prepared at room temperature. Complexes 1 – 5 were characterized by elemental analyses, single crystal X‐ray diffractions, powder X‐ray diffractions, infrared spectra, thermogravimetric analyses, fluorescence spectra and magnetic susceptibility measurements. Complexes 1 and 2 possess the linear trinuclear Co²⁺ structures. Complexes 3 and 4 exhibit similar one‐dimensional (1D) chains. Complex 5 comprises the 1D helical chain. The change of anion in cobalt salt from CH₃COO^? to Cl^? to SO₄^2? leads to the structural evolution from the linear trinuclear Co²⁺ structure to the 1D chain to the 1D helical chain. Complexes 1 – 5 exhibit the Hpmad‐based emissions. The magnetic properties of 1–5 were also investigated.     

Theoretical investigations on analytical potential energy function and spectroscopic parameters for the state b3Πu of dimer 7Li2

The SAC‐CI (symmetry‐adapted‐cluster configuration‐interaction) method presented in Gaussian 03 program package is applied to investigate the adiabatic potential energy curves (PECs) of ⁷Li₂(b³Π_u). These calculations are performed at numbers of basis sets, such as 6‐311++G(3df,3pd), 6‐311++G(2df,2pd), 6‐311++G(df,pd), D95V++, D95(3df,3pd), D95(d,p), cc‐PVTZ, 6‐311++G and 6‐311++G(d,p). All the ab initio calculated points are fitted to the analytic Murrell‐Sorbie functions and then used to compute the spectroscopic parameters. The analytic potential energy function (APEF) for this b³Π_u state is reported. By comparison, the spectroscopic parameters reproduced by the APEF attained at 6‐311++G(2df,2pd) are found to be very close to the latest experimental findings. With the APEF obtained at the SAC‐CI/6‐311++G(2df,2pd) level of theory, a total of 62 vibrational states is found when J = 0. The complete vibrational levels, classical turning points, inertial rotation and centrifugal distortion constants for these vibrational states are also reported. The reasonable dissociation limit for this state is deduced using the calculated results at present. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Kinetics of hydrazine exchange in the pentacyanohydrazineferrate(II) ion

Summary Rate constants and activation parameters for the release of hydrazine (both neutral and protonated) coordinated to the Fe(CN) ₅ ^3– moiety are reported, together with the corresponding stability constants. The rates of formation of the complexes have been evaluated indirectly. The results confirm the general dissociative mechanism for pentacyano(ligand) ferrate(II) ions but some significant differences are found on comparing the data with those from several coordinated diamines. The acidity of complexed N₂H ₅ ⁺ is enhanced byca. two orders of magnitude with respect to free N₂H ₅ ⁺ , its rate of release being greater than that of N₂H₄ by a factor of 31. A solvation controlled model for the dissociation of [Fe(CN)₅L]^n– is shown to be operative in its general features, although specific charge effects are shown to influence the rate of both the dissociation and the formation reactions.     

Kinetics of the solvolysis of the cis-chloroazidobis-(1,2-diaminoethane)cobalt(III) ion in water+t-butanol mixtures

Summary Rate constants for the solvolysis of the complex ioncis-[Coen₂N₃Cl]⁺ have been determined for a range of temperatures in mixtures of water witht-butanol ranging up to 50% v/v of the latter. Linear plots of log (rate constant) against the reciprocal of the absolute temperature are obtained at all concentrations oft-butanol and the variations of the enthalpy and entropy of activation with solvent composition show extrema at compositions where extrema occur in the physical properties of the mixture which are influenced by solvent structure. The importance of solvent structure is shown by the curved plot found for log (rate constant) against reciprocal of dielectric constant at constant temperature. The application of a free energy cycle to the free energies of activation in water and in the mixture and the free energies of transfer of individual ionic species between water and the mixtures shows that the effect of changes in solvent structure on the reacting cation in the transition state dominates over that in the initial state.     

C-Br Bond Dissociation Mechanisms of 2-Bromothiophene and 3-Bromothiophene at 267 nm

C-Br bond dissociation mechanisms of 2-bromothiophene and 3-bromothiophene at 267 nm were investigated using ion velocity imaging technique. Translational energy distributions and angular distributions of the photoproducts, Br(²P_3/2) and Br^*(²P_½), were obtained and the possible dissociation channels were analyzed. For these two bromothiophenes, the Br fragments were produced via three channels: (i) the fast predissociation following the intersystem crossing from the excited singlet state to repulsive triplet state; (ii) the hot dissociation on highly vibrational ground state following the internal conversion of the excited singlet state; and (iii) the dissociation following the multiphoton ionization of the parent molecules. Similar channels are involved for photoproduct Br^* of the 2-bromothiophene dissociation at 267 nm; whereas for the photoproduct Br^* of 3-bromothiophene, the dissociation channel via internal conversion from the excited singlet state to highly vibrational ground state became dominating and the fast predissociation channel via the excited triplet state almost disappeared. Informations about the relative contribution, energy disposal, and the anisotropy of each channel were quantitatively given. It was found that with the position of Br atom in thienyl being far from S atom, the relative ratios of products from channels (i) and (ii) decreased obviously and the anisotropies corresponding to each channel became weaker.     

Substitution of azide atNNN″N″-tetraethyldiethylenetriaminechlorogold-(III) in binary aqueous solvent mixtures

Summary Rate constants are reported and discussed for the reaction of [Au(Et₄dien)Cl]²⁺ with HN₃ (pH = 1) and of [Au(Et₄dien-H)Cl]⁺ with N ₃ ^– (pH = 7) in several binary aqueous solvent mixtures.     

13C NMR and structural study of solid state naphthol-1 and naphthol-2 arylazo-derivatives

Solid state ¹³C NMR spectra of a series of naphthol-1 and naphthol-2-arylazo-derivatives were studied and compared with respective results in solutions. Signals of carbon nuclei of naphthalene ring were assigned. Tautomeric forms of compounds were determined. It was shown that 4-(p-NO₂C₆H₄)-azonaphthol-1 and 1-(p-NO₂C₆H₄)-azonaphthol-2 in solid state existed exclusively in quinohydrazone form. The other two compounds—1-(C₆F₅)-azonaphthol-2 and 2-(p-CH₂C₆H₄)-azonaphthol-1 in solid state were not found in the form of individual tautomeric mixtures but in respective tautomeric equilibrium form. Thus, during transition from liquid to solid state, the tautomeric equilibrium was practically unchanged. In solid state it was found that rotation around aryl-N bond was hindered in the time scale of NMR spectroscopy.     

Kinetics and mechanism of aqua ligand substitution reactions from cis-diaqua-bis(bipyridine)ruthenium(II) ion by pyridine-2-aldoxime in aqueous medium

Summary The kinetic behaviour of cis-[Ru(bipy)₂(H₂O)₂]²⁺ towards the anating ligand pyridine-2-aldoxime as a function of temperature, ligand concentration, substrate complex concentration and pH is reported and the rate expression Rate = k ₁ k ₂[Ru(bipy)₂(H₂O)₂]²⁺ [LL]/(k _-1 + k ₂[LL]) is established where k ₁ is the water dissociation rate constant for the slow step, k _-1 is the rate constant for the aquation, k ₂ is the ligand-capturing rate constant of the five-coordinate intermediate [Ru(bipy)₂(H₂O)]²⁺ and LL is pyridine-2-aldoxime. The reaction is pH-dependent in the pH range 3.65–5.50. The enthalpy and entropy of activation were obtained using Eyring plots. The results are in conformity with a dissociative mechanism.     

Kinetics of solvolysis and cyanide attack at 2-benzoylpyridine-iminemolybdenum(0) tetracarbonyl and related compounds

Summary The kinetics of reaction of [Mo(CO)₄(bpami)], where bpami is the Schiff base derived from 2-benzoyl pyridine and ammonia, with cyanide in several solvents show parallel solvolysis and cyanide attack. Rate laws and activation parameters are consistent with an associative mechanism for the predominant pathway, cyanide attack. From kinetic and solubility measurements it is possible to analyse solvent effects on reactivity for these reactions into initial state and transition state contributions. Rate constant trends in binary mixtures of non-aqueous solvents can be understood in terms of preferential solvation of the molybdenum compound and of cyanide. Finnlly, ligand effects on reactivity have been established for a number of [Mo(CO)₄(diimine)] complexes; over a 5×10⁴ times range in rate constants for cyanide attack there is a marked correlation of reactivity with MLCT frequencies for the metal to diimine charge-transfer band.     

High‐temperature dissociation of ethyl radicals and ethyl iodide

The decomposition of ethyl iodide and subsequent dissociation of ethyl radicals have been investigated behind incident shock waves in a diaphragmless shock tube by laser‐schlieren (LS) densitometry (1150–1870 K, 55 ± 2 Torr and 123 ± 3 Torr). The LS density‐gradient profiles were simulated assuming that the initial dissociation of C₂H₅I proceeded by 87% C–I fission and 13% HI elimination. Excellent agreement was found between the simulations and experimental profiles. Rate coefficients for the C–I scission reaction were obtained and show strong falloff. Gorin model RRKM (Rice, Ramsperger, Kassel, and Marcus) calculations are in excellent agreement with the experimental data with E₀ = 55.0 kcal/mol, which is in very good agreement with recent thermochemical measurements and evaluations. However, E₀ is approximately 2.7 kcal/mol higher than previous estimates. First‐order rate coefficients for dissociation of C₂H₅I were determined to be k_55Torr = 8.65 × 10⁶⁸ T^?16.65 exp(?37,890/T) s^?1, k_123Torr = 3.01 × 10⁶⁹ T^?16.68 exp(?38,430/T) s^?1, k_∞ = 2.52 × 10¹⁹ T^?1.01 exp(?28,775/T) s^?1. Rates of dissociation for ethyl radicals were also obtained, and these are in very good agreement with theoretical predictions (Miller J. A. and Klippenstein S. J. Phys Chem Chem Phys 2004, 6, 1192–1202). The simulations show that at low temperatures ethyl radicals are consumed through recombination reactions as well as dissociation, whereas at high temperatures, dissociation dominates. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 433–443, 2012     

Ab initio potential energy surface and excited vibrational states for the electronic ground state of Li2H

A 285-point multi-reference configuration-interaction involving single and double excitations (MRS-DCI) potential energy surface for the electronic ground state of Li₂H is determined by using 6-311G (2df, 2pd) basis set. A Simons-Parr-Finlan polynomial expansion is used to fit the discrete surface with a X² of 4.64 × 10^-6. The equilibrium geometry occurs at R_e =0.172 nm and <LiHLi =94.10. The dissociation energy for reaction Li₂H(²A)⇑ Li₂(¹⌆_g)+H(²S) is 243.910 kJ/mol. and that for reaction Li₂H(²A)⇑HLi(¹be)+Li(²S) is 106.445 kJ/mol. The inversion barrier height is 50.388 kJ/mol. The vibrational energy levels are calculated using the discrete variable representation (DVR) method. Project supported by the National Natural Science Foundation of China (grant No. 29673029) and by the Special Doctoral Research Foundation of the State Education Commission of China.     

Solvent effect on the initial state and transition state of anation of pentaammine-aquachromium(III) ion by thiocyanate

Summary Solvent effects on rate constants and activation enthalpies and entropies for the anation of [Cr(NH₃)₅H₂O]²⁺ by SCN^- in aqueous mixtures of MeOH, EtOH, i-PrOH and t-BuOH were analysed into initial and transition state contributions. From kinetic measurements, solubilities of the complex salt and enthalpies of solution, the thermodynamic transfer functions G _inft ^supo , H _inft ^supo and S _inft ^supo , corresponding to the transfer of reactants and activated complex from water to aqueous-organic mixtures, were evaluated. The very small difference in solvation of initial and transition states, indicated by the analysis, seemed to be consistent with the proposed interchange mechanism.