Quantitative measure for the "nakedness" of fluoride ion sources

A quantitative measure for the donor strength or "nakedness" of fluoride ion donors is presented. It is based on the free energy change associated with the transfer of a fluoride ion from the donor to a given acceptor molecule. Born-Haber cycle calculations were used to calculate both the free energy and the enthalpy change for this process. The enthalpy change is given by the sum of the fluoride ion affinity of the acceptor (as defined in strict thermodynamic convention) and the lattice energy difference (DeltaU(POT)) between the fluoride ion donor and the salt formed with the acceptor. Because, for a given acceptor, the fluoride affinity has a constant value, the relative enthalpy (and also the corresponding free energy) changes are governed exclusively by the lattice energy differences. In this study, BF(3), PF(5), AsF(5), and SbF(5) were used as the acceptors, and the following seven fluoride ion donors were evaluated: CsF, N(CH(3))(4)F (TMAF), N-methylurotropinium fluoride (MUF), hexamethylguanidinium fluoride (HMGF), hexamethylpiperidinium fluoride (HMPF), N,N,N-trimethyl-1-adamantylammonium fluoride (TMAAF), and hexakis(dimethylamino)phosphazenium fluoride (HDMAPF). Smooth relationships between the enthalpy changes and the molar volumes of the donor cations were found which asymptotically approach constant values for infinitely large cations. Whereas CsF is a relatively poor F(-) donor [(U(POT)(CsF) - U(POT)(CsSbF(6))) = 213 kJ mol(-)(1)], when compared to N(CH(3))(4)F [(U(POT)(TMAF) - U(POT)(TMASbF(6))) = 69 kJ mol(-)(1)], a 4 times larger cation (phosphazenium salt) and an infinitely large cation are required to decrease DeltaU(POT) to 17 and 0 kJ mol(-)(1), respectively. These results clearly demonstrate that very little is gained by increasing the cation size past a certain level and that secondary factors, such as chemical and physical properties, become overriding considerations.     

Lattice potential energy estimation for complex ionic salts from density measurements

This paper is one of a series exploring simple approaches for the estimation of lattice energy of ionic materials, avoiding elaborate computation. The readily accessible, frequently reported, and easily measurable (requiring only small quantities of inorganic material) property of density, rho(m), is related, as a rectilinear function of the form (rho(m)/M(m))(1/3), to the lattice energy U(POT) of ionic materials, where M(m) is the chemical formula mass. Dependence on the cube root is particularly advantageous because this considerably lowers the effects of any experimental errors in the density measurement used. The relationship that is developed arises from the dependence (previously reported in Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg. Chem. 1999, 38, 3609) of lattice energy on the inverse cube root of the molar volume. These latest equations have the form U(POT)/kJ mol(-1) = gamma(rho(m)/M(m))(1/3) + delta, where for the simpler salts (i.e., U(POT)/kJ mol(-1) < 5000 kJ mol(-1)), gamma and delta are coefficients dependent upon the stoichiometry of the inorganic material, and for materials for which U(POT)/kJ mol(-1) > 5000, gamma/kJ mol(-1) cm = 10(-7) AI(2IN(A))(1/3) and delta/kJ mol(-1) = 0 where A is the general electrostatic conversion factor (A = 121.4 kJ mol(-1)), I is the ionic strength = 1/2 the sum of n(i)z(i)(2), and N(A) is Avogadro's constant.     

Ionic hydrates,M(p)X(q).nH(2)O: lattice energy and standard enthalpy of formation estimation

This paper is one of a series (see: Inorg. Chem. 1999, 38, 3609; J. Am. Chem. Soc. 2000, 122, 632; Inorg. Chem. 2002, 41, 2364) exploring simple approaches for the estimation of lattice energies of ionic materials, avoiding elaborate computation. Knowledge of lattice energy can lead, via thermochemical cycles, to the evaluation of the underlying thermodynamics involving the preparation and subsequent reactions of inorganic materials. A simple and easy to use equation for the estimation of the lattice energy of hydrate salts, U(POT)(M(p)X(q).nH(2)O) (and therefore for solvated salts, M(p)X(q).nS, in general), using either the density or volume of the hydrate, or of another hydrate, or of the parent anhydrous salt or the volumes of the individual ions, is derived from first principles. The equation effectively determines the hydrate lattice energy, U(POT)(M(p)X(q).nH(2)O), from a knowledge of the (estimated) lattice energy, U(POT)(M(p)X(q)), of the parent salt by the addition of ntheta(U) where theta(U)(H(2)O)/kJ mol(-1) = 54.3 and n is the number of water molecules. The average volume of the water molecule of hydration, V(m)(H(2)O)/nm(3) = 0.0245, has been determined from data on a large series of hydrates by plotting hydrate/parent salt volume differences against n. The enthalpy of incorporation of a gaseous water molecule into the structure of an ionic hydrate, [Delta(f)H degrees (M(p)X(q).nH(2)O,s) - Delta(f)H degrees (M(p)X(q),s) - nDelta(f)H degrees (H(2)O,g)], is shown to be a constant, -56.8 kJ (mol of H(2)O)(-1). The physical implications with regard to incorporation of the water into various types of solid-state structures are considered. Examples are given of the use of the derived hydrate lattice energy equation. Standard enthalpies of formation of a number of hydrates are thereby predicted.     

Born-Haber-Fajans cycle generalized: linear energy relation between molecules, crystals, and metals

Classical procedures to calculate ion-based lattice potential energies (U(POT)) assume formal integral charges on the structural units; consequently, poor results are anticipated when significant covalency is present. To generalize the procedures beyond strictly ionic solids, a method is needed for calculating (i) physically reasonable partial charges, delta, and (ii) well-defined and consistent asymptotic reference energies corresponding to the separated structural components. The problem is here treated for groups 1 and 11 monohalides and monohydrides, and for the alkali metal elements (with their metallic bonds), by using the valence-state atoms-in-molecules (VSAM) model of von Szentpály et al. (J. Phys. Chem. A 2001, 105, 9467). In this model, the Born-Haber-Fajans reference energy, U(POT), of free ions, M(+) and Y(-), is replaced by the energy of charged dissociation products, M(delta)(+) and Y(delta)(-), of equalized electronegativity. The partial atomic charge is obtained via the iso-electronegativity principle, and the asymptotic energy reference of separated free ions is lowered by the "ion demotion energy", IDE = -(1)/(2)(1 - delta(VS))(I(VS,M) - A(VS,Y)), where delta(VS) is the valence-state partial charge and (I(VS,M) - A(VS,Y)) is the difference between the valence-state ionization potential and electron affinity of the M and Y atoms producing the charged species. A very close linear relation (R = 0.994) is found between the molecular valence-state dissociation energy, D(VS), of the VSAM model, and our valence-state-based lattice potential energy, U(VS) = U(POT) - (1)/(2)(1 - delta(VS))(I(VS,M) - A(VS,Y)) = 1.230D(VS) + 86.4 kJ mol(-)(1). Predictions are given for the lattice energy of AuF, the coinage metal monohydrides, and the molecular dissociation energy, D(e), of AuI. The coinage metals (Cu, Ag, and Au) do not fit into this linear regression because d orbitals are strongly involved in their metallic bonding, while s orbitals dominate their homonuclear molecular bonding.     

Structure and reactivity of V(2)O(5): bulk solid, nanosized clusters, species supported on silica and alumina, cluster cations and anions

Vanadyl bond dissociation energies are calculated by density functional theory (DFT). While the hybrid (B3LYP) functional results are close to the available reference data, gradient corrected functionals (BP86, PBE) yield large errors (about 50 to 100 kJ mol(-1)), but reproduce trends correctly. PBE calculations on a V(20)O(62)H(24) cluster model for the (001) surface of V(2)O(5) crystals virtually reproduce periodic slab calculations. The low bond dissociation energy (formation of oxygen surface defect) of 113 kJ mol(-1)(B3LYP) is due to substantial structure relaxations leading to formation of V-O-V bonds between the V(2)O(5) layers of the crystal. This relaxation cannot occur in polyhedral (V(2)O(5))(n) clusters and also not for V(2)O(5) species supported on silica or alumina (represented by cage-type models) for which bond dissociation energies of 250-300 kJ mol(-1) are calculated. The OV(OCH(3))(3) molecule and its dimer are also considered. Radical cations V(2)O(5)(+) and V(4)O(10)(+) have very low bond dissociation energies (22 and 14 kJ mol(-1), respectively), while the corresponding radical anions have higher dissociation energies (about 330 kJ mol(-1)) than the neutral clusters. The bond dissociation energies of the closed shell V(3)O(7)(+) cation (165 kJ mol(-1)) and the closed shell V(3)O(8)(-) anion (283 kJ mol(-1)) are closest to the values of the neutral clusters. This makes them suitable for gas phase studies which aim at comparisons with V(2)O(5) species on supporting oxides.     

The reaction of Li[Al(OR)4] R = OC(CF3)2Ph, OC(CF3)3 with NO/NO2 giving NO[Al(OR)4], Li[NO3] and N2O. The synthesis of NO[Al(OR)4] from Li[Al(OR)4] and NO[SbF6] in sulfur dioxide solution

NO[Al(OC(CF(3))(2)Ph)(4)] 1 and NO[Al(OC(CF(3))(3))(4)] 2 were obtained by the metathesis reaction of NO[SbF(6)] and the corresponding Li[Al(OR)(4)] salts in liquid sulfur dioxide solution in ca 40% (1) and 85% (2) isolated yield. 1 and 2, as well as Li[NO(3)] and N(2)O, were also given by the reaction of an excess of mixture of (90 mol%) NO, (10 mol%) NO(2) with Li[Al(OR)(4)] followed by extraction with SO(2). The unfavourable disproportionation reaction of 2NO(2)(g) to [NO](+)(g) and [NO(3)](-)(g)[DeltaH degrees = +616.2 kJ mol(-1)] is more than compensated by the disproportionation energy of 3NO(g) to N(2)O(g) and NO(2)(g)[DeltaH degrees =-155.4 kJ mol(-1)] and the lattice energy of Li[NO(3)](s)[U(POT)= 862 kJ mol(-1)]. Evidence is presented that the reaction proceeds via a complex of [Li](+) with NO, NO(2)(or their dimers) and N(2)O. NO(2) and Li[Al(OC(CF(3))(3))(4)] gave [NO(3)(NO)(3)][Al(OC(CF(3))(3))(4)](2), NO[Al(OC(CF(3))(3))(4)] and (NO(2))[Al(OC(CF(3))(3))(4)] products. The aluminium complex [Li[AlF(OC(CF(3))(2)Ph)(3)]](2) 3 was prepared by the thermal decomposition of Li[Al(OC(CF(3))(2)Ph)(4)]. Compounds 1 and 3 were characterized by single crystal X-ray structural analyses, 1-3 by elemental analyses, NMR, IR, Raman and mass spectra. Solid 1 contains [Al(OC(CF(3))(2)Ph)(4)](-) and [NO](+) weakly linked via donor acceptor interactions, while in the SO(2) solution there is an equilibrium between the associated [NO](+)[Al(OC(CF(3))(2)Ph)(4)](-) and separated solvated ions. Solid 2 contains essentially ionic [NO](+) and [Al(OC(CF(3))(3))(4)](-). Complex 3 consists of two [Li[AlF(OC(CF(3))(2)Ph)(3)]] units linked via fluorine lithium contacts. Compound 1 is unstable in the SO(2) solution and decomposes to yield [AlF(OC(CF(3))(2)Ph)(3)](-), [(PhC(CF(3))(2)O)(3)Al(mu-F)Al(OC(CF(3))(2)Ph)(3)](-) anions as well as (NO)C(6)H(4)C(CF(3))(2)OH, while compound 2 is stable in liquid SO(2). The [small nu](NO(+)) in 1 and [NO](+)(toluene)[SbCl(6)] are similar, implying similar basicities of [Al(OC(CF(3))(2)Ph)(4)](-) and toluene.     

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.     

Volume-based thermodynamics: estimations for 2:2 salts

The lattice energy of an ionic crystal, U(POT), can be expressed as a linear function of the inverse cube root of its formula unit volume (i.e., Vm(-1/3)); thus, U(POT) approximately 2I(alpha/Vm(1/3) + beta), where alpha and beta are fitted constants and I is the readily calculated ionic strength factor of the lattice. The standard entropy, S, is a linear function of Vm itself: S approximately kVm + c, with fitted constants k and c. The constants alpha and beta have previously been evaluated for salts with charge ratios of 1:1, 1:2, and 2:1 and for the general case q:p, while values of k and c applicable to ionic solids generally have earlier been reported. In this paper, we obtain alpha and beta, k and c, specifically for 2:2 salts (by studying the ionic oxides, sulfates, and carbonates), finding that U(POT)[MX 2:2]/(kJ mol(-1)) approximately 8(119/Vm(1/3) + 60) and S degree [MX 2:2]/(J K(-1) mol(-1)) approximately 1382V(m) + 16.     

Solid-state energetics and electrostatics: Madelung constants and Madelung energies

The Madelung constants of ionic solids relate to their geometry and electrostatic interactions. Furthermore, because of issues in their evaluation, they are also of considerable mathematical interest. The corresponding Madelung (electrostatic, coulomb) energy is the principal contributor to the lattice energies of ionic systems, and these energies largely influence many of their physical properties. The Madelung constants are here defined and their properties considered. A difficulty with their application is that they may be defined relative to various lattice distances, and with various conventions for inclusion of the charges, leading to possible confusion in their use. Instead, the unambiguous Madelung energy, E(M), is to be preferred in chemistry. An extensive list of Madelung energies is presented. From this data set, it is observed that there is a strong linear correlation between the lattice energies of ionic solids, U(POT), and their Madelung energies: U(POT)/kJ mol(-1) = 0.8519E(M) + 293.9. This correlation establishes that the lattice energy, U(POT), for ionic solids is about 15% smaller than the attractive Madelung energy, the difference arising from the repulsions unaccounted for by the solely coulombic Madelung energy calculation. Correlations of U(POT) against E(M) for alkali metal hydrides and transition metal compounds, each having considerable covalency, show much reduced Madelung contributions to the lattice energy. These correlations permit ready estimation of lattice energies, and are the first to be based on actual data rather than a broad analysis. The independent volume-based thermodynamic (VBT) method, which relies on a separate correlation with the formula unit volume of the ionic material, complements these correlations.     

Evidence for the blue 10 pi S62+ dication in solutions of S8(AsF6)2: a computational study including solvation energies

The energetics of dissociation reactions of S(8)(2+) into stoichiometric mixtures of S(n)(+), n = 2-7, and S(m)(2+), m = 3, 4, 6, 10, were investigated by the B3PW91 method [6-311+G(3df)//6-311+G] in the gas phase and in solution, with solvation energies calculated using the SCIPCM model and in some cases also the COSMO model [B3PW91/6-311+G*, dielectric constants 2-30, 83, 110]. UV-vis spectra of all species were calculated at the CIS/6-311G(2df) level and for S(4)(2+) and S(6)(2+) also at the TD-DFT level (BP86/SV(P)). Standard enthalpies of formation at 298 K were derived for S(3)(2+) (2538 kJ/mol), S(6)(2+) (2238 kJ/mol), and S(10)(2+) (2146 kJ/mol). A comparison of the observed and calculated UV-vis spectra based on our calculated thermochemical data in solution suggests that, in the absence of traces of facilitating agent (such as dibromine Br(2)), S(8)(2+) dissociates in dilute SO(2) solution giving an equilibrium mixture of ca. 0.5S(6)(2+) and S(5)(+) (K approximately 8.0) while in the more polar HSO(3)F some S(8)(2+) remains (K approximately 0.4). According to our calculations, the blue color of this solution is likely due to the pi-pi transition of the previously unknown 10 pi S(6)(2+) dication, and the previously assigned S(5)(+) is a less important contributor. Although not strictly planar, S(6)(2+) may be viewed as a 10 pi electron Hückel-aromatic ring containing a thermodynamically stable 3p(pi)-3p(pi) bond [d(S-S) = 2.028 A; tau(S-S-S-S) = 47.6 degrees ]. The computations imply that the new radical cation S(4)(+) may be present in sulfur dioxide solutions given on reaction of sulfur oxidized by AsF(5) in the presence of a facilitating agent. The standard enthalpy of formation of S(6)(AsF(6))(2)(s) was estimated as -3103 kJ/mol, and the disproportionation enthalpy of 2S(6)(AsF(6))(2)(s) to S(8)(AsF(6))(2)(s) and S(4)(AsF(6))(2)(s) as exothermic by 6-17 kJ/mol. The final preference of the observed disproportionation products is due to the inclusion of solvent molecules, e.g., AsF(3), that additionally favors the disproportionation of 2S(6)(AsF(6))(2)(s) into S(8)(AsF(6))(2)(s) and S(4)(AsF(6))(2)(AsF(3))(s) by 144 kJ/mol.     

Is the enthalpy of fusion of tris(acetylacetonato)metal(III) complexes affected by their potential energy in the crystal state?

In this Article we present enthalpies of fusion and melting points obtained from new thermochemical measurements of tris(acetylacetonato)metal(III), M(acac)(3), complexes (M = Fe, Al, Cr, Mn, Co) using differential scanning calorimetry (DSC) and evaluate them in relation to their different values found in the literature. An enthalpy of fusion of 27.67 kJ mol(-)(1) was derived for Mn(acac)(3) from a symmetrical DSC thermogram captured for the first time. The enthalpy value was indirectly confirmed with the solubility measurements of Mn(acac)(3) in acetylacetone. A hypothesis has been stated that the enthalpy of fusion and the potential energy of M(acac)(3) in the crystal state may be related. To calculate molecular in-crystal potential energy, in this Article we proposed a molecular mechanics model for the M(acac)(3) class of compounds. Nine X-ray crystal structures of M(acac)(3) complexes (M = Fe, Al, V, Mn, Co, Cr, Sc) were included in the modeling. The conformational potential energy was minimized for a molecule surrounded by other molecules in the crystal lattice. The partial charges from two schemes, the electrostatic potential (ESP) fit and the natural population analysis (NPA), were used to construct two types of force fields to examine which force field type would yield a better fit with the experimental thermal properties. The final force fields were named FF-ESP and FF-NPA. Both force field sets reproduced well the experimental crystal data of nine M(acac)(3) complexes as well as of tris(3-methyl-2,4-pentanedionato-O,O')cobalt(III). Only in-crystal potential energies derived by FF-NPA yielded a significant correlation (correlation coefficient R = -0.71) with the measured enthalpies of fusion. The enthalpy of fusion for Co(acac)(3) could not be determined experimentally because of simultaneous decomposition and fusion, and it is predicted to be 33.2 kJ mol(-)(1) from the correlation regression line.     

Crystal structures, lattice potential energies, and thermochemical properties of crystalline compounds (1-C(n)H(2n+1)NH3)2ZnCl4(s) (n = 8, 10, 12, and 13)

As part of our ongoing project involving the study of (1-C(n)H(2n+1)NH(3))(2)MCl(4)(s) (where M is a divalent metal ion and n = 8-18), we have synthesized the compounds (1-C(n)H(2n+1)NH(3))(2)ZnCl(4)(s) (n = 8, 10, 12, and 13), and the details of the structures are reported herein. All of the compounds were crystallized in the monoclinic form with the space group P2(1)/n for (1-C(8)H(17)NH(3))(2)ZnCl(4)(s), P21/c for (1-C(10)H(21)NH(3))(2)ZnCl(4)(s), P2(1)/c for (1-C(12)H(25)NH(3))(2)ZnCl(4)(s), and P2(1)/m for (1-C(13)H(27)NH(3))(2)ZnCl(4)(s). The lattice potential energies and ionic volumes of the cations and the common anion of the title compounds were obtained from crystallographic data. Molar enthalpies of dissolution of the four compounds at various molalities were measured at 298.15 K in the double-distilled water. According to Pitzer's theory, molar enthalpies of dissolution of the title compounds at infinite dilution were obtained. Finally, using the values of molar enthalpies of dissolution at infinite dilution (Δ(s)H(m)(∞)) and other auxiliary thermodynamic data, the enthalpy change of the dissociation of [ZnCl(4)](2-)(g) for the reaction [ZnCl(4)](2-)(g)→ Zn(2+)(g) + 4Cl(-)(g) was obtained, and then the hydration enthalpies of cations were calculated by designing a thermochemical cycle.     

Proton exchange kinetics from the bound waters on the oxo-centered rhodium(III) trimer [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+: a variable pH and temperature 1H NMR study

Proton exchange from the bound to the bulk waters on the oxo-centered rhodium(III) trimer, [Rh(3)(micro(3)-O)(micro-O(2)CCH(3))(6)(OH(2))(3)](+)(abbreviated as Rh(3)(+)), was investigated over the temperature range of 219.1-313.9 K using a (1)H NMR line-broadening technique. By solving the modified Bloch equations for a two-site chemical exchange, lifetimes (tau) for proton transfer at pH = 2.7, 3.6, and 7.0 ([Rh(3)(+)]= 26 mM, T= 298 K) were determined to be 0.3 (+/-.08) ms, 2 (+/-0.3) ms, and 0.2 (+/-0.2) ms, respectively. From the temperature dependence of the rate, the activation parameters were determined to be DeltaH(++)= 16.2 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 123 (+/-2) J mol(-1) K(-1), DeltaH(++)= 14.9 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 141 (+/-2) J mol(-1) K(-1), and DeltaH(++)= 45 (+/-2) kJ mol(-1) and DeltaS(++)=- 22 (+/-5) J mol(-1) K(-1) for pH = 2.7, 3.6 and 7.0, respectively. All results are reported for a mixed solvent system [acetone : 250 mM NaClO(4)(aq)(3:1)], which was necessary to depress the freezing point of the solution so that the (1)H NMR signal due to bound water could be observed. The pK(a) of Rh(3)(+) was measured to be 8.9 (+/-0.2) in the mixed solvent, which is near the pK(a) for an aqueous solution (8.3 (+/-0.2)). Surprisingly, the lifetimes for protons on Rh(3)(+) are close to those observed for the Rh(OH(2))(6)(3+) ion, in spite of the considerable difference in structure, Br?nsted acidity of the bound waters and average charge on the metal ion.     

Structure and dynamics of binary and ternary lanthanide(III) and actinide(III) tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione] (TTA)-tributylphosphate (TBP) complexes. Part 3, the structure, thermodynamics and reaction mechanisms of 8- and 9-coordinated binary and ternary Y-TTA-TBP complexes studied by quantum chemical methods

Possible mechanisms for intermolecular exchange between coordinated and solvent water in the complexes Y(TTA)(3)(OH(2))(2) and Y(TTA)(3)(TBP)(OH(2)) and intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(OH(2))(HTTA) and Y(TTA)(3)(TBP)(HTTA) have been investigated using ab initio quantum chemical methods. The calculations comprise both structures and energies of isomers, intermediates and transition states. Based on these data and experimental NMR data (Part 2) we have suggested intimate reaction mechanisms for water exchange, intramolecular exchange between structure isomers and intermolecular exchange between free HTTA and coordinated TTA. A large number of isomers are possible for the complexes investigated, but only some of them have been investigated, in all of them the most stable geometry is a more or less distorted square anti-prism or bicapped trigonal prism; the energy differences between the various isomers are in general small, less than 10 kJ mol(-1). 9-coordinated intermediates play an important role in all reactions. Y(TTA)(3)(OH(2))(3) has three non-equivalent water ligands that can participate in ligand exchange reactions. The fastest of these exchanging sites has a QM activation energy of 18.1 kJ mol(-1), in good agreement with the experimental activation enthalpy of 19.6 kJ mol(-1). The mechanism for the intramolecular exchange between structure isomers in Y(TTA)(3)(OH(2))(2) involves the opening of a TTA-ring as the rate determining step as suggested by the good agreement between the QM activation energy and the experimental activation enthalpy 47.8 and 58.3 J mol(-1), respectively. The mechanism for the intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(HTTA) and Y(TTA)(3)(TBP)(HTTA) involves the opening of the intramolecular hydrogen bond in coordinated HTTA followed by proton transfer to coordinated TTA. This mechanism is supported by the good agreement between experimental activation enthalpies (within parenthesis) and calculated activation energies 68.7 (71.8) and 35.3 (38.8) kJ mol(-1). The main reason for the difference between the two systems is the much lower energy required to open the intramolecular hydrogen bond in the latter. The accuracy of the QM methods and chemical models used is discussed.     

Periodic trends in hexanuclear actinide clusters

Four new Th(IV), U(IV), and Np(IV) hexanuclear clusters with 1,2-phenylenediphosphonate as the bridging ligand have been prepared by self-assembly at room temperature. The structures of Th(6)Tl(3)[C(6)H(4)(PO(3))(PO(3)H)](6)(NO(3))(7)(H(2)O)(6)·(NO(3))(2)·4H(2)O (Th6-3), (NH(4))(8.11)Np(12)Rb(3.89)[C(6)H(4)(PO(3))(PO(3)H)](12)(NO(3))(24)·15H(2)O (Np6-1), (NH(4))(4)U(12)Cs(8)[C(6)H(4)(PO(3))(PO(3)H)](12)(NO(3))(24)·18H(2)O (U6-1), and (NH(4))(4)U(12)Cs(2)[C(6)H(4)(PO(3))(PO(3)H)](12)(NO(3))(18)·40H(2)O (U6-2) are described and compared with other clusters of containing An(IV) or Ce(IV). All of the clusters share the common formula M(6)(H(2)O)(m)[C(6)H(3)(PO(3))(PO(3)H)](6)(NO(3))(n)((6-n)) (M = Ce, Th, U, Np, Pu). The metal centers are normally nine-coordinate, with five oxygen atoms from the ligand and an additional four either occupied by NO(3)(-) or H(2)O. It was found that the Ce, U, and Pu clusters favor both C(3i) and C(i) point groups, while Th only yields in C(i), and Np only C(3i). In the C(3i) clusters, there are two NO(3)(-) anions bonded to the metal centers. In the C(i) clusters, the number of NO(3)(-) anions varies from 0 to 2. The change in the ionic radius of the actinide ions tunes the cavity size of the clusters. The thorium clusters were found to accept larger ions including Cs(+) and Tl(+), whereas with uranium and later elements, only NH(4)(+) and/or Rb(+) reside in the center of the clusters.     

Rates of water exchange for two cobalt(II) heteropolyoxotungstate compounds in aqueous solution

Polyoxometalate ions are used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co(II) atoms in two polyoxotungstate sandwich molecules using the (17)O-NMR-based Swift-Connick method. The compounds were the [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) and the larger αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) ions, each with two water molecules bound trans to one another in a Co(II) sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT-IR, UV-Vis, and EXAFS spectroscopy, ESI-MS, single-crystal X-ray crystallography, and potentiometry. For [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) at pH 5.4, we estimate: k(298)=1.5(5)±0.3×10(6) s(-1), ΔH(≠)=39.8±0.4 kJ mol(-1), ΔS(≠)=+7.1±1.2 J mol(-1) K(-1) and ΔV(≠)=5.6 ±1.6 cm(3) mol(-1). For the Wells-Dawson sandwich cluster (αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-)) at pH 5.54, we find: k(298)=1.6(2)±0.3×10(6) s(-1), ΔH(≠)=27.6±0.4 kJ mol(-1) ΔS(≠)=-33±1.3 J mol(-1) K(-1) and ΔV(≠)=2.2±1.4 cm(3) mol(-1) at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S=3/2 Co(II) species (such as the [Co(H(2)O)(6)](2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.     

Nitrosyl induces phosphorous-acid dissociation in ruthenium(II)

The trans-[Ru(NO)(NH(3))(4)(P(OH)(3))]Cl(3) complex was synthesized by reacting [Ru(H(2)O)(NH(3))(5)](2+) with H(3)PO(3) and characterized by spectroscopic ((31)P-NMR, δ = 68 ppm) and spectrophotometric techniques (λ = 525 nm, ε = 20 L mol(-1) cm(-1); λ = 319 nm, ε = 773 L mol(-1) cm(-1); λ = 241 nm, ε = 1385 L mol(-1) cm(-1); ν(NO(+)) = 1879 cm(-1)). A pK(a) of 0.74 was determined from infrared measurements as a function of pH for the reaction: trans-[Ru(NO)(NH(3))(4)(P(OH)(3))](3+) + H(2)O ? trans-[Ru(NO)(NH(3))(4)(P(O(-))(OH)(2))](2+) + H(3)O(+). According to (31)P-NMR, IR, UV-vis, cyclic voltammetry and ab initio calculation data, upon deprotonation, trans-[Ru(NO)(NH(3))(4)(P(OH)(3))](3+) yields the O-bonded linkage isomer trans- [Ru(NO)(NH(3))(4)(OP(OH)(2))](2+), then the trans-[Ru(NO)(NH(3))(4)(OP(H)(OH)(2))](3+) decays to give the final products H(3)PO(3) and trans-[Ru(NO)(NH(3))(4)(H(2)O)](3+). The dissociation of phosphorous acid from the [Ru(NO)(NH(3))(4)](3+) moiety is pH dependent (k(obs) = 2.1 × 10(-4) s(-1) at pH 3.0, 25 °C).     

Syntheses and Properties of New Layered Alkali-Metal/Ammonium Vanadium(V) Methylphosphonates: M(VO(2))(3)(PO(3)CH(3))(2) (M = NH(4), K, Rb, Tl). Single-Crystal Structures of K(VO(2))(3)(PO(3)CH(3))(2) and NH(4)(VO(2))(3)(PO(3)CH(3))(2)

The hydrothermal syntheses of a family of new alkali-metal/ammonium vanadium(V) methylphosphonates, M(VO(2))(3)(PO(3)CH(3))(2) (M = K, NH(4), Rb, Tl), are described. The crystal structures of K(VO(2))(3)(PO(3)CH(3))(2) and NH(4)(VO(2))(3)(PO(3)CH(3))(2) have been determined from single-crystal X-ray data. Crystal data: K(VO(2))(3)(PO(3)CH(3))(2), M(r) = 475.93, trigonal, R32 (No. 155), a = 7.139(3) ?, c = 19.109(5) ?, Z = 3; NH(4)(VO(2))(3)(PO(3)CH(3))(2), M(r) = 454.87, trigonal, R32 (No. 155), a = 7.150(3) ?, c = 19.459(5) ?, Z = 3. These isostructural, noncentrosymmetric phases are built up from hexagonal tungsten oxide (HTO) like sheets of vertex-sharing VO(6) octahedra, capped on both sides of the V/O sheets by PCH(3) entities (as [PO(3)CH(3)](2-) methylphosphonate groups). In both phases, the vanadium octahedra display a distinctive two short + two intermediate + two long V-O bond distance distribution within the VO(6) unit. Interlayer potassium or ammonium cations provide charge balance for the anionic (VO(2))(3)(PO(3)CH(3))(2) sheets. Powder X-ray, TGA, IR, and Raman data for these phases are reported and discussed. The structures of K(VO(2))(3)(PO(3)CH(3))(2) and NH(4)(VO(2))(3)(PO(3)CH(3))(2) are compared and contrasted with related layered phases based on the HTO motif.     

On the parallel mechanism of the dissociation of energy-selected P(CH3)3+ ions

Energy selected trimethyl phosphine ions were prepared by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. This ion dissociates via H, CH(3), and CH(4) loss, the latter two involving hydrogen transfer steps. The ion time-of-flight distribution and the breakdown diagram are analyzed in terms of the statistical RRKM theory, which includes tunneling. Ab initio and DFT calculations provide the vibrational frequencies required for the RRKM modeling. CH(3) loss could produce both the P(CH(3))(2)(+) by a simple bond dissociation step, and the more stable HP(CH(2))CH(3)(+) ion by a hydrogen transfer step. Quantum chemical calculations are extensively used to uncover the reaction scheme, and they strongly suggest that the latter product is exclusively formed via an isomerization step in the energy range of the experiment. The data analysis, which includes modeling with the trimethyl phosphine thermal energy distribution, provides accurate onset energies for both H (E(0K) = 1024.1 +/- 3.5 kJ/mol) and CH(3) (E(0K) = 1024.8 +/- 3.5 kJ/mol) loss reactions. From this analysis, we conclude that the Delta(f)H(298K) degrees [HP(CH(2))(CH(3))(+)] = 783 +/- 8 kJ/mol and Delta(f)H(298K) degrees [P(CH(2))(CH(3))(2)(+)] = 711 +/- 8 kJ/mol.     

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.