Kinetics of silicate exchange in alkaline aluminosilicate solutions

In strongly alkaline aqueous KOH solutions containing SiIV in large excess over AlIII, the kinetics of exchange of monomeric silicate with small acyclic aluminosilicate solute species is much more rapid than with either cyclic aluminosilicates or any all-silicate anions. Selective inversion recovery 29Si NMR studies of homogeneous solutions of stoichiometric composition 3.0 mol kg-1 of SiO2, 0.1 mol kg-1 of Al2O3, and 8.0 mol kg-1 of K2O in 60-75% D2O gave rate constants of 2.0 +/- 0.2 kg mol-1 s-1 and 17 +/- 4 s-1 for the forward and reverse reactions of monomeric silicate with (HO)3AlOSiOn(OH)(3-n)(n+1)- (n = 2 or 3) at 0 degree C. These rate constants are more than 10(4)-fold faster than those extrapolated from 60 to 90 degrees C for comparable reactions of silicate anions. The greater lability of acyclic aluminate centers relative to silicate is ascribed partly to the availability of HO- groups for condensation reactions on Al and mainly to the ease of expansion of the coordination number of AlIII beyond 4. The latter attribute is diminished when AlIII is constrained to be tetrahedral in cyclic structures. With respect to the mechanism of formation of zeolites from alkaline aqueous media, it is suggested that small, labile AlOSi units add rapidly to growing zeolitic structures "on demand", whereas the more kinetically inert cage or ring structures cannot. This would explain why a silicate or aluminosilicate structure that is dominant among solute species at equilibrium in the presence of a particular cation may bear little or no geometric relation to the zeolitic framework promoted kinetically by that same cation.     

Phase transitions of octamethylcyclotetrasiloxane confined inside aluminosilicate and silicate nanoporous matrices

We report the melting behaviour of a dipolar cyclic siloxane liquid: octamethylcyclotetrasiloxane (OMCTS) confined in three mesoporous silica matrices: Al-SBA-15, SBA-15 and CPG glasses, using differential scanning calorimetry and dielectric spectroscopy. We investigate the influence of acid sites on the adsorptive properties of mesoporous silica materials, which were synthesized by applying Pluronic-type polymers as pore-creating agents. Aluminosilicate matrices have been synthesized by direct synthesis procedure using aluminium chloride. These materials characterized by N₂ sorption measurements, and the small-angle X-ray scattering data exhibit the same hexagonal P6 mm structure with a mean mesopores size of 4.6 nm (Al-SBA-15) and 4.9 nm (SBA-15). The controlled pore glasses used in this experiment have pores of mean diameter of 7.5 nm. For all systems studied, the OMCTS melting point in pores has been found to decrease with decreasing pore diameter. This result is in qualitative agreement with that obtained in molecular simulation where the adsorbate-wall interactions are weak compared to the adsorbate–adsorbate interactions.     

Density functional theory for strongly-interacting electrons: perspectives for physics and chemistry

Improving the accuracy and thus broadening the applicability of electronic density functional theory (DFT) is crucial to many research areas, from material science, to theoretical chemistry, biophysics and biochemistry. In the last three years, the mathematical structure of the strong-interaction limit of density functional theory has been uncovered, and exact information on this limit has started to become available. The aim of this paper is to give a perspective on how this new piece of exact information can be used to treat situations that are problematic for standard Kohn-Sham DFT. One way to use the strong-interaction limit, more relevant for solid-state physical devices, is to define a new framework to do practical, non-conventional, DFT calculations in which a strong-interacting reference system is used instead of the traditional non-interacting one of Kohn and Sham. Another way to proceed, more related to chemical applications, is to include the exact treatment of the strong-interaction limit into approximate exchange-correlation energy density functionals in order to describe difficult situations such as the breaking of the chemical bond.     

Unexpected stable dimerisation of an anionic imidopyrrolecarboxylate in polar solution

The imidopyrrolecarboxylate 3(-) unexpectedly forms stable dimers (K(ass) = 130 M(-1) in CHCl(3)/DMSO, 1?:?1, v/v) despite the fact that two anions have to interact. The dimer is more stable than an analogous neutral amidopyrrolecarboxylic acid dimer (K(ass) < 10 M(-1)) underlining the importance of charged H-bonds compared to neutral ones.     

Density functional theory predicts the barriers for radical fragmentation in solution

[reaction: see text] N-Methoxypyridyl radicals formed by one-electron reduction of the corresponding cationic heterocycles undergo N-O bond cleavage. Experimental activation free energies for a series of these bond fragmentations are compared to corresponding barriers determined from electronic structure calculations. The DFT barriers agree well with those from experiment, being smaller than the latter values by an average value of ca. 1 kcal/mol, for rate constants varying over almost 3 orders of magnitude, or within ca. 3 kcal/mol over 8 orders of magnitude of rate constant. For a model compound, the B3PW91/6-31+G hybrid density functional method is also found to be in good agreement with the MCSCF-MRMP2 method. One of the reactions is found by DFT to have no minimum for the reactant radical, consistent with a truly barrierless reaction.     

Density functional study of the lithiation of cyclic vinyl ethers in solution

Lithiation of three cyclic vinyl ethers—2,3-dihydrofuran, 3,4-dihydro-2H-pyran, and 2,3-dihydrooxepin in ethereal solution are investigated at the density functional theory level of B3LYP/6-31++G(d,p). Several solvation models were used, including the microsolvation model, the continuum models such as the Onsager model, the polarized continuum models, and the isodensity polarized continuum model, and the mixed discrete-continuum model. Both the microsolvation and the mixed discrete-continuum model gave results consistent with experiments. Theoretical calculations also indicate that lithiation of 2,3-dihydrooxepin undergoes allylic lithiation concomitant with ring opening.     

Co-precipitation behaviour of titanium-containing silicate solution

The co-precipitation behaviour of a simulated Al₂(SO₄)₃-TiOSO₄-Na₂SiO₃ solution that imitated the lixivium of Ti-bearing blast furnace slag (Ti-slag) leached by sulphuric acid was investigated in this study. Various chemical analyses were employed to study the selective precipitation of multiple target components. Based on the high-added-value applications of Ti-slag, a new method was developed to prepare aluminium titanate composites from titanium-containing silicates. The findings demonstrate that the onsets of Ti and Al precipitation occur at pH values of 3.5 and 5.0, respectively, followed by Si precipitation. The particle sizes of the co-precipitates were greatly influenced by the precipitants, pH and the initial Al/Ti mole ratio. The results also show that the precipitation ratio of Ti, Al and Si generally increases with the pH and temperature, regardless of the Al/Ti mole ratio. The Si-O-Al, Ti-O-Al, and Ti-O-Si bonds that were formed were dependent on the pH and the initial Al/Ti mole ratio. There was a synthesis path for β-Al2TiO5 (AT) from the solid-state reaction between rutile and α-Al₂O₃ at 1362.5°C. The AT composites were successfully prepared by sintering the co-precipitates at 1450°C, which exhibited good thermal stability as estimated by the XRD measurements of the sample annealed at 1200°C for 4 hours.     

Density functional theory for efficient ab initio molecular dynamics simulations in solution

We present a density functional for first-principles molecular dynamics simulations that includes the electrostatic effects of a continuous dielectric medium. It allows for numerical simulations of molecules in solution in a model polar solvent. We propose a smooth dielectric model function to model solvation into water and demonstrate its good numerical properties for total energy calculations and constant energy molecular dynamics.     

Density functional theory-based prediction of the formation constants of complexes of ammonia in aqueous solution: indications of the role of relativistic effects in the solution chemistry of gold(I)

A prediction of the formation constants (log K1) for complexes of metal ions with a single NH3 ligand in aqueous solution, using quantum mechanical calculations, is reported. DeltaG values at 298 K in the gas phase for eq 1 (DeltaG(DFT)) were calculated for 34 metal ions using density functional theory (DFT), with the expectation that these would correlate with the free energy of complex formation in aqueous solution (DeltaG(aq)). [M(H2O)6]n+(g) + NH(3)(g) = [M(H2O)5NH3]n+(g) + H2O(g) (eq 1). The DeltaG(aq) values include the effects of complex changes in solvation on complex formation, which are not included in eq 1. It was anticipated that such changes in solvation would be constant or vary systematically with changes in the log K(1) value for different metal ions; therefore, simple correlations between DeltaG(DFT) and DeltaG(aq) were sought. The bulk of the log K1(NH3) values used to calculate DeltaG(aq) were not experimental, but estimated previously (Hancock 1978, 1980) from a variety of empirical correlations. Separate linear correlations between DeltaG(DFT) and DeltaG(aq) for metal ions of different charges (M2+, M3+, and M4+) were found. In plots of DeltaG(DFT) versus DeltaG(aq), the slopes ranged from 2.201 for M2+ ions down to 1.076 for M4+ ions, with intercepts increasing from M2+ to M4+ ions. Two separate correlations occurred for the M3+ ions, which appeared to correspond to small metal ions with a coordination number (CN) of 6 and to large metal ions with a higher CN in the vicinity of 7-9. The good correlation coefficients (R) in the range of 0.97-0.99 for all these separate correlations suggest that the approach used here may be the basis for future predictions of aqueous phase chemistry that would otherwise be experimentally inaccessible. Thus, the log K1(NH3) value for the transuranic Lr3+, which has a half-life of 3.6 h in its most stable isotope, is predicted to be 1.46. These calculations should also lead to a greater insight into the factors governing complex formation in aqueous solution. All of the above DFT calculations involved corrections for scalar relativistic effects (RE). Au has been described (Koltsoyannis 1997) as a "relativistic element". The chief effect of RE for group 11 ions is to favor linear coordination geometry and greatly increase covalence in the M-L bond. The correlation for M+ ions (H+, Cu+, Ag+, Au+) involved the preferred linear coordination of the [M(H2O)2]+ complexes, so that the DFT calculations of DeltaG for the gas-phase reaction in eq 2 were carried out for M = H+, Cu+, Ag+, and Au+. [M(H2O)2]+(g) + NH3(g) = [M(H2O)NH3]+(g) + H2O(g) (eq 2). Additional DFT calculations for eq 2 were carried out omitting corrections for RE. These indicated, in the absence of RE, virtually no change in the log K1(NH3) value for H+, a small decrease for Cu+, and a larger decrease for Ag+. There would, however, be a very large decrease in the log K1(NH3) value for Au(I) from 9.8 (RE included) to 1.6 (RE omitted). These results suggest that much of "soft" acid behavior in aqueous solution in the hard and soft acid-base classification of Pearson may be the result of RE in the elements close to Au in the periodic table.     

铜蛋白质的配位化学模拟

概述了生物酶中铜蛋白质配位化学模拟的研究进展，介绍了铜离子配合物生物酶活性中心的重要铜蛋白质的结构与功能的研究方法及最新进展。     

The novel dimerisation of radical anions generated electrochemically in aqueous solution from maleimide

The one-electron reduction of maleimide yields the novel compound 1,2,3,4-butane-tetracarboxylic 1,2:3,4 diimide exclusively. Substrate-substrate hydrogen bonding was found to have a profound influence upon the pathways and kinetics of this unusual reaction.     

Density functionals for inorganometallic and organometallic chemistry

We present a database of 21 bond dissociation energies for breaking metal-ligand bonds. The molecules in the metal-ligand bond energy database are AgH, CoH, CoO+, CoOH+, CrCH3+, CuOH2+, FeH, Fe(CO)5, FeO, FeS, LiCl, LiO, MgO, MnCH3NiCH2+, Ni(CO)4, RhC, VCO+, VO, and VS. We have also created databases of metal-ligand bond lengths and atomic ionization potentials. The molecules used for bond lengths are AgH, BeO, CoH, CoO+, FeH, FeO, FeS, LiCl, LiO, MgO, RhC, VO, and VS and the ionization potentials are for the following atoms: C, Co, Cr, Cu, Ni, O, and V. The data were chosen based on their diversity and expected reliability, and they are used along with three previously developed databases (transition metal dimer bond energies and bond lengths and main-group molecular atomization energies) for assessing the accuracy of several kinds of density functionals. In particular, we report tests for 42 previously defined functionals: 2 local spin density approximation (LSDA) functionals, 14 generalized gradient approximation (GGA) methods, 13 hybrid GGA methods, 7 meta GGA methods, and 8 hybrid meta GGA methods. In addition to these functionals, we also examine the effectiveness of scaling the correlation energy by testing 13 functionals with scaled or no gradient-corrected correlation energy, and we find that functionals of this kind are more accurate for metal-metal and metal-ligand bonds than any of the functionals already in the literature. We also present a readjusted GGA and a hybrid GGA with parameters adjusted for metals. When we consider these 57 functionals for metal-ligand and metal-metal bond energies simultaneously with main-group atomization energies, atomic ionization potentials, and bond lengths we find that the most accurate functional is G96LYP, followed closely by MPWLYP1M (new in this article), XLYP, BLYP, and MOHLYP (also new in this article). Four of these five functionals have no Hartree-Fock exchange, and the other has only 5%. As a byproduct of this work we introduce a convenient diagnostic, called the B1 diagnostic, for ascertaining the multireference character in a bond.     

Theoretical surface-enhanced Raman spectra study of substituted benzenes I. Density functional theoretical SERS modelling of benzene and benzonitrile

This paper reports a DFT modelling of SERS spectra for benzene and benzonitrile on the basis of a simple noncoordinate substrate-adsorbate model. Assignment of normal modes was obtained from internal force constants and potential energy distribution matrices and used to identify, according the SERS selection rules, the orientation of the optimized molecules on the metallic surface. Calculated band enhancements are in good agreement with experimental observations. The optimized geometry parameters of the molecule-Ag system, changes of HOMO-LUMO energies are discussed to give insight in the different SERS mechanisms for both molecules.     

Density functional theory study of uranium(VI) aquo chloro complexes in aqueous solution

Mixed uranyl aquo chloro complexes of the type [UO2(H2O)xCly]2-y (y = 1, 2, 3, 4; x + y = 4, 5) have been optimized at the BLYP, BP86, and B3LYP levels of density functional theory in vacuo and in a polarizable continuum modeling bulk water (PCM) and have been studied at the BLYP level with Car-Parrinello molecular dynamics (MD) simulations in the gas phase and in explicit aqueous solution. Free binding energies were evaluated from static PCM data and from pointwise thermodynamic integration involving constrained MD simulations in water. The computations reveal significant solvent effects on geometric and energetic parameters. Based on the comparison of PCM-optimized or MD-averaged uranyl-ligand bond distances with EXAFS-derived values, the transition between five- and four-coordination about uranyl is indicated to occur at a Cl content of y = 2 or 3.     

Density functional theory study of 1:1 glycine-water complexes in the gas phase and in solution

We present a systematic study of 1:1 glycine-water complexes involving all possible glycine conformers. The complex geometries are fully optimized for the first time both in the gas phase and in solution using three DFT methods (B3LYP, PBE1PBE, X3LYP) and the MP2 method. We calculate the G3 energies and use them as the reference data to gauge hydrogen bond strength in the gas phase. The solvent effects are treated via the integral equation formalism-polarizable continuum model (IEF-PCM). Altogether, we locate fifty-two unique nonionized (N) structures and six zwitterionic (Z) structures in the gas phase, and fifty-five N structures and thirteen Z structures in solution. Both correlation and solvation are shown to be important in geometry determination. We found that in the gas phase, a water molecule binds more strongly to the carboxylic acid group of glycine than to its amine group, whereas in solution phase the reverse is true. The most stable Z structure is isoenergetic with the most stable N structure.     

Density functional theory and molecular clusters

Density functional theory (DFT) methods, including nonlocal density gradient terms in the exchange and correlation energy functionals, were applied to various types of molecular clusters: H-bonded, ionic, electrostatic, and London. Reliable results on the structure and stabilization energy were obtained for the first two types of cluster as long as Becke3LYP and Becke3P86 functionals and basis sets of at least DZ + P quality were used. DFT methods with currently available functionals failed completely, however, for London-type clusters, for which no minimum was found on the potential energy surfaces. DFT interaction energy exhibits the same basis set extension dependence as the Hartree-Fock (HF) interaction energy. Therefore, the Boys-Bernardi function counterpoise procedure should be employed for elimination of the DFT basis set superposition error. © 1995 John Wiley & Sons, Inc.     

Water versus acetonitrile coordination to uranyl. Density functional study of cooperative polarization effects in solution

Optimizations at the BLYP and B3LYP levels are reported for mixed uranyl-water/acetonitrile complexes [UO(2)(H(2)O)(5-n)(MeCN)(n)](2+) (n = 0-5), in both the gas phase and a polarizable continuum modeling acetonitrile. Car-Parrinello molecular dynamics (CPMD) simulations have been performed for these complexes in the gas phase, and for selected species (n = 0, 1, 3, 5) in a periodic box of liquid acetonitrile. According to structural and energetic data, uranyl has a higher affinity for acetonitrile than for water in the gas phase, in keeping with the higher dipole moment and polarizability of acetonitrile. In acetonitrile solution, however, water is the better ligand because of specific solvation effects. Analysis of the dipole moment of the coordinated water molecule in [UO(2)(H(2)O)(MeCN)(4)](2+) reveals that the interaction with the second-shell solvent molecules (through fairly strong and persistent O-H···N hydrogen bonds) causes a significant increase of this dipole moment (by more than 1 D). This cooperative polarization of water reinforces the uranyl-water bond as well as the water solvation via strengthened (UO(2))OH(2)···NCMe hydrogen bonds. Such cooperativity is essentially absent in the acetonitrile ligands that make much weaker (UO(2))NCMe···NCMe hydrogen bonds. Beyond the uranyl case, this study points to the importance of cooperative polarization effects to enhance the M(n+) ion affinity for water in condensed phases involving M(n+)-OH(2)···A fragments, where A is a H-bond proton acceptor and M(n+) is a hard cation.     

Efficient adsorbents of nanoporous aluminosilicate monoliths for organic dyes from aqueous solution

Growing public awareness on the potential risk to humans of toxic chemicals in the environment has generated demand for new and improved methods for toxicity assessment and removal, rational means for health risk estimation. With the aim of controlling nanoscale adsorbents for functionality in molecular sieving of organic pollutants, we fabricated cubic Im3m mesocages with uniform entrance and large cavity pores of aluminosilicates as highly promising candidates for the colorimetric monitoring of organic dyes in an aqueous solution. However, a feasible control over engineering of three-dimensional (3D) mesopore cage structures with uniform entrance (~5 nm) and large cavity (~10 nm) allowed the development of nanoadsorbent membranes as a powerful tool for large-quantity and high-speed (in minutes) adsorption/removal of bulk molecules such as organic dyes. Incorporation of high aluminum contents (Si/Al=1) into 3D cubic Im3m cage mesoporous silica monoliths resulted in small, easy-to-use optical adsorbent strips. In such adsorption systems, natural surfaces of active acid sites of aluminosilicate strips strongly induced both physical adsorption of chemically responsive dyes and intraparticle diffusion into cubic Im3m mesocage monoliths. Results likewise indicated that although aluminosilicate strips with low Si/Al ratios exhibit distortion in pore ordering and decrease in surface area and pore volume, enhancement of both molecular converges and intraparticle diffusion onto the network surfaces and into the pore architectures of adsorbent membranes was achieved. Moreover, 3D mesopore cage adsorbents are reversible, offering potential for multiple adsorption assays.