Broken symmetry approach to calculation of exchange coupling constants for homobinuclear and heterobinuclear transition metal complexes

The application of broken symmetry density functional calculations to homobinuclear and heterobinuclear transition metal complexes produces good estimates of the exchange coupling constants as compared to experimental data. The accuracy of different hybrid density functional theory methods was tested. A discussion is presented of the different methodological approaches that apply when a broken symmetry wave function is used with either Hartree–Fock or density functional calculations. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1391–1400, 1999     

Kinetic modeling of the thermal degradation of polyethylene and polystyrene mixtures

One possible process for recovering valuable chemical and petrochemical products from plastic waste is the stepwise thermal degradation of polymer mixtures. This potentially allows the step by step simultaneous separation of the different product fractions generated by the polymers of the blend. The aim of this paper is to investigate the effect of the mixing scale of the polymers and their interactions in the melt. Several thermogravimetric analyses were performed on small samples of polyethylene (PE) and polystyrene (PS) mixtures. Two types of operating conditions were adopted: the first one is a dynamic analysis with a linear increase of the temperature over time, the latter consists of two sequential isothermal steps. The experimental results confirm that if the mixing scale is poor, the decomposition of each polymer behaves independently of the presence of the other one. Conversely, when the mixing of the two polymers reaches the molecular scale, a co-pyrolysis takes place with partial interactions. A two phase system is assumed: one phase characterized by a larger PS fraction, the other one by a prevailing PE amount. In order to properly predict the kinetic interactions typical of the mixed phases, it was necessary to extend the detailed kinetic model already developed and validated for the single polymers. The resulting two phase model gives a satisfactory explanation of several experimental data from the thermal degradation of PE–PS mixtures.     

One-Dimensional Intercalation Compound 2 HgS⋅SnBr2: Ab Initio Electronic Structure Calculations and Molecular Dynamics Simulations

The one-dimensional intercalation compound 2 HgS⋅SnBr₂ (figure; white: Hg, light gray: S, dark grey: Sn, and black: Br) is an example of the ability of weak host–guest interactions to induce structural modifications in the HgS host. The stability and dynamic behavior of the intercalation compound and its precursors HgS and SnBr₂ have been analyzed by means of DFT/plane wave calculations. The molecular dynamics simulations facilitate the understanding of the structural rearrangements during the intercalation process.     

Magneto-Structural Correlations in Trinuclear Cu(II) Complexes: A Density Functional Study

Summary.  Density functional theoretical methods have been used to study magneto-structural correlations for linear trinuclear hydroxo-bridged copper(II) complexes. The nearest-neighbor exchange coupling constant shows very similar trends to those found earlier for dinuclear compounds for which the Cu–O–Cu angle and the out of plane displacement of the hydrogen atoms at the bridge are the two key structural factors that determine the nature of their magnetic behavior. Changes in these two parameters can induce variations of over 1000 cm⁻¹ in the value of the nearest-neighbor coupling constant. On the contrary, coupling between next-nearest neighbors is found to be practically independent of structural changes with a value for the coupling constant of about −60 cm⁻¹. The magnitude calculated for this coupling constant indicates that considering its value to be negligible, as usually done in experimental studies, can lead to considerable errors, especially for compounds in which the nearest-neighbor coupling constant is of the same order of magnitude. Corresponding author. E-mail: eliseo.ruiz@qi.ub.es Received June 28, 2002; accepted July 22, 2002     

Guest Modulation of Spin‐Crossover Transition Temperature in a Porous Iron(II) Metal–Organic Framework: Experimental and Periodic DFT Studies

The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin‐crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)₄]} ( 1 ) with five‐membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin‐crossover transition with hysteresis loops 14–29 K wide and average critical temperatures T_c=201 K ( 1?fur ), 167 K ( 1?pyr ), and 114.6 K ( 1?thio ) well below that of the parent compound 1 (T_c=295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1?fur , while 1?pyr and 1?thio show 50 % spin transition. For 1?fur the transformation between the HS and IS (middle of the plateau) phases occurs concomitantly with a crystallographic phase transition between the tetragonal space groups P4/mmm and I4/mmm, respectively. The latter space group is retained in the subsequent transformation involving the IS and the LS phases. 1?pyr and 1?thio display the tetragonal P4/mmm and orthorhombic Fmmm space groups, respectively, in both HS and IM phases. Periodic calculations using density functional methods for 1?fur , 1?pyr , 1?thio , and previously reported derivatives 1?CS₂ , 1?I, 1?bz (benzene), and 1?pz (pyrazine) have been carried out to investigate the electronic structure and nature of the host–guest interactions as well as their relationship with the changes in the LS–HS transition temperatures of 1?Guest . Geometry‐optimized lattice parameters and bond distances in the empty host 1 and 1?Guest clathrates are in general agreement with the X‐ray diffraction data. The concordance between the theoretical results and the experimental data also comprises the guest molecule orientation inside the host and intermolecular distances. Furthermore, a general correlation between experimental T_c and calculated LS–HS electronic energy gap was observed. Finally, specific host–guest interactions were studied through interaction energy calculations and crystal orbital displacement (COD) curve analysis.     

Development of a procedure for the multiresidue analysis of pesticides in vineyard soils and its application to real samples

A procedure for multiresidue analysis was developed for the extraction and determination of 17 pesticides, including herbicides, fungicides, and insecticides, as well as certain degradation products, in vineyard soils from La Rioja region (Spain). Different solvents and mixtures were tested in spiked pesticide‐free soils, and pesticides were comparatively evaluated by gas chromatography with mass spectrometry and liquid chromatography with mass spectrometry. Recoveries >70%, with relative standard deviations <9%, were obtained when a mixture of methanol/acetone or a mixture of methanol/CaCl₂ 0.01 M for the most polar compounds was selected as the extraction solvent. Method validation was accomplished with acceptable linearity (r² ≥ 0.987) within the concentration range of 0.005–1 μg/mL corresponding to 1.667–333.4 μg/kg and 0.835–167.1 μg/kg for liquid chromatography with mass spectrometry and gas chromatography with mass spectrometry, respectively, and detection limits <0.4 μg/kg for the compounds were studied. The extraction method was applied to 17 real vineyard soil samples, and terbuthylazine and its metabolite desethylterbuthylazine were the most ubiquitous compounds, as they were detected in the 100% of the soils analyzed. The presence of fungicides was also high, and the presence of insecticides was lower than other pesticides. The results confirm the usefulness of the optimized procedure for monitoring residues in vineyard soils.     

Squarato-metal(II) complexes. 2. unusual bonding mode for a squarato-bridged trinuclear copper(II) complex

Herein, we report the structural characterization and magnetic properties of the unique squarato-bridged-tricopper(II) complex, [Cu3(pmap)3(micro1,2,3-C4O4)](ClO4)(4).2H 2O (1), based on the tripod tripyridylamine ligand bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine (pmap). Each of the three copper centers is penta-coordinated by four N atoms of a pmap ligand and one bridging O atom of the central squarato dianion. This complex is the first example of a non-polymeric X-ray structurally characterized trimeric transition metal complex with the three metal cations being bridged by a single squarato ligand in a micro1,2,3 coordination mode. The magnetic properties of the complex were measured over the temperature range 2-300 K. The complex exhibits moderate bulk antiferromagnetic interaction. The three magnetic exchange pathways have J values of -27.8, -20.8, and -31.9 cm(-1). The DFT calculations corroborate the relatively strong antiferromagnetic couplings obtained from the fitting of the experimental magnetic susceptibility data and allow an assignment of the fitted J values. Several geometrical parameters have been analyzed using theoretical calculations to establish magnetostructural correlations for complex 1.     

Stereochemistry and spin state in four-coordinate transition metal compounds

A systematic DFT computational study of the stereochemistry associated with each spin state for first transition series four-coordinate d(n) (n = 0-10) homoleptic metal complexes is presented. The stereochemistry of [MMe4](x-) complexes in the 21 spin configurations analyzed can be predicted from the d orbital occupation in the ideal tetrahedral geometry, grouped in three families with tetrahedral, square planar, or intermediate structures that can be described in some cases as sawhorses. The effect of the following factors on the spin state and stereochemical preferences has also been studied: (a) substitution of the sigma-donor methyl ligands by pi-donor chlorides, (b) a high (+4) oxidation state of the metal, and (c) substitution of the metal atom by a second transition series one. Through those factors, low-spin tetrahedral structures can be achieved, as summarized by a magic cube.