Theoretical calculations of the kinetics and mechanisms of the gas phase elimination of primary,secondary, and tertiary 2‐hydroxyalkylbenzenes

Theoretical study of the elimination kinetics of 2‐phenylethanol, 1‐phenyl‐2‐propanol, and 2‐methyl‐1‐phenyl‐2‐propanol in the gas‐phase has been carried out at the MP2/6‐31G(d,p), B3LYP/6‐31G(d,p), B3LYP/6‐31++G(d,p), MPW1PW91/6‐31G(d,p), MPW1PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), and PBEPBE/6‐31++G(d,p) levels of theory. The three substrates undergo two parallel elimination reactions. The first elimination appears to proceed through a six‐membered cyclic transition state to give toluene and the corresponding aldehyde or ketone. The second parallel elimination takes place through a four‐membered cyclic transition state producing water and the corresponding unsaturated aromatic hydrocarbon. Results from MP2/6‐31G(d,p) and MPW1PW91/6‐31++G(d,p) methods were found to be in good agreement with the experimental kinetic and thermodynamic parameters in the formation of toluene and the corresponding carbonyl compound. However, the results for PBEPBE/6‐31G(d,p) were in better agreement with the experimental data for the second parallel reaction yielding water and the corresponding unsaturated aromatic hydrocarbon. The charge distribution differences in the TS related to the substitution by methyl groups in the substrates can account for the observed reaction rate coefficients. The synchronicity parameters imply semi‐polar transition states for these elimination reactions. Copyright © 2009 John Wiley & Sons, Ltd.     

Networks based on hydrogen-bonds containing phosphorus anions and tris(3,5-dimethylpyrazolyl)borate nickel(II) moieties

Five structural kinds of nickel hydrogen-bonded networks containing hydrotris(3,5-dimethylpyrazolyl)borate ligands (Tp^∗) have been elucidated by X-ray diffraction, [Tp^∗Ni(OH₂)₃][(p-NO₂C₆H₄O)₂PO₂] (4), [Tp^∗Ni(OH₂)₃][Me₂PO₂]·Me₂P(O)OH (5), [Tp^∗Ni(OH₂)₃][(nBuO)₂PO₂]·0.5H₂O (6), [(Hpz)Tp^∗Ni(OH₂)₂][(Ph)PO₂OH] (7) and [Tp^∗Ni(OH₂)₂(Me₂PO₂)] (8). The most relevant supramolecular feature of complexes 4-8 is all of them form coordination networks based on hydrogen bonds between water molecules and phosphate, phosphonate or phosphinate anions. These hydrogen bonds are formed within the monomer units in addition to connect monomers along the chains. Their behaviors in solution were investigated by one- and two-dimensional ¹H NMR techniques.     

Defective dicubane-like tetranuclear nickel(II) cyanate and azide nanoscale magnets

Four tetrameric nickel(II) pseudohalide complexes have been synthesized and structurally, spectroscopically, and magnetically characterized. Compounds 1-3 are isostructural and exhibit the general formula [Ni(2)(dpk·OH)(dpk·CH(3)O)(L)(H(2)O)](2)A(2)·2H(2)O, where dpk = di-2-pyridylketone; L = N(3)(-), and A = ClO(4)(-) for 1, L = NCO(-) and A = ClO(4)(-) for 2, and L = NCO(-) and A = NO(3)(-) for 3. The formula for 4 is [Ni(4)(dpk·OH)(3) (dpk·CH(3)O)(2)(NCO)](BF(4))(2)·3H(2)O. The ligands dpk·OH(-) and dpk·CH(3)O(-) result from solvolysis and ulterior deprotonation of dpk in water and methanol, respectively. The four tetramers exhibit a dicubane-like core with two missing vertexes where the Ni(II) ions are connected through end-on pseudohalide and oxo bridges. Magnetic measurements showed that compounds 1-4 are ferromagnetic. The values of the exchange constants were determined by means of a theoretical model based on three different types of coupling. Thus, the calculated J values (J(1) = J(2), J(3), and D) were 5.6, 11.8, and 5.6 cm(-1) for 1, 5.5, 12.0, and 5.6 cm(-1) for 2, 6.3, 4.9, and 6.2 cm(-1) for 3, and (J(1), J(2), J(3), and D) 6.9, 7.0, 15.2, and 4.8 cm(-1) for 4.     

Kinetics of the Gas‐Phase Elimination Reaction of Benzyl Chloroformate and Neopentyl Chloroformate

The gas‐phase eliminations of benzyl chloroformate (475–523 K, 31–103 Torr) and neopentyl chloroformate (563–622 K, 37–70 Torr), in a deactivated static reaction vessel, and in the presence of a free radical suppressor, are homogeneous, unimolecular, and follow a first‐order rate law. The rate coefficients are expressed by the following Arrhenius equations: Benzyl chloroformate Neopentyl chloroformate Formation of neopentyl chloride: Formation of 2‐methylbutenes: The derived kinetic and thermodynamic parameters for benzyl chloroformate decomposition indicate the reaction proceeds through a concerted four‐membered cyclic transition state to give benzyl chloride and CO₂ gas. Neopentyl chloroformate undergoes a parallel reaction, where neopentyl chloride formation may arise from a polar‐concerted four‐membered cyclic transition state, whereas the mixture of olefins, 2‐methyl‐2‐butene, and 2‐methyl‐1‐butene appears to be produced from a carbene intermediate. This intermediate seems to be originated from a concerted five‐membered cyclic transition state of the neopentyl substrate.     

Dinuclear cobalt(II) and copper(II) complexes with a Py2N4S2 macrocyclic ligand

The interaction between Co(II) and Cu(II) ions with a Py(2)N(4)S(2)-coordinating octadentate macrocyclic ligand (L) to afford dinuclear compounds has been investigated. The complexes were characterized by microanalysis, conductivity measurements, IR spectroscopy and liquid secondary ion mass spectrometry. The crystal structure of the compounds [H(4)L](NO(3))(4), [Cu(2)LCl(2)](NO(3))(2) (5), [Cu(2)L(NO(3))(2)](NO(3))(2) (6), and [Cu(2)L(μ-OH)](ClO(4))(3)·H(2)O (7) was also determined by single-crystal X-ray diffraction. The [H(4)L](4+) cation crystal structure presents two different conformations, planar and step, with intermolecular face-to-face π,π-stacking interactions between the pyridinic rings. Complexes 5 and 6 show the metal ions in a slightly distorted square-pyramidal coordination geometry. In the case of complex 7, the crystal structure presents the two metal ions joined by a μ-hydroxo bridge and the Cu(II) centers in a slightly distorted square plane or a tetragonally distorted octahedral geometry, taking into account weak interactions in axial positions. Electron paramagnetic resonance spectroscopy is in accordance with the dinuclear nature of the complexes, with an octahedral environment for the cobalt(II) compounds and square-pyramidal or tetragonally elongated octahedral geometries for the copper(II) compounds. The magnetic behavior is consistent with the existence of antiferromagnetic interactions between the ions for cobalt(II) and copper(II) complexes, while for the Co(II) ones, this behavior could also be explained by spin-orbit coupling.     

Mild hydrothermal synthesis, crystal structure, thermal behaviour, spectroscopic and magnetic properties of (NH4)0.80Li0.20[Fe(AsO4)F]

The (NH₄)_0.80Li_0.20[Fe(AsO₄)F] compound has been synthesized under mild hydrothermal conditions. The compound crystallize in the orthorhombic Pna2₁ space group, with cell parameters a=13.352(9), b=6.7049(9), c=10.943(2) Å and Z=8. The compound belongs to the KTiO(PO₄) structure type, with chains alternating FeO₄F₂ octahedra and AsO₄ tetrahedra, respectively, running along the “a” and “b” crystallographic axes. The diffuse reflectance spectrum in the visible region shows the forbidden electronic transitions characteristic of the Fe(III) d⁵-high spin cation in slightly distorted octahedral geometry. The Mössbauer spectrum at room temperature is characteristic of iron (III) cations. The ESR spectra, carried out from room temperature to 200 K, remain isotropic with variation in temperature; the g-value being 1.99(1). Magnetic measurements indicate the predominance of strong antiferromagnetic interactions.     

Theoretical study of neighboring carbonyl group participation in the elimination kinetics of chloroketones in the gas phase

The gas‐phase elimination of kinetics 4‐chlorobutan‐2‐one, 5‐chloropentan‐2‐one, and 4‐chloro‐1‐phenylbutan‐1‐one has been studied using electronic structure methods: B3LYP/6‐31G(d,p), B3LYP/6‐31++G(d,p), MPW91PW91/6‐31G(d,p), MPW91PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), PBEPBE /6‐31++G(d,p), and MP2/6‐31++G(d,p). The above‐mentioned substrates produce hydrogen chloride and the corresponding unsaturated ketone. Calculation results of 4‐chlorobutan‐2‐one suggest a non‐synchronous four‐membered cyclic transition state (TS) type of mechanism. However, in the case of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one, the carbonyl group assists anchimerically through a polar five‐membered cyclic TS mechanism. The polarization of the C? Cl bond, in the sense of C^δ+…Cl^δ?, is a rate‐determining step in these elimination reactions. The significant increase in rates in the elimination of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one is attributed to neighboring group participation due to the oxygen of the carbonyl group assisting the C? Cl bond polarization in the TS. Copyright © 2010 John Wiley & Sons, Ltd.     

A new partially deprotonated mixed-valence manganese(II,III) hydroxide-arsenate with electronic conductivity: magnetic properties of high- and room-temperature sarkinite

A new three-dimensional hydroxide-arsenate compound called compound 2 has been synthesized by heating (in air) of the sarkinite phase, Mn(2)(OH)AsO(4) (compound 1), with temperature and time control. The crystal structure of this high-temperature compound has been solved by Patterson-function direct methods. A relevant feature of this new material is that it is actually the first member of the adamite-type family with mixed-valence manganese(II,III) and electronic conductivity. Crystal data: a = 6.7367(5) ?, b = 7.5220(6) ?, c = 9.8117(6) ?, α = 92.410(4)°, β = 109.840(4)°, γ = 115.946(4)°, P1?. The unit cell content derived from Rietveld refinement is Mn(8)(O(4)H(x))(AsO(4))(4). Its framework, projected along [111], is characterized by rings of eight Mn atoms with the OH(-)/O(2-) inside the rings. These rings form an almost perfect hexagonal arrangement with the AsO(4) groups placed in between. Bond-valence analysis indicates both partial deprotonation (x ? 3) and the presence of Mn in two different oxidation states (II and III), which is consistent with the electronic conductivity above 300 °C from electrochemical measurements. The electron paramagnetic resonance spectra of compound 1 and of its high-temperature form compound 2 show the presence of antiferromagnetic interactions with stronger magnetic coupling for the high-temperature phase. Magnetization measurements of room-temperature compound 1 show a complex magnetic behavior, with a three-dimensional antiferromagnetic ordering and magnetic anomalies at low temperatures, whereas for compound 2, an ordered state is not reached. Magnetostructural correlations indicate that superexchange interactions via oxygen are present in both compounds. The values of the magnetic exchange pathways [Mn-O-Mn] are characteristic of antiferromagnetic couplings. Notwithstanding, the existence of competition between different magnetic interactions through superexchange pathways can cause the complex magnetic behavior of compound 1. The loss of three-dimensional magnetic ordering by heating of compound 1 could well be based on the presence of Mn(3+) ions (d(4)) in compound 2.     

Some Homological Properties of Skew PBW Extensions

We prove that if R is a left Noetherian and left regular ring then the same is true for any bijective skew PBW extension A of R. From this we get Serre's Theorem for such extensions. We show that skew PBW extensions and its localizations include a wide variety of rings and algebras of interest for modern mathematical physics such as PBW extensions, well-known classes of Ore algebras, operator algebras, diffusion algebras, quantum algebras, quadratic algebras in 3-variables, skew quantum polynomials, among many others. We estimate the global, Krull and Goldie dimensions, and also Quillen's K-groups.