Kinetics of hydrolysis of 4-methoxyphenyl-2,2-dichloroethanoate in binary water-cosolvent mixtures; the role of solvent activity and solute-solute interactions

Rate constants are reported for the pH-independent hydrolysis of 4-methoxyphenyl-2,2-dichloroethanoate in aqueous solution as a function of the concentration of added cyanomethane (acetonitrile), polyethylene glycol (PEG 400) and tetrahydrofuran (THF). The concentration of water was varied between ca. 25 and 55.5 M. It was found that the variation in water activity yields only a minor contribution to the observed variation in rate constants. Interestingly, for both cyanomethane and PEG 400 log(k) varies approximately linearly with the molar concentration of water. Medium effects in highly aqueous solutions ([H(2)O] > 50 M) of ethanol, 1-propanol, 2-propanol, 1-butanol and 2-methyl-2-propanol have also been determined. Unexpectedly, in this concentration range the alcohols induce significantly smaller effects per unit volume than cyanomethane. The present results are discussed in terms of pairwise interaction parameters. Isobaric activation parameters have been determined and reveal remarkable differences in the nature of the induced medium effects.     

Reduced and excited states of (bpym)[PtCl(2)](n)() (bpym = 2,2'-bipyrimidine; n = 1, 2): experiments and DFT calculations

The complexes (bpym)PtCl(2) (1) and the new (micro-bpym)[PtCl(2)](2) (2), bpym = 2,2'-bipyrimidine, were synthesized and, in the case of 1, crystallized in solvent-free form for X-ray diffraction. The molecules 1 exhibit two different kinds of stacking motifs in the crystal with an interstack CH--N interaction. Complexes 1 and 2 were found to be sufficiently soluble for cyclic voltammetry, spectroscopy (absorption and emission), and spectroelectrochemical studies (UV-vis, EPR). As a result of single or double coordination of the strongly sigma-accepting [PtCl(2)] fragment to bpym, the paramagnetic anions 1(*)(-)() and 2(*)(-)() and the dianions 1(2-) and 2(2-) could be reversibly generated, despite the presence of metal-halide bonds. DFT calculations of A((195)Pt) and g tensor components confirm that the singly occupied MOs of the monoanionic species have mainly pi(bpym) character with nonnegligible platinum d orbital participation. The assignments of the electronic absorption and emission and resonance Raman spectra for both complexes are supported by DFT calculations.     

A kinetic study of 1,3-dipolar cycloadditions in micellar media

The kinetics of the 1,3-dipolar cycloadditions (DC) of benzonitrile oxide with a series of N-substituted maleimides in micellar media have been investigated. Surfactants studied include anionic sodium dodecyl sulfate, cationic cetyltrimethylammonium bromide, and a series of nonionic alkyl poly(ethylene oxide) surfactants (CxEy). The kinetic data have been analyzed by using the pseudo-phase model for bimolecular reactions. Much larger micellar accelerations (up to a factor of 17) were observed for these reactions than was previously found for Diels-Alder (DA) reactions (J. Org. Chem. 2002, 67, 7369-7377). This is explained by the smaller solvent sensitivity of DC reactions, which causes the micellar rate constants to be much closer to the value of water (km/kw approximately 0.25-0.45 for DC reactions vs 0.02-0.05 for DA reactions). Further evidence is presented, that a water/1-propanol mixture ([H2O] ca. 15 M) is a fairly good mimic of the micellar reaction environment for these reactions. Isobaric activation parameters have been determined for the reaction in the micellar phase of C16E20, using micellar rate constants. They correspond well to values obtained for the aforementioned micelle mimic.     

Light-induced metastable states in oxalatenitrosylruthenium(II) and terpyridinenitrosylruthenium(II) complexes

Two extremely long lived metastable states (SI and SII) can be accessed by irradiation with light in the blue-green spectral range at temperatures below 200 K in Cs(2)[Ru(ox)(NO)Cl(3)], [Ni(cyclam)][Ru(ox)(NO)Cl(3)].3H(2)O, and [Ru(terpy)(NO)(OH)Cl][PF(6)]. The crystal structures of the ground states of the oxalate-containing compounds are presented, and the influence of the atomic distances of the cations/anions is discussed with respect to the decay temperatures. The radiationless thermal decay of the metastable states is detected by differential scanning calorimetry (DSC) for the three compounds. Both metastable states decay exponentially in time under isothermal conditions. The excited states are energetically separated from the ground state by potential barriers given by the activation energy of the Arrhenius law. In [Ni(cyclam)][Ru(ox)(NO)Cl(3)].3H(2)O the enthalpy maximum of the thermal decay of SII appears at 182 K, which is a relatively high decay temperature for SII. The reason for this strong temperature shift compared to those of the other compounds could be due to the polarization effect of Ni(2+) on the electron density at the Ru site via the Cl atom.     

Synthesis, crystal structure, and transistor performance of tetracene derivatives

The substitution of chloro or bromo groups in tetracene gives rise to the change of crystal structure, having a substantial effect on carrier transport. Halogenated tetracene derivatives were synthesized and grown into single crystals. Monosubstituted 5-bromo- and 5-chlorotetracenes have the herringbone-type structure, while 5,11-dichlorotetracene has the slipped pi stacking structure. Mobility of 5,11-dichlorotetracene was measured to be as high as 1.6 cm2/V.s in single-crystal transistors. The pi stacking structure, which enhances pi orbital overlap and facilitates carrier transport, may thus be responsible for this high mobility.     

A law of the iterated logarithm for kernel estimators of regression functions

Summary For fixed design regression data kernel estimation is widely used to estimate _(v)(t), thev-th derivative of (t). Denoting such an estimator by this paper is concerned with the almost sure convergence of . It is shown that under several inserting procedures a law of iterated logarithm holds.     

Understanding the role of sodium during adsorption: a force field for alkanes in sodium-exchanged faujasites

We have developed a united atom force field able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. This force field successfully reproduces experimental adsorption properties of n-alkanes over a wide range of sodium cation densities, temperatures, and pressures. The force field reproduces the sodium positions in dehydrated FAU-type zeolites known from crystallography, and it predicts how the sodium cations redistribute when n-alkanes adsorb. The cations in the sodalite cages are significantly more sensitive to the n-alkane loading than those in the supercages. We provide a simple expression that adequately describes the n-alkane Henry coefficient and adsorption enthalpy as a function of sodium density and temperature at low coverage. This expression affords an adequate substitute for complex configurational-bias Monte Carlo simulations. The applicability of the force field is by no means limited to low pressure and pure adsorbates, for it also successfully reproduces the adsorption from binary mixtures at high pressure.     

Dehydration of synthetic and natural vivianite

Thermal analyses of synthetic and natural vivianite (Fe²⁺)₃(PO₄)₂·8H₂O) were determined using a high-resolution thermal analyser coupled to a mass spectrometer.Five dehydration weight loss steps were observed for the natural vivianite at 105, 138, 203, 272 and 437 °C. The first weight loss step involves the reaction (Fe²⁺)₃(PO₄)₂·8H₂O→(Fe²⁺)₃(PO₄)₂·3H₂O+5H₂O. The TGA/MS for the synthetic vivianite gave similar results to that of the natural sample. Mass spectrometry shows that water is lost up to 450 °C and after this temperature oxygen is lost. Changes in the structure of vivianite were followed using infrared emission spectroscopy. A model is proposed for the dehydration of vivianite.     

Thermal decomposition of ammonium jarosite (NH4)Fe3(SO4)2(OH)6

Thermogravimetry combined with mass spectrometry has been used to study the thermal decomposition of a synthetic ammonium jarosite. Five mass loss steps are observed at 120, 260, 389, 510 and 541°C. Mass spectrometry through evolved gases confirms these steps as loss of water, dehydroxylation, loss of ammonia and loss of sulphate in two steps. Changes in the molecular structure of the ammonium jarosite were followed by infrared emission spectroscopy (IES). This technique allows the infrared spectrum at the elevated temperatures to be obtained. IES confirms the dehydroxylation to have taken place by 300°C and the ammonia loss by 450°C. Loss of the sulphate is observed by changes in band position and intensity after 500°C.