Molecular orbital description of the polythiazyl polymer Part 1. MNDO calculations of models of (SN)x

Semiempirical MNDO calculations are performed on various models for the polythiazyl polymer with increasing chain sizes. Population analysis, geometry optimization and the evolution of the frontier orbital energies are discussed. A low-lying broken symmetry structure with (SN)₂ repeating units is detected.     

Fine structure in the hydrogen atom boxed in a spherical impenetrable cavity

The spectrum of the hydrogen atom confined in a spherical impenetrable box of radius R_c has been investigated by many authors up to date, but not at the level of relativistic corrections. It is well known that, as R_c diminishes, all energy levels and the pressure increase very rapidly, whereas the polarizability goes to zero. In this report, we have computed the relativistic corrections that underlie the fine structure of the confined hydrogen atom, as a function of R_c. Such corrections correspond to relativistic kinetic energy, spin‐orbit coupling and the Darwin term, which are calculated in the frame of time‐independent perturbation theory, for which, use was made of the exact confined hydrogen atom wave functions. We show that for a confinement radius of 0.5 au the relativistic corrections increase up to three orders of magnitude with respect to those corresponding to the free atom. As R_c decreases, the kinetic energy correction and the spin‐orbit coupling for become negative whereas their absolute value and the Darwin term, which is positive, increase very rapidly.     

Mechanistic studies of carbonate macrocyclization

The kinetics of phenylchloroformate (PCF) reactions have been used to model some of the key chemical events in carbonate macrocyclization. Three reactions have been studied using stopped-flow FT-IR spectroscopy: formation of acyl ammonium salt from PCF and three different trialkylamines, the conversion of acyl ammonium salt to urethane, and the condensation reaction between acyl ammonium salt and 4-isopropylphenol. The rate dependence was studied for triethylamine (TEA), diethylmethylamine (DEMA) and tri-n-butylamine (TBA) at 0°C in anhydrous CH₂Cl₂. The reactivity order for acyl ammonium salt formation for TBA: TEA: DEMA is 1 : 2.7 : >444. By contrast, condensation and urethane formation are not sensitive to the structure of the amine. The rate of condensation is comparable to the rate of acyl ammonium salt formation for TEA and TBA, while the rate of urethane formation is the slowest process for all three amines. These results are consistent with the view that the yield of macrocyclic polycarbonates is related to the concentration of the acyl ammonium salt. The optimum amine concentration for obtaining high yields of cyclics varies with the amine structure and parallels the difference in the rates of acyl ammonium salt formation. © 1994 John & Sons, Inc.     

Crystallization of ZSM-12 Zeolite with Different Si/Al Ratio

In this work the acid properties of a series of HZSM-12 zeolites with different Si/Al ratio were studied. The ZSM-12 crystals were synthesized by the hydrothermal method starting from a gel with the following molar composition: 20MTEA:10Na₂O:x Al₂O₃:100SiO₂:2000H₂O, with x = 0.50, 0.67, 1, 1.25 and 2, respectively. The gels were crystallized at 140^∘C for 6 days, then washed, dried and calcined to remove the MTEA template. The samples were ion-exchanged with an ammonium chloride solution and calcined again to obtain the zeolites in the acid form. The materials thus obtained were characterized by XRD, SEM, BET, TG and n-butylamine adsorption. The Si/Al ratio in the reaction mixture affects the amount of zeolite produced and the size of the particles. The XRD analysis indicated that the ZSM-12 zeolite crystallizes in a pure form only with Si/Al ratio above 33. The SEM analysis showed the presence of crystallites with very well defined prismatic shapes. The removal of the MTEA of the pores of the ZSM-12 by TG indicated that there are two kinds of internal sites occupied by MTEA inside the structure. The BET area of the ZSM-12 decreases proportionally with the crystallinity of materials. The desorption of n-butylamine showed that the acid site density is proportional to aluminum content, but the Si/Al ratio shows little influence on the relative strengths of these sites.     

Reactivity of M(II) metal-substituted derivatives of pig purple acid phosphatase (uteroferrin) with phosphate

The Fe(II) of the binuclear Fe(II)Fe(III) active site of pig purple acid phosphatase (uteroferrin) has been replaced in turn by five M(II) ions (Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)). An uptake of 1 equiv of M(II) is observed in all cases except that of Cu(II), when a second more loosely bound Cu(II) is removed by treatment with edta. The products have been characterized by different analytical procedures and by UV-vis spectrophotometry. At 25 degrees C, I = 0.100 M (NaCl), the nonenzymatic reactions with H(2)PO(4)(-) give the mu-phosphato product, and formation constants K/M(-1) show an 8-fold spread at pH 4.9 of 740 (Mn), 165 (Fe), 190 (Co), 90 (Ni), 800 (Cu), 380 (Zn). The variations in K correlate well with stability constants for the complexing of H(2)PO(4)(-) and (CH(3)O)HPO(3)(-) with M(II) hexaaqua ions. At pH 4.9 with [H(2)PO(4)(-)] > or = 3.5 mM rate constants k(obs) decrease, and an inhibition process in which a second [H(2)PO(4)(-)] coordinates to the dinuclear center is proposed. The mechanism considered accounts for most but not all of the features displayed. Thus K(1) values for the coordination of phosphate to M(II) are in the range10-60 M(-1), whereas K(2) values for the bridging of the phosphate to Fe(III) are in the narrower range 7.8-12.4. From the fits described K(i) approximately 10(3) M(-1) for the inhibition step, which is independent of the identity of M(II). Values of k(obs) decrease with increasing pH, giving pK(a) values which are close to 3.8 and independent of M(II) (Fe(II), Zn(II), Mn(II)). The acid dissociation process is assigned to Fe(III)-OH(2) to Fe(III)-OH(-), where OH(-) is less readily displaced by phosphate.     

Study of the Adsorption Properties of MCM-41 Molecular Sieves Prepared at Different Synthesis Times

The variation of surface properties of SiMCM-41 and AlMCM-41 nanoporous materials as function of synthesis time was examined. The main properties studied were: surface area, pore diameter, pore volume, mesoporous parameter, and wall thickness. Siliceous MCM-41 molecular sieves were synthesized starting from hydrogels with the following molar compositions: 4.58SiO₂:0.435Na₂O:1 CTMABr:200 H₂O for SiMCM-41, and 4.58SiO₂:0.485 Na₂O:1 CTMABr:0.038 Al₂O₃:200 H₂O, for AlMCM-41. Cetyltrimethylammonium bromide (CTMABr) was used as the structural template. The crystallographic parameters were obtained from XRD data and by nitrogen adsorption using the BET and BJH methods. The results obtained showed a significant variation of the surface properties of the MCM-41 materials as a function of the synthesis time reaching silica wall thickness of ca. 2 nm on the fourth day.     

Immobilized microorganisms in the reduction of ethyl benzoylacetate

The enantioselective reduction of ethyl benzoylacetate (EBA) into ethyl (S)-3-hydroxy-3-phenylpropanoate (S-HPPE) by nine yeast strains and three filamentous fungi strains is described. The conversion obtained was in the range 0-89% and the enantiomeric excess was 100% in many cases. Conversion levels were higher when the reduction was performed with microorganisms immobilized in calcium alginate and enantioselectivity remained excellent. Some reaction’s conditions of bioreduction by immobilized cells of Rhodotorula rubra were studied using a 2^5-2 fractional factorial design.