Matrix isolation infrared studies of complexes formed between substituted phenols and trimethylamine

Infrared spectroscopy and matrix isolation technique have been used to study the 1 : 1 complexes formed between 2,4,5-trichlorophenol (TCP), pentachlorophenol (PCP) or 2-chloro-4,6-dinitrophenol (CNP) and trimethylamine (TMA) isolated in solid argon. The results were analyzed in relation to the type of complex formed. Depending on the proton-donor ability of the phenol three different types of hydrogen bonded complexes have been identified in argon matrices. The weakest phenol in the series, TCP (pK_a = 6.72), forms a strong molecular hydrogen bonded complex with TMA as indicated by the broad ν(OHN) absorption with a maximum at 2490 cm⁻¹ and a band at 811 cm⁻¹ due to the ν_s(C₃N) mode of the perturbed amine. The strongest phenol, CNP (pK_a = 2.01), interacts with TMA in an argon matrix to form ionic complex with the proton transferred to the base molecule. This is evidenced by the presence of the ν(NH⁺---O⁻) absorption between 3000−1800 cm⁻¹, by the ν_as(C₃N⁺) and ν_s(C₃N⁺) absorptions due to the protonated amine and by numerous product bands due to the relatively strongly perturbed modes of the phenol ring. The interaction between TMA and a phenol of intermediate strength, PCP (pK_a = 4.74), in solid argon probably leads to the formation of two types of hydrogen bonded complexes: an ionic complex with the proton transferred to the amine molecule and a pseudosymmetric one with the proton more or less equally shared between the phenol and amine molecules. In this case the protonic absorption consists of two broad features situated in the 3000–1600 cm⁻¹ and 950–400 cm⁻¹ regions due to the ν(NH⁺O⁻) and ν(OHN) modes, respectively.     

An ab initio analysis of the electronic structure and harmonic frequencies of nickel porphyrin

Ab initio calculations are performed to understand the geometry, electronic structure, and vibrational frequencies of nickel porphyrin (NiP). Hartree-Fock (HF) and second-order perturbation (MP2) theories are applied with polarized basis sets. The calculated geometrical parameters are in very good agreement with the crystal structure determination. The electronic structure and bonding are analyzed in terms of complexation and correlation effects. Not unexpectedly, the HF depiction of the metal-porphyrin interaction is rather ionic while ligand σ donation is dominant at the MP2 level. Scaled HF frequencies of NiP and its isotopomers are in very good agreement with observed infrared and resonance Raman data. Received: 7 January 1997 / Accepted: 6 May 1997     

Synthesis of new polycyclic systems containing the pyrimido[2,1-a]phthalazine skeleton

The synthesis of new polyheterocycles containing the pyrimido[2,1-a]phthalazine system 5, 6 and 7 , obtained by condensation of phthalic anhydride with the appropriate hydrazides 2, 3 and 4 , respectively, are reported. It was also synthesized the 14H-[1]Benzothieno[3′,2′:4,5]pyrimido[2,1-a][1,2,4]triazolo[4,3-c]phthalazin-14-one (16).     

Regioselectivity in SNH reactions of some 3-nitro-1,8-naphthyridines with chloromethyl phenyl sulfone

A number of 2-X-3-nitro-1,8-naphthyridines (X= H,D,OH,Cl,NH₂, OEt) react with the anion of chloromethyl phenyl sulfone exclusively into 2-X-3-nitro-4-(phenylsulfonylmethyl)-1,8-naphthyridines in high yield. The reaction is found by quantum chemical calculations to be controlled by the interactions of the HOMO of the nucleophile with the LUMO of the substrate, and not by charge.     

Titanosilicates as Supports for an Enzymatic Alcoholysis Reaction

Novel [Cr^III(amp)(bipy)(Cl)] (1) (H₂amp = N-(hydroxyphenyl)salicyldimine; bipy = 2,2-bipyridyl) and [Cr^III(app)(bipy)(Cl)]⁺ (2) (H₂app = N-(hydroxyphenyl)pyridine-2-carboxaldimine; bipy = 2,2-bipyridyl) complexes have been synthesized and characterized by physico-chemical methods. Complexes 1 and 2 have been employed as catalysts in the oxidation of both saturated and unsaturated hydrocarbons using tert-butylhydroperoxide (t-BuOOH). The significance of the results with respect to oxo-functionalization of C-H bonds both in unsaturated and saturated hydrocarbons is noted.     

Genomic clustering of the Trypanosoma cruzi nonlong terminal L1Tc retrotransposon with defined interspersed repeated DNA elements

We have analyzed the genomic distribution and organization of the long interspersed nucleotide element (LINE) L1Tc, a nonlong terminal repeat (LTR) retrotransposon of Trypanosoma cruzi. The results indicate that the L1Tc element is dispersed along the parasite genome and that in some regions it is organized in tandem repeats. The data allowed us to define the existence of short direct-repeated sequences flanking the genomic L1Tc elements. Relevant is the finding that the LINE L1Tc is located in genomic regions rich in short interspersed nucleotide elements (SINE)-like sequences. In particular, the L1Tc element is found associated to E13-related sequences, redefined in this work and renamed RS13Tc, and to a newly described RS1Tc sequence. The RS1Tc sequence is present, per haploid genome, in about 3,200 copies. Northern blot analysis showed that the RS1Tc is being transcribed into RNAs of different sizes. The analysis of the chromosomal distribution of these elements in various strains of T. cruzi suggested that this type of clustering might be a common feature of the genome of these parasites.     

Effect of the oxygen transfer coefficient on xylitol production from sugarcane bagasse hydrolysate by continuous stirred-tank reactor fermentation

The effect of the oxygen transfer coefficient on the production of xylitol by biocon version of xylose present in sugarcane bagasse hemicellulosic hydrolysate using the yeast Candiada guilliermondii was investigated. Continuous cultivation was carried out in a 1.25-L fermentor at 30°C, pH 5.5, 300 rpm, and a dilution rate of 0.03/h, using oxygen transfer coefficients of 10,20, and 30/h. The results showed that the microbial xylitol production (11 g/L) increased by 108% with the decrease in the oxygen volumetric transfer coefficient from 30 to 20/h. The maximum values of xylitol productivity (0.7g/[L…h]) and yield (0.58 g/g) were obtained at k _L a 20/h.     

Sol-Gel Nanosized Semiconducting Titania-Based Powders for Thick-Film Gas Sensors

Pure, 5 at%, and 10 at% Ta- or Nb-doped TiO₂ nanosized powders were prepared by the sol-gel method. The powders heated to 400°C have the crystalline anatase structure. While the pure TiO₂ powder heated to 850°C has the rutile structure, the addition of Ta and Nb inhibited the anatase-to-rutile phase transformation at this temperature. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired at 650°C and 850°C for 1 h. SEM observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO₂. The addition of Ta and Nb is effective to keep the TiO₂ grain size at the nanometric level even at 850°C. Conductance measurements showed that a good gas response is observed only for the nanostructured titania-based films. The CO response of these materials is only slightly affected by humidity.     

Non-linear dielectric effect in the acetone-benzene and acetophenone-benzene systems

The non-linear dielectric effect has been studied for acetone and acetophenone, and their solutions in benzene. With the NDE results obtained in this work and the dielectric polarization calculated from literature values of the permittivity, density and refractive index, the mean values of the square and the fourth power of the dipole moment were calculated. Assuming dipolar association of the acetone and acetophenone molecules to be restricted to dimerization, the Gibbs free energy ΔG^o of the dimerization process is determined.