The electronic effects of substituents at the acyl carbon in the gas-phase elimination kinetics of tert-butyl α-substituted acetates

The gas-phase eliminations of several tert-butyl esters, in a static system and in vessels seasoned with allyl bromide, have been studied in the temperature range of 171.5–280.1°C and the pressure range of 23–98 torr. The rate coefficients for the homogeneous unimolecular elimination of these esters are given by the following Arrhenius equations: for tert-butyl pivalate, log k₁(s^?1) = (13.44 ± 0.30) ? (169.1 ± 3.1) kJ · mol^?1 (2.303RT)^?1; for tert-butyl trichloroacetate, log k₁(s^?1) = (12.41 ± 0.08) ? (141.1 ± 0.7) kJ · mol^?1 (2.303RT)^?1; and for tert-butyl cyanoacetate log k₁(s^?1) = (11.31 ± 0.44) ? (137.8 ± 4.1) kJ · mol^?1 (2.303RT)^?1. The data of this work together with those reported in the literature yield a good linear relationship when plotting log k/k₀ vs. σ* values (ρ* = 0.635, correlation coefficient r = 0.972, and intercept = 0.048 at 250°C). The positive ρ* value suggests that the movement of negative charge to the acyl carbon in the transition state is rate determining. The present results along with previous investigations ratify the generalization that electron-withdrawing substituents at the acyl side of ethyl, isopropyl, and tert-butyl esters enhance the elimination rates, while electron-releasing groups tend to reduce them. The negative nature of the acyl carbon and the polarity in the transition state increases slightly from primary to tertiary esters.     

CI calculations concerning the styrene cis-trans photoisomerization

Using minimal basis sets, the ionic and diradical excited states of the twisted styrene molecule have been calculated through extended CI, after appropriate SCF procedures. The lowest excited state is a diradical, and the photoisomerization should not involve the ionic state.     

Ionic liquids in vacuo; solution-phase X-ray photoelectron spectroscopy

The in situ monitoring of catalysis in Room Temperature Ionic Liquids (RTILs) is fundamental to the understanding of catalytic processes and the role of RTILs in catalytic turnover; we describe how XPS can be used to give information on both pure RTILs and catalytically-active RTIL-based solutions.     

Bioactive eremophilanolides from Senecio poepigii

A new bioactive eremophilanolide, 1alpha-tigloyloxy-8betaH,10betaH-eremophil-7(11)-en-8alpha,12-olide, was isolated from Senecio poepigii and its structure was elucidated by spectral analysis. 1alpha-Angeloyloxy-8beta-methoxy-10betaH-eremophil-7(11)-en-8alpha,12-olide was also isolated. Antifungal and insect antifeedant properties were evaluated.     

Stability of steady states of the Cauchy problem for the exponential reaction-diffusion equation

We consider the Cauchy problem

     

Isotope dilution GC-MS routine method for the determination of butyltin compounds in water

A standard GC-MS instrument with electron impact ionisation has been used to develop a fast, simple and reliable method for the simultaneous determination of monobutyltin (MBT), dibutyltin (DBT) and tributyltin (TBT) in water samples. Isotope dilution analysis (IDA) is used for the determination of species, taking advantage of a commercially available spike solution containing a mixture of MBT, DBT and TBT enriched in ¹¹⁹Sn. Method detection limits for 100-mL samples were between 0.18 and 0.25 ng L⁻¹ for the three butyltin compounds with typical RSD between 2 and 4% at levels between 100 and 10 ng L⁻¹, respectively. Recovery of tin species in spiked samples (natural water, wastewater and seawater) was quantitative. The stability of butyltin compounds in collected seawater samples was also studied. The addition of a 1% (v/v) glacial acetic acid preserved tin species in the samples for at least 5 days at room temperature. The IDA method was finally implemented in a routine testing laboratory and it was subsequently accredited by the Spanish National Accreditation Body according to the requirements of UNE-EN ISO/IEC 17025.     

A highly enantioselective abiotic receptor for malate dianion in aqueous solution

The highly enantioselective molecular recognition of the malate dianion by a synthetic receptor in aqueous solution has been studied by potentiometric titrations, mass spectrometry (ESI-MS), diffusion measurements (PGSE NMR) and molecular modeling.     

Highly diastereoselective amplification from a dynamic combinatorial library of macrocyclic oligoimines

Cadmium promoted diastereoselective amplification of a single member from a dynamic combinatorial library of stereoisomeric oligoimines of different sizes allows the efficient preparation of a new macrocyclic polyamine.