The Diels–Alder additions of formaldehyde, formaldehyde imine, (Z)-diazene, (E)-diazene and nitrosyl hydride to oxazole have been investigated by frontier molecular orbital theory and by ab initio methods. The former cannot account for steric and other electronic interactions in the transition states and is of limited value in predicting the reactivity of these dienophiles. The transition state structures of all possible isomers for the concerted cycloadditions were located at the restricted Hartree Fock 3-21G level and the energies evaluated by means of the second-order Møller–Plesset perturbation theory with the 6-31 G* basis set. The heteroatom lone pairs play a decisive role in directing the regioselectivity of these additions.

The capability and activation of thiophene as a diene for the Diels–Alder reactions has been investigated by ab initio methods. The reactants and transition structures are optimized with RHF/3-21 + G*, RHF/6-31 + G* and MP2/6-31 + G*. Energies are evaluated with the same level of theory and by single point calculations employing MP2/6-31 + G*//RHF/6-31 + G* and MP3/6-31 + G*// MP2/6-31 + G* methods. The ab initio calculated geometries are compared with experimental data, where available. The relative reactivity of buta-1,3-diene, thiophene and the S-methylthiophenium ion were estimated by comparing their FMO energy gap with ethene as the dienophile. The predicted activation energies are in full agreement with the qualitative determination of reactivity and suggest that thiophene is not a suitable diene for the Diels–Alder reaction. For the S-methylthiophenium ion, the predicted activation energy may be reached under normal reaction conditions. Thus S-methylthiophenium should be a suitable diene for the Diels–Alder reactions.

The kinetics and the mechanism of degradation of the mesotrione herbicide by three electrochemical advanced oxidation processes (EAOPs) systems, namely electro-Fenton (EF) with Pt anode (EF-Pt), anodic oxidation with BDD anode (AO) and EF-boron doped diamond anode (EF-BDD), were investigated in acidic aqueous solutions. The degradation of mesotrione obeyed apparent first-order reaction kinetics, and its absolute rate constant value with hydroxyl radicals at pH 3.0, determined by the competitive kinetics method, was found to be 8.20 × 108 L mol–1 s–1. Fourteen different mesotrione degradation products were separated and/or identified using HPLC / MSn analyses. A rationalized scheme was proposed for the reaction pathways of mesotrione degradation in the EF process. The mesotrione mineralization yield values ranged between about 58 and 97% for a 6-h electrolysis time, according to the type of EAOP system and the electrolysis current intensity value. The evolution of the toxicity of mesotrione aqueous solutions with electrolysis time (t) was investigated during treatment by the EF-Pt and EF-BDD systems. Toxicity was measured with the Microtox® method, based on the luminescence inhibition of marine bacteria Vibrio fischeri. The curves % inhibition vs. time were found to depend on the toxicity of formed mesotrione degradation products.

The aim of the study was to monitor volatile organic compounds (VOCs) in outdoor air in nine cities/towns across the Republic of N. Macedonia and the Republic of Kosovo for nine consecutive months, using the Radiello® passive and diffusive samplers, and gas chromatography-mass spectrometry (GC-MS). In parallel, employing static headspace gas chromatography-mass spectrometry (SHS-GC-MS) technique, VOCs from different types of commercially available gasoline and diesel fuels in the monitored cities were analyzed in order to obtain the chemical profiles and to evaluate the presence of their components in ambient air. GC-MS analyses indicate that the gasoline fuel components (BTEX: benzene, toluene, ethylbenzene, and xylenes) and diesel fuel components (n-undecane, n-dodecane, n-tridecane, n-tetradecane, and other linear alkanes) account for approximately 60% of VOCs in the outdoor air in the sampling sites. The bulk of the VOCs in the all-sampling locations regardless of the season come from fossil fuels (automotive fuels) and only minor fraction comes from biogenic emission which is dominated by monoterpenes. Furthermore, the ratios of benzene to toluene (B/T) and xylenes to ethylbenzene (X/E) were used to assess the origins of VOCs emissions in the outdoor urban air at the monitored sites.