A theoretical study on the molecular and electronic structures and the enthalpies of formation of several polychlorinated dibenzo-p-dioxins, PCDDs, has been carried out using DFT at the B3LYP/6-31G(d) and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) levels of theory. The calculated molecular structures are planar in all of the studied PCDDs, in agreement with the available X-ray diffraction data. NBO charges, and HOMO and LUMO energies have also been obtained and discussed. The enthalpies of formation have been calculated through isodesmic reactions, and the values obtained have been compared with the few available experimental values and with values estimated by other authors using semiempirical methods or group additivity approaches.

Theoretical studies on the thermolysis in the gas phase of ethyl methyl and diethyl carbonates were carried out using ab initio theoretical methods at the MP2/6-31G(d) and MP2/6-311þþG(2d,p) levels of theory. Two possible pathways were studied, one of them via a six-membered cyclic transition state and the other one in two steps, via a six-and a four-membered cyclic transition state, respectively, with the formation of an alcohol, ethylene and carbon dioxide. The nucleophilic substitution to give ethers was also studied. The results indicate that the reactions occur via a mechanism in two steps. The progress of the reactions was followed by means of the Wiberg bond indices. The kinetic parameters calculated for the studied reactions agree well with the available experimental results.

ABSTRACT:The gas-phase thermal decomposition of 3-hydroxypropionitrile, 3-hydroxybutyronitrile, and 3-hydroxy-3-methylbutyronitrile has been studied at the MP2/6-31G(d) level of theory at 683.15 K and 0.06 atm. Results based both in energy and structure data seem to indicate a favorable route of decomposition via a six-membered cyclic transition state (similar to those suggested for thermal decomposition of other related compounds, such as ␤-hydroxyketones, ␤-hydroxyalkenes, and ␤-hydroxyalkynes) rather than a four-membered cyclic transition state or even a quasiheterolytic pathway. Wiley Periodicals, Inc. Int J Quantum Chem 91: 618 -625, 2003

BackgroundProtozoan parasites such as Giardia duodenalis, Cryptosporidium spp., Cyclospora cayetanensis, Toxoplasma gondii and Entamoeba histolytica represent a great challenge to the systems producing water for human consumption because their cystic forms are persistent in the environment and resist to the disinfection methods conventionally used for their control. In this study, we investigated the presence of these protozoan pathogens in both raw and treated water samples used for the production of drinking water in Nariño Department, southwest Colombia. We collected 110 water samples (10 lof each sample) and analyzed them with real-time PCR (qPCR). qPCR-positive samples were genotyped with PCR and DNA sequencing.ResultsGiardia duodenalis was detected in 35/110 (31.8%) of the samples and Cryptosporidium spp. in 9/110 (8.2%) of the samples; no sample was positive for T. gondii, E. histolytica or C. cayetanensis. Giardia duodenalis was detected in samples of both raw water (Drinking Water Treatment Plants (DWTP): 47.83%;Drinking Water Rural Plants (DWRP): 18.42%) and water collected either after conventional physicochemical treatment (26.09%) or after disinfection by chlorine (50%), whereas Cryptosporidium spp. were only detected in raw waters (DWTP: 17.39%; DWRP: 13.16%). The two pathogens were detected in both types of treatment plants supplying water to urban areas and to rural zones. Analysis of gdh and tpi markers identified assemblages AI, AII and H of G. duodenalis, while analysis of the small subunit rRNA and gp60 markers of Cryptosporidium-positive samples identified C. parvum (Subtype IIcA5G3c), C. galli, C. molnari, Cryptosporidium sp. genotype II of bats and Cryptosporidium sp. genotype VIII of birds.ConclusionsThe results obtained demonstrate the presence of protozoan parasites in the water of the study region, and the need to improve the surveillance systems for these pathogens and identify the corresponding sources of contamination.Electronic supplementary materialThe online version of this article (10.1186/s13071-018-3147-3) contains supplementary material, which is available to authorized users.