- Molecular modelling of the isothiazolo[5,4-b]pyridin-3(2H)-one derivatives
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The performance of several semiempirical (MNDO, AM1, PM3, and SAM1) and ab initio (HF and MP2/6-31G*) methods for describing the structural and electronic features of a series of isothiazolopyridines, some of them bearing a hypervalent sulphur, is compared. Most of semiempirical methods calculate reasonable molecular structures, as compared with X-Ray structures, even in the case of S-oxides and S,S-dioxides. However, dipole moments are barely reproduced by these methods, even in the case of SAM1, which includes d orbitals. Hartree-Fock ab initio calculations do not lead to good dipole moment values in the case of S,S-dioxides. The agreement with experimental values is much better in the case of second order Moller-Plesset calculations, but this seems to be due to the systematic differences found between HF and MP2 values.
- Martinez-Merino, Victor,Garcia, Jose I.,Mayoral, Jose A.,Gil, Maria J.,Zabalza, Jose M.,Fayet, Jean P.,Vertut, Marie C.,Carpy, Alain,Gonzalez, Alberto
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- Nicosulfuron: Alcoholysis, chemical hydrolysis, and degradation on various minerals
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Alcoholysis (methanol or ethanol) and hydrolysis (pH ranging from 4 to 11) of the herbicide nicosulfuron at 30 °C principally involves the breakdown of the urea part of the molecule. A high yield of the corresponding carbarnate was obtained along with aminopyrimidine during alcoholysis. Hydrolysis led to both aminopyrimidine and pyridylsulfonamide. The latter compound may be easily cyclized (pH ≥ 7). First-order kinetics describe the rates of alcoholysis and hydrolysis well. The rate constants (0.44 days-1 for methanolysis) decreased from 0.50 to 0.002 days-1 as pH increased from 4 to 8, then remained stable under alkaline conditions. In acidic or neutral solution, the hydrolysis path appeared prevalent (≥70%), whereas in an alkaline medium it decreased when pH increased. The chemical degradation of nicosulfuron on various dry minerals (calcium bentonite, kaolinite, silica gel, H+ bentonite, montmorillonite K10, and alumina) was investigated at 30 °C. The best conditions for the degradation are obtained on acidic minerals after herbicide deposition using the liquid method. Under these conditions an acceptable correlation with pseudo-first-order kinetics was observed, and the major degradation path is similar to that proposed for chemical hydrolysis. Conversely, alumina seemed to favor other unknown degradation processes. The hydrolysis paths of nicosulfuron and rimsulfuron appeared to be different.
- Sabadie, Jean
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p. 526 - 531
(2007/10/03)
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