Ab initio and DFT modeling of stereoselective deamination of aziridines by nitrosyl chloride

The stereochemical course of the deamination of cis‐2,3‐dimethylaziridine by nitrosyl chloride was investigated at the QCISD/6‐31G(d) level. Calculations reveal that the reaction takes place in two steps. In the first step, the reactants form a pre‐reactive complex, followed by a spiro‐type bicyclic transition state, which on dissociative cycloelimination gives the N‐nitrosoaziridine intermediate. In the second step, this intermediate undergoes cycloreversion through a slightly asynchronous concerted transition state to form an alkene with the same stereochemistry, which is in total agreement with experiment. In the whole reaction, the denitrosation step is found to be rate‐determining. For comparison, geometry optimizations and energies were also obtained at the B3LYP/6‐31G(d) level. It was found that the B3LYP energy results differed significantly from the QCISD ones. To analyze the reason for this difference, B3LYP calculations were repeated by varying the contribution of exact exchange in the Becke functional. With respect to the QCISD results, it has been shown that the functional with 0% exact exchange yields the best activation barriers, whereas the functional with 30% exact exchange is the most suitable one to carry out the complexation and reaction energy calculations. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Ring cleavage of aziridines by difluoroamine: mechanistic insights from ab initio and DFT study

cis-2,3-Dimethylaziridine reacts with difluoroamine to give the corresponding alkene and nitrogen with retention of configuration. We have carried out a DFT study of this reaction to clarify the reaction mechanism by considering a multistep reaction pathway with possible intermediacy of several three- and four-membered cyclic intermediates and transition states (TSs). The energetics of this reaction shows that the reaction takes place in four steps including a three-membered azamine intermediate. Both the energetics and the stereochemical outcome of this reaction rule out the formation of a four-membered diazetine intermediate during the reaction. Although the first N-N bond formation step is rate determining, the final step, asynchronous concerted cleavage of the azamine intermediate, explains the stereochemistry of this reaction. The asynchronous nature of the final step makes the reaction Woodward-Hoffmann allowed, as reported by Yamabe and Minato (J. Phys. Chem. A 2001, 105, 7281). Computations at HF and MP2 levels confirm the same trends in energetics. Single point energy computations at B3LYP, MP2, and QCISD levels with the 6-311++G(d,p) and cc-pVTZ basis sets show that the larger basis sets predict higher free energies of activation and less negative free energies of reaction. Intrinsic reaction coordinate (IRC) analyses reveal the asynchronous nature of the first and the last steps of the reaction. The deamination of trans-2,3-dimethylaziridine was shown to follow a course of reaction similar to that of the cis isomer.     

Olive oil and its phenolic constituent tyrosol attenuates dioxin-induced toxicity in peripheral blood mononuclear cells via an antioxidant-dependent mechanism

Olive oil (OO) and its phenolic compounds are reported to possess many potential biological effects, which are ascribed to its powerful antioxidant property. In this study, we have assessed whether OO and its phenolic compound tyrosol (TY) could mitigate 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced oxidative damages in peripheral blood mononuclear cells (PBMC). The results showed that exposure of PBMC to 10 nM TCDD caused significant cell death and elevated cellular concentrations of reactive oxygen species and lipid peroxidation. Comet assay indicated that OO and TY protected DNA damage against dioxin toxicity. In addition, alterations in levels of antioxidant enzymes were substantially prevented by OO and TY. TCDD-induced CYP1A1 activity and loss of mitochondrial membrane potential were significantly reduced by the administration of OO and TY. The results suggested that dietary modifications incorporating diets rich in OO and associated phenolics could prove beneficial in protecting individuals against toxicity induced by dioxins.     

Estimation of239Pu in urine,influence of Sulkowich reagent

Plutonium is known to be co-precipitated with Sulkowich reagent as calcium ammonium oxalate. In adopting this technique for bio-assay of plutonium, its accuracy depends on the self-absorption of the resulting precipitate in each urine sample. Pu recovery experiments were carried out with varying concentration of Ca and Mg, using different volumes of Sulkowich reagent. When the sample volume is 500 ml, Pu in urine can be estimated with an accuracy and precision of 74.38%±7.4%; with a detection limit of 0.06 Bq (1.6 pCi) per dm³. An attempt was made to improve the detection limit, accuracy and precision values. The results obtained are presented in this paper.     

Ring opening of boriranes vis‐à‐vis aziridines: An ab initio and DFT probe on the mechanisms

Borirane undergoes ring opening reaction with NOCl and HNF₂ yielding the corresponding alkenes. Ab initio and density functional investigations of this reaction with cis‐ and trans‐2,3‐dimethylboriranes reveal that these reactions take place in a single step through the formation of a prereactive complex and a transition state giving the alkene with the same stereochemistry. Calculations clearly show that the concerted cleavage of C? B bonds leads to retention of stereochemistry. Further, it shows that HNF₂ cleaves boriranes more efficiently than does NOCl. Intrinsic reaction coordinate analyses and bond order analysis describe the nature of the transition state very well and fix the reaction mechanism. Solvent effect calculations through PCM model, with acetonitrile and CCl₄ as solvents, do not alter the gas phase results significantly. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Oxaphosphetane versus betaine formation in epoxide ring opening by PPh3: a mechanistic probe by ab initio and DFT modeling

Ab initio and density functional investigations on the deoxygenation of cis-2,3-dimethylepoxide by PPh₃ reveal a two-step mechanism. Simultaneous ring-opening and C-C bond rotation facilitates oxaphosphetane formation thereby ruling out the formation of a betaine intermediate and is consistent with the inversion of stereochemistry observed in the reaction.     

Some improvements in the estimation of137Cs in urine by the AMP-chlorostannate method

An accurate and reliable method has been developed for the estimation of radiocesium in urine. Initially cesium is adsorbed on ammonium phosphomolybdaste (AMP) precipitate and separated by ion exchange from other contaminants. Cesium thus separated is estimated as cesium chlorostannate, Cs₂SnCl₆, from a 50 (v/v)% solution of concentrated HCl in ethyl alcohol. While the results are in good agreement with the values obtained by γ-spectrometry using a Marinelli beaker in its range of applicability, the present method has a much lower detection limit. It is observed that the method has significant advantages over the methods available with respect to analysis time, accuracy and detection limits.     

A fast and simple method for the estimation of natural uranium in urine

Natural uranium (U) in urine was co-precipitated with Ca₃(PO₄)₂ and then with BiPO₄ after reducing uranyl ions from +6 to +4 state. U separated from BiPO₄ was then quantified by measuring its absorbance after complexing it with Arsenazo-III. The interference caused by ions such as Fe³⁺, Bi³⁺, Zn²⁺ and PO₄ ³⁻ was studied during the measurement of U absorbance. The percent recovery of U was 81.4±2.6 for 5 to 30 μg U present in 500 ml urine. The minimum detectable concentration of U was 2 μg/l and the time required to complete the analysis was 2 days.