Determination of plutonium and americium in moss and lichen samples

A simple, sensitive and selective method is described for the simultaneous determination of plutonium and americium in lichen and moss samples which can be used as the atmospheric radioactivity bioindicators. Plutonium is separated from a HCl leaching solution by a Microthene-TNOA column; americium is separated by a KL-HDEHP column and purified by PMBP-TOPO extraction. A special attention has been paid to the decontamination of plutonium and americium from²¹⁰Po. Ten lichen and 12 moss samples from tree trunks have been analyzed: starting from 2 g sample, the average yields and the detection limits were 70.2±12.5% and 28 mBq/kg for plutonium and 70.0±15.1% and 34 mBq/kg for americium. The concentrations (mBq/kg) ranged from 28 to 4960 for^239,240Pu, from 28 to 171 for²³⁸Pu and from 34 to 1930 for²⁴¹Am, respectively.     

Selective Catalytic Reduction of Nitric Oxide by Ammonia over Ag and Zn‐Exchanged Cuban Natural Zeolites

The catalytic selective reduction of NO over metal‐exchanged (Zn^II, Ag^I) natural zeolites (mordenite and clinoptilolite) from Cuba using NH₃ as a reducing agent in the presence of excess oxygen was studied. Both transition metals slightly improve the catalytic performance for the NO reduction. Zn^II‐exchanged zeolites exhibit a moderate catalytic activity, with conversions of NO of ≈58 % and high selectivity to N₂ at high temperatures.     

Glucose–Nucleobase Pseudo Base Pairs: Biomolecular Interactions within DNA

Noncovalent forces rule the interactions between biomolecules. Inspired by a biomolecular interaction found in aminoglycoside–RNA recognition, glucose‐nucleobase pairs have been examined. Deoxyoligonucleotides with a 6‐deoxyglucose insertion are able to hybridize with their complementary strand, thus exhibiting a preference for purine nucleobases. Although the resulting double helices are less stable than natural ones, they present only minor local distortions. 6‐Deoxyglucose stays fully integrated in the double helix and its OH groups form two hydrogen bonds with the opposing guanine. This 6‐deoxyglucose‐guanine pair closely resembles a purine‐pyrimidine geometry. Quantum chemical calculations indicate that glucose‐purine pairs are as stable as a natural T‐A pair.     

Not Carbon s–p Hybridization,but Coordination Number Determines C−H and C−C Bond Length

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C−H and C−C bonds as the carbon atoms involved vary, in s–p hybridization, along sp³ to sp² to sp. Our quantum chemical bonding analyses based on Kohn–Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above-mentioned shortening in C−H and C−C bonds is not determined by an increasing amount of s-character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp³ to sp² to sp.     

One-electron reduction of 8-bromo-2-aminoadenosine in the aqueous phase: radiation chemical and DFT studies of the mechanism

Two tautomeric forms of one-electron oxidized 2-aminoadenosine (2AA) have been produced by reactions of hydrated electrons (e aq-) with 8-bromo-2-aminoadenosine (8-Br-2AA) at natural pH, whereas only one tautomer is formed by oxidation of 2AA. Tailored experiments by pulse radiolysis and time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations allowed the definition of the reaction mechanism in some detail. The electron adducts of 8-Br-2AA protonated at C8 eject Br- and produce the two short-lived tautomers (8 and 9). The first observable species decays by first-order kinetics to produce the second intermediate, which is also obtained by oxidation of 2AA by SO4*-. The rate of tautomerization (k taut = 4.5 x 104 s-1) is strongly accelerated by phosphate and is retarded in D2O (kinetic isotope effect 7). B1B95/6-31+G** calculations showed that the tautomerization is a water-assisted process. In acidic or basic solutions, the "instantaneous" formation of one-electron oxidized 2AA or its deprotonated forms has been produced by reactions of e aq- with 8-Br-2AA. gamma-Radiolysis of 8-Br-2AA in aqueous solutions followed by product studies led to the formation of 2AA as a single product.     

Watson-crick base pairs with thiocarbonyl groups: How sulfur changes the hydrogen bonds in DNA

We have theoretically analyzed mimics of Watson-Crick AT and GC base pairs in which N-H···O hydrogen bonds are replaced by N-H···S, using the generalized gradient approximation (GGA) of density functional theory at BP86/TZ2P level. The general effect of the above substitutions is an elongation and a slight weakening of the hydrogen bonds that hold together the base pairs. However, the precise effects depend on how many, and in particular, on which hydrogen bonds AT and GC are substituted.. Another purpose of this work is to clarify the relative importance of electrostatic attraction versus orbital interaction in the hydrogen bonds involved in the mimics, using a quantitative bond energy decomposition scheme. At variance with widespread believe, the orbital interaction component in these hydrogen bonds is found to contribute more than 40% of the attractive interactions and is thus of the same order of magnitude as the electrostatic component, which provides the remaining attraction.      

Enantiomeric specific determination of hexabromocyclododecane by liquid chromatography-quadrupole linear ion trap mass spectrometry in sediment samples

This paper describes the determination and quantitation of hexabromocyclododecane (HBCD) enantiomers by liquid chromatography-quadrupole linear ion trap mass spectrometry (LC-QqLIT-MS). The method is based on the use of a chiral chromatographic column Nucleodex beta-PM (200 mm x 4.0 mm, 5 microm), which allows a good separation between HBCD enantiomers [(+/-)alpha, (+/-)beta and (+/-)gamma] in less than 15 min and the detection is performed by a Q-Trap instrument. Linearity was checked between 0.05 and 25 injected ng. Limits of detection (LODs) were in the range of 0.3-1.5 pg, limits of quantification (LOQs) were between 1 and 6 pg, and both values are lower than those published in the literature applying LC-MS-MS methods. The method was applied to sediment samples collected along the Cinca River, a tributary of the Ebro River (northeast of Spain). Samples were extracted and purified following a pressurized liquid extraction method. LODs of the method were between 0.12 and 5.61 ng/g and LOQs, from 0.38 to 1.87 ng/g. Total HBCD levels in these sediments ranged from not detected to 2660 ng/g dry weight. Enantiomeric fractions (EFs) were calculated and compared with EF obtained from standard injections. It is important to note that calculated EFs were corrected using (2)H(18)-labeled HBCD standards, in order to compensate matrix effect. EFs obtained in sediment samples suggested a higher presence of (+)alpha-HBCD and (+)gamma-HBCD in technical mixture, this suggests that it is not a racemic mixture.     

Theoretical calculations of homogeneous catalysis in the gas phase: elimination kinetics of 2,2-dimethoxypropane in the presence of HCl,F3CCOOH and CH3COOH

The mechanisms of gas-phase elimination kinetics of 2,2-dimethoxypropane in the presence of hydrogen chloride, trifluoroacetic acid and acetic acid were studied using Moller Plesset, ab initio combined method Complete Basis Set (CBS)-QB3 and various density functional theory methods with 6-311G(d,p) and 6-311++G(d,p) basis sets. The M06-2X/6-311++G(d,p) method provided reasonable agreement with the experimental enthalpy and energy of activation. Formation of 2-methoxypropene and methanol products occurs through six-membered cyclic ring transition state (TS) structure. The TS was characterised by single imaginary frequency, and confirmed through intrinsic reaction coordinate (IRC) calculations. The IRC calculations suggest the development of a van der Waal complex between the 2, 2-dimethoxy propane and the acid catalyst, leading to the TS formation. The process of decomposition in the absence of the acid catalyst requires much higher temperature with an energy of activation above 200 kJ/mol. This fact appears to be a consequence of a four-membered cyclic TS-type of mechanism in the non-catalysed reaction. Structural parameters, analyses of natural bond orbital charges and bond orders of the acid-catalysed elimination reactions in this study suggest that the polarisation of the C–O bond, in the direction C^δ+—O^δ?, is rate-determining in the TS. These reactions are non-synchronous concerted polar in nature.