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11.
The fragmentation patterns of 3-methyl-2-butyl trifluoroacetate and 2-methyl-2-butyl trifluoroacetate were investigated by GC/MS/MS, with electron impact and collision-induced dissociation, on regular and isotope-labeled (deuterium and 18O) esters. The atoms found in various fragments could be traced back to specific positions in the parent molecules. In this way, molecular rearrangements potentially occurring during the formation of esters by trifluoroacetolysis of 3-methyl-2-butyl p-toluenesulfonate or trifluoroacetic acid addition to various 2-methylbutenes could be shown. Rearrangements also occurred during the fragmentation, particularly during the expulsion of the small fragments CO, C2H4 and F2CO. For the decompositions of oxygen-containing ions these rearrangements were highly specific. By contrast, alkyl cations lead to fragments that are fully scrambled (statistical label distribution). Alkene radical cations ([CnH2n]+ ˙) fragment to daughter ions that are extensively, but less than statistically scrambled. Hydrogen scrambling may also occur in fluoroalkyl cation fragments. 相似文献
12.
The magnetic susceptibilities of cobaltocene and chromocene have been measured between 83 and 293 K; for cobaltocene the results suggest an appreciable orthorhombic splitting of the 2∏(σ2πδ4 ground state, whilst for chromocene the data provide further support for the 3Δ(σδ3) ground state previously deduced. 相似文献
13.
14.
Wiseman G 《Journal of AOAC International》2002,85(3):792-796
Consequential to the implementation of European Commission (EC) Regulation 1139/98, EC Regulation 49/2000, and EC Regulation 50/2000 has been the need to measure accurately the levels of the genetically modified (GM) species Roundup Ready Soya and Bt 176 Maize that are present in food. Analytical methods to detect and quantitate these transgenic species have received much attention particularly with respect to the deminimus threshold of 1% for their presence in materials derived from non-GM identity-preserved (IP) supplies. The relative advantages and limitations of threshold analysis by double-competitive polymerase chain reaction (PCR) and quantitative real-time PCR are discussed in their application to the quantitative analysis of processed foods. Consideration is also given to other factors involved in the analyses that affect the performance of quantitative procedures, and to the many uncertainties involved in the precision of a reported analytical result. 相似文献
15.
Edie M. Rasmussen Peter Willett Terence Wilson Gordon A. Manson George A. Wilson 《Analytica chimica acta》1990
The use of two types of parallel computer hardware for increasing the efficiency of processing in chemical structure data bases is discussed. The distributed array processor can be used for the clustering of 2-D chemical structure data bases by using the Jarvis—Patrick clustering method and for the ranking of output in an experimental system for substructure searching in the 3-D macromolecules in the Protein Data Bank. The Inmos transputer can be used in the construction of PC-based systems for 2-D substructure searching and in the identification of the maximal substructures common to pairs of 3-D molecules. 相似文献
16.
Asymmetric induction was achieved in the photoaddition of 3-methyl-2-cyclohexenone to optically active enoates 1a–c and the absolute configuration of the major adduct was determined. 相似文献
17.
The effect of mixing (or “splicing”) extended and minimal basis sets on molecular properties such as geometries, Mulliken charges, dipoles, and internal rotation barriers was studied for several test molecules. The effect is gauged by comparison with full extended basis set calculations. It is found that splicing improves most properties relative to full minimal basis set calculations, and little accuracy is lost if the splicing is done in a judicious manner. 相似文献
18.
Abstract— Chlorophyll, pheophytin and bacteriochlorophyll sensitize a one-electron transfer in the presence of quinones in ethanol, to produce a ternary complex of ground state porphyrin analog, alcohol cation radical and semiquinone anion radical as the primary photo-product at low temperature. A similar photo-oxidation of alcohol to produce a binary complex is caused by direct excitation of quin-one in the absence of chlorophyll. The mechanisms of these reactions and their implications for photosynthesis are discussed. 相似文献
19.
Regioselective ortho lithiation of 2-, 3-, and 4-halopyridines is achieved with lithium diisopropylamide (?78°, tetrahydrofuran) to afford, upon quenching with electrophilic reagents, 2,3- and 3,4-disubstituted pyridines in good to excellent yield. 相似文献
20.
Photocycloadditlon of (+)-isoplperltenone and cyclobutene-1-carboxylic acid gives an adduct which upon reduction with NaCNBH3 followed by thermolysis yields an isomer of isoaristolactone in an overall yield of 26%. 相似文献