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'Isotope effects' in keywords Facet   section ZfN Section A:Volume 044  [X]
Facet   Publication Year 1989  [X]
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1Author    L. Xie, W. H. SaundersRequires cookie*
 Title    Is  
 Abstract    o to p e E ffe c ts in K in e tic E n o la te F o r m a tio n fro m 2 -P e n ta n o n e a n d 2 -P e n t a n o n e -l , l , l -d 3 This paper is dedicated to Professor Jacob Bigeleisen on the occasion of his 70th birthday 2-Pentanone and 2-pentanone-l,l,l-d3 were treated with three-fold excesses of lithium diiso-propylamide (LDA) or lithium hexamethyldisilazide (LHMDS) in tetrahydrofuran (THF) with and without hexamethylphosphoric triamide (HMPA, 3 mol per mol of base) at temperatures ranging from 24 to — 70 °C. The deuterium kinetic isotope effects calculated from the product ratios (mea­ sured by GLC as trimethylsilyl enol ethers) showed a range of temperature dependences: none (LDA in THF), attenuated with A^jA^ = 2.53 (LHMDS in THF), and normal with /1H//1D~0.6 indicating moderate tunneling (LDA and LHMDS in THF-HMPA). The variation in temperature dependence is attributed to reaction via multiple base species in which HMPA affects the equilibria between the base species. 
  Reference    Z. Naturforsch. 44a, 413 (1989); received January 13 1989 
  Published    1989 
  Keywords    Isotope Effects, Kinetic Enolates, Temperature Dependence, Lithium Dialkylamide Bases 
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 TEI-XML for    default:Reihe_A/44/ZNA-1989-44a-0413.pdf 
 Identifier    ZNA-1989-44a-0413 
 Volume    44 
2Author    Peder Svensson, Nils-Äke Bergman, Per AhlbergRequires cookie*
 Title    Is A Theoretical Study using the Ab Initio STO -3G Potential Energy Surface  
 Abstract    o to p e E ffe c ts a n d C o n c e rte d T w o -H y d ro n -T r a n s f e r b e tw e e n F o rm a m id in e M o le c u le s Dedicated to Professor Jacob Bigeleisen on the occasion of his 70 th birthday With the intention to assist mechanistic studies of two-hydron-transfer reactions using lH/2H isotope effects we have carried out theoretical studies of the two-hydron-transfer reaction between formamidine molecules. The ab initio STO-3G potential energy surface has been calculated and all stationary points characterized by their force constant matrices. With this basis set the two hydrons are transferred concertedly and synchronously. The transition state (TS) has D2h symmetry. 'H/2H isotope effects were calculated using the Bebovib-IV program. Our results deviated from predictions made from common simple vibrational models. The isotope effects are dominated by the zero-point-energy contribution. The effect due to the stretching vibrations of the initial state is to a small extent counteracted by stretching of the TS. The bending vibrations appear to play a dominant role in reducing the magnitude of the isotope effect for the two-hydron-transfer reaction to a value expected for a one-hydron-transfer reaction. 
  Reference    Z. Naturforsch. 44a, 473—479 (1989); received May 18 1989 
  Published    1989 
  Keywords    Two-hydron-transfer, Formamidine, Ab initio, STO-3G, Isotope effects 
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 TEI-XML for    default:Reihe_A/44/ZNA-1989-44a-0473.pdf 
 Identifier    ZNA-1989-44a-0473 
 Volume    44