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1998 (1)
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1Author    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 
2Author    Michal Jaszuński, Antonio RizzoRequires cookie*
 Title    Ab initio Study of Nitrogen-14 Nuclear Quadrupole Coupling and NMR Signal Linewidths in Some Azoles  
 Abstract    Ab initio values of Nitrogen l4 N Nuclear Quadrupole Coupling Constants (NQCC's) are calcu-lated for a series of methyl-substituted azoles in the Multiconfigurational SCF (MCSCF) approx-imation. The four triazoles and two tetrazoles studied here are all isoelectronic. This enabled us to use the same level of approximation -basis set and active space -for all the molecules. The computed NQCC's are used to estimate the relative widths of the U N NMR signals, assuming an identical effect of molecular tumbling for all the nuclei in a molecule and neglecting solvent effects. The linewidths for the unsubstituted N atoms are, in agreement with experiment, much larger than for the methyl-substituted N atom. For the 1-methyl-tetrazole we present also the NMR shielding and spin-spin coupling constants and discuss in some more detail the dependence of all calculated NMR properties on the basis set and correlation effects. 
  Reference    Z. Naturforsch. 53a, 362—369 (1998); received December 31 1997 
  Published    1998 
  Keywords    14 N NQCC, Azoles, MCSCF, Ab initio, Electron Correlation 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0362.pdf 
 Identifier    ZNA-1998-53a-0362 
 Volume    53 
3Author    N. Sathyan, V. Santhanam, J. SobhanadriRequires cookie*
 Title    Conformational Analysis of Chloromethylenimine and its Hydrogen-Bonded Dimers with Water from the Study of Nuclear Quadrupole Interactions  
 Abstract    A molecular conformation study on N-Chloromethylenimine and its 1:1 dimeric form with water has been carried out using the ab-initio method at 6-31 G and 6-31 G* basis set. We consider the two most stable conformers of the N-chloromethylenimine -water binary mixture involving double hydrogen bonds. In all cases the proton affinity has been calculated. Each system considered in this work has the nuclear quadrupole interactions of the nitrogen and chlorine resonant nuclei which have been calculated and compared. It is found that the nuclear quadrupole coupling constant for the nitrogen nucleus increases in the hydrogen bonded complexes and decreases for the chlorine nucleus compared to the monomeric values. The influence of proton affinity is reflected in the nuclear quadrupole coupling constant. 
  Reference    Z. Naturforsch. 51a, 534—536 (1996); received November 20. 1995 
  Published    1996 
  Keywords    N-Chloromethylenimine, Water, Hydrogen bond Nuclear Quadrupole Coupling Con-stant, Proton Affinity, Ab-initio 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0534.pdf 
 Identifier    ZNA-1996-51a-0534 
 Volume    51