| | 1 | Author
| Peder Svensson, Nils-Äke Bergman, Per Ahlberg | Requires 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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| default:Reihe_A/44/ZNA-1989-44a-0473.pdf | | | | Identifier
| ZNA-1989-44a-0473 | | | | Volume
| 44 | |
| 2 | Author
| Michal Jaszuński, Antonio Rizzo | Requires 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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| default:Reihe_A/53/ZNA-1998-53a-0362.pdf | | | | Identifier
| ZNA-1998-53a-0362 | | | | Volume
| 53 | |
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