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Measurement and calculation of 13C and 15N NMR chemical-shift tensors of a push-pull ethylene

Amini, S. K ; Sharif University of Technology | 2009

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  1. Type of Document: Article
  2. DOI: 10.1139/V09-018
  3. Publisher: 2009
  4. Abstract:
  5. Methyl 3-dimethylamino-2-cyanocrotonate (MDACC) has a remarkably weak carbon-carbon double bond. It has strong electron-withdrawing groups on one end and electron-donating groups on the other: a so-called push-pull ethylene. To investigate this unusual electronic structure, we have determined the crystal structure and measured both the 13C and 15N NMR chemical-shift tensors. These measurements are supplemented by shielding-tensor calculations done with density functional methods. The large difference (approximately 100 ppm) between isotropic chemical shifts of the two alkenyl carbons reflects a large charge release from the electron-donating side of C=C double bond to the electron-withdrawing groups. Comparison of the calculated orientations of the principal components of the alkenyl carbons obtained from ab initio calculations shows that the primary changes in charge density occur in the molecular plane. On the other hand, smaller charge density changes above and below the plane of the C=C double bond establish the conjugation of donor and acceptor groups with π and π molecular orbitals of the central double bond, respectively, which lowers the barrier to rotation about this bond
  6. Keywords:
  7. Crystal structure ; CSA ; Density functional methods ; NMR ; Push-pull ethylenes ; Shielding anisotropy ; Ab initio calculations ; Alkenyl ; C-C double bonds ; Carbon-carbon double bonds ; Charge release ; CSA ; Density change ; Density functional methods ; Donor and acceptor ; Double bonds ; Electron withdrawing group ; Electron-donating ; Electron-donating group ; Isotropic chemical shifts ; Molecular planes ; NMR ; Principal Components ; Push-pull ethylenes ; Shift tensors ; Anisotropy ; Chemical bonds ; Chemical shift ; Density functional theory ; Electronic structure ; Electrons ; Ethylene ; Magnetic anisotropy ; Molecular orbitals ; Nuclear magnetic resonance ; Shielding ; Tensors ; Crystal structure
  8. Source: Canadian Journal of Chemistry ; Volume 87, Issue 4 , 2009 , Pages 563-570 ; 00084042 (ISSN)
  9. URL: https://www.nrcresearchpress.com/doi/abs/10.1139/V09-018?mobileUi=0#.XjgDA4ihW1s