Molecular electronic structure and its relation to chemical shift in 13C NMR

Author(s)

Renan Vidal Viesser

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Química
• Defense date: 2017-09-06

Examining board members

Cláudio Francisco Tormena; Luciano Nassif Vidal; Ataualpa Albert Carmo Braga; Miguel Angel San Miguel Barrera; Igor Dias Jurberg

• Advisor: Cláudio Francisco Tormena

Abstract

The present thesis focuses on understanding how electronic effects can affect the 13C NMR chemical shift in two groups of organic molecules: 1) NH2- and NO2-substituted benzenes, and 2) cis- and trans-1,2-dihaloethene isomers. Halogen effects (from F to I) on the shielding of carbon nuclei were evaluated highlighting the heavy atom effect presents in bromine and iodine derivatives. Effects of electron-donating (NH2) and electron-withdrawing (NO2) groups on the 13C NMR chemical shift bonded to halogen were also investigated, as well as, the influence of substituent position (ortho, meta, and para). The 13C shielding tensor and its diamagnetic (dia), paramagnetic (para), and spin-orbit (SO) components were decomposed into NLMO (Natural Localized Molecular Orbitals) contributions and analyzed in terms of principal components (11, 22, and 33). These analyses allowed a deep investigation concerning the shielding/deshielding mechanisms and the identification of which orbitals were responsible for the experimental 13C data. The sigma framework of orbitals was responsible for the shielding/deshielding mechanisms caused by NH2 and NO2 groups instead of the pi orbitals, as suggested in the literature. The results for 1,2-dihaloethenes showed the importance of steric effects and hyperconjugative interactions on the magnitude of para and SO components. The steric hindrance between halogens of cis isomer affected the SO relativistic effect, taking part in the deshielding behavior of that isomer relative to trans isomer of iodine compounds. In summary, the results of this thesis showed that the classical association between electronic density and nuclear shielding is not simple, since the latter is a response property of magnetic interactions between occupied and unoccupied orbitals (AU)

• FAPESP's process: 12/12414-1 - Relationship between molecular electronic structure and heavy atom effect on 13C NMR chemical shift
• Grantee: Renan Vidal Viesser
• Support Opportunities: Scholarships in Brazil - Doctorate