Counterintuitive deshielding on the 13C NMR chemical shift for the trifluoromethyl anion

The trifluoromethyl anion (CF3−) displays 13C NMR chemical shift (175.0 ppm) surprisingly larger than neutral (CHF3, 122.2 ppm) and cation (CF3+, 150.7 ppm) compounds. This unexpected deshielding effect for a carbanion is investigated by density functional theory calculations and decomposition analy...

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Bibliographic Details
Published inMagnetic resonance in chemistry Vol. 58; no. 6; pp. 540 - 547
Main Authors Viesser, Renan V., Tormena, Cláudio F.
Format Journal Article
LanguageEnglish
Published Bognor Regis Wiley Subscription Services, Inc 01.06.2020
Subjects
13C
Anions
carbanion
Carbon
Chemical equilibrium
chemical shift
Density functional theory
DFT
Electron density
Fluorine
Molecular orbitals
NMR
Nuclear magnetic resonance
Shielding
Tensors
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Summary:The trifluoromethyl anion (CF3−) displays 13C NMR chemical shift (175.0 ppm) surprisingly larger than neutral (CHF3, 122.2 ppm) and cation (CF3+, 150.7 ppm) compounds. This unexpected deshielding effect for a carbanion is investigated by density functional theory calculations and decomposition analyses of the 13C shielding tensor into localized molecular orbital contributions. The present work determines the shielding mechanisms involved in the observed behaviour of the fluorinated anion species, shedding light on the experimental NMR data and demystify the classical correlation between electron density and NMR chemical shift. The presence of fluorine atoms induces the carbon lone pair to create a paramagnetic shielding on the carbon nucleus. The 13C NMR chemical shift in CF3- is larger than in CHF3 and CF3+ compounds. Fluorine promotes a magnetic interaction with the carbon lone pair causing a deshielding effect in the carbon nucleus of CF3-.
ISSN:0749-1581
1097-458X
DOI:10.1002/mrc.4958