Counterintuitive Deshielding On The C-13 Nmr Chemical Shift For The Trifluoromethyl Anion

The trifluoromethyl anion (CF3-) displays C-13 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 C-13 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.