US EPA

6586794 

Journal Article 

ORIGIN OF THE STABILITY OF CARBON TETRAFLUORIDE - NEGATIVE HYPERCONJUGATION REEXAMINED 

Wiberg, KB; Rablen, PR; , 

1993 

Yes 

Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 

AMER CHEMICAL SOC 

WASHINGTON 

115 

2 

614-625 

10.1021/ja00055a034 

WOS:A1993KJ68900034 

The energetic preference for multiple fluorine substitution at carbon has been examined theoretically. Both the stabilization and bond shortening with increasing fluorine substitution may be attributed to Coulombic interactions between the negatively charged fluorines and the increasingly more positively charged carbon. This conclusion leads to the prediction that multiple silyl substitution should also lead to stabilization, and it was confirmed by calculations. Conversely, FCH2SiH3, in which the carbon will be close to neutral because of the opposing electron demand of the substituents, has negligible stabilization. Multiple cyano substitution leads to destabilization, and this may be attributed to Coulombic interactions between the positively charged carbons of the cyano groups and the increasingly more positively charged central carbon. The same is found with multiple nitro substitution. Multiple chlorine substitution has little effect, in accord with the smaller difference in electronegativity between carbon and chlorine. The question of negative hyperconjugation in carbon tetrafluoride was explored by the calculation of delocalization indices for the fluorine lone pairs, and no significant interactions were found. Deformation density plots also were examined and showed that increasing fluorine substitution led to reduction in charge density only at the backside of the C-F bonds, as expected for electron polarization due to the increasing positive charge at carbon. A case in which negative hyperconjugation is more likely to be important ((fluoromethyl)amine) also was examined, and some evidence for nitrogen lone pair donation was found.