Steric deshielding of fluorine

As indicated by the two 4-t-butyl-l-trifluoromethylcyclohexanes in Scheme 5.2, a CF3 group in the equatorial position is more greatly shielded than that in an axial position. This may be another example of what is known as steric deshielding of fluorine atoms. 

Notice that the CF2 fluorines of the Z isomer of 4,4-difluoro-2-butene are significantly deshielded relative to those of the E isomer. This is another probable example of steric deshielding by a proximate, in this case cis-alkyl group (see Chapter 2, Section 2.2.1). 

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