Fluorine anomalous behavior

As seen in Scheme 5.3, the trend of anomalous behavior of fluorine compared to other halogens with respect to its effect upon the chemical shift of hydrogens bound to the same carbon continues in the series of tri-halomethanes. It is seen that a CC13 group has a greater inductive effect on the chemical shifts of /1-hydrogens as well. 

In contemporary terms, one might also see the differences between fluorine and the other halogens as resulting from the phenomenon of first-member anomaly, whereby the uppermost element in the main-group elements shows anomalous behavior when compared with other group members. 

Next we focus on fluorinated cyclopropanes where we discuss, for example, the fact that 1,1-difluorocyclopropane and hexafluorocyclopropane show unusual behavior which in part or whole reflects extra strain. Contrariwise, cyclopropanes and other strained molecules substituted by perfluoroalkyl groups such as CF3 are anomalously stable despite having some slight extra strain. 

---