Fluorine is most preferred when the halogens are substituted on an alkyl or alkoxy group (e.g. CF3O and CF3CH2O).By ???oxo??? is meant the functional group ??????O???, such as, for example, (1) ???C???(O)???, that is a carbonyl group; (2) ???S???(O)???, that is, a sulfoxide group; and (3) ???N???(O)???, that is, an N-oxide group, such as pyridyl-N-oxide.Compounds described herein may contain one o