Three C2 axes containing each B-F bond lie in the plane of the molecule
perpendicular to the three-fold axis. The rotation axis of highest order (i.e., C3) is called the
principal axis of rotation. When the principal Cn axis has n even, then it contains a C2 operation
associated with this axis. Perpendicular C2 axes and their associated operations must be denoted
with prime and double prime superscripts.