“…The selection rules are ⌬J ϭ 0, Ϯ1; ⌬k ϭ Ϯ3; ⌬ ϭ 0. This method has been used to measure "forbidden" rotational energy differences in symmetric rotors without internal rotation, both for polar molecules such as PH 3 (7) and OPF 3 (8,9), and for non-polar systems such as BF 3 (10), PF 5 (11), and SO 3 (12). However, studies of internal rotation in symmetric rotors have not (to our knowledge) been previously published.…”