The Possible Microwave Absorption in the Molecules belonging to the Point Groups D2d ≡ Vd and Td

It is shown that pure rotational absorption in the molecules belonging to the point group D2d ≡ Vd is possible if the molecule is in the excited, preferably lowest, doubly degenerate vibrational state. The approximate positions of these microwave absorption lines, as calculated from the B value obtained from the infrared data, are given in the case of allene. It is also shown that the pure rotational absorption is possible in the molecules belonging to the point group Td if they are in the excited triply degenerate (F2) vibrational states. The matrix element of dipole moment corresponding to our transition is about the same as that of the overtone vibrational bands, in both cases.