Group theoretical treatments found in the literature for other molecules, based on double-groups of appropriate permutation–inversion groups, are modifed slightly and applied to H2O2. This permits a more unified view of many theoretical results derived in various earlier studies of the molecule. Briefly, if no effects due to internal rotation tunneling were observed in the H2O2 spectrum, the molecule could be treated using the C2 point group of its equilibrium geometry. If effects due to internal rotation tunneling through only the trans barrier are observed (as is presently the case), the molecule should be treated using the C2h point group of its trans planar conformation at the top of the tunneling barrier. If effects due to internal rotation through both the trans and cis barriers can be detected, it will be necessary to treat the molecule using a double group [Formula: see text] of the C2h, point group. Vibrational, torsional, and rotational basis function symmetry species, energy level diagrams, selection rules for electric dipole transitions and perturbations, etc. are presented here for the [Formula: see text] double group, but these results can easily be converted to those for the C2h point group by dropping all s and d subscripts. An empirically discovered successful fitting procedure for the ground state rotational levels, reported in a treatment of microwave and millimetre wave measurements on H2O2, can be rationalized on the basis of the present theoretical results.

Hougen, J. T. (1984). Summary of group theoretical results for microwave and infrared studies of H2O2. Canadian Journal of Physics, 62(12), 1392–1402.