Distortion dipole effect in symmetric molecules

The theory of frame distortion in polyatomic molecules is reconsidered with the aid of a diagonal Herman-Wallis-type concept applied to nominally forbidden pure rotational transitions. Watson's formula for electric dipole forbidden transition moments is obtained using the formalism of extraneous quantum numbers [K. V. Kazakov, Quantum Theory of Anharmonic Effects in Molecules (Elsevier, Amsterdam, 2012)]. A similar consideration is permitted to derive the transition moments for magnetic dipole absorption in account of rovibrational interaction. The modified rotational selection rules and some numerical estimates are obtained by taking trial examples of highly symmetric molecules. The distortion electric dipole spectra of methane simulated for various temperatures are compared with the pertinent experimental data. Particular attention is given to trihydrium, for which rovibrational effects in both electric and magnetic dipole spectra are examined. It is shown that, first, the distortion magnetic moment of H3+ is generated solely by electronic motion if the g factor is diagonal, while the nuclear motion manifests itself through the off-diagonal inclusion of g, and, second, the smallness of this magnetic contribution to the line strength is quenched by the strong dependence on the rotational quantum number.