Rotation-vibration energy cluster formation in XH2D and XHD2 molecules (XÂ =Â Bi, P, and Sb)

We investigate theoretically the energy cluster formation in highly excited rotational states of several pyramidal XH2D and XHD2 molecules (X = Bi, P, and Sb) by calculating, in a variational approach, the rotational energy levels in the vibrational ground states of these species for J⩽70. We show that at high J the calculated energy levels of the di-deuterated species XHD2 exhibit distinct fourfold cluster patterns highly similar to those observed for H2X molecules. We conclude from eigenfunction analysis that in the energy cluster states, the XHD2 molecule rotates about a so-called localization axis which is approximately parallel to one of the X-D bonds. For the mono-deuterated XH2D isotopologues, the rotational spectra are found to have a simple rigid-rotor structure with twofold clusters.