Spectroscopic signatures of bond-breaking internal rotation. II. Rotation-vibration level structure and quantum monodromy in HCP

The rotation-vibration level structure of ground electronic state HCP is in vestigated at vibrational energies approaching and exceeding that of the li near CPH saddle point. With respect to energies above the saddle point, we investigate possible spectroscopic manifestations of strong Coriolis intera ctions between the hindered, bond-breaking internal rotation of the hydroge n about the CP core and the rotation of the molecule in the space-fixed axi s system. With respect to energies below the saddle point, we provide new i nterpretations, from quantum and semiclassical points of view, of previousl y observed anomalously large B (rotational) and g(22) (energy dependence on the vibrational angular momentum) constants for the large-amplitude pure b ending states of HCP (referred to elsewhere as "isomerization" or saddle no de states). We also predict similar anomalies in other spectroscopic consta nts, including the "centrifugal distortion" constant D and the "rotational l-resonance" parameter q(2). These changes in the effective spectroscopic r otation-vibration constants are shown to be a direct consequence of the sph erical pendulum topology of the HCP bend/internal rotor system, which is as sociated with a phenomenon called quantum monodromy, defined as the absence of a smoothly valid set of quantum numbers for all states. Our semiempiric al model for the HCP bend/internal rotor mode is derived using principles o f semiclassical inversion and provides new insights into the breakdown in t he ability of rovibrational effective Hamiltonians to model highly vibratio nally excited states of HCP. (C) 2001 American Institute of Physics.