Intermolecular bending states and tunneling splittings of water trimer from rigorous 9D quantum calculations: II. Characterization of the states

In a recent work we reported on the computation of intermolecular bending states in water trimer, a 9D problem when the monomers are treated as rigid and the intermonomer distances are fixed. In this paper we present the results of an effort to ascertain the vibrational character of those states. The large-amplitude, highly-coupled nature of these vibrations presents significant challenges in respect to their assignment. We introduce a new methodology, based on the use of symmetry coordinates for all nine of the torsional and librational degrees of freedom that constitute the intermolecular bending modes in the trimer, which addresses those challenges. The approach ultimately yields expressions for the bend eigenstates as expansions over small numbers of physically meaningful terms, from which the contributions of the torsional and librational excitations to a bending state can be identified and quantified. Results for approximately one hundred bending states with excitation energies below ∼ 560 cm−1 are presented.