  In this letter we study dynamical tunneling in highly excited symmetric
molecules. The role of classical phase space structures like resonances and
chaos on the tunneling splittings are illustrated using the water molecule as
an example. It is argued that the enhancements in the splittings due to
resonances (near-integrable phase space) and due to chaos (mixed phase space)
are best understood away from the fluctuations associated with avoided
crossings. In particular we provide an essential difference between the two
mechanisms in terms of high order perturbation theory. The analysis, apart from
testing the validity of a perturbative approach, suggests such systems as prime
candidates for studying dynamical tunneling.
