Dynamical tunneling in molecules: role of the classical resonances and chaos

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.