Knowledge Bank

The lowest electronic transitions of tetrahydropyran (THP), 1,4-dioxane, and 1,4-dioxane-$d8$ cooled in a free jet expansion have been examined by $2+1$ REMPI spectroscopy. The transition in dioxane is not allowed in one photon and in fact has not been reported previously. These $(n->3s)$ spectra, which consist primarily of Q-branches, are extremely sharp and intense. The absorption intensity ration for circularly and linearly polarized light is less than 0.2 which indicates that the transition is between states of the same symmetry, $A\prime ->A\prime$ in the case of THP, and $A->A$ for the dioxanes. These assignments were made with the aid of ab initio molecular orbital calculations. Additionally, the Hartree Fock calculations show that the 3s Rydberg orbital is primarily comprised of 3s atomic orbitals from all the carbon atoms in THP and dioxane. Several of the low frequency ring vibrations in the Rydberg spectra of THP and dioxane have been assigned. It is proposed that the relaxation of the C-O-C ether bond is the major consequence of electronic excitation.