Vibrational Spectra and Structure of Coumaran and Its Ring-Puckering Potential Energy Functions in the S$_{0}$ and S$_{1}(\pi $,$\pi $*) Electronic States

The far-infrared (IR), jet-cooled fluorescence excitation (FES), single vibronic level fluorescence (SVLF), and ultraviolet (UV) absorption spectra of coumaran have been recorded and analyzed. The assignment of those spectra has allowed a detailed energy map of both the S$_{0}$ and S$_{1}(\pi $,$\pi $*) electronic states of the ring-puckering ($\nu _{45})$ vibration to be determined. A one-dimensional potential energy function for the ring-puckering vibration very nicely predicts the experimentally determined energy level spacings for both electronic states. In the S$_{0}$ ground state the barrier to planarity is 154 cm$^{-1}$ and the puckering dihedral angle is 25\r{ }. In the S$_{1}(\pi $,$\pi $*) excited state the corresponding values are 34 cm$^{-1}$ and 14\r{ }. The decreased barrier in the electronic excited state results from decreased angle strain in the five-membered ring.