Far-Infrared and Raman Spectra and Vibrational Potential Energy Surface for the Ring-Puckering and Ring-Flapping of Coumaran

The far-infrared and Raman spectra of coumaran vapor have been recorded and analyzed. Single, double, and triple quantum jump transitions (Δ v P = 1, 2, 3) were observed in the 60−270 cm -1 region for the ring-puckering vibration in both the ground and first excited states of the ring-flapping vibration. These were utilized to assign the ring-puckering quantum states for both flapping states, allowing both one- and two-dimensional potential energy functions to be determined. The one-dimensional function in terms of the ring-puckering coordinate ( x 1 ) has the form V (cm -1 ) = (7.92 × 10 5 ) x 1 4 − (2.99 × 10 4 ) x 1 2 and has a barrier to planarity of 279 cm -1 (3.32 kJ/mol) with energy minima corresponding to dihedral angles of ±30°. The two-dimensional surface in terms of the puckering and flapping coordinates shows that there is extensive interaction between these two vibrations. The barrier for the two-dimensional surface is 275 cm -1, and the dihedral angle is 37°. The coumaran barrier is considerably higher than that for 2,3-dihydrofuran (83 cm -1 ), which does not have the attached benzene ring.