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 (Delta vp = 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 x 10(5))x(1)(4) - (2.99 x 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 degrees. 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 degrees. The coumaran barrier is considerably higher than that for 2,3-dihydrofuran (83 m(-1)), which does not have the attached benzene ring.