E. Bondoc et al., Far-infrared and Raman spectra and vibrational potential energy surface for the ring-puckering and ring-flapping of coumaran, J PHYS CH A, 104(2), 2000, pp. 275-279
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 v
ibration in both the ground and-first excited states of the ring-flapping v
ibration. These were utilized to assign the ring-puckering quantum states f
or both flapping states, allowing both one- and two-dimensional potential e
nergy 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 flap
ping coordinates shows that there is extensive interaction between these tw
o vibrations. The barrier for the two-dimensional surface is 275 cm(-1), an
d the dihedral angle is 37 degrees. The coumaran barrier is considerably hi
gher than that for 2,3-dihydrofuran (83 m(-1)), which does not have the att
ached benzene ring.