Knowledge Bank

The structure of coumaran (2,3-dihydrobenzofuran) is characterized by a non-planar skeleton, due to the position of the most far apart carbon in the dihydrofuran ring. We have measured 133 a, b and c type lines of the rotational spectrum in the millimeter-wave region (59-78GHz), characterized by a doubling typical of an inversion motion such as the ring-puckering of the five-membered ring. By using a coupled Watson's Hamiltonian we found an energy splitting $E=93682.019(22)$ MHz; this value is quite in contrast with a previous far-infrared and Raman $studya$. In order to better interpret this large amplitude motion we performed also quantum chemical calculation $(MP26-311++G\ast \ast )$ and used Meyer's flexible model.