$^{a}$E. Bondoc, T. Klots, J. Laane, J. Phys. Chem A 104, 275 (2000) ; Author Institution: Dipartimento di Chimica ``G. Ciamician'', Universit\`{a} di Bologna ; 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 $study^{a}$. In order to better interpret this large amplitude motion we performed also quantum chemical calculation $(MP2 6-311++G^{\ast\ast})$ and used Meyer's flexible model.