The rotational spectrum of coumaran has been investigated by free-jet absorption millimeter-wave spectroscopy. All transitions are split into two component lines due to the low barrier to ring puckering of the five-membered ring. The ground state splitting (Delta E-01 = 93682.02(2) MHz, 3.124895(1) cm(-1)) is much larger than the value inferred from the infrared spectrum [0.7 cm(-1). E. Bondoc, T. Klots, J. Laane, J. Phys. Chem. A 104 (2000) 275]. Such a discrepancy affects the estimate of the anomeric effect, as discussed in the text. (c) 2005 Elsevier B.V. All rights reserved.