The entrance channel leading to the addition reaction between the hydroxyl radical and acetylene has been examined by spectroscopic characterization of the asymmetric CH stretching band of the pi-hydrogen bonded OH-acetylene reactant complex. The infrared action spectrum observed at 3278.6 cm(-1) (origin) consists of seven peaks of various intensities and widths, and is very different from those previously reported for closed-shell HF/HCl-acetylene complexes. The unusual spectrum arises from a partial quenching of the OH orbital angular momentum in the complex, which in turn is caused by a significant splitting of the OH monomer orbital degeneracy into (2)A' and (2)A" electronic states. The magnitude of the (2)A'-(2)A" splitting as well as the A rotational constant for the OH-acetylene complex are determined from the analysis of this b-type infrared band. The most populated OH product rotational state, j(OH)=9/2, is consistent with intramolecular vibrational energy transfer to the nu(2) Cequivalent toC stretching mode of the departing acetylene fragment. The lifting of the OH orbital degeneracy and partial quenching of its electronic orbital angular momentum indicate that the electronic changes accompanying the evolution of reactants into products have begun to occur in the reactant complex. (C) 2004 American Institute of Physics.