We identified the H-bonded structure of the pyrrole-acetone 2-1 cluster by IR spectroscopy and quantum chemical calculations. The observed NH stretches at 3335 and 3406 cm(-1) can be reproduced by the calculated ones of the cyclic structure formed by N-H center dot center dot center dot O=C and N-H center dot center dot center dot pi H-bonds. We discuss the donor-acceptor interactions of the N-H center dot center dot center dot O=C H-bonds in cyclic clusters based on natural bond orbital analysis. The n sigma* interaction makes a dominant contribution to the H-bond in the 2-1 cluster. This is in sharp contrast to the 1-2 cluster, in which the pi sigma* interaction also plays a role in H-bond formation. (C) 2013 Elsevier B. V. All rights reserved.