Cyclic structure of pyrrole–acetone binary clusters: Comparative study of the N–H⋯OC hydrogen bond

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⋯OC and N–H⋯π H-bonds. We discuss the donor–acceptor interactions of the N–H⋯OC H-bonds in cyclic clusters based on natural bond orbital analysis. The nσ∗ 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 πσ∗ interaction also plays a role in H-bond formation.