The N–H⋯OC hydrogen bond (H-bond) plays a key role in stabilizing the geometry and energy of biomolecules such as protein folding and DNA double strand. To investigate N–H⋯OC H-bonds in a microscopic view, we apply IR cavity ringdown spectroscopy (IR-CRDS) and density functional theory (DFT) calculation to pyrrole–diethyl ketone (Py–Dek) clusters in the gas phase. Dek exhibits a pentane carbon chain, which provides various conformations such as anti, gauche, and their mixtures. An introduction of the carbon-chain flexibility to Py–Dek clusters is expected to cause a diversity of the N–H⋯OC H-bond formation. In the observed IR spectra, there are seven prominent bands of the NH stretches due to Py–Dek clusters. These bands are classified into three groups: one for Py₁–Dek₁, two for Py₁–Dek₂, and four for Py₂–Dek₁. Stable structures and their harmonic frequencies obtained by DFT calculations provide the proper NH band assignments and appropriate cluster structures. Py₁–Dek₁ exhibits a single isomer, which is formed by an ordinary N–H⋯OC H-bond between Py and anti-conformation of Dek (Dek(a)) with a linear carbon-chain. Py₁–Dek₂ shows two isomeric structures, in which both isomers are commonly constructed by the N–H⋯OC H-bond for the first Dek and by the stacking interaction between π electrons of Py and the second Dek. Both isomers exhibit the Dek(a) for the stacking interaction, but are distinguished between Dek(a) and gauche-conformation Dek (Dek(g)) for the N–H⋯OC H-bond. Py₂–Dek₁ shows a triangular cyclic structure, which is formed by the N–H⋯OC H-bond, the N–H⋯π H-bond, and the stacking interaction between Py and Dek. The observed four bands are assigned to two N–H⋯OC and two N–H⋯π H-bonds for two isomeric structures due to Dek(a) and Dek(g). Not only smaller clusters but also higher hetero-tetramers are characterized based on the architecture of smaller clusters. In particular, Py₂–Dek(a)₂(I) was the first to be found with a highly symmetric (C_i) cyclic structure. Calculated potential energy surfaces of Py–Dek clusters shed light on the impact of Dek flexibility on N–H⋯OC H-bond diversity. Selective formation of isomeric structures for Py–Dek clusters is discussed in terms of a mechanism of a two- and three-body collision process in a supersonic expansion.