Contribution of the π electron to the N–H⋯OC hydrogen bond: IR spectroscopic studies of the jet-cooled pyrrole–acetone binary clusters

Abstract

To investigate the π bonding electron contribution to N–H⋯OC hydrogen-bond (H-bond) formation, we applied IR cavity ringdown spectroscopy to jet-cooled pyrrole–acetone (Py–Ac) binary clusters. The observed NH stretching vibrations were analyzed by density functional theory (DFT), in which the energetically optimized structures, harmonic frequencies, and interaction energies were calculated for various sizes of binary clusters. We observed three NH stretching vibrations, ascribed to binary clusters at 3406, 3388, and 3335 cm⁻¹. These were assigned to H-bonded NH stretches of the Py₂–Ac₁, Py₁–Ac₁, and Py₁–Ac₂ clusters, respectively. The Py₁–Ac₁ cluster has a single N–H⋯OC H-bonded structure with C_s symmetry, while the Py₁–Ac₂ cluster has a cyclic structure formed by a single N–H⋯OC H-bond, dipole–dipole interactions, and weak CH H-bonds. A natural bond orbital (NBO) analysis was performed to reveal the H-bond strength in Py–Ac binary clusters. For the Py₁–Ac₂ cluster, we found that the donor–acceptor interactions are not only the n → σ* type (O atom lone pair to the NH anti-bonding orbitals), but also the π → σ* type (the CO π bonding to the NH anti-bonding orbitals). By analyzing the relationship between the frequency shift and the stabilization energy in donor–acceptor interactions, we concluded that larger red-shift of the NH stretching vibration in the Py₁–Ac₂ can be explained by not only the lone pair and the π electron contributions to the N–H⋯OC H-bond, but also the dipole-interaction between Py and non-H-bonded Ac. We also discussed the structures of Py₂–Ac₁ clusters.