Density functional theory (DFT) and natural bond orbital (NBO) investigation of intramolecular hydrogen bond interaction and excited-state intramolecular proton transfer (ESIPT) reaction in a five-membered hydrogen-bonding system 2-(1H-pyrazol-5-yl)pyridi

► DFT-based theoretical modeling of ESIPT and water-mediated ESPT processes. ► Five-membered hydrogen bonding ring system (2-(1H-pyrazol-5-yl)-pyridine). ► ESIPT with a finite barrier is explored in an archetypal example of pyrrole-pyridine model system. ► Nature and strength of hydrogen bonding interaction from the Natural Bond Orbital (NBO) perspectives. ► Directional character of hydrogen bonding interaction is exploited from the NBO standpoint.