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

The present work aims at exploring the excited-state intramolecular proton transfer (ESIPT) reaction in a five-member hydrogen bonding ring system, viz., 2-(1H-pyrazol-5-yl)pyridine (2Pz–5Py), in which the ESIPT process is found to be associated with an appreciably large barrier, on the lexicon of a Density Functional Theory (DFT)-based computational approach. Nevertheless the feasibility of ESIPT has been argued from the changeover of relative stability patterns of the enol (E) and the proton-transferred (PT) forms on photoexcitation from the S0-PES to the S1-PES. Critical evaluation on the modulations of geometrical parameters other than the PT reaction coordinate has also been undertaken. The intramolecular hydrogen bonding (IraMHB) interaction in 2Pz–5Py has been explored by calculation of the hyperconjugative charge transfer interaction from the lone electron pair of ring nitrogen atom to the σ∗ orbital of N–H bond, under the provision of Natural Bond Orbital (NBO) analysis. Further, we have also endeavoured to cast light on the important issue of water-mediated ESPT in 2Pz–5Py. Apart from examining the impact of water mediacy on the barrier for ESPT process, the substantial influence of the solvent (water) reaction field on the energetic and geometric aspects of the molecule are also explored through calculations based on polarizable continuum model (PCM). Again the applicability of NBO analysis has been extended to characterize the nature and strength of IMHB in the hydrated cluster of 2Pz–5Py. The directional nature of hydrogen bonding interaction is also unveiled at this juncture.