Cationic and excited state characterization of NH⋯X (X = N, O) interaction in benzimidazole complexes with ammonia and water using ZEKE and FDIR spectroscopy

Abstract

In this study we have used various laser spectroscopic studies to investigate the neutral excited state and cationic ground state of the NH···N and NH···O bound complexes of an N–heterocyclic aromatic compound, benzimidazole (BIM). The complexes of BIM with ammonia and water were formed using supersonic jet expansion method. The excited state of the complexes was characterized using the fluorescence depletion infrared (FDIR) spectroscopy and the zero kinetic energy (ZEKE) photo-electron spectroscopy was used to characterize the cationic state. This is the first report of a ZEKE spectrum for an NH···N bound hydrogen bonded complex. The adiabatic ionization energies (A.I.E.) for the BIM-ammonia and BIM-water complexes were found to be 63621 ± 5 cm−1 and 64410 ± 5 cm−1, respectively. These were red shifted by 3925 cm−1 and 3136 cm−1 with respect to that of the BIM monomer. The results re-emphasize the fact that ammonia is a better hydrogen bond acceptor than water due to its higher proton affinity. The experimental observations are supported by extensive calculations using various DFT functionals and MP2 method. The shifts in the A.I.E. values and the binding energies in the cationic state computed using the CAM-B3LYP and ωB97X-D functionals were found to match better with the experimentally determined values compared to those computed at the B3LYP, B3LYP-D3 as well as MP2 level. Assignments of the intermolecular and intramolecular modes were made by comparison with calculated frequencies. We also report the ZEKE spectrum of the BIM monomer and revise some of the assignments made in an earlier report of the mass analyzed threshold ionization (MATI) spectrum of the same.