Journal of Physical Chemistry A, Vol.114, No.32, 8323-8330, 2010
Infrared-Optical Double Resonance Spectroscopic Measurements on 2-(2'-Pyridyl)benzimidazole and its Hydrogen Bonded Complexes with Water and Methanol
The H2O and MeOH complexes of 2-(2'-pyridyl)benzimidazole (213131) were investigated using laser induced fluorescence and IR-UV double resonance spectroscopic techniques. The 08 band for the SI So electronic transition of 2PBI was observed at 31 616 cm(-1), and corresponding transitions for the H2O and MeOH complexes show substantial red shifts of 1059 and 1203 cm(-1), respectively. A long progression up to v' = 5 in the 92 cm(-1) vibrational mode was observed for bare 2PBI, which is considerably shortened to v' = 2 and v' = I for the H2O and MeOH complexes, respectively. The combined experimental and theoretical results suggest that both H2O and MeOH form cyclic complex with 2PBI incorporating N-H center dot center dot center dot O and O-H center dot center dot center dot N hydrogen bonds. Comparison with other known hydrogen-bonded H2O complexes of bifunctional aza-heteroaromatic molecules suggest that 2PBI-H2O is probably one of the strongest of all known complexes. Further, the experimental data in combination with calculations suggests that the hydrogen atom/proton transfer leading to tautomeric form might be thermodynamically spontaneous in the electronic excited state, due to explicit solvent (H2O and MeOH) participation.