The Becke, three-parameter, Lee-Yang-Parr exchange-correlation functional have been used to study the geometry, relative energy, frequency and intensity of the vibrational bands of the 4-azabenzimidazole (4AB) tautomers and the most stable tautomer's homodimers. FT-IR and Raman spectra of the 4AB have been measured in the regions 4000-100 cm(-1) and 3500-100 cm(-1), respectively for the first time. The stability of 4AB tautomers were reported. All vibrational frequencies assigned in detail with the help of total energy distribution (TED) and isotopic shifts. The energy and atomic charges were discussed. N-H center dot center dot center dot N type intermolecular hydrogen bonding interactions were suggested for 4AB dimeric forms. H-1 and C-13 NMR properties have been calculated for the most stable two tautomeric forms using the gauge independent atomic orbital (GIAO) method. (C) 2012 Elsevier B.V. All rights reserved.