Normal vibrational mode analysis and assignment of benzimidazole by ab initio and density functional calculations and polarized infrared and Raman spectroscopy

Molecular structure and fundamental vibrational frequencies of benzimidazole (BZI) are reported using ab initio-Hartree-Fock (HF) and density functional theory (DFT) methods at different levels of calculation. Observed polarized IR and Raman fundamentals of polycrystalline, single crystal, KBr-BZI disks, and gaslike samples are analyzed and assigned by comparison to the computed values. The assignment of fundamentals shows a one-to-one correspondence between the observed and calculated fundamentals using the BLYP/6-31G* level of calculation without applying any scaling factor except for XH stretching (X represents C or N), where a 0.995 scaling factor has been used. Hydrogen bonding and its effects on some normal vibrational modes are discussed. The results show that the nonscaled BLYP/6-31G* level of calculation may be used as a reference for assessing the intermolecular hydrogen bonding effect.