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.

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