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• One dimensional potential energy scanning has been performed. • Effect of HCl on the vibrational spectra has been computed. • Electronic structure and HOMO–LUMO energies were computed. • NBO analysis has been performed. We have performed a comprehensive analysis of the conformational and vibrational spectroscopic properties of Diphenhydramine in the gas phase using potential energy scanning calculations at DFT/B3LYP/6-31++G(d, p) level of theory which identified three different conformational structures. The influence of hydrochloride (HCl) on the energy profile and vibrational frequencies of the most stable conformational structure was investigated. The presence of HCl showed a decrement in the energy by -460.8312 a.u. confirming DPH HCl structurally more stable than isolated DPH. Normal coordinate analysis (NCA) was employed to compute the total potential energy distribution for both the most stable conformational structure and the HCl complex. The results indicate that the DFT-computed vibrational frequencies are in good agreement with previously reported values. HOMO and LUMO was conducted to study the transition profile of the molecule. Additionally, NBO calculations were performed to ensure the stability of electronic structures at the same level of theory. [Display omitted] [ABSTRACT FROM AUTHOR]