DFT and Harmonic Force Fields of Species Derived from Potent Opioid Analgesic Hydrochloride Fentanyl Drug

In this work, B3LYP/6-311++G** and harmonic force fields calculations were combined to predict structural and vibrational properties of species derived from potent opioid fentanyl drug in the gas phase (GP) and in aqueous solution (AS). Hence, the expected 153, 156 and 159 normal vibration modes of base, cation and hydrochloride fentanyl were completely assigned and its scaled force constants reported. The studies of atomic charges evidence positive MK charges on N2 atoms confirming the protonation of cation and hydrochloride on the piperazine rings. A low gap value reveals that the hydrochloride form is the most reactive in AS (4.4545 eV) while in GP the cation is the less reactive (5.4940 eV). The absence in the experimental spectrum of a very strong IR band predicted for the hydrochloride form at 1913 cm-1, suggests the presence of cation in the solid phase, as supported by the predicted UV-Vis spectra.