Local anesthetics like lidocaine (N-(2,6-dimethylphenyl)-N2,N2-diethylglycinamide) and pirlocaine ((RS) N-(2-methylphenyl)Npropylalaninamide) are amide local anesthetic and lidocaine is used topically to relieve itching, burning andpain from skin inflammations, injected as a dental anesthetic or as alocal anesthetic forminor surgery. Prilocaine is applied in dentistry and is a sodium channel blocker Theirs common forms are salts and insoluble in water which have less toxic effects. Mass spectrometric methods are most commonly used. Unfortunately, these methods are destructive and the sample is no longer available for re-analysis by other laboratories. IR and Raman spectroscopy provide alternatives for non-invasively analyzing drug crystals. Furthermore, these methods are accurate and high sensitive for lower amounts of these materials [2]. The Raman and IR spectra of the titled compound are obtained and by using quantum chemistry calculations the frequencies are assigned to the normal modes [1]. The aim of this study is investigation on the structure, stable conformer, and vibrational spectroscopy in solid, CCl4 and CH3CN solutions of Prilocaine and lidocaine with using DFT calculations at B3LYP/6-31G** level of theory. 4 different stable conformers were obtained for Lidocaine and Prilocaine. The structures of the most stability of these conformers and atom numbering of the system are shown in Fig 1. The relative stabilities of the lidocaine conformers are in the range of 3.5-4.97 kcal/mol and those for prilocaine are in 3.5-5.1 kcal/mol range, with respect to the most stable conformer. Dihedral angle between Ph ring and N2C6 for prilocaine conformers is about 40-60º and more than those in lidocaine, so there is no resonance between them that it confirmed with calculated N-Ph bond length. The dihedral angles of O=C-CH-N1 in prilocaine and lidocaine are 77.25° and 154.37°, respectively. The difference is due to the presence of two ethyl groups bounded to N1 in Lidocaine while there is only one n-propyl group bounded to N1.

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