Propriedades espectroscópicas da cocaína: um estudo experimental e teórico

Abstract

Cocaine is a natural substance, extracted from the Erythroxylon coca plant. The cocaine is an alkaloid that acts on the central nervous system, having anaesthetic and euphoric actions. Between two chemical forms of cocaine, i.e., ionic and neutral, the latter one corresponds to the crack, which is produced from base paste of coca. The identification of cocaine has been performed using colorimetric tests, but the pseudo positive results are often, indicating the need of the confirmatory analyses in laboratories. Chromatographic analyses are common, but few information can be found in the literature regarding the detailed characterization of the spectroscopic profiles of cocaine. In this context, the present study was developed by experimental and theorical systematic analyses, utilizing infrared (IR) and Raman vibrational spectroscopies, as well as the nuclear magnetic resonance spectroscopy (NMR), to evaluate both ionic and neutral forms of cocaine. The theoretical study was based on the time independent and dependent Density functional theories (DFT e TD-DFT). The experimental analyses were performed with cocaine samples, which were provided and supervised by civil police agents of Minas Gerais. The main theoretical IV, Raman e RMN results have shown a good agreement to experimentally measured spectra. Furthermore, the computed results provide correct assignments and interpretations of the experimental bands, indicating the main differences to distinguish the two forms of cocaine. The Raman and IR results have proven the capability to identify chromophore groups responsible to differentiate the two chemical forms of cocaine. The symmetric and asymmetric N-H stretches, localized at 2500-3200 cm-1, were decisive to identify the ionic form of cocaine, while C-H vs (sp3 and sp2) stretch at 2979,1 cm-1 appears with notable intensity only in the Raman and IR spectra in neutral CD form. The M06/Def2-TZVP results of RMN, specifically the chemical displacement of 13C and 1H in the gas phase and in alcoholic solution, have shown a good linear correlation with the experimental values, showing R2 values, which varies between 0,992 to 0,998 for 13C and 0,947 to 0,958 for 1H for both forms of cocaine. The present results indicate that theoretical and experimental collaboration is a useful tool for assignment and interpretation of the IV, Raman and NMR spectra so that important features can be found to distinguish the two chemical forms of cocaine.