SERS and DFT study of 5F‐PB‐22

Abstract

AbstractIn this work, a joint experimental and theoretical study on the synthetic cannabinoid, 5F‐PB‐22 (1‐(5‐fluoropentyl)‐8‐quinolinyl ester‐1H‐indole‐3‐carboxylic acid) is reported. The molecular vibrations of 5F‐PB‐22 were investigated by Raman and surface‐enhanced Raman spectroscopy (SERS) spectroscopies. In parallel, quantum chemical calculations based on density functional theory (DFT) with the hybrid B3LYP exchange‐correlation functional coupled with the standard 3‐21G basis set were used to determine the geometrical and vibrational characteristics of 5F‐PB‐22 and to predict the adsorption geometry of the molecule on the silver colloidal surface. The SERS spectrum of 5F‐PB‐22 was recorded using a 532‐nm laser line and a hydroxylamine phosphate reduced silver colloid as the SERS substrate. SERS and Raman spectral bands have been assigned by comparison with the DFT calculated spectrum. A self‐aggregation spectrum was obtained, but in order to obtain a spectrum with NaCl as the aggregating agent, a two‐step procedure was developed. It was found that 10‐mM NaCl can be used as a trigger for aggregation before adding 5F‐PB‐22 followed by the addition of 20‐ to 100‐mM NaCl to complete aggregation. Using this procedure, 5F‐PB‐22 was easily detected at 2.12 × 10−8 M. Based on a predicted molecular adsorption geometry from the calculated molecular electrostatic potential and the SERS spectrum, it is proposed that the quinoline moiety of the molecule lies in a preponderant parallel orientation to the silver surface with the plane of the indole ring being perpendicular to the surface. The molecular orientation is affected by the surface coverage by 5F‐PB‐22 and the presence of Cl−.