Vibrational spectra of ketamine hydrochloride and 3, 4-methylene-dioxymethamphetamine in THz range

In recent years, many countries have paid more attention to the contraband of different types of drugs. Therefore identification of drugs using THz-TDS is greatly important and necessary for security. The THz absorption and refractive index spectra of ketamine hydrochloride have been present using THz-TDS in the range of 0.2-2.6 THz. The absorption spectrum exhibits strong absorption features at 1.16, 1.35, 1.52, 1.93 and 2.40THz. Corresponding to every absorption peak, a visible index decrease with frequency can be observed in the refractive index spectrum, which means that the abnormal dispersion is always accompanied with absorption. In order to better understand the absorption spectrum, theoretical calculation was carried out using DFT with Gaussian03 package. Fully geometry optimizations and frequency analysis of optimized structures were performed at B3LYP/6-311+G* and B3LYP/6-31G* levels for ketamine hydrochloride and 3, 4-methylenedioxymethamphetamine (MDMA) , respectively. Comparisons of experimental and theoretical results for ketamine hydrochloride and MDMA were shown in Figure 1. The results show that, for ketamine hydrochloride, three absorption peaks can be found at the frequencies of 1.17, 1.86, and 2.44 THz in the calculated spectrum. The absorption peaks located at 1.35 and 1.52 THz are absent in the calculation, which can possibly be attributed to the intermolecular vibration or phonon modes other than an isolated-molecular vibration. While for MDMA, the calculated and experimental results agree better. The calculated absorption peaks which located at 1.11, 1.78, and 2.60 THz lit well with the experimental absorption bands peaked at 1.20, 1.83, 2.57 THz except frequency shift. The identification of the two molecules' vibrational modes was assigned based on DFT calculation. The calculated results also indicate that the whole vibration modes are accompanied by the torsion of the whole structure, and the magnitude of vibration increases with the inc- ease of frequency.