The terahertz(THz) fingerprint spectra of Ephedrine Hydrochloride and Papaverine Hydrochloride have been measured
using THz time-domain Spectroscopy (THz-TDS) system in the region of 0.2~2.6 THz. To explain the spectra, both
gas-phase simulation methods and solid-state simulation methods were performed in the efforts to extract pictures of the
molecular interior vibrational modes. By comparing the results of various gas-phase simulation methods, It was found
that using the semi-empirical theory is more applicable than the density functional theory (DFT) for some chemical
compounds. In the solid-state calculations, solid-state density functional theory (DFT) was employed to obtain the
vibration frequencies and Difference-Dipole Method (DDM) was used to calculate the corresponding infrared (IR)
intensity. In the process of calculating the IR intensity of Papaverine Hydrochloride in terahertz range, we found that the
results by Hirshfeld partitioning method agree better with the experiments than the ones derived from Mulliken atomic
charges. Moreover, the accuracy of simulation results depends on the basis sets and grid size being chosen.
Experimental measurement and theoretical analysis of THz spectrum for methylenedioxy amphetamine are introduced.
The refractive index and absorption coefficient of the sample were observed by terahertz time-domain spectroscopy
(THz-TDS) technique in the range of 0.2~2.6 THz. It exhibits obvious absorption feature at 1.40 THz and weak THz
absorption at around 1.68 and 2.21 THz. The spectral absorption characteristic in THz band was useful for the inspection
and identification of drugs using THz-TDS. The theoretical calculation was performed using Density functional theory
(DFT) with the GAUSSIAN 03 software package. Fully geometry optimization and frequency analysis of the optimized
structure were performed at the B3LYP/6-21G levels. The simulated absorption spectrum was in agreement with the
experimental data, and can hence be used for the assignment of observed THz spectrum. The theoretical simulation result
showed that absorption peaks mainly result from intra-molecule and inter-molecule vibrations, different absorption peaks
are corresponding to different vibrational modes and intensity. So the combination of the THz-TDS and DFT is an
effective way to investigate characteristic spectra of illicit drugs.