Study on Different Conformation Raman Spectra of Methamphetamine Based on Density Functional Theory

As a new-type drug, methamphetamine has been spreading rapidly in recent years and its social harm has become increasingly serious, which has brought severe challenges to the relevant regulatory authorities. How to provide a non-destructive, fast and accurate drug detection method has important practical and application value. Based on the above conditions, Raman spectroscopy is a novel method. However, due to the differences of methamphetamine molecules, the Raman spectra detected are different, which affect the on-site methamphetamine detection, and even cause misjudgment, and bring great difficulties to the establishment of the database of Raman spectra of methamphetamine. Therefore, according to the density functional theory, the Becke-3-Lee-Yang-Parr (B3LYP) hybrid functional method was used to determine the three dihedral angles of the methamphetamine molecules φ1, φ2 and φ3 on the 6-31G basis. In the range of 0°~360°, the potential energy surface scanning was performed in steps of 10° respectively, and 12 different stable conformations of methamphetamine molecule were obtained based on minimal energy points. Besides those, four lower energy conformations were further optimized and their vibration frequencies were calculated on the 6-31G++(d, p) basis set. Finally the theoretical Raman spectra obtained were compared with the experimental spectra. The results showed that the differences of methamphetamine conformations produced various shifts on peak positions at 746, 837 and 1 356 cm-1 in Raman spectra, however, the Raman peaks positions at 632, 1 003, 1 180 and 1 312 cm-1 were basically unaffected. Therefore, when we identify suspicious samples, those unaffected peaks could be used as the identifier spectra of methamphetamine, and the matching method by identifier Raman spectra is obviously faster than most traditional correlation coefficient matching algorithms. Meanwhile, in the four lower energy conformations selected, the calculated results from the conformation IX were the closest to the experimental values. Therefore, in order to assign the experimental Raman identifier spectra of methamphetamine, the IX' result was combined with the potential energy distribution of each vibration frequency and related literature. In those results, 1 003 cm-1 was the strongest identifier peak of methamphetamine, which was assigned to the aromatic respiration vibration. The Raman peak at 837 cm-1 was assigned to NH rocking vibration. In addition, the Raman peak at 1 180 cm-1 was attributed to CN stretching vibration and 1 312 cm-1 belonged to CH2 wagging vibration. These researches have the potential to provide useful references for drug detection, database of drug Raman spectra establishment and theoretical calculation of Raman spectroscopy of drug molecular in future. © 2020, Peking University Press. All right reserved.