The enantioseparation of two cathinone homologues— methylone and pentylone—by HPLC with UV–Vis detection has been successfully carried out using different methods of chiral separation for individual substances.
Furthermore, a detailed structural analysis of both drugs of abuse has been achieved by the methods of electronic and vibrational spec- troscopy, specifically conventional UV and IR absorption with ECD and VCD in combination with DFT calculations at the B3LYP/6-311++G or B3PW91/6-311++G levels of theory, including solvent effects. According to this study, these homologues can be difficult to distinguish solely by ECD or UV absorption. VCD spectroscopy appeared to be the best of the employed methods for distinguishing struc- turally similar chiral substances, not only in a small spec- tral range such as IR absorption spectroscopy provides, but essentially across the spectrum, especially in combination with quantum chemical calculations.
Structural analysis sug- gested the existence of 5 and 9 stable conformers of methy- lone and pentylone, respectively, in an aqueous solution. Finally, very good agreement between the experimental and the corresponding simulated spectra was achieved.
Thus, we were able not only to determine the absolute configuration of the individual conformers of methylone and pentylone, but also to describe the molecular structure of both compounds.
In conclusion, chiroptical spectroscopic methods confirmed the high potential for structural analysis of molecules sig- nificant in the forensic sciences.
These data can be helpful in considering the forensic evidence in criminal proceedings. Moreover, the structural analysis performed by the quantum chemical calculations can be used for the understanding of the biological activity and toxicity of the NPS.
A detailed knowledge of the 3D structure of the substances may allow not only the detailed study of the binding properties, metab- olism, transport or distribution of these substances in the human organism, but also the development of antagonist drugs for overdoses, addictions or poisoning.
The shape of the experimental IR absorption spectra of meth- ylone and pentylone in the spectral range 1750–1250 cm −1 was very similar with a total number of 11 and 9 bands for methylone and pentylone, respectively.
For the cor- rect assignment of the individual bands and their respective vibrational modes, the spectra were scaled by a factor of 0.99 and 0.98 and resulted in a very convincing index of spectra similarity: 0.85 and 0.91 for methylone and pentylone, respectively. Looking at the individual spectra of the conformers, it is worth noting the large influ- ence of the dihedral angle α 1 on the spectral shape. Band 1 was shifted to the lower wavenumbers in the spectra of all the conformers with a positive value of angle α 1 compared with the spectra of the other conformers.
Band 2 was apparent only in the spectra of the conformers with the positive value of angle α 1, hence the intensity of this band was lower in the averaged spectra and appeared only as a shoulder.
In contrast, bands 9 in the spectra of methylone and 8 in the spectra of pentylone were not present in the spectra of those conformers.
The bands in this area reflected the bending vibration of the C–H group, including the umbrella bending vibration of –CH 3 . Band 7 in the spec- tra of methylone and band 6 in the spectra of pentylone were more intense in the case of the conformers with positive value of α 1 . Band 10 in the spectra of methylone reflected the bending vibration of the C–H group in CH–CH 3 and was correctly predicted by the DFT calculations. This group was not present in the structure of pentylone, therefore we did not expect a band reflecting the bending vibration of the C–H group in CH–CH 3 to occur in its IR spectra. This was confirmed by the experimental spectra as well.
Further- more, slight differences in the spectral shapes were observed between the experimental IR absorption spectra of methyl- one and pentylone hydrochloride, particu- larly in the spectral range of 1420–1280 cm −1 . Therefore, this method can be used to distinguish structurally similar substances, but, unfortunately, with low sensitivity.
The experimental VCD spectra were pre- sented in the spectral range of 1750–1263 cm −1 because of the high level of VCD noise caused by overabsorption at lower wavenumbers. The simulated VCD spectra were scaled by a factor of 0.98 and 0.99 for methylone and pentylone, respec- tively, for the correct assignment of the individual bands.
The resulting index of spectra overlap reached convincing values of 0.86 and 0.70 for methylone and penty- lone, respectively.
In the experimental VCD spectra of-methylone and-pentylone hydrochloride, there were several areas in which individual substances could be reliably distinguished, mainly in a spectral range of 1400–1263 cm −1 , where the molecules not only had a different number of bands but also different spectral shapes.
Moreover, that was predicted correctly by the quantum chemical simulations. In addition, in the spectral range of 1700–1580 cm −1 , there were different intensities of bands 1 and 2, which was also correctly predicted by the simulations. Our results dem- onstrating a higher propensity of distinguishing between two structurally similar compounds by VCD than ECD are in very good agreement with the literature. The conformers of methylone and pentylone with a positive value of dihedral angle α 1 again had a distinct spectral shape different from the rest of the conformers, e.g.