## 2 Results and Discussion
### Experimental and Theoretical IR Spectra  
The IR spectra are an indispensable tool for quickly determining the present substances in seized samples, especially if they are in appreciable quantities. There is no need to analyze the whole spectrum but only for several intense bands originating from carbonyl vibration or some other highly polar groups. Marking agents are often added to cover these bands or induce their shift through intermolecular bonding. The following section presents the comparison between the experimental and theoretical IR spectra of LSD and LSD tartrate. Some of the practical aspects are outlined. The IR spectrum of LSD has been calculated during the optimization process. The vibrational spectrum can be visualized by the GaussView [22] program and further analyzed in the VEDA software [23]. As LSD is a low-symmetry molecule, all vibrations are active in the IR spectrum (141 normal modes). When the highly symmetric molecule is analyzed, it is beneficial to obtain both IR and Raman spectra because of the exclusion principle. In our case, the calculated wavenumbers were overestimated, and the scaling factor was calculated by comparing experimental and theoretical wavenumbers. The scaling factor can be obtained as the ratio between the observed and calculated wavenumbers of the same vibration, most commonly C=O vibration. The band belonging to this vibration in the experimental spectrum is positioned at 1625 nm, while in the theoretical spectrum, it is at 1747 cm\({}^{-1}\) (Figure 2). The scaling factor calculated based on these values is 0.930. The experimental, scaled, and unscaled theoretical values are presented in Table S3. Theoretical values are shown only in the observed range (3500 to 500 cm\({}^{-1}\)). Figure 2 shows the experimental and scaled IR spectrum of LSD base and LSD tartrate. The IR spectrum of LSD can be divided into three distinct regions. The first one is a high-frequency region between 3500 and 2500 cm\({}^{-1}\) characterized by the peaks belonging to the N\(-\)H and C\(-\)H stretching vibrations. The N\(-\)H stretching vibration is located at 3200 cm\({}^{-1}\) in the experimental spectrum [24; 25]. In the theoretical spectrum, this band \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **B3 LYP** & **B3 LYP-D3 BJ** & **CAM-B3 LYP** & **B3 PW91** & **M05-2X** & **M06-2X** \\ \hline Bond lengths (Å) & 0.0423 & 0.0430 & 0.0444 & 0.0439 & 0.0436 & 0.0437 \\ Bond angles (\({}^{\circ}\)) & 2.88 & 2.82 & 2.83 & 2.85 & 2.70 & 2.72 \\ \hline \end{tabular}
\end{table} 
Table 1: Mean absolute errors for the bond lengths and angles for structures optimized with various functionals in conjunction with 6-31++G(d,p) level of theory. Figure 2: Experimental and theoretical (at M05-2X/6-311++G(d,p) level of theory) IR spectra of (a) LSD base and (b) LSD tartrate. is positioned at 3446 cm\({}^{-1}\). This difference is a consequence of the physical state of the sample, as the theoretical calculations were performed for the isolated molecule in a vacuum. The other prominent bands include the sp\({}^{2}\) hybridized carbon atom C\(-\)H vibrations at 3097 and 3046 cm\({}^{-1}\). The sp\({}^{3}\) hybridized carbon atoms C\(-\)H vibrations can be assigned to the bands at 2920, 2883, 2839, 2810, and 2774 cm\({}^{-1}\). These values are well reproduced in the theoretical spectrum (Table S3). As these vibrations are not particularly helpful for the LSD analysis, they are not discussed in detail. The most noticeable band located at 1625 cm\({}^{-1}\) belongs to the C=O stretching vibration in both spectra. This band (between 1600 and 1630 cm\({}^{-1}\)) is typical for all the tertiary LSD amides and their analogs, although its position depends on the neighboring substituents [8; 9; 11; 24]. Another intense band, assigned as C\(-\)C stretching vibration, is positioned at 1432/1379 cm\({}^{-1}\) in the experimental/theoretical spectrum. The bands between 1600 and 1000 cm\({}^{-1}\) can be attributed to mixed vibrational modes, including stretching and bending vibrations.