## 2 Results and Discussion
### Experimental and Theoretical UV-VIS Spectra  
The UV-VIS spectra are not helpful in the chemical characterization of substances due to the very broad peaks and spectra similarity for compounds that do not share the same structure. Electronic absorption of several functional groups can be assigned to the bands in the UV-VIS spectrum. However, substituents, elongated delocalization of \(\pi\)-electrons, solvent, pH, and temperature can influence their position. These spectra are essential in forensic analysis when the absorption wavelength is determined for the UV-VIS detectors coupled with chromatography columns, especially in liquid chromatography. Even when spectra are known, selecting wavelengths not shared with other compounds is of utmost interest, as this would induce an error in quantification. The UV-VIS spectrum of LSD consists of two wide peaks with maxima around 220 and 315 nm (Table 3), of which the one with a longer wavelength is used for quantification [29]. These spectra were recorded in water, as shown in the previous reference. A maximum at this wavelength is common for the compounds containing the aromatic core with the additional ring structures and electron-donating groups. There are slight deviations in the UV-VIS spectra of LSD's analogs, especially when another carboxyl group is present [8; 29]. The prediction of the wavelength for the novel LSD derivatives would be of the highest importance, especially when their quantification in seized samples is required. Several methods can be applied for the prediction of spectra, and they are discussed in the following paragraphs using the LSD's electronic spectrum as an example. The simplest method for the spectra prediction is the compound's optimization in the desired solvent by the mentioned CPCM method (or any other suitable for the prediction). Then, the TD-DFT method is applied to calculate the electronic transitions. Ten transitions with the lowest energy (longest wavelength transitions) were calculated. In the studied example, two transitions with significant oscillator strength were observed. The first one is at 288 nm, significantly different from the experimental one. The second wavelength is well reproduced (226 nm). The first transition is assigned as HOMO\(\rightarrow\)LUMO with an oscillator strength of 0.3793. The second can be denoted as HOMO\(\rightarrow\)LUMO+2. The calculated energy values represent only the energy difference between the bottoms of the excited state and ground state energy surfaces, while the experimental spectrum contains the adjacent vibrational and rotational transitions responsible for the peak width. The bathochromic shift in the observed spectrum compared to the theoretical one is probably due to the stabilization effect of the solvent on the excited state. The HOMO, LUMO, and LUMO+1 densities of LSD are shown in Figure 3. The HOMO orbital is mainly localized on the indole core of LSD, covering the adjacent ring's double C=C bond. On the other hand, the LUMO orbital includes the indole moiety and the carbonyl group as the most polar group present in the structure. Most LSD molecules consist of carbon-carbon bonds, and this difference in electron density distribution only slightly changes the possible interactions with solvent molecules. The LUMO+1 is similar to the HOMO orbital and includes the same functional groups (indole ring). Based on these electron density distributions, the solvent molecules should stabilize the LUMO orbital more than the other two through interactions with the carbonyl group. These interactions