## 3 Results and Discussion
### Frontier Molecular Orbitals  
The quantum chemical calculation results in a set of molecular orbitals for the molecule under consideration. These orbitals represent the spatial distribution of electrons within the molecule. Frontier molecular orbitals (FMOs) are of particular interest from a chemical perspective because they play a crucial role in various chemical reactions and the properties of the molecules [63]. The HOMO (Highest Occupied Molecular Orbital) is the molecular orbital with the highest energy level that is occupied by electrons. It represents the outermost electrons in a molecule and is involved in electron donation during chemical reactions. The LUMO (Lowest Unoccupied Molecular Orbital) is the molecular orbital with the lowest energy level that is unoccupied. It represents an area where electrons can be accepted during chemical reactions. The HOMO and LUMO are often considered the most important molecular orbitals because they influence the reactivity and properties of a molecule. For example: The HOMO influences electron-donating abilities, nucleophilic reactions, and interactions with electron acceptors. The LUMO influences electron-accepting abilities, electrophilic reactions, and interactions with electron donors. Understanding the FMOs helps predict and interpret various chemical phenomena, such as reaction mechanisms, molecular stability, and electronic transitions. In summary, the quantum chemical calculation of molecules provides information about their molecular orbitals. A decrease in the HOMO-LUMO energy gap (Equation (1)) signifies a possible charge transfer interaction taking place within a molecule because of the strong electron-accepting ability of the electron acceptor groups. Approximate ionization energy (_IE_) and electron affinity (_EA_) calculations (Equations (2) and (3)) based on these ideas are known as the so-called Koopmans theorem. The HOMO and LUMO energies are used to define the global chemical reactivity descriptors [64], where \(\eta\) denotes the global chemical hardness (Equation (4)), \(\zeta\) denotes the global chemical softness (Equation (5)), and \(\mu\) represents the electronic chemical potential (Equation (6)), which describes the charge transfer within a system in its ground state. Compounds with greater values of chemical potential are more reactive than those with small electronic chemical potentials. The global electrophilicity index, \(\omega\) (Equation (7)), is a concept used in theoretical chemistry to assess the electrophilic character of a molecule or a chemical species. A higher electrophilicity index indicates a higher tendency of a system to accept electrons and, therefore, a higher  
Figure 9: Comparison of infrared spectra of DIT-SAL, dithranol, salicylic acid, and molecules. electrophilic character. This index is often used in the context of the Parr function [65], which is part of the conceptual density functional theory (CDFT). \[E_{g}=E_{LUMO}-E_{HOMO} \tag{1}\]  
\[EA=-E_{LUMO} \tag{2}\]  
\[IP=-E_{HOMO} \tag{3}\]  
\[\eta=\frac{IP-EA}{2} \tag{4}\]  
\[\zeta=\frac{2}{IP-EA} \tag{5}\]  
\[\mu=-\frac{IP+EA}{2} \tag{6}\]  
\[\omega=\frac{\mu^{2}}{2\eta} \tag{7}\]  
The HOMO and LUMO isosurface maps of DIT-SAL, and, for comparison, also two constituent molecules, are presented in Figures 10-12. As can be seen from Figure 10, the HOMO of DIT-SAL is distributed mostly on the salicylic acid part of the molecule, and on the other hand, the LUMO is localized mostly on the dithranol fragment of DIT-SAL. Interestingly, for HOMO-1, as well as for LUMO+1, this localization of the orbitals is exchanged. The first FMO delocalized across the whole DIT-SAL molecule is HOMO-2. The energy gap of DIT-SAL (Table 2) is 3.915 eV, almost the same as for dithranol (4.062 eV), indicating that the SAL-DIT molecule is chemically stable and may be considered a strong electrophile because its \(\omega\) (4.772 eV) is substantially greater than 1.5 eV [66]. This finding is also supported by its rather high chemical hardness, which was found to be 1.958 eV.