Calixarenes (CA) and thiacalixarenes (TCA) are macrocyclic compounds. Calixarenes (CA) and thiacalixarenes (TCA) structure include the upper and lower rims. These molecules can be in cone conformation, partial cone, 1,2-alternate and 1,3-alternate. An important feature of CA and TCA is the ability to functionalize molecules by substitution of upper and / or lower rims by different functional groups, substitution of methylene bridges for S, SO, SO2, N, NO bridges, etc., and by changes the conformation of molecules. This makes it possible to widely change the selectivity and binding efficiency of various substrates , which allows them to be used in extraction processes, and opens up great opportunities for the use of CA and TCA molecules in various fields of chemistry, biology, and physics.
Accordingly, the question arises of the effect of conformation on the electronic structure and, as a consequence, on the physical and chemical properties of CA and TCA molecules. The methods of X-ray emission (XES), X-ray absorption spectroscopy (XANES, EXAFS) and X-ray photoelectron spectroscopy (XPS), as well as quantum-chemical methods are highly characteristic and effective methods for studying the electronic structure of various chemical compounds.
In this paper, we studied the electronic structure of TCA molecules by XES, XPS, XANES, and quantum chemistry. The influence of molecular conformation on RES and XANES was studied. The X-ray absorption spectra of the acyclic molecule TCA and cone conformation and 1,3-alternate were obtained. Quantum-chemical calculations of TCA molecules electronic structure in the cone 1,3-alternate conformation were carried out by DFT in the ADF software package. Carbon XANES spectra were calculated in the ground state of the molecules and the Z+1 model by ADF software package and by FMDNES program. The experimental K-edges of carbon absorption of TCA molecules cone, 1,3-alternate and acyclic molecules are compared and compared with the results of quantum chemical calculations. Based on the carried out theoretical calculations, it was shown that, the LUMO of the studied TCA molecules were constructed with the participation of 2p AO carbon atoms of CH, CS, CO fragments constituent TCA. It is also shown that, when passing from cone conformation to 1,3-alternate conformation, the LUMO structure changes and the contribution of 2p AO CH fragments becomes dominant.