GEOMETRICAL AND ELECTRONIC STRUCTURE OF PROPYL-, TETRAMETHYL-, DIMETHYLETHYL- AND BUTYLADAMANTANES AND THEIR THERMODYNAMIC CHARACTERISTIC ACCORDING TO DFT

Propyladamantanes were synthesized by alkylation of adamantane with isopropyl alcohol in the temperature range from 5 to 40 °C in the presence of 96% sulfuric acid. Tetramethyl- and dimethylethyladamantanes were synthesized by isomerization of perhydroanthracene in the presence of aluminium oxide catalyst on the setup of the flow type. Isomers of butyladamantanes were obtained by the reaction of alkylation of the adamantane with isooctane. For each molecule, the optimization of the geometric parameters of atoms was carried out using analytical calculation methods. By calculating the frequencies of normal vibrations using the second derivatives, it was confirmed that the stationary points determined by optimizing the geometry correspond to the minima of the potential energy surface. The structure of 1-n-propyladamantane (I), 1-isopropyl-adamantane (II), 2-n-propyladamantane (III), 1,2-di-n-propyladamantane (IV), 1,3-dimethyl-5-ethyladamantane (V), 1,3,5,6-tetramethyladamantane (VI), 1,3,5,7-tetramethyladamantane (VII), perhydroanthracene (VIII), 1-n-butyladamantane (IX), 1-isobutyladamantane (X), 1-sec-butyladamantane (XI) has been studied using the DFT method with the Becke–Lee–Yang–Parr hybrid energy functional of electron density with the 6-31G* basis set. The geometric and electronic characteristics of the compounds and their total energy, normal vibration frequencies have been calculated. It has been shown that the calculated Gibbs free energies of formation for the perhydroanthracene isomerization products are in qualitative agreement with the experimental product composition of the isomerate and alkylation of adamantane with isopropyl alcohol are in qualitative agreement with the experimental composition of the products. A good agreement of calculated and experimental data on the composition of equilibrium mixtures was obtained. The theoretical geometry of the synthesized alkyladamantanes with Td symmetry very well agrees with the results of electron diffraction. Closest to the results obtained experimentally, the geometry was predicted by B3LYP, in which the lengths of C-C and C-H bonds are close to 1.544 and 1,100Ả, respectively, and the C-Csec-C and C-Cter-C angles are 109°. The results of the calculation using the B3LYP method are in good agreement with the experimental data. There is no definite relationship between the size of the molecules and the convergence of the calculated and experimental data. A practically important conclusion arising from the results of this and previous studies is that the use of the calculation method leads to “chemically accurate” data.