Energy of Solvation and a Quantum-Chemical Model of the Structure of 18-Crown-6 Ether in Nonaqueous Solvents

A quantum chemical study of the structure of different molecular configurations of 18-crown-6 ether (18C6) in the free state, as well as in methanol, ethanol, dimethylsulfoxide, dimethylformamide, and acetonitrile media is performed using the GAUSSIAN 03 software package in the B3LYP version of the electron density functional theory using the cc-pVTZ basis set. The structure of molecules in the solvated state is studied by means of a self-consistent reaction field in the polarizable continuum model (PCM). Based on the calculation results, it is concluded that the configuration of the macrocycle of the C-i symmetry is in all cases the one most stable. The geometrical parameters and the charges on the atoms in the free and solvated states are calculated for this configuration of 18C6. To provide details of the intermolecular interactions between the solvent and solute molecules in the studied solvents, enthalpies of solvation Delta H-solv(0) (18C6) are calculated via quantum-chemical simulations using the PCM model. It is established that the solvation energy of 18C6 in alcohols is mostly determined by the universal interactions between the crown-ether and solvent molecules, and the contributions from the universal and specific interactions to the solvation enthalpy of the macrocycle are virtually equal in aprotic solvents.